Ill handling decelleration
 

My partner and I took his freshly built Vega to the track Saturday for shake down runs and discovered a problem.

The car rolls from side to side on decelleration. It goes straight but it can be a little unnerving ay 140+ mph. This happens as soon as you let off the throttle, before braking. Brake application doesn't seem to affect the rolling. It keeps up the motion until you're almost stopped.

The car accelerates hard and straight all the way down the track, but as soon as you let off of it, the rolling begins.

We thought it might be spring/shock adjustment, but everything is adjusted properly (tripple checked). Front and rear spring rates measure out evenly and so to the shocks. There does seem to be more suspension travel on the right front corner which we can't account for and that may be at the heart of the problem.

Anyone have any thoughts?

Topping the shocks out is very wild on the big end. look to see at ride height how much shock is showing between the bumper and shock body. If you have 3" travel shocks or near that I never run more than 3/4" of ram showing. I use the travel for tire application not bump which the bumper does plenty well by it self without much shock travel in the bump or compression. If that checks out the next it a broken or weak spring in the front. A scale report will detect this easilly. Excessive preload put in the car will permit the car to behave seeming well under acceleration than wild under decell. Is the driver 500#? Could your car be built lob-sided, are the shock mounts square to each other and square to the rear suspension brackets? If you are within .030" that would be ok.... check these things including tire pressure front and rear too and you should have the problem by the neck.javascript:emoticon(':shock:') I hope this gives you something to start with.

Another thing you could check is the shock valving on each shock. Take each shock off and if it's a coilover remove the spring and fully compress then fully extend, change your shock settings and see if it changes if not you may have a bad shock. I had this happen once brand new shocks and one had no resistance on rebound compared to the other. Just a thought.

Beast477 is correct, I forgot about that issue potential. having them adjusted differently side to side will raise Hell to. Sounds like the answer is in the question. The right side travel being greater needs not to be glossed over.

IT COULD ALSO BE FRONT END ALIGNMENT WHEN YOU LET OFF QUICKLY IT THROWS THE CASTER/ CAMBER OFF ENOUGH TO START A VIOLENT MOVEMENT-YOU MIGHT ALSO CHECK FOR A CRACKED BRACKET WELD ON THE SUSPENSION[ SAME THING IT WON'T SHOW UNTIL THE CAR DROPS BACK DOWN]
UNEVEN AIR PRESSURE ON REAR SLICKS OR FRONT TIRES

I WOULD CHECK EVERY POSSIBILTY BEFORE YOU PUT IT IN THE WALL OR ON IT'S ROOF!!!!!!!!!!!!!!!!!!!!!!!!!!!
SOMETIMES A SIMPLE LITTLE THING CAN KILL YOU VERY QUICKLY
LIVELY

x3 on the shock setting, or bad shock.

Sounds like bump steer to me. If this "freshly built vega" included any mods to the front steering/suspension bump steer is the likely culprit. If the front end geometry isn't correct the toe will change as the ride height of the car changes. When this happens it will steer the car in different directions as the ride height changes.

though about bump steer, seems it would happen at gear change also. could be an acute case of tank slap, or death wobble, caused by low air pressure in the slicks also.

Thanks to everyone for your thoughts and suggestions. We have a lot of things to check.

Did you check that right front shock off the car? Even if they are adjustable, if there is a bad seal they will do this.

Other than that, Tell us about your suspension. Stock, Ladder, 4link? Are spring/shocks set narrow in the rear? What kind of lateral link do you have, and how high or low is it? Anti roll bar? Pre loaded weight on cross corner springs? Spring rates?

With out knowing much about the car, my list of maybes would be bad shock, preloaded right rear and left front spring, no anti roll bar front or rear, too low a instant roll center in the rear for the center of gravity.

I would have to agree with gearhead. I think its going to be bump steer. I don't know why but I've seen more Vegas have this problem than any other. It may be because of the placement of the rack n pinion. I had one we fixed that when you jacked it up you would have 3/4" on tow out. The car also had 1/2 tow in if two healthy men sat on each fender. The tie rods where at too much of an angle. We used dirt track tie rod adapter stud. You got to start somewhere. Let us know what you find.

