View Poll Results: Opinions-Criticisms-or Appreciation.
Very Helpful
Maybe Helpful
Already knew This
Helped me with my car
An utter waste of reading time!
Voters: 138. You may not vote on this poll

4 Link Basics. Theory & Help Instructions

Old 03-25-2007, 12:57 PM
Senior Member
Thread Starter
Join Date: Feb 2006
Location: VA Hospital, Dallas, Tx (214 302 1924) cell-972-464-7400
Posts: 540
Default 4 Link Basics. Theory & Help Instructions

I was asked to make this post here, by one of the Moderators.
Previously I had begun this on another board and it got too large. I have since added quit a bit more to it.

Introduction to theory and descriptions.
Part 1

My first and very real suggestion to you, and to all racers is to buy some books on basic racecar chassis design and adjustments.

I will post three of the links to sources, which may be helpful for you to purchase. You will discover that if you research and read writings by different authors and builders, that they all do not use the same theories or the same terminology.

This can get confusing if you do not understand that some of the different terms are describing exactly the same thing.
Examples are;
Tire or chassis Separation = Anti-Squat = Tire Hit.
Negative separation = Squat = Not enough tire hit.
Neutral Line = Line of Theoretical Instant Center

Many years ago Alston Engineering, Chris Alston, developed his own theories and his ways of explaining. He also sold many chassis training tapes and also conducted chassis schools all over the country and the students came out with words and theories and descriptions which were different than what the other chassis men like Don Ness, Willie Reils, Don Hardy, Wolverine (Tom Smith), Ron Butler, Gateway, Paul Peyton, were using. Those guys used the same terminologies that Most of the older guys like me were used to, so we were all talking on the same page. We all could actually talk and discuss suspension tech on the phone and the other person knew what each other was meaning.

This is not Trashing Mr. Chris Alston, far from it. He has contributed much into racing suspension and chassis from where we all were years ago. Were it not for his clinics and schools, it is likely there would still not be many good chassis shops around. I give him thanks and appreciation for what he has brought to us all.

Now there is so many different ways to explain theories that I too get lost in their explanations. I have still never seen any benefit to using the Percentage of Rise theory. Yes it does work once you can figure what is your theoretical Percentage of Rise. None of the theories take into consideration the individual car's power differences or gearing differences etc. So to me it is all fictional or loose theories of what is an actual "Squat/No Squat/Anti Squat Line". No matter what fictitious percentages you call it the bottom line is you are still moving your Instant Center, up or down or forward or backward, just like we have been doing for ages. [Note; this is only my opimions.]

Now days, many of the chassis builders are using the words originally started by Chris Alston, and also some of the Chrysler Engineers, and now being frequently used by many who were schooled in them, and later picked up by Dave Morgan and others. It uses a theory called "Percentage of Rise" and the words of Anti-Squat". Anti-Squat is easy to understand.
"Percentage of rise is not so easy". The theoretical perfect Peercentage of rise isn't.

You can make either theory to work and to work right but I am more familiar with just using the Imaginary line of theoretical Instant Center (similar to Chris Alston's and others' Neutral Line. or the lower but, Parallel running imaginary, Squat/Anti Squat/No Squat Line than I am with using the Percentage of Rise, to explain things. I also can add that as big as Alston Engineering is that they have never had a Pro Stock car using their designs to ever be a winner or champion to my knowledge.. One time many, many years ago, they had a ladder bar car owned and run by the Yuill Brothers, that did win one Pro Stock race. However they did also do some great work on Dave Riolo’s car when it was first re-done years back and it worked very well. So you can do whatever you wish with that piece of worthless info. .


ETC Racing Programs


Tim McAmis chassis plotting software.
"Chassis Master"

Jerry Bickel Chassis plotting software.
"4 Link Wizard"
Plus get and read this treasure book;
Complete Guide to Chassis Performance Manual
"The Chilton Manual Of Drag Racing"

Dave Morgan
BOOK: Door slammers: The Chassis Book
Door slammers Chassis Tuning Package
(some good info here and some bad info too)


There are basic similarities when adjusting Ladder Bar suspensions, but you can only adjust Instant Center to be moved either up or down.

It is very beneficial for you to know or to learn just what happens when you adjust or locate any component of your rear suspension by using basic laws of physics. Also know and remember that with a 4 link that all suspension angles, measurements and adjustments rapidly or instantly change the moment you begin the launch applying forces of torque. Then it is almost as if the right and the left are seeing different adjustments because of the change in bar angles as the right rear tries to lift inside the right fender.

Anti-Roll Bars;
Anti-Roll bars do help a lot but they are another integral part of the entire rear suspension equation that needs to work in a balanced force with all other components of that rear suspension. Each piece is as critical as any other piece. Always remember that Anti-Roll Bars were not intended for adjusting Pre-Load. They are not named Pre-Load Bars, They are named Anti-Roll Bars, but with a few factory suspension cars like the Chevelle, Malibu, Cutlass etc and Mustangs the Anti-Roll bars can help when spring adjustments are not enough, and any bar adjusting would upset the rear alignment. On all other cars it is my opinion that they should always be adjusted to be neutral. I do not believe that Anti-Roll bars can be any benefit with a correctly set up and adjusted Ladder Bar car. I know there are many who disagree. There is no stronger torsion bar than a rear end housing if it is welded and is braced.

The other theory which is more commonly used is more simple in description and establishes a theoretical ideal "Point of Instant Center" or "Neutral Line" that uses an imaginary line from the rear tire contact point center that will be diagonal to the actual point of the car's real center of gravity. This is what we will use.

The Percentage of rise theory moves down below the actual center of gravity and then draws their imaginary
line to the tire contact point. For this article I completely will do away with Percentage of rise except just to talk against or to
compare to it.

