Question about suspension dynamics
#1
Coolest Guy Evar!
Thread Starter
Question about suspension dynamics
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
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
#2
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
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
Other than that, I'm all ears. As, I would love to know the answer to this question also
#3
Coolest Guy Evar!
Thread Starter
You're not too far off. Here's the response I got on a different forum.
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.
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.
#5
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St. Jude Donor '08
That DJWorm guy is one very knowledgeable person.
I learned a lot reading his posts.
Hitting bumps or dips in the road causes vertical suspension movement which leads to changes in toe. This is referred to as bump steer. When this happens the car changes direction, which is not good.
I know places like Pfadt and others sell bump steer kits to reduce bump steer.
I learned a lot reading his posts.
Hitting bumps or dips in the road causes vertical suspension movement which leads to changes in toe. This is referred to as bump steer. When this happens the car changes direction, which is not good.
I know places like Pfadt and others sell bump steer kits to reduce bump steer.
#6
Coolest Guy Evar!
Thread Starter
Bump steer is not the right term. Here is the first writeup I found on google. http://www.longacreracing.com/articl...sp?ARTID=13#Q1
Randy
Randy
-RR
#7
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"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."
Either I am misunderstanding what he is saying or he is flat wrong.
Scrub radius has to do with wheel width and suspension pivot point, NOT ride height. If the ball joint pivot is inboard of the wheel centerline, then you have more scrub radius than with the BJ pivot point located outboard of the wheel centerline. C5s & C6s have almost no scrub because the pivot is close to the centerline of the rim.
Bump steer isn't transmitted through a spring either! As Randy's article points out, bump steer is caused by the tie rod not following the same arc as the suspension, and these two arcs cause a toe change.
Most Corvettes wiggle in bumpy turns because the rear suspensions are out of alignment.
Out of over 250 Corvettes that I align each year, only a handful are within reasonable alignment settings. It is a common happening that I receive phone calls after one of our alignments, touting that the mid-turn wiggle is gone. Most thought that it was a trait of the cars suspension design, not a trait of the suspension alignment.
If your c5 or C6 Corvette wiggles in the middle of a bumpy turn, then go get a quality 4 wheel alignment. Don't blame the suspension design. The bump steer in these models is very small.
Either I am misunderstanding what he is saying or he is flat wrong.
Scrub radius has to do with wheel width and suspension pivot point, NOT ride height. If the ball joint pivot is inboard of the wheel centerline, then you have more scrub radius than with the BJ pivot point located outboard of the wheel centerline. C5s & C6s have almost no scrub because the pivot is close to the centerline of the rim.
Bump steer isn't transmitted through a spring either! As Randy's article points out, bump steer is caused by the tie rod not following the same arc as the suspension, and these two arcs cause a toe change.
Most Corvettes wiggle in bumpy turns because the rear suspensions are out of alignment.
Out of over 250 Corvettes that I align each year, only a handful are within reasonable alignment settings. It is a common happening that I receive phone calls after one of our alignments, touting that the mid-turn wiggle is gone. Most thought that it was a trait of the cars suspension design, not a trait of the suspension alignment.
If your c5 or C6 Corvette wiggles in the middle of a bumpy turn, then go get a quality 4 wheel alignment. Don't blame the suspension design. The bump steer in these models is very small.
#8
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St. Jude Donor '08
Autocross & Roadrace Forum Sponsor
Most Corvettes wiggle in bumpy turns because the rear suspensions are out of alignment.
Out of over 250 Corvettes that I align each year, only a handful are within reasonable alignment settings. It is a common happening that I receive phone calls after one of our alignments, touting that the mid-turn wiggle is gone. Most thought that it was a trait of the cars suspension design, not a trait of the suspension alignment.
If your c5 or C6 Corvette wiggles in the middle of a bumpy turn, then go get a quality 4 wheel alignment. Don't blame the suspension design. The bump steer in these models is very small.
Out of over 250 Corvettes that I align each year, only a handful are within reasonable alignment settings. It is a common happening that I receive phone calls after one of our alignments, touting that the mid-turn wiggle is gone. Most thought that it was a trait of the cars suspension design, not a trait of the suspension alignment.
If your c5 or C6 Corvette wiggles in the middle of a bumpy turn, then go get a quality 4 wheel alignment. Don't blame the suspension design. The bump steer in these models is very small.
In addition to that, even a proper alignment can go way out of whack due to cornering forces. The deflection in the control arm bushings is significant. That deflection itself can cause a 'wiggle' feeling, as well as the change of alignment that occurs from the deflection.
Certain tires also contribute to the feeling. The much maligned runflats are at least very good in the sidewall stiffness department.
-Aaron
#9
Le Mans Master
Scrub radius has to do with wheel width and suspension pivot point, NOT ride height. If the ball joint pivot is inboard of the wheel centerline, then you have more scrub radius than with the BJ pivot point located outboard of the wheel centerline. C5s & C6s have almost no scrub because the pivot is close to the centerline of the rim.