IF it is a 12 bolt GM the tubes could be slipping in punkin. Later J.Smith 4621 racing

poss. tire balance
 

had problem at one time after installing larger engine and car went faster found rear tires out of balance., got balanced and corrected problem , we had pulled complete front end apart,sent parts all over the country to have checked. and it came down to tire balance. some times we look for the hard problem, when its the little one we forget. good luck

Ill handle in decel
 

I see some comments about bump steer. If it were a bump steer problem wouldn't it show up when the front end settles after the launch? I am very interested in what you find your problem is because I have a car doing the same thing. I did find on mine the left lower control arm was very tight. I haven't had it back out yet to see if this fixed my problem. I'll let everyone know if it does.

Thanks TS1955

Re: Ill handle in decel
 

Your car lifts and falls during launch but is still lifted higher under power than setting while stopped. You magnify that when you lift at the finish line. Not only do you have air pushing down on your hood but your car may fall abruptly when you lift. If your tie rods have to much rake they will cause toe out or toe in as your front end goes up or down.


Most car builders design their cars so that the effects of bump steer are minimal. However, you must still take care to bolt on your suspension carefully so as not to create unwanted bump steer. Make sure that you are always using the correct components for a particular car. Bump steer must be designed into the car and cannot be adjusted out if improper parts are used or if pivot points are moved without considering bump steer design principles.

In order to accomplish zero bump the tie rod must fall between an imaginary line that runs from the upper ball joint through the lower ball joint and an imaginary line that runs through the upper a-arm pivot and the lower control arm pivot. In addition, the centerline of the tie rod must intersect with the instant center created by the upper a-arm and the lower control arm.

The instant center is an imaginary point that is created by drawing a line from the upper a-arm ball joint through the a-arm pivot where it is intersected by an imaginary line that extends from the lower ball joint through the inner control arm pivot. Where the two imaginary lines intersect is the instant center.


Sounds complicated? Really it is very simple. To achieve zero bump the front end must be designed correctly. The tie rod must travel on the same arc as the suspension when the car goes through travel. Simply matching lengths and arcs to prevent any unwanted steering of the front tires.

To exaggerate, if the tie rod were only 10" long and the suspension were 20" long then when the suspension traveled the tie rod angle would shorten much quicker than the suspension arc. In this scenario the tie rod would shorten much quicker through travel than the suspension and the car would toe in drastically over bumps. The shorter arc of the tie rod would pull on the spindle and toe it in through travel.

Bump Simplified

When designing a car, if the centerline of the outer tie rod lines up with the centerline of the lower ball joint, and the inter tie rod lines up with the lower pivot point then the length and angle of the tie rod and suspension will be the same resulting in zero bump. Most car builders design their cars in this way.

^^^^^^^^^^^^^^

good post.

dparker,
Thanks for the explanation on bump steer. I looked at my car and it seems to be OK. The outer tie rod is in the center line of the ball joints & the inner is in line with the control arm pivot point and is parallel with the lower control arm.

Thanks TS1955

I agree with the bump steer answer. heres the thing. check the pivot point of the rack at the inside of the rod. if it is bending(piviting) at the same point of the inside lower contol arm then that part is correct, offen a rack is to short or too long for the frame width.(not in line with the lower control arm ) if this bending point is the same, then check the angle of the tie rod from pivot to steering arm. if this rod rises or falls at a different angle to the lower control arm it will toe in or toe out the tire on deceleraration. there are tie rod spacer kits to correct the angle and lenth of the rod. second thing to check is if the control arms and shock have been installed correctly. meaning they need to be tilted back 7 to 10 degrees, simular to the factory. if they were welded on level(which I have seen) then it will pitch the front end forward ahead of virtical center.tossing the weight on the front end causing the back end to wash out when you lift at the top end.