Percentage of Rise (PR)
Anti-Squat (AS)
Neutral line (NL)

These are all words phrased in Chris Alston's Chassis Classes years ago and are now used by some shops that were trained in using them to describe what was taking place in suspension settings.

What happens when Torque hits Rear End from Driveline;
Know and understand that anytime torque is applied that your left rear tire will be trying to push down and your right rear tire will be trying to rise looking from the rear of car. All cars in any kind of racing or activity will exhibit this twisting motion if there is any form of friction coefficient existing between the tire rolling surface and the ground/track/road. The tendency will still be there to twist the rear with torque even with no tires or wheels. It is when you start to hook that it really shows up.

If your rear end was not attached with brackets, when the drive shaft rotates in the counter-clockwise movement (looking from rear of car), the entire rear end assembly would rotate counter-clockwise like a propeller if it were suspended in the air and had room to move. The drive shaft rotates in the direction that engine torque is applied to it. Even attached with bars and suspension it still wants to rotate.

Always keep in mind that As greater torque is applied the rotational force will be greater so as power increases the car will want to pull to the right, which we counteract by suspension preloading in adjustments. If doing any bar preloading do know that even only 1/2 turn is an extreme adjustment from neutral load position. We usually preload bars in increments of portions of a nut flat when we are close. 1/2 of a flat can make a car go straight that was trying to veer to the right. Too much bar preload will pull us to the left and quickly can get us into the wall.

Corner weight adjusting methods;

Physically moving weight
adjusting weight preload with springs or spring adjustments
adjusting preload with 4 link or ladder bars
adjusting preload with an anti-roll bar (not preferred).

A lot of times what is considered to be an ideal weight bias on the corners with car sitting in a shop is not close to ideal weight bias during launch and going down the track.

Funny thing about weight bias and preload;
What can make a straight launch can also cause some down track problems and unequal unloading while stopping. There has to be a balance of what launches good, drives good, and stops straight.

To further complicate it all is that the more power you launch with the more the car will drive to the right, so we add preload into the right rear and the left front to act as a counter force.

To make it even trickier is that if you have a car just right to launch good and stop and drive good, and you lose power or reduce tune-up etc, then the car will drive to the left.

The higher the horsepower is, the trickier it all gets.

Bar Pre-load ecample;
How many of you noticed that when Jeff Naiser was sorting his car out and running in the middle of the pack and at times not qualifying, that then his car was always driving initially to the left and would then get straight further down track. BUT when he was finally able to get his full power to the track and ran his string of low 4.50's and 4.40's that his car launched and ran and drove arrow straight. You see, his car was set up to run the number. When he was off on power it drove left. How could they have known what preload would work best when the car was able to do full power? Several reasons. Good educated guessing with much 4-link experience with that same car and lower powered engine. The first few times out the car ran and hooked pretty good and gave then indications of what to expect with it. Robert Fulmer is a very good suspension man.

Ladder Bars or 4-Link;
I personally prefer a 4-link set up to anything, IF I am the one doing the tuning and adjusting. If not I usually do a different suspension set-up so customers can make simple changes. Ladder bars are very simple and are easy to teach a customer how to adjust and when to adjust them.

Truth is there are many racers using 4 link that really have not a lot of understanding of physics and why to adjust what. I have seen racers adjust bars up and down going far too radical of a change pretty much with all adjustments. After I saw a question and answers to those questions in a Forum between two different owners of Pro Mod cars that were professionally built and I saw that the answer given could cause a serious crash is when I began writing this.

Most of the time minute adjusting can make huge differences. Simply a small change in rear ride height will alter the bar angles and the active point (Actual) of instant center. Slightly moving the front of the bottom bar and top bar down (lowering actual instant center, and at same time not moving it forward) can at times make a big difference in a car that tries to lift the left rear of the car like in a barrel roll, and also can help to keep the front tires from lifting as quick. This applies to cars with equal length bars. This quickie rule of thumb does not apply equally to cars with two different length bars. The effect will be similar but the Instant Center will also move rearward at the same time.

Contrary to what many of your chassis shops will set and their adjustments, what you really want is in most instances the bottom bar will be near to level while under hard torque and full acceleration, not when the car is sitting still with no torque applied. Sitting still it will likely have the front of bottom bar lower than the rear of the same bar, by just a few degrees.

Bottom Bar on 4 Links;
If the bottom bar is close to level before the launch and the left front side of car is rising and the tire is planting lifting the rear, there can be a very negative effect called "Barrel Roll". That is because with all the suspension movements the bottom bar on the left can end up pushing upward and the bottom bar on the right side is pushing downward. Oops! Can you say Whoopie!. Animal Jim's car was a good example of this, also Charlie Bucks, car exhibited that trait. I also had one that did that until I changed the bottom bar. Simply lowering the actual point of instant center, which would also place the bottom bar in a downward angle, can help eliminate this. Anti-Roll bars now also help this. If lowering Instant Center is the only desire in change while keeping actual distance of Instant Center the same, you must move both bottom and top bars the same amount in the same direction. You can also move the rear of both bars up one hole if it looks close to the same IC length when you plot it out on paper, computer or however you plot out your suspension positions. There are now computer programs that can help you quite a lot as I mentioned earlier in this thread.

If you will look under any top 16 Pro Stock car you can notice that the bars usually have not been in more then two to three different holes even from the very beginning. Most of the adjusting between the holes comes with the initial base lining while the car is new. Once a good overall baseline is found, most other tuning and adjusting to differing track conditions is minimal. Even slight adjusting changes make a big difference.

Bar Spread Bar Mounting Locations;

Someone asked about rear bar mounting positions and if it is important. on another board.

