5 bar and toe change rate

[71roadster]

I built a five bar rear suspension similar to the C4 corvette model. I used the longest toe control rods I could fit and set them for the minimal amount of toe-in on compression as possible. By measurement I get about 0.8 degrees of toe in from a normal resting position to full compression. which on 25 in diameter tire gives about 0.175" of toe change from ride height to full compression.

Unfortunately this seems to be far too much. When I hit a bump, the resultant toe steer is so strong it really pushes the rear of the car around. A bump on the right side pushes the rear left and vice versa. It is scary actually and I can't leave it like this. There is limited room to decrease the toe change a little more, but everything I have read, (and that is a lot - so much for self study!) says toe-in on compression is a desirable thing. And I think if I set the toe control rod closer to parallel at rest, I risk going to toe-in at partial compression to toe-out at full compression. So I really don't know where to go at this point and I don't really like driving it the way it is.

Here is a shot of mine.
https://static1.pt-content.com/images/noimg.gif

Here are shots of a couple of others that really take the toe change to opposite ends of the extremes.

This one would have a LOT of toe-in on compression
https://static1.pt-content.com/images/noimg.gif

And this one would have toe-out on compression
https://static1.pt-content.com/images/noimg.gif

Maybe somebody here knows how much toe change a stock C4 suspension produces? I checked the C4 forums and there does not seem to be too much hard technical discussion over there. Any ideas?
https://static1.pt-content.com/images/pt/2007/05/user_online-1.gif https://static1.pt-content.com/images/pt/2007/05/report-1.gif (http://forums.corvetteforum.com/report.php?p=1560421208) https://static1.pt-content.com/images/pt/2007/05/progress-2.gif https://static1.pt-content.com/images/pt/2007/05/edit-1.gif (http://forums.corvetteforum.com/editpost.php?do=editpost&p=1560421208)

[David Pozzi]

I'm sure someone who has worked on one of these will post some good info.

I have no experience with this suspension but while you are waiting for "good" answer, the general rule would be if the upper toe link is close to the drive shaft in height, it should be placed pretty close to the same angle and length as the shaft. If it were the height of the lower link, it should match that length and angle. Anywhere in-between and it's a combination of the two. The ends of the toe links would be along lines drawn vertically from top pivot to bottom pivot of the suspension, both inners and outers. IF the front pivots of the trailing arms is directly in line with the rear pivots. Which is not how this suspension is laid out, the front pivots are outboard, - way outboard.

I know GM placed the two steering pivots close together. I used to have a book that said why it was done that way but can't find it right now. I believe if you looked at the rear suspension from above in "plan" view, and drew a line from upper trailing link pivot on the frame to the inner driveshaft U joint it would extend to the toe link pivot, so looking at it in two dimensions from the rear doesn't give the whole picture for this type suspension.

Have you tried the inner toe link mount lower?
David

[LowBuckX]

I remember a few details of rear alignment from working at a corvette shop for a short time that may be playing a small roll in what you are exsperiancing.. or maybe not.

Static alignment should start at a slight Toe- in to start and on accel and steady state driving the forces on the suspension will pull your tires/uprights to a 0 or near 0 toe. If you start with a 0 or near 0 toe The working forces will pull you into a toe out situation which maybe your problem to start. Just a thought to consider.

[chicane67]

My advice would be to call Guldstrand himself... if he's in the building, he'll talk.

The 84 to early 88 Y-body toe sucked. Then in late 88... GM finally made the change to the toe bracket to correct for their oversight (thanks to dickiepoo guldstrand and his years spent in the Corvette Challenge Series.) Guldstrand has offered a toe change bracket (and heim/rods in kit form) for some 23+ years. If I remember correctly, as it has been a few years since I actually looked at the orignal drawings, it knocked the toe change under 0.025"... from designed ride height to bump and was still very close to that with our normal street alignment settings. Noting that it requires the dog bones and camber rods to be of the same construction. However, in a bandaid effort, the OE dog bones with the Z51 durometer rubber were better at controlling tortional rotation, which in effect, limits link sloppiness and the resluted scrub (thrust) toe.

https://static1.pt-content.com/images/noimg.gif

Toe in on compression is better than toe out, that is for sure. If the rear squats at all, like during acceleration, it toe steers itself all over the place. On the stock chassis' we actually would dial zero to 1/16 toe out... just to compensate for torque loading and component flex. It was still rather squirrely rolling neutral or under decel... but on power it was right where you needed it. Dangerous is the only thing that comes to mind with that. On the modified chassis' we would set it for zero to 1/8 to in... because the taperd tube/heim rods were so ridgid, if you needed the 1/8th... you had better of dialed in an 1/8th.

By sight alone, the toe rods were set slightly positive, with the camber rods being set parallel to the plane of the earth. Remember... the initial camber setting will effect toe change in rate. The more static camber... the less it will effect the toe change when set up as stated above.

[Norm Peterson]

I think it comes down to making the toe control rod axis aim as closely to the FVIC as possible over some range of suspension position. But since the toe link swings in a much shorter arc than the FVSA it will not always intersect there. However, you probably do want it as long as possible for minimum 'error'. You don't want it parallel to the half-shaft (that is the "upper arm" in this suspension, IIRC). Holding the hub carrier pickup points fixed, I'd guess that inclining the toe links at a bit more than 1/4 of the way between the halfshaft and lower control arm inclinations would put you in the ballpark. IOW, and this is intended as example only as I have not even attempted to scale any angles or other dimensions, if your halfshaft inclination is 1° uphill toward the chassis and the LCA inclination is 5° uphill to the chassis, maybe shoot for something between 2° and 3° uphill toward the chassis for the toe link. A little less initial toe change in bump will come around to closer to zero toe at full bump.

You'll probably end up setting it such that there is a slight error (and slight bumpsteer/toe steer) at static ride height so that things WON'T go crazy as the suspension moves further from the static position, and 2° uphill might be too aggressive. FWIW, it's a 3-D problem, so fore/aft chassis bracket location and caster/camber gains matter. As, I think, is the matter of just how hard it gets driven.

How does the toe steer vary with suspension position? Are there any toe measurements vs bump/rebound positions other than static ride height and max bump? I think that sometimes the rate at which toe steer changes is worse than the actual direction that it's in. IOW, a very mild change in toe steer that's oversteer-ish may not be as bad as a sharply diminishing understeer-ish steer. And for that you need a plot with more than two data points.


Norm

[71roadster]

Thanks for the very informative responses. Especially the one abouth the amount of desired toe change. Quantitative data. I love it!

I had an epiphany while trying to work this out. What I forgot to account for was the fact that the halfshaft swinging up causes more toe-in than the toe-control rod ever will (the toe control rod being significantly longer). I incorrectly attributed all toe change to the toe control rod and so placed it to cause toe in. Taking in to account the half shaft causing toe in at a greater rate, I think it is apparent the toe control rod should be in or close to the same orientation as the half shaft. I will try this and measure the results.

[Norm Peterson]

Be careful.

The lower control arm is also moving in a manner that affects toe, as it forces the bottom of the upright to move in an arