Here is what the car tends to do: Start to turn and it seems not to respond, then darts into the corner (Non-linear turn in response). That is unusual and new to me and the car always transitioned well. Understeer was manageable with the gas. Not sure why.

I just put in solid bushing (steel) in the upper and lower control arms. I also changed the rims and tires. I went to 17 inch minilite replicas from PS engineering and Michelin Pilots from and rear.

Ideas?

 

How healthy is the rest of the front suspension? Have you inspected the steering components and tie rod ends? How about the pump, valve, and cylinder? Have any of them been replaced or serviced to your knowledge?

 

If the car is lowered could be a case of "bump steer"

 

Manual or power steering?

 

OK, some replies: Yes, most of the suspension is brand new. I took out the Daytona springs and replaced them with the F-41 springs. New ball joints, tie rods with heavy duty sleeves for the Tie Rods. All Moog heavy duty stuff. New shocks, Bilstein Shocks, aluminum Sway Bar Bushings, Heim Jointed Sway Bar Links, new Stainless steel cylinder…A lot of new stuff.

 

Power steering

 

I have installed a bump steer kit

 

Did the car get aligned after all of the new suspension equipment?

 

Yes

 

We've narrowed it down then. If it's not suspension it's steering. Suspects can include the pump which may be failing, the valve which may have issues, the cylinder, or the box. At this point though we'd be guessing which without more in depth inspection into these components from you but it's a good narrow field to start checking things.

 

Jack up front end...wheels off ground; start engine and exercise steering wheel throughout its entire range. You are looking for any binding that may occur somewhere in that travel. If the control valve senses a rise in resistance, it will try to compensate by increasing the force to overcome it.

If you can't get enough 'feel' with the engine on, turn it off and try that test again.

P.S. If you need to do so, you can balance the control valve while you have the front end jacked up.

 

What kind of condition is the rear differential in? If it is in poor shape with lots of side yoke play, your getting "rear steer" problems. What are alignment settings front/rear?

 

After you do that, check the lash.

 

Thank you for all the replies. I checked the steering box and there is no excessive play. Plus the non-linear turn in response happened after all my modifications. Power valve was balanced on installation.

Also, the components in the rear end are sound. I did remove and replace the rear control arms and replaced the rubber bushings in the front of the arm (where it inserts in the frame) with Spherical Metal Joints & Sleeves to hopefully reduce some of the trailing throttle oversteer. I also replaced the rubber camber rods with Camber heim joint Rod Kit.

I have an interesting PM I like to share only because I think this email may serve to help others: It said:

“first suspect would be the tires, but what is your front toe setting?

The bump steer kit will make the car more stable under braking.
as well as the bushings. The only change that is significant, with
respect to geometry is changing rims, did you change the rim centerline
with respect to the spindle, or in technical terms the scrub radius?

Where is the center line of the rim located with respect to the centerline
off the spindle?”

So everyone knows, the front off-set is Dead center in the front. I have flairs, so I could accommodate the wider rim and 17 inch tires.

The rear, I had to have same offset as stock rims, 4 inches or I would have needed the special control arms.

I may get the car re-aligned. I do not remember my toe setting. I think I may have asked for 1/16 of and inch toe-IN. I tried toe-out once and still remember what it was like trying braking at the end of the main straight at Bridgehampton Race Circuit.

I am wonder if too much toe in would do this?

 

Is this a problem in slow, medium or fast turns?

Was the bumpsteer kit already on the car before this became noticeable, or also a new mod?

Any other geometry mods? ...raised diff, for instance?

Z and D heights?

Rear static toe setting?

Bilstein Sports or HD's, and what previous shocks?

What previous tires?

And, are you a trail-braker?

 

The problem is at slow speeds. Not sure at higher speeds and I have not put the car on track yet. The seat belts were old and the orig seats a disaster, so I am putting in new seats and 6 point harness

The bumpsteer is a new modification. The geometry is the same. The F-41 (now) and Daytona springs both lower the car one inch in the front from stock. No other geometry mods.