Yes it is but in my opinion at times some people make too big a fuss over bar spread in it's priority of importance.

A correct set of rear 4 link brackets will measure shorter from axle center from top bracket holes and longer on bottom for bottom bracket holes.

This is because of mechanical reaction time of top vs bottom and because of mechanical force or leverage of top vs bottom which is a multiplying factor of how much torque gets appied to either bottom or top bars.

A correct rear 4 link top bracket will also have holes for adjusting Starting at bottom hole forward of vertical axle center and each additional hole will be farther from horizontal and verticle center of axle housing. Meanwhile at the bottom the bottom bracket will have all adjusting holes to be in a straight vertical lineeither at vertical center, but usually slightly forward of vertical center.

The longer leverage at bottom will react quicker but not as much immediate force. The shorter leverage at top will react slower but with much greater instant force.

So yes' It is important.


Shocks for Drag Racing

I feel that good quality Double Adjustable shock absorbers are one of the most valuable suspension tuning aids there is and is money well spent. Cars running quicker than a 9.00 in the 1/4 really need them.
Side Note;
Coilover shocks well suited to street usage (no brand names) are not Drag Racing shocks. Shocks, and the good shocks that are primarily designed and built and tuned for Drag Racing absolutely will out perform any designed and built for the street coilover shock. You can still use the Drag Racing shock on street/strip vehicles though.
The 90/10 shcoks were designed to hook a 500 to 600 horsepower car on 7 inch wide old school tires on old tracks. hey in my opinion have no place on any modern day Dag Car including in the Stock Classes. Better ways to get better weight transfer when it is needed and not have to deal with the nose in the air position all the way down the track catching maximum wind resistance. Check the Stock Suspension Section.


The Chassis design software by Bickel Race cars & QuarterMax, should help you to understand better and to see on your computer what can change and the reaction when even small changes are made in bar adjustments.

I cannot tell you in a forum how to adjust the suspension of your particular car. No one can.

This will end the introduction to 4 link basics. Next are instructions to find your car's Center of Gravity without scales.
Cont --> to Part 2

4 Link suspension basics

Cont from Part1

Finding Car's Center of Gravity;

IN part of this section, I shall try to teach you how to determine what your actual front to rear weight distribution is by using stuff you already have, without having to go pay a chassis shop to scale your car if you are on a tight budget or there are no scales near you. Of course you are better off to scale the car but we have not always had individual wheel scales in drag racing so here is an old fashioned method that has worked for years in some of the baddest cars there was.

With 4 wheel scales it is easier to find the center of gravity and center of balance. You take the instructions I gave and use them in reverse to find where it is from the front to the rear. So if you know the total weight and the weight on each end then you will be able to calculate the percentages on each end with that info. Then you just use your wheelbase and figure for example 45 % of weight on the rear translates to 45 % of your wheel base from the rear would be the center of the car so that should be the balance point.

Write all of this info down so you will keep it in a log. Any adjustment changes you will write down and the results of them always.

where is your actual center of gravity of the car.

What is your actual wheelbase

what is your rear tire diameter

Place a weight in the driver's seat equal to the weight of driver. Load fuel tank, water reservoir and make the car balance the same, as it would be weighted while at the starting line.

get two jack stands. (I actually even place short angles on them so there is an actual point instead of the larger top of jack stand).

Now you will think I am nuts.

You are going to balance the entire car on only two jack stands, one on each side. Do not give up. The car will perfectly balance. Mark that place with a piece of thin "3M FinishLine Tape" (Buy at Paint and body supply) attached to each side of your car in a level vertical line. That is the point of front to rear balance of your car. Good to write this down by measuring from balance point to the center of both front and center of rear wheel hub.


Make a mark where a level vertical line is at the center of balance, to the ground or floor. Mark a level vertical line where the rear-housing center is. Mark a level vertical line where the front hub center is. All of these three vertical lines will be used to determine your actual front/rear weight distribution.


Measure your exact wheelbase from center of rear housing marked vertical line to center of front hub marked vertical line. Write it down.


Divide the distance measured horizontally level on the ground/floor from the rear housing center to the vertical level line that is at the car's point of balance by the measured wheelbase. That will = your rear weight percentage or percentage of weight on the rear wheels. Write it down.


Divide the distance measured horizontally level on the ground/floor from the center of front hub to the marked vertical line at car balance point by the measured wheelbase. That will = your front weight percentage or percent of weight on front wheels. Write it down.

Lower car to the ground and bounce it a few times to get it to be at normal ride height.

measure from ground level to center of camshaft. That should be close to the height of center of gravity. Write this measurement down.

At the exact place previously established as the car's balance point, at the height of the center of camshaft, will be your
theoretical point of center of gravity of your car. Mark that spot on the door or fender by crossing two thin pieces of tape in an
X. Take a picture of side of car with tape on it.

Measure all the discovered dimensions, including the measurements from front axle and rear axle to that place where the car
perfectly balanced. Write all this down in a notebook to keep.

With someone helping stretch the thin tape from the place you determined was the center of gravity of your car, in a downward angle to the bottom and center of where your rear tire is contacting the pavement. This will be the Theoretical Instant Center line (Neutral Line). You will keep this imaginary line in your mind forever once you begin using it. It is only a point of reference for you. Take a pic of tape positions on car from the side.

Take a picture of the side of your car with the tape showing the angular line from the bottom of rear tire to the place marked to show cam height at the balance point that we are calling the car's center of gravity.

That imaginary line is to be used as a reference as theoretical "Line of Instant Center". Some call it "Neutral Line". The actual (or active) point of instant center is the actual point where a straight line extended from your top bar and your bottom bar will intersect in an imaginary line looking like a triangle shaped like a Ladder Bar.

