Ill handling decelleration

Old 01-13-2009, 06:54 AM
  #11  
dparker
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Originally Posted by gearhead1011
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.
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.
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Old 01-14-2009, 07:37 PM
  #12  
JimmySmith
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IF it is a 12 bolt GM the tubes could be slipping in punkin. Later J.Smith 4621 racing
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Old 01-20-2009, 05:50 PM
  #13  
jmo4x5x
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Default 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
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Old 02-21-2009, 11:21 AM
  #14  
TS1955
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Default 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
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Old 02-22-2009, 07:17 AM
  #15  
dparker
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Default Re: Ill handle in decel

Originally Posted by TS1955
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
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.
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Old 02-22-2009, 07:28 AM
  #16  
Tod74
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^^^^^^^^^^^^^^

good post.
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Old 02-22-2009, 12:38 PM
  #17  
TS1955
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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
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Old 04-09-2009, 06:33 AM
  #18  
tigwelder
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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.
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