The have changed. The tires were BFG Comp T/A’s and I went to Pilot Sport PS2
Now the BFG were 15 inch rims….Sorry 4 AM, so I’ll double check…..something like 245/50X15 from and 275/50X15 rear. The Michelins are 245/40-17 on a 17X9.5 rim. Rears are 335/35X17 mounted on 17X11 rims. Shocks are Bilstein Sports.

The rear toe-in setting was 1/16. Again, with the solid bushings, I was hoping to remove changes when braking.

 

1/16" rear toe is too little, your car should have at least 3/16 to 1/4" rear toein. Depending on where your halfshafts sit, you could be going into toeout at the rear, this will exhibit the EVIL handling vices you are describing. Also dont use too much negative camber at the rear as it will tend to loose traction on corner exit, about 3/4 to 1 degree negative camber maximum at the rear.

 

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.

 

This helps a lot an gave me a lot to think about…..Let me touch on some of the points you brought up:

raising the diff in the chassis to put your geometry onto a better part of the toe-steer curve. That is a great idea, if I could figure out who could machine the alloy blocks locating your rear crossmember and then raise it .5 to .375 that would help.

lowered front spring rates.... actually delay the car taking its set.
Yes but I lowered it from a ridiculously high rate (860 inch pounds) to something more manageable (550 inch pounds). That is more than the stock 280 inch pound springs. Softening springs rates will cause more grip not less grip-but the car will take longer to set. This is something I probably never noticed with #860 springs and now you notice it with the #550 . So, its a question of degrees. IMHO, The #860 were too stiff. The suspension never really worked.

disproportionate change in tire sizes you've made, which would shift balance to understeer.Yes, I was sort of worried how this is was going to work. I really would have liked to see 275 or 285 in the front but they would not fit. I was thinking of getting race tires they will go on my American Racing 200s 15x10 rims that I saved.

550# front coils just aren't up to the task of being pressed very hard on modern rubber. If I understand, you think these springs are not too soft but I should resist the temptation to go softer? If that is it…. yes I agree.
Remember, these are the rates GM raced our Corvettes with back in the day. Though, as tires got bigger and better some teams when up to #650-#700 range.

Transient response is addressed with shock and toe settings.
First think I going to re-check is front toe setting and set it to 0 toe front. If all the rubber removed, I should get no toe changes even under hard braking (no toe-out)

Second think I would suspect is tires. This is my first set of Michelins. Previously I always ran B.F. Goodrich and would of again if they had the sizes I wanted.

Third, after reading your reply, is to consider cranking in more compression setting in the shocks (I guess I would have to send them back to Bilstein.) I do not know if I saved my Koni’s.


trail-braking force into corner entry.I need to learn now. I stink at it. If you read Mark Donahue’s book The Unfair Advantage, I have a “European” style of driving.

increased Ackerman due to the bumpsteer blocksI need to investigate this. Try to see what the front bumpsteer curves look like before and after. Making changes with no data sort of sucks

 

Of springs, bars and shocks, IMCO one should do their best to determine the optimum front spring rate for their purposes and then tune the rest of the lot as necessary to get the most out of them.

Spring rates do factor into roll couple distribution, and thus balance, but they do not change the amount of total weight transfered (bars do). Shocks regulate how quickly weight transfer will occur. Weight transfer tends to increase with available traction. As a point of reference while considering all of this, 550# coils require ~56% more compression than do 860's to reach the same weight transfer loading, whether lateral or longitudinal. 700's only ~23% more...

It might mean another trip to the drawing board, but if you're otherwise unable to get your car sorted to your satisfaction, I'd take a long, hard look at the combination of front springs and relative shock valving. In any event, collecting springs, bars and shocks is more often than not all part of the process of getting dialed in, so by all means don't get locked into a particular setup that isn't working for you.