Non-linear turn in response
Thread Starter
Drifting
Joined: Nov 2007
Posts: 1,695
Likes: 270
From: New York
Agreed. I don't want to tell you how many sway bars I have. Getting it dialed in is unfortunately a lot of trial and error (and money).
I have to get some track time, on the new set-ups. Thanks for all the replies!
I have to get some track time, on the new set-ups. Thanks for all the replies!
Thread Starter
Drifting
Joined: Nov 2007
Posts: 1,695
Likes: 270
From: New York
Of course, you've broken the time honored rule of thumb never to make more than one change at a time, which will make speculative diagnosis a bit more of a challenge, but let's try to narrow things down so you might better formulate a good testing plan for identifying and solving the issue at hand.
IMOE (and I have run it before) rear toe-out, in and of itself, should tend to rotate the rear of the car into corners more quickly (usually too quickly) rather than cause corner entry understeer. But, in any event, given the C2/C3's inherent rear toe-steer characteristics I'd never advise a setup on one which allowed rear toe to travel into toe-out at any point. Since our toe-steer moves increasingly towards (or into) toe-out once the half shafts dip below level with the road, I'd chart your rear toe-steer curves from static ride height into as much bump as you'll likey ever encounter to make sure you're not going past 0 at any time. Do keep in mind that 1) what the rear tires are doing when loaded is most important, and 2) rear toe-steer isn't linear. Fortunately, since you have sphericals, you can be precise here without having to worry about overcompensating with additional toe-in for bushing compliance. If your half-shafts are at or near level in static ride height, you're a good candidate for raising the diff in the chassis to put your geometry onto a better part of the toe-steer curve.
As for things which should more likely contribute to corner entry understeer (could be one or more), the first thing that jumps out at me is the disproportionate change in tire sizes you've made, which would shift balance to understeer. However, that should yield a constant throughout all phases of handling, and from your description once the car transitions into a set I'm left to assume cornering balance is apparently good from there on out (notice I didn't say "neutral".)
Tire construction itself could possibly be part of the problem. Of the Michelin's I've raced on (both street tires and purpose-built slicks), how they've built cornering force has varied widely, some impressively quick and some not so much. There's also the possibility of new tread squirm at work, which is why shaving street tires down is a common practice for racing on them. Also, few Michelin's I've run have worked very well when carrying very much trail-braking force into corner entry. If that's your style of attacking turns you may have to dial it back a notch (if not altogether) to get the car pointed, as difficult and as boring that may be.
Then, as you've significantly lowered front spring rates, perhaps you've reduced the front spring rate too much, and thus overly lessened overall roll stiffness, which can cause sluggish response and actually delay the car taking its set. One thing at work in this case is when spring rates are so low relative to shock dampening that they need too much time to become fully loaded. Softer shocks would speed up this process, but more readily facilitating what may well be too much dive/roll/bump isn't exactly a step forward. Sorry, increasing front bar isn't the answer either, as that would shift overall balance towards understeer throughout cornering. (Again, I'm assuming corner entry understeer is the only issue. If the car oversteers once it points, then we're chasing our tails here.)
Added to that, bars don't do a thing for anti-dive or anti-squat, so you could likely have more than one issue going on where 550# front coils just aren't up to the task of being pressed very hard on modern rubber. Despite what the BBSS (big bars, soft springs) school may believe, advise to run the softest springs you can doesn't always mean softer springs are going to work better. And, for the sake of outright performance, comfort isn't a valid argument.
As for front toe, zero or a little toe-out can usually help turn in to low-speed corners, but if you don't wish to change settings back and forth for street duty you're pretty much stuck with the minimum amount of toe-in which you find stable enough for the street. That said, you do have increased Ackerman due to the bumpsteer blocks, which should reduce the need for any radical front toe settings.
Unless there's something amiss or out of the ordinary going on, the above are the prime suspects I'd investigate. There are a couple of more sinister things possible, but I'd strive to eliminate the above before going there. In the mean time, get a tire pyrometer and think about some adjustable shocks.
