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Question about suspension characteristic

1K views 3 replies 3 participants last post by  No Doubt  
#1 ·
I was having a conversation with a friend who owns a Mustang and he asked a pretty good question that I could not answer. He wanted to know why (C-5) Corvettes have that characteristic side wiggle over bumps - especially noticeable in corners. He remarked that he noticed in a test drive that Corvettes which have IRS and Mustangs (not including 99-04 Cobras) which have solid axle suspensions, exibited the same handling characteristic.

I have owned many Mustangs from the 99-03 model years to include the IRS Cobras and know exactly what he means. However, I didn't have an explanation to give him why an IRS Corvette behaves this way.
In my own experience, I haven't been in C5 or C6 that did not behave this way including those with stock through modified with coilovers. I did not experience this type of suspension dynamic with the Ferrari I once owned or a in a Mercedes or Porsche that I rode in.


Can someone explain why the suspension behaves this way and what can be done to control/correct it?

Thanks

-RR :pop:
 
#2 · (Edited)
If a car "adjusts" it's track when hitting a surface imperfection it is due to Bump Steer. If it adjusts during acceleration it is Torque Steer.

All cars bump steer to some extent. Stiffer suspensions will reflect bump steer more harshly.

As it comes from the factory the Corvette suspension is a compromise between good street handling on relatively rough roads and competition handling on smooth track surfaces. It is biased towards safe street HP handling with understeer built in. Corvette engineers tuned out most of the bump steer when they went with a Zero Scrub Radius.

Lowering increases the scrub radius but you can tune it by widening the track to get the scrub radius back to Zero. The transverse spring will conduct the bump steer directly to the opposite wheel.

Other things we do to improve handling, such as increasing negative camber, more positive caster and installing stiffer bushings will transmit the bump steer more harshly.

Fortunetely Bump Steer can be tuned to a minimum using a bump steer gauge and making adjustments. Torque steer can be also tuned out using stiffer Motor mounts and Tranny/Diff mounts and by adjusting corner weights.

There are of course many other variables that contribute to the "wiggle" such as shock Rebound valving, Spring rates,Rake setting, F/R weight ratio and Cross Corner % Difference to name a few. All can be tuned to reduce it.

The other thing to consider is that excessive Bump Steer and/or Torque Steer is frequently mis-diagnosed due to poor tire grip, poor surface grip, the steering not being straight as power is applied or the power being applied in an unsmooth manner. Improper braking and steering inputs can "upset" the suspension and then power is not belng applied smoothly or when the suspension is not "set". Ie. Driver error is the cause. This can also be "tuned out" by educating the driver or getting a new, more knowledgable one.
 
#4 ·
The issue in this case is at least partly due to shock rebound valving. Specifically, excessive rebound damping according to GM engineering. The issue was corrected with a minor change in rear shock damping characteristics in late '07 cars.

The cause for earlier model years (the '01-'03 Z06) was the same and thus the '04 Z06 shocks helped reduce this phenomenon.