Use the narrow 3M Finishline tape, which you will find at auto paint and body supply houses or paint stores. If you use a magic marker, or pen or pencil to mark on your car you will have a nightmare removing the marks. Finishline tape will stick on and will easily peel off.

Place the 3M Finishline tape in a vertical level line on the car at that point of balance. Which is easier when you have scales to weigh the ends with. At the camshaft ht place a piece of the 3M Finishline tape in a horizontal line at camshaft ht. That X where the two pieces of tape cross should be your Center of Gravity of the car. That is Center of Gravity. That is not where you want your Instant Center. I use Center of Gravity as my reference point for pretty much everything related to suspension adjustments.

Adjust Instant Center & WHY;
As you move your actual instant center further forward or below that Theoretical "Line of Instant Center" (Neutral Line) (It is an imaginary
angled line, where you had your tape) The rear suspension hit will decrease, but the front will have a tendency to rise on it's suspension. Likewise, If you move your actual "Instant Center" back or up toward the Theoretical "Line of Instant Center", the rear suspension hit will increase.

As you move your actual or active Instant Center point forward of the Imaginary line of Instant Center (Neutral Line) you will promote more front suspension lift and less rear tire hit.

As you move the actual or active Instant Center to the rearward you will give it more tire hit and greater rear suspension reaction and less front suspension lift.

As you adjust for increased rear tire hit, you will be at the same time be adjusting from less effect of actual bar lift in the front.

When you adjust for decreased rear tire hit, you will at the same time be adjusting for more effect of actual bar lift in the front.

If you reach the Ideal, as the tires hook the car will rotate on the rear axle and will also try to lift the front, as the car is moving forward.

Hopefully you will know that a low torque engine will be needing more adjusted Hook or Tire plant than will a high torque BB car. As the power level at launch increase you will need less and less adjusted hook in your suspensions. Sounds strange but that is the way it is.



Contradicting my words;
That simple explanation is and can be misleading. Best example is the old Chevy factory 4 link. According to suspension theory that car should be very wheel stand prone, because the point of actual instant center is far forward in front of the car and is also below the Theoretical Line of Instant center. Problem is it is so far to the extreme forward that only a tremendous amount of power with huge tires to hook could possibly get it up. That car's factory tendency is to actually lift the rear tires under hard acceleration is the reason it squats. Only changing the upper bar angle to a lower front position, or moving the front of bottom bar up or the rear of bottom bar down, will give major rear tire hit on that car. Now you could also make a way to change the forward and/or rear lower bar mounting position and in essence end up with a factory but modified adjustable 4 link. Lol. Rear shock dampening control is needed after making any extreme bar angle changes to that style of suspension. You can also help those cars by lowering the rear ride ht. That does change the bar angles some.

There is so much more to say but there is just not enough room in a forum to get much deeper than I have.

Times past in suspension adjusting & plotting
In the older days we used to draw out all the dimensions I just discussed on a shop wall or on a shop floor to plot out the different bar angles. Now you can use computer programs. I do still do it to scale on a piece of paper though when no computer is present. Actually by now it is pretty much in my head of what to do based on results observed by Video Camera seen in slow single frame advance or with data provided on shock travel. In really extreme bad conditions watching the car can tell somewhat it needs, but be aware that things are happening quicker than your eye can see. Best is a video shot at an angle covering whole car from left rear corner. A good quality video shot from left rear at an angle will tell you much when play back one frame at a time.

Please try to learn as much as you can about this subject if you are tuning your own 4 link. It can get dangerous if you go too extreme.

Also remember also that after your run the 4 link adjustment and rear components are also important. You do not want severe rear suspension unload when you back off it either. When you cross the traps you do not want to hit the brakes and experience severe rear suspension unload or if you pull a chute. We may later get into using the actual car's center of gravity calculated at speed to determine where to place the chute tether attachment point.

One other point to make.
If you set your preload with a top bar do not get wild and crazy. Before you set preload with all weight in the car you should be able to easily slide the front bolt out after removing the nut. One top bar will be used to set and adjust pinion angle. The other will be used to adjust preload to preload the left front/right rear weight bias. With this adjustment a little can make a big difference. 1/2 turn of bar pre-load is extreme.

If you are using the top right bar to adjust pinion angle, the top left bar will be your pre-load bar. In this example you will
lengthen the top left pre-load bar to pre-load weight to the left front and right rear tires. Be careful just a 1/2 turn of pre-load makes a huge difference. Usually even a fraction of a nut flat can be noticed in your handling.

If you are using the top left bar to set pinion angle, then the top right bar will be your pre-load bar. To jack weight in
pre-loading using the top right bar you will make it shorter. Again just 1/2 turn is a huge difference.

Bottom bar length adjustments are used only to center the rear in alignment with the chassis

The rear and the front springs should be no stronger/heavier than what is needed to support the weight of the car while sitting at starting line. With strong and fast cars rear double adjustable shocks are a must.

How do we know if we have the correct spring rates;

To begin with we need to understand tha the spring in a drag car has only two functions.

Support the weight of the car up to the starting line.
Give as smooth of a ride aas possible while going down the track.
Possibly a third function is to support the additional weight during weight transfer.

If you can not adjust lower spring adjustment enough for there to be 75% maximum, to 62.5% minimum of the total shock travel inside the shock while car is sitting at rest with all weight in it including driver, you have the wrong springs for the car.

Usually a 2900 to 3000 lb car will want between a 130# to a 145# rear spring on it, but that is never absolute. I know Outlaw 3,000 lb cars working well with 150# springs even. You can not go by just the rear weight to guess what springs you need. Too many other thing figure in, such as the exact shock mount placement in relation to center of axle, Shock mounting angles if any,

You let the car tell you what it wants and you can do that without ever going down the track. At all times you want to keep enough compression available for car to sink on shocks some when front weight is transfered to rear by wheelstands and/or accelleration forces which shove a bunch of weight to the rear even if no wheelstand. You also need enough compresion to allow a good smooth ride down the track. Bottoming or topping a shock out is a sure fire way to unload tires and if at high speed to crash.