On a side note, I'd still like to know your specific Z and D heights. Highly suspect you may well benefit from lowering your rear RC and lengthening your rear IC's, especially with those 335's, and regardless of the current issue.
Hope something here helps you find a solution.
IMOE (and I have run it before) rear toe-out, in and of itself, should tend to rotate the rear of the car into corners more quickly (usually too quickly) rather than cause corner entry understeer. But, in any event, given the C2/C3's inherent rear toe-steer characteristics I'd never advise a setup on one which allowed rear toe to travel into toe-out at any point. Since our toe-steer moves increasingly towards (or into) toe-out once the half shafts dip below level with the road, I'd chart your rear toe-steer curves from static ride height into as much bump as you'll likey ever encounter to make sure you're not going past 0 at any time. Do keep in mind that 1) what the rear tires are doing when loaded is most important, and 2) rear toe-steer isn't linear. Fortunately, since you have sphericals, you can be precise here without having to worry about overcompensating with additional toe-in for bushing compliance. If your half-shafts are at or near level in static ride height, you're a good candidate for raising the diff in the chassis to put your geometry onto a better part of the toe-steer curve.
As for things which should more likely contribute to corner entry understeer (could be one or more), the first thing that jumps out at me is the disproportionate change in tire sizes you've made, which would shift balance to understeer. However, that should yield a constant throughout all phases of handling, and from your description once the car transitions into a set I'm left to assume cornering balance is apparently good from there on out (notice I didn't say "neutral".)
Tire construction itself could possibly be part of the problem. Of the Michelin's I've raced on (both street tires and purpose-built slicks), how they've built cornering force has varied widely, some impressively quick and some not so much. There's also the possibility of new tread squirm at work, which is why shaving street tires down is a common practice for racing on them. Also, few Michelin's I've run have worked very well when carrying very much trail-braking force into corner entry. If that's your style of attacking turns you may have to dial it back a notch (if not altogether) to get the car pointed, as difficult and as boring that may be.
Then, as you've significantly lowered front spring rates, perhaps you've reduced the front spring rate too much, and thus overly lessened overall roll stiffness, which can cause sluggish response and actually delay the car taking its set. One thing at work in this case is when spring rates are so low relative to shock dampening that they need too much time to become fully loaded. Softer shocks would speed up this process, but more readily facilitating what may well be too much dive/roll/bump isn't exactly a step forward. Sorry, increasing front bar isn't the answer either, as that would shift overall balance towards understeer throughout cornering. (Again, I'm assuming corner entry understeer is the only issue. If the car oversteers once it points, then we're chasing our tails here.)
Added to that, bars don't do a thing for anti-dive or anti-squat, so you could likely have more than one issue going on where 550# front coils just aren't up to the task of being pressed very hard on modern rubber. Despite what the BBSS (big bars, soft springs) school may believe, advise to run the softest springs you can doesn't always mean softer springs are going to work better. And, for the sake of outright performance, comfort isn't a valid argument.
As for front toe, zero or a little toe-out can usually help turn in to low-speed corners, but if you don't wish to change settings back and forth for street duty you're pretty much stuck with the minimum amount of toe-in which you find stable enough for the street. That said, you do have increased Ackerman due to the bumpsteer blocks, which should reduce the need for any radical front toe settings.
Unless there's something amiss or out of the ordinary going on, the above are the prime suspects I'd investigate. There are a couple of more sinister things possible, but I'd strive to eliminate the above before going there. In the mean time, get a tire pyrometer and think about some adjustable shocks.
On a side note, I'd still like to know your specific Z and D heights. Highly suspect you may well benefit from lowering your rear RC and lengthening your rear IC's, especially with those 335's, and regardless of the current issue.
Hope something here helps you find a solution.
The good. While most cars do not turn in well, this car turns in very well. The car like being under power and you better not lift in a corner. The Michelin Pilots have a tenacious grip. No real issues even after all the rebuilding.
The bad. The car was squirrely. I used this as a shakedown run and found what I saw at low speed worse at high speed. My friend started to left foot brake and smoothed things out. I tended to prefer to brake in a straight line and power through corners.
