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Discussion Starter · #1 ·
Anyone ever run into a severe wiggle in the rear while running drag radials? My car is shaking side to side in the rear starting about half track and it continues until I lift at the stripe. It feels awful in the car and can be seen just watching the car from the rear. Another racer suggested adding more pinion angle, thinking the pinion was going over center. I added two degrees to make a total of 6 and the issue went away, until I started to put power back in it. I've added air the the tires (as high as 20psi) and tried softening and stiffening the shocks with no improvement. A local stock suspension mustang guy thought my corner weights might be way off so I scaled it last night. I didn't think they looked too bad.

lf- 1095 rf- 1015

lr- 918 rr-885

I have the ability to adjust instant center and what not. The anti roll bar was set neutral with the driver in the car. The tires are 325/50 Hoosiers on a 10'' rim. They have been balanced.

Any Ideas?
 

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Does it do it on a chassis dyno? I'm far from an expert but that could tell you if its an aerodynamic thing or a suspesion thing.
 

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how is the alignment ? Axle straight in the car ? Does the car dog track ?

Anyone ever run into a severe wiggle in the rear while running drag radials? My car is shaking side to side in the rear starting about half track and it continues until I lift at the stripe. It feels awful in the car and can be seen just watching the car from the rear. Another racer suggested adding more pinion angle, thinking the pinion was going over center. I added two degrees to make a total of 6 and the issue went away, until I started to put power back in it. I've added air the the tires (as high as 20psi) and tried softening and stiffening the shocks with no improvement. A local stock suspension mustang guy thought my corner weights might be way off so I scaled it last night. I didn't think they looked too bad.

lf- 1095 rf- 1015

lr- 918 rr-885

I have the ability to adjust instant center and what not. The anti roll bar was set neutral with the driver in the car. The tires are 325/50 Hoosiers on a 10'' rim. They have been balanced.

Any Ideas?
 

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Discussion Starter · #6 ·
I run a high trac 9 from auto weld chassis. Its about half way down the page. All rod ends no bushings and they are all tight. It also has a homebuilt anti roll bar.

http://www.autoweldchassis.com/vbi.ivnu


I had the car on an alignment rack recently. We put as much caster in it as I could and checked toe at both ride height and with the front raised. There was not a whole lot of change. The car does not dog track and looks level from the back. The nose sits a little lower than the rear. The rear thrust angle and rear toe looked to be dead on. The car launches straight with the wheels up about a 12'' or so and drives really nice to the 1/8 mile. The wiggle gets worse all the way to the stripe but as soon as I lift it flattens right out with no drama slowing down at all.

I'm starting to wonder if its the hoosier tires. They have a pretty soft sidewall and seem to squirm around alot if you push the car side to side. Its been a while but I do not remember it doing this with the M\T radials on it.
 

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Try more air in the tires, should add some stability.
 

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I run a high trac 9 from auto weld chassis. Its about half way down the page. All rod ends no bushings and they are all tight. It also has a homebuilt anti roll bar.


You stated that all bars have rod ends, do you have a wish bone to keep the rearend centered in the car. I have found with rod ends this will allow the car to sway a little due to rearend moving side to side
 

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3945# race weight and a tire with a marginal sidewall?????

Have you ever tried stiffwall slicks on this car???????

Or maybe going back to the M/T`s and trying those for a comparison????????
 

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Discussion Starter · #11 ·
Thanks for the ideas so far.

I do have some poly bushings that I could replace the rod ends with. Maybe do just the uppers first and see if it stops. I'm going for a test day next friday. I will be bringing a pair of M\T radials to try also.
 

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I remember watching a few nova's with stock suspension do this wiggle thing at the top end. Troy Pirez has to know about it. His nova was one of them when they had BFG's. It looked like it would start to wobble after the 1/8. I always thought it was from them pedaling the car.
 

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at what MPH does tha car start to wiggle ? Could be some aerodynamics problem. I am spoiled with late model cars but got to thinking that a 70 chevelle has the aerodynamics of a school bus. How tall does the car sit ? Is the ride hieght really high ?
 

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Discussion Starter · #14 ·
I'd say it starts around 120 or so.

The front sits pretty low at rest but its up a bit going down track

The strange part is that the problem is new this year. Last year I made a bunch of passes in the 155-159 range and never had an issue. I'm really leaning towards it being rear suspension or the tire just because it seems to go away even if I just lift a little. The car will maintain the nose up attitude and is still accelerating but with the reduced power the wiggle is gone. As soon as I reapply the power it starts again.
 

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I got the same problem. I put screws in my rims this week.Gonna try that.I run 15.5 lbs. in the 315 m/t.
 

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Try adjusting the car to sit lower going down the track. Sounds crazy but with the radials being that down on air pressure alot of wierd things coulde be happening. From what I have seen of the hoosiers they do seem to like more tire pressure. Could be the radial cap is cupping at high speed.


I'd say it starts around 120 or so.

The front sits pretty low at rest but its up a bit going down track

The strange part is that the problem is new this year. Last year I made a bunch of passes in the 155-159 range and never had an issue. I'm really leaning towards it being rear suspension or the tire just because it seems to go away even if I just lift a little. The car will maintain the nose up attitude and is still accelerating but with the reduced power the wiggle is gone. As soon as I reapply the power it starts again.
 

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I went thru the same ordeal a few years back with a street car on regular BFG rdaila T/As on a fox-body mustang. In an effort to eliminate the violent swaying, I replaced both the upper and lower control arms and the rear springs. Condition remained. I then took it to a couple of local chassis shops with one saying that my aftermarket torque box replacement were welded in slightly off. He "fixed" this and it still swayed (I figured it would because it didn't begin swaying unitl quite some time after I got the torque boxes replaced). In my case the ultimate solution came in a last ditch effort by replacing the shocks of all things. Not sure if the same thing is causing your problems, but $1K later this ended being my problem. See if someone has a an extra set of shocks you can borrow and test it out.

-Jason
 

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I run a high trac 9 from auto weld chassis. Its about half way down the page. All rod ends no bushings and they are all tight. It also has a homebuilt anti roll bar.


You stated that all bars have rod ends, do you have a wish bone to keep the rearend centered in the car. I have found with rod ends this will allow the car to sway a little due to rearend moving side to side
The upper control arms are triangulated from the housing to the chassis on his chevelle, this should keep the rearend from moving side to side
 

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I had the same problem with my Vega and my brother had the same problem with his Monza using Hoosier 29rx15x9 drag radials, tried air pressure from 15-20 lbs, adjusted shocks, I was told that I should have went up to 23-24 lbs. of air and the problem would have went away. Sold tires with 24 runs on them and went back to Bias ply Hoosiers and problem went away.
 
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