Support car on all 4 wheels so you can get under it. Disconnect anti_roll bar on one side (if equipped) just to be sure it is not interfering. adjust the bottom spring adjusters all the way even on both sides, doesn't matter right now how high or low. Measure how much of the shock ram is sticking out of the shock. If more than 50% of the total travel is outside the shock, you need to adjust spring adjusters down so that you will only have appx 25% to 37.5% of the total shock travel sticking out. If you can't and at all the way down on adjusters, that means the spring is too stiff. If you have adjusted the lower spring adjusters as high as you can and you still can not get at least 37.5% to 25% of the total shock travel to stick out of the shock, that spring is far too light and the shock will bottom out while lanuching and while going down track when it encounters any sort of bumps.

The more rear travel the shocks have available the bettter it is to tune. At one time people were using shocks with 5 inches total travel in the rear. That does not allow much room for any error. 6 inches is far better and over 6 inches is even better. Problem is that the longer travel shocks do require more room to mount.

I will use the 6 inch travel shock as my example;

Ideal will be 1.500 to 2.25 inches of ram inside the shock I can not accept anything else if I am tuning the car.

This tolerance does allow it to work with more than one spring rate spring becuase of the lower spring mount adjustability.

I do believe I will be adding this into my 4 link article at Racing Junk. I did touch on it but not this much.

Hope this helps you.

Oh' A ladder Bar car will need to hit the tires a little harder than a 4 link car will becuase of the difference in how they work.

If you are also expermenting with power adders etc. Know this, no matter how crazy and backwards it wiill sound. The more you increase power to the rear wheels the less suspension action you want front and rear.



With any rear suspension setup, changing the ride height does change the way the car reacts.

Fortunately now there are several computer programs to aid in plotting 4 link adjustments, and those who are using them are getting good results if they can just understand the program and how they work. That is the easiest way for a novice.

Long or short bars. Longer in my opinion is better, But years ago Larry Kopp running out of Wayne County Speed Shop showed up with an Oldsmobile Firenza with a different cutting edge range of ideas all on the same car. He had 4 link bars that were quite a bit longer than the normal 20 - 22 inch bars, and the car did not work very well. I do not think it was because of bar length. I think it was that the complete over all set up was so radical that other parts of his suspension were working against him. They ran that car only one year. I have so many photos of that car. I do think several of the ideas on that car were really good but bottom line is it could qualify but could not win. They went to a more conventional car the next year and began winning again. Amongst everything else they had the rear coilovers operating in a configuration like a rocker arm like a Can Am car.


The lower the Instant Center is the less effect it has on rear suspension torque reaction. The higher it is, the greater effect it has. By theory the further forward the Instant Center is the more the front will lift and the further back the less it will lift. As I said that is just theory that in extremes just doesn't work. Weight Transfer does still work by the front rising but in most cars today we do not want the front to rise on it's suspension any more than it is necessary. I promise you that when the front tires are off the track you have all the weight transfer you can handle. Spend more time on making the rear to work right. Once you have that you can do some extra fine-tuning on the front if you need it. Most True 10.5 cars and Radial Tire cars do need some lift of the front for weight transfer. Bigger tire cars don't.


I have just mentioned some of the beginning basics here. Hope I helped some.
Did that answer some your questions or did I get too far off the topic?

4 Link suspension basics.


One thing I forgot to suggest to you.

When you do get a good suspension baseline on your car I imagine that the point of actual instant center will be either right above the ground, a few inches above, or it could even be buried several inches into the ground and behind your rear engine plate, probably somewhere around the rear of the door and near the ground or between the ground and your frame height. Time will tell.

The measurements I told you to write down about from the point of balance to the rear axle and to the front axle you can use to figure what percentage of your wheel base is forward of the point of balance and what percentage is to the rear of it. Once you know these two percentages and you know the vehicle weight you will then know the actual percentage of weight on the rear and on the front and the amount of weight on each end of your car.

By experience I can tell you that 55 % on the rear is not a good idea. Lol. Not unless you want a 140 mph wheel stand when you change gears. . My opinion is 47 % to 50 % is much easier to tame. I know guys with serious Outlaw cars that are placing ballast forward to even bring theirs to 46 %, trying to keep the front down some. What is right for your car is what you find that works.

Soon I will tell you how you can altar your front/rear weight distribution by the way you install your shocks. Don't be surprised if some of this sounds different. Lol. Maybe we will get even deeper into this if you all wish for us to do it?


Do understand that I am not a Chassis Man and only work with chassis of my customers that are local. For years I did my own too. My experiences came as a racer in what was at one time considered to be a high horsepower car. I did go through an Alston Chassis Clinic years ago, but was mostly in disagreement with them the whole class.

I had a car that was a handful at the time and It did not take me real long to learn the things to not do.

I also was blessed to have at that time in my life some friends in Warren Johnson and Larry Olsen that helped me along quite a lot. That car had a tendency to pic the front end up when I shifted to high gear at around 140 to 144 mph. Although not real high, I can promise you that loosing sight of the track at 140 mph plus is just not a fun type of entertainment. BIG PROBLEM with it was the torque and power was violent even just on motor and the rear to front weight distribution was asinine and set up too much to the rear by my ignorance. I had 55 % on rear tires when that car wanted 48%. It was very wheel stand prone and initially also wanted to do a barrel roll like those of Charlie Buck and Animal Jim.