My thanks to SkunkWorks, following your advice, the rear half shafts were not quite parallel to the ground. I found my D height was off and I have ordered longer rear spring bolts to get the back down and get the proper D height.
By bringing the rear down and getting the proper D height, will get the car re-aligned. I agree with Solid LT1 and will try 3/16 toe-in
By bringing the rear down, I believe that may also help with the castor in the front?
Anyway, thanks for all who took the time to reply for your help and support.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Glad you found that helpful.
FWIW, a more straightfoward baseline from which to work than found in my earlier advice would be to just start by setting static rear ride height to where the inner U's are 1/2" higher than the outers, and lower the inner camber struts to 1/2" further below the diff than stock, as a default setup. While changing diff height relative to the ground from this baseline might possibly locate a specific vehicle's geometry to a better place on the toe-steer curve for a given circuit, finding that particular point could prove a laborious task. So, I wouldn't worry too much about that until/unless indications are that there's something left to be gained by the effort. And, whether you fiddle with this or not, should you desire to lower the rear ride height furhter, I'd do so by relocating the diff in the chassis and maintain the diff's height relative to the road.
As for playing with inner strut heights, which alters rear negative camber gain, while you NEVER want to lower them far enough to induce positive gain, with wide, modern rubber and lots of power you may possibly find less gain than the above baseline works for a given AX/RR circuit (drag racing aside). But, as with diff height, unless the stopwatch, the tire pyrometer or your backside indicate otherwise, this baseline probably ought to be your default setting.
Hope that's less confusing to follow when setting things up.
TSW
FWIW, a more straightfoward baseline from which to work than found in my earlier advice would be to just start by setting static rear ride height to where the inner U's are 1/2" higher than the outers, and lower the inner camber struts to 1/2" further below the diff than stock, as a default setup. While changing diff height relative to the ground from this baseline might possibly locate a specific vehicle's geometry to a better place on the toe-steer curve for a given circuit, finding that particular point could prove a laborious task. So, I wouldn't worry too much about that until/unless indications are that there's something left to be gained by the effort. And, whether you fiddle with this or not, should you desire to lower the rear ride height furhter, I'd do so by relocating the diff in the chassis and maintain the diff's height relative to the road.
As for playing with inner strut heights, which alters rear negative camber gain, while you NEVER want to lower them far enough to induce positive gain, with wide, modern rubber and lots of power you may possibly find less gain than the above baseline works for a given AX/RR circuit (drag racing aside). But, as with diff height, unless the stopwatch, the tire pyrometer or your backside indicate otherwise, this baseline probably ought to be your default setting.
Hope that's less confusing to follow when setting things up.
TSW
Thread Starter
Drifting
Joined: Nov 2007
Posts: 1,695
Likes: 270
From: New York
I was discussing your reply with a friend. It seems GM did re-located the camber struts on the C4-S2 I think was ’88 or '89 and higher and the sell the brackets to retrofit the older cars .
After thinking about all this, for now I will just set the rear height to spec. 1.65" and make sure it’s the same on both sides. I will also increase the rear toe-in total from .125 to .250. To follow the “one change at a time” (actually that is two changes) rule, I will adjust those settings before making further changes. Driving the car on track is always a big help.
It would be very easy to do a camber curve analysis of my car, all I have to do is unbolt the rear spring end and move the suspension through its normal travel range. A friend has a camber gauge.
After thinking about all this, for now I will just set the rear height to spec. 1.65" and make sure it’s the same on both sides. I will also increase the rear toe-in total from .125 to .250. To follow the “one change at a time” (actually that is two changes) rule, I will adjust those settings before making further changes. Driving the car on track is always a big help.