(My apologies if that's what you meant.)
Agreed. A properly designed SLA suspension with zero bump steer, the lower control arm and the tie rod should both be parallel to the ground. Ask any T1 racer why they don't lower their cars.
#11
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I agree. He's wrong.
Not quite. Scrub radius (also called Kingpin offset) is the distance between the steer axis (the imaginary line going through the upper and lower balljoints) of the suspension where it strikes the ground and the center of the contact patch, in front view.
(My apologies if that's what you meant.)
That is what I meant
Agreed. A properly designed SLA suspension with zero bump steer, the lower control arm and the tie rod should both be parallel to the ground. Ask any T1 racer why they don't lower their cars.
Not quite. Scrub radius (also called Kingpin offset) is the distance between the steer axis (the imaginary line going through the upper and lower balljoints) of the suspension where it strikes the ground and the center of the contact patch, in front view.
(My apologies if that's what you meant.)
That is what I meant
Agreed. A properly designed SLA suspension with zero bump steer, the lower control arm and the tie rod should both be parallel to the ground. Ask any T1 racer why they don't lower their cars.
#13
If bumpsteer is corrected then lowering the car should not change anything. In addition, anyone setting up bumpsteer should do so at whatever ride height the car will be run at. That includes cycling the suspension.
-V
-V
#14
Former Vendor
I wish it where that easy. Now you have to look at camber change, castor change, shock movement, shock angle, anti dive and fun things like that. When building a full out racecar. We can spend up to 25 days alone on suspension during a build. The best part is it's never over, cars will keep getting faster.
And my wife has the nerve to ask why I spend 90 percent of my awake life with cars.
Randy
#15
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Any suggestions on where to find or how much they run?
Thanks!!!!
#16
I've been setting up my race cars for several years now and never run into much problem setting bumpsteer at multiple ride heights and castor settings. I am assuming that in T-1 factory tie rod ends are required. Otherwise I cannot understand why any bumpsteer could not be corrected when lowering the car. The only issue that I have ever come accross, and this may apply to t-1, is that when running a lot of positive castor there is not enough room with smaller diameter wheels to lower the steering arm enough for proper correction.
-V
-V
#17
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IMHO you need to decide, do you want a low car because it looks like a race car, or do you want a real race car?
#18
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Some SpeedGT teams are lowered so much they run custom uprights.
Last edited by BrianCunningham; 09-25-2007 at 11:44 PM.
#20
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St. Jude Donor '08
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Trying to distill this information down to a few 'rules' like don't lower the car more than x or whatnot oversimplifies the problem at hand.
There are at least 100 different factors in suspension design that are affected and important to various degrees. The rules of your sanctioning body, your budget and your fabrication and setup skills will determine which factors are feasible to mess with.
Even with a clean sheet of paper like in F1 there is still no consensus on what constitutes perfect suspension design.
With a Corvette, most people are not going to change suspension pickup points (at least to a major degree) and therefor the roll centers are going to move, potentially to an undesirable location with an over lowered car. Other effects have been mentioned, like camber change through the travel range, effective spring rate (especially with coil-overs), bump steer etc... Those effect don't happen in a vacuum either, the magnitude of those effects are influenced by suspension factors we have control over like spring rates, roll stiffness (sway bars) tire choices, shock damping, wheel travel etc...
For your average racer, and even for most teams with a lot of budget the answer is testing. You have to understand the systems (or hire someone who does) and get your starting point. From there testing is the main way to determine if a particular change makes you car faster or slower. The interactions are so numerous that even the best theory can sometimes not hold true. Change one thing at a time and see the effects. Another important factor is consistant driving, because you are often looking for tenths of a second, not seconds.
-Aaron
There are at least 100 different factors in suspension design that are affected and important to various degrees. The rules of your sanctioning body, your budget and your fabrication and setup skills will determine which factors are feasible to mess with.
Even with a clean sheet of paper like in F1 there is still no consensus on what constitutes perfect suspension design.
With a Corvette, most people are not going to change suspension pickup points (at least to a major degree) and therefor the roll centers are going to move, potentially to an undesirable location with an over lowered car. Other effects have been mentioned, like camber change through the travel range, effective spring rate (especially with coil-overs), bump steer etc... Those effect don't happen in a vacuum either, the magnitude of those effects are influenced by suspension factors we have control over like spring rates, roll stiffness (sway bars) tire choices, shock damping, wheel travel etc...
For your average racer, and even for most teams with a lot of budget the answer is testing. You have to understand the systems (or hire someone who does) and get your starting point. From there testing is the main way to determine if a particular change makes you car faster or slower. The interactions are so numerous that even the best theory can sometimes not hold true. Change one thing at a time and see the effects. Another important factor is consistant driving, because you are often looking for tenths of a second, not seconds.
-Aaron