The theories or fact I have written came from a lot of doing it wrong and learning what was needed instead. I learned what to adjust and the effects of the adjustments to get the barrel roll out. That was before there were any Anti-Roll Bars.

I learned too quickly by busting a new high dollar Mark Williams Aluminum Rear about what Dead Hook with too much traction can do with high horsepower. I learned by experience how to not do a 4-wheel stand. Lol. That was not fun either because it too cost me a lot of money for repairs. I also learned that there is more than one way to do a 4-wheel stand too. I also learned the thrill of a 3-wheel stand. Not good when that other rear tire catches traction again. Can you say INSTANT TURN?

I have not driven a racecar in years but now and for several years my learning curve has improved by working out bugs with customer cars.

Something that seems strange but is fact. It takes less rear suspension reaction with a high horsepower car than it does with a lower powered car. So for example if your car is launching perfect with a lower powered engine. When you put the big power to it the car will want to over react and you will need to tone down the rear suspension action by bar adjustments to take hook out or by shock tightening or by maybe even both. It sounds odd on the face that a lower powered car will work better with more radical suspension adjustments but that is the way it is because the higher powered car is transferring a ton more of torque into the suspension and tires with not a lot of reaction.

Another pet peeve of mine is cars that slam down hard on wheelie bars. If the suspension is right the wheelie bars will ride along as a safety item and will barely touch down if at all. Do not try to control your wheel stands with lowering the wheelie bars. Adjust hook out. I do like wheelie bars and am against the race rules that do not allow them but it is for safety reasons in a just in case situation.

I ran my mouth enough today.


The dangers of internet suspension advice.

This just came up on another board and I was freaking. Well intentioned people were trying to give someone 4 link advice by telling them which exact holes in the brackets to move the bottom and the top bars to for adjustments. The advice was ranging from, put the top bar in the 4th hole down and move the bottom bar in the end hole. And several other well meaning but very dangerous pieces of advice.

Not all 4 link bars or brackets are made or mounted the same. Some bar rod ends and bracket holes are 3/4, some are 5/8 and some are even 1/2 inches. All bolt spacing between the holes in the brackets are not the same. All rear brackets are not the same. All bar lengths are not the same. Some bars have shorter top and longer bottom bars. No one can give exact advice to adjust blindly by his or her knowledge or by guessing.

Radical wrong adjustments in a 4-link car with high horsepower can literally kill you.

Now here was my reply and I hope I made no one mad there.
Subject: No one can tell you how to adjust your 4 link on a forum

[Edit] Date Posted: 13:31:40 12/06/05 Tue

Except possibly the one who is familiar with your individual set up and they know the car and where everything is mounted and aligned.

I could look at a video tape of your car up close, frame by frame, launching and be able to give advice about how or where in relation to the place everything is now mounted and adjusted, but there is no way that I or anyone can tell you which hole in your setup can work or which bar or which end needs to be adjusted.

I can suggest moving your point of active instant center forward, backward, up or down. I can suggest even shock adjustments by viewing a video tape but I will clearly state, I nor anyone can tell you which hole position will accomplish anything by guessing and on the internet.

Sorry if anyone gets angry with me about this but messing with 4 link in the dark without knowledge of all facts is one of the most dangerous things than can be done to a high powered race car. The wrong move too radical by mistake or following mistaken advice can
kill you.


one ? i have is all the same theroies apply with ic and everything with a ladder bar right???__________________
Up or down I.C. would be the same basic concept.

With a Ladder Bar you cannot move I.C. forward or backward except when first building the car and setting up the bars. You can buy ladder bars in different lengths though. Also with a good braced rear end and using Ladder Bars you have the world's strongest anti-roll bar in my opinion. I know that some chassis guys do not agree but I see no reason for anti-roll bar on a ladder bar car. You can adjust left to right pre-load in a ladder bar car. It is just not as easy to do. You can adjust spring tension on any suspended car no matter what kind of bars you have.

The Torque Arm cars do still use the same laws of physics and do also react the same to I.C. Changes, but the location of where the I.C. is is not figured the same.

Figuring the center of gravity is the same on any vehicle. Easier done with scales but can be done with no special equipment.

Jake & Others about the Parachute Tether location;
I feel the the tether should be at cam ht or slightly below cam ht if at all possible. It is correct to think that it should be near the center of weight ht of the car. I have seen too many cars unload the front or the rear when the chute pops. I hope that everyone knows to drive into the chute under power? Then you let off , or hit neutral or whatever and begin braking. Hit the chute prior to the finish line to give you a little extra time in case it does not open and you must use brakes. If your brakes are a hard pedal or do not stop the car well, that is another subject we all need to discuss. I have seen more than one car built with incorrect brake components and even incorrect pedal ratio. A car should be capable of stopping in emergency with no chute. The Chute makes it safer. Boy, did I learn that the hard way.

If you have a car or are checking a customer's car get a vid or several fast frame stills of the car as the chute is deployed and opens. Watch the front suspension and watch the rear suspension and the tires to see what is taking place. Nothing should unload or actually change just from the chute opening as far as suspension goes. The vids and pics will tell you what the car needs.

Torque Arm Suspensions I.E. Late GM F Body and G Body w/Torque Arm.

Question was asked of Mr. Billy Shipe, and I began the responses;

I am not Mr. Billy, but I can tell you that it does.

Torque Arm suspension modifications are a different sort of animal and are different form of suspension technology but still using the same laws of physics.

They still utilize what we call an Instant Center, but moving it and adjusting it is different. Also ways to control rotation around rear axle is different.

Let me see what Mr. Billy has to offer before I run my mouth anymore. Despite he and I not agreeing on many things, he is very intelligent and knowledgable. This I know.