It would be very easy to do a camber curve analysis of my car, all I have to do is unbolt the rear spring end and move the suspension through its normal travel range. A friend has a camber gauge.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Of course, this isn't a C4. For a C3 baseline, I'd stick with those defaults I mentioned most recently, which came from John Greenwood (a C3 guru if ever there were one). It's worth noting even the General didn't recommend stock ride heights for road racing; rather 1-1.25" Z and 1.25" D (along with raising the diff). Also, GM lowered the inner strut links from the C2 to the C3 to lower rear RC, reduce jacking and reduced rear camber gain. Lowering the struts an additional 1/2" is recommended to account for tire improvements since.
Also, tho having the half-shafts level is undesirable, you don't want the diff located any higher relative to the ground than necessary to sufficiently counter adverse toe-steer (primarily in bump or squat). Sorry if my older previous post on this confused the issue more than it helped.
From the pics you sent, and given the above, I'd at least follow Chevy Power recommendations, if not Greenwood's more recent ones, for a baseline. And, you might be a candidate for tall upper ball joints up front.
Also, tho having the half-shafts level is undesirable, you don't want the diff located any higher relative to the ground than necessary to sufficiently counter adverse toe-steer (primarily in bump or squat). Sorry if my older previous post on this confused the issue more than it helped.
From the pics you sent, and given the above, I'd at least follow Chevy Power recommendations, if not Greenwood's more recent ones, for a baseline. And, you might be a candidate for tall upper ball joints up front.
Thread Starter
Drifting
Joined: Nov 2007
Posts: 1,695
Likes: 270
From: New York
According to my Corvette assembly manual, you want the have shaft parallel to the ground. What am I missing?
You mention John Greenwood and I went to his site, but could not find anything on his recommendations. Can you please share the link?
You mention John Greenwood and I went to his site, but could not find anything on his recommendations. Can you please share the link?
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
The AIM isn't the correct reference point for C3 AX/RR chassis prep and setup info from the factory. Instead, tho several years out of print and more akin to Cliff Notes than a textbook, you'll find this far more relevant to tracking...
http://www.corvettefaq.com/c3/ChevyPower.pdf
It must be kept in mind that having the half-shafts parallel to the ground is not a goal, rather a result of lowering rear ride height, CG and RC which can actually worsen C2/C3 inherent toe-steer if not also addressed by relocating the diff to compensate. Greenwood goes a bit further into this than does the Chevy Power book's brief mention, as well as covering several other issues, here...
http://www.corvettefaq.com/c3/vip.pdf
FWIW, note that there have been further tire and bushing advances since those articles were first published, race prep in and of itself isn't the only topic, and many hardcore types have found rear bars are NOT necessarily required. However, those articles still offer a wealth of knowledge from which any serious C3 enthusiast can benefit. And, tho different settings may prove optimum for a particular car on a particular track on a particular day, I believe his advice regarding diff height and camber strut location are as sound a baseline from which to work as any.
It goes without saying that there's much, much more to it than this, as you can fill a small library with books on vehicle dynamics. And, do bear in mind you can also break the bank learning to apply what you come to understand about it. (My ex will back me up on both counts.)
http://www.corvettefaq.com/c3/ChevyPower.pdf
It must be kept in mind that having the half-shafts parallel to the ground is not a goal, rather a result of lowering rear ride height, CG and RC which can actually worsen C2/C3 inherent toe-steer if not also addressed by relocating the diff to compensate. Greenwood goes a bit further into this than does the Chevy Power book's brief mention, as well as covering several other issues, here...
http://www.corvettefaq.com/c3/vip.pdf
FWIW, note that there have been further tire and bushing advances since those articles were first published, race prep in and of itself isn't the only topic, and many hardcore types have found rear bars are NOT necessarily required. However, those articles still offer a wealth of knowledge from which any serious C3 enthusiast can benefit. And, tho different settings may prove optimum for a particular car on a particular track on a particular day, I believe his advice regarding diff height and camber strut location are as sound a baseline from which to work as any.
It goes without saying that there's much, much more to it than this, as you can fill a small library with books on vehicle dynamics. And, do bear in mind you can also break the bank learning to apply what you come to understand about it. (My ex will back me up on both counts.)
Last edited by TheSkunkWorks; Aug 12, 2012 at 06:46 PM.
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