Location: Orlando, FL
Posted: Mon Jun 18, 2007 7:27 am Post subject:


Thanks, Ed.

Page 27 of my blog

has a spreadsheet for determining the IC location for a torque arm. Pages 4, 11, and 28 help in making sense out of the results.


There are many companies making aftermarket Torque Arms for your car. Many of those are no different suspension wise except are made to be stronger.

I like the ones made by Steve Spohn and Spohn Engineering and those made by Brian at Madman Racecars or Thunder Racecars in Louisiana best. The one made by Wolfe Racecars is similar to that one of Spohn Engineering but for much more miney.

S&W Racecars Torque Arm for your car is 12 inches shorter than the factory one is. All of the good ones are shorter than the factory ones are. BMR has one that doesn't look bad too.

If you do buy an aftermarket Torque Arm do not buy one that is a replacemnet for the factory one. You do not want the Torque Arm appliying torque to the transmission like the factory does.

Most everyone that sells Torque Arms also sells adjustable bottom control bars and also pieces you can use to modify the rear of those bars bracket mounting positions.

Steve Spohn races a Gen 3 car himslef. Brian at Madman or Thunder Racing builds and tunes his bars to his customer's cars.


New member Unk for their protection


Am I missing something here? I don't own a car with a torque arm suspension but, correct me if I'm wrong. Doesn't the rear mount of the torque arm on the housing attach above and below the axle centerline? If so, doesn't that mean the I/C is dictated by the location of the front mount of the torque arm and the lower outside trailing arms have nothing to do with the I/C location? The forces of the housing rotation are directed through the torque arm like a ladder bar, correct? .



You are correct. Almost.
Earlier I said the Torque Arm Suspensions are using the same laws of physics but we apply them different, or something like that.

The torque arm is applying all force originating from a central or center part of the rear axle housing. The outside of the housing is where the tire friction resistance is and it is the outer parts of the housing that will react to that force by pushing forward on the lower bars.

I know it doesn't make sense at first. In real practile usage with the same torque arm, same shocks and the only change is the mounting ht of the rear of the lower bars, by changing them to be lower in the rear it will give a harder hit on the rear tires. Go figure lol. That is why rear re-positioning brackets are made and sold. They make them bolt in or weld in. Only moving the lower bar mounting positions does also change the Instant Center.


Mike Peters
New Member


Thanks Ed. I appreciate the insight. I assumed the lower links acted with force vector you're describing but, assumed the I/C was dictated by the front mount location of the torque arm. The lower span of the torque arm would still have a force vector acting upon the launch characteristics though possibly not near the influence of the lower outboard trailing arms. Seems like this could be a source of suspension bind due to the unequal lengths and plane differences, no?



In an earlier post, I simply referred to a spreadsheet at my site which calculates the IC position. But, of course, the spreadsheet doesn't give any explanation.

The IC of a torque arm suspension is located at the intersection of a vertical line through the torque arm contact point and another line through the pivot points of the lower links. So, as Ed pointed out, if you lower the rear pivots for the lower links (or, for that matter, raise the front pivots), the IC will move up. This means more anti-squat, or, as Ed put it, a harder hit on launch.

Mike Peters
New Member

Agreed. Just like the SSM bars for the factory 4-link cars. Does the force transmitted through the lower span of the torque arm not come into the equation? Is it negligible because it is located on the centersection of the housing?



My theory. Not scientific fact;
Mike as you pointed out, The bottom ataching point is above the axle centerline and the lower attaching point is below the axle centerline. BUT BARELY.

Many people do not take this into any consideration with 4 link or ladder bars or 3 link suspension. Leverage Means a lot. A typical 4 link will have the rear attaching point of the top bar a few inches above the axle center but will have the attaching point of the bottom bar several more inches below the axle center.

Consider the axle center as a pivot point;
The further away from the pivot point you put attaching points, the less leverage the twisting center will exert on that point of force. It will move quicker but with less force. I.E. Bottom bar of a proper 4-link or bottom bar side of ladder bar or bottom bar os 3-link.

Again consider center of axle as a pivot point;
Now the attaching points are much closer to center than the previous example I.E. Top bar of 4-link or 3-link. The force exerted on that closer in to the center attaching point for bolts, will be much greater and will move slower as the axle is trying to twist in the suspension joints.

Now again consider center of axle as a pivot point;
Bottom attaching point is far from the axle center but the top attaching point is somwhat close to the axle center, I.E. Ladder Bars. The torque force exerted on the top part of the ladder bar is not very great with little movement and the bottom force exeted in the axle twisting motion is tremendous pushing it as a solid bolted on part to swing radically upward as axle rotates.

Lastly we have out little forgotten Step-child of Rear Suspensions the Single Torque Arm [Which I do like once the mods are done and better bars used] ;

Rear top attaching point is barely above axle center, so when axle tries to rotate not much force or speed is exerted on that attaching point. Bottom raer attaching point is just slightly lower than axle center, usually near the lower part of front axle section. Not much torque force is exerted on the bottom section, either but more than the top section, The solid mounted Torque Arm willo move as one with the rotation of the axle center section, but not with as much leverage force on the lower section of it, like a ladder bar would have.

Anyone forgot the Lower Bar on the Torque Arm sYSTEM?
nOT ONLY DOES MOVING THE MOOUNTING POINT OF THE BAR CHNAGE THE EFFECTIVE ic WHEN YOU LOWER IT IN THE REAR YOU ARE TAKING SOME OF THE TWISTING FORCE LEVERAGE AWAY AND TRANSFERING (Caps) that same force back into the Torque arm because if one suspension member loses leverage over the other that one will gain an increased leverage advantage. BUT with the now higher IC because of the lowering of the rear of the bottom bar the movement it willl exert will be a quicker action.

Factory Type Front Suspensions;

If you must use the factory stock suspension components;

Ditch the 90/10 's. If it still has a front anti-sway bar remove it. If you need front lift, remove control arms and measure length of bushing with teeth. Grind teeth off of bushings. Hone inside of bushings so the bolt is a light slip fit. Lube it all and install control arms with modified bushings using shims or washers as shims to take up the space the teeth you removed occupied. A good Variable Rate front coil as made by MOOG etc for the SB car should also help.

For shocks a good adjustable front from Strange is great but you can also do it poor boy style by either buying the lightest duty cheap shock you can find or by going to junk yard and trying the shocks by hand until you find two with similar feel that is not worn out but is not super stiff either.

All of that extra work will give you much quicker front suspension rise if you need it than the 90/10 gave and you can better control the car when it comes down and it will not unlad the rear tires plus it will handle much better all the way down the track.


I hope I am not further confusing you in all of this.

Think Instant Center. Think Bar leverages. It is all the same thing whether we are talking about ladder bars, 4 links, 3 links Torque Arms, as far as how certain laws of physics actually control it all, we just need to learn new tricks to stay in the game as technologies change.


edvancedengines is offline  
Old 04-24-2007, 04:50 PM
bjuice's Avatar
Join Date: Apr 2003
Location: Greenville s.c.
Posts: 3,236

AWESOME ARTICLE !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!


bjuice is offline  
Old 04-27-2007, 12:54 AM
Senior Member
Thread Starter
Join Date: Feb 2006
Location: VA Hospital, Dallas, Tx (214 302 1924) cell-972-464-7400
Posts: 540

Thank you very much Brian.
I really do appreciate that.

edvancedengines is offline  
Old 04-27-2007, 07:19 AM
Senior Member
Join Date: Dec 2006
Posts: 272

Well Mr. Vanceengines.....
Looks like you have been around the block a time or two yourself.
It really amazes me that an engine builder can give out such good information and has a good handle on chassis setups and what they are doing. More so than a lot of "chassis builders " I have run into in my days..

I have printed out your article to read, 17 pages, as my eyes are not as good at reading computer screens too long, and have looked through it quickly, but will re read it more in depth this weekend.

There is not a lot of information out there as there should be on chassis setups and designs, and anyone who is serious about these cars should pick up as many books and videos and any good articles as they can find.

I do agree with you that it is very hard to tune chassis over the internet, and care must be taken giving such advise. Someone who comes on a forum and asks," my ladderbar car goes to the right when i launch on trans brake, what should I do?" , has no simple answer. Not seeing the car, it could be as simple as a little preload, or the whole chassis could be out of wack and need a major overhaul.
But I find most guys these days want quick simple answers to all their car problems, and are unwilling to read and learn. These guys will always be running in the back of the pack trying to catch up in all areas of their cars.

Anyway, thanx for posting this bit of information, with your permission, I would like to print it out and give it to my customers who want it, if that is ok with you.

I see you also post over on the yellow bullet forums, a place were "angels fear to tread", :shock: I will see you there somethime.

And as for your signature sir, "may the Lord bless you and keep you, all the days of your life"
mcracecars is offline  
Old 04-28-2007, 07:03 PM
Senior Member
Join Date: Mar 2007
Location: Texas
Posts: 525

edvancedengines if I comprehend 1/3 of your post I will be headed to the front of the pack. That is very good reading and after printing it out i have my first book on chassis and suspension. Thanks for the grest info. Wesley
woodsman is offline  
Old 06-15-2007, 10:02 AM
Junior Member
Join Date: Jun 2007
Posts: 1
Default Re: 4 Link Basics. Theory & Help Instructions

edvancedengines, you have hit the nail on the head with your info on 4link setup. Good job!

I'm not a chassis guy. My company developed the original 4-Link Wizard program and some others, plus the original edition of the Bickel chassis book (in conjunction with JBRC) and the Reher-Morrison engine books (in conjunction with RMRE).

Your explanation of how the car might behave with an IC in front of or behind the front-to-rear balance point is the clearest I have heard. Should be very helpful to those in need of a starting point.
softwareguy is offline  
Old 06-15-2007, 11:21 AM
Senior Member
Join Date: Apr 2007
Location: Mississippi Gulf Coast
Posts: 124


Thanx, ED. Obviously you do it all, and VERY well! A racer's Racer!
alscoupe is offline  
Old 11-17-2007, 09:07 PM
Join Date: Sep 2006
Posts: 62

as said as hp and torque increases, the car will pull to the right. exactly what my car has done when going from 800 to 1000hp. the queation i have is when adjusting the preload from a neautal position by half a flat, do you mean shorten or lengthen the bar to acheive the preload?
deepockets is offline  
Old 11-18-2007, 02:45 PM
Senior Member
Thread Starter
Join Date: Feb 2006
Location: VA Hospital, Dallas, Tx (214 302 1924) cell-972-464-7400
Posts: 540

the queation i have is when adjusting the preload from a neautal position by half a flat, do you mean shorten or lengthen the bar to acheive the preload?
If top left bar is setting pinion angle;
You make the top right bar shorter to increase right rear and left front pre-load.

You may not need 1/2 nut flat even. Only you can tell how hard it is pulling.

If top right bar is setting pinion angle;
You use the top left to adjust pre-load on the right rear and left front tire by increasing the length.

edvancedengines is offline  
Old 05-07-2008, 05:59 PM
Senior Member
fla1976's Avatar
Join Date: Aug 2007
Posts: 1,523
Default Chassis

Great article. It clearly defines how to go about setting up a chassis. I printed it out also as a reference for chassis setup. Thanks for the effort and information.
fla1976 is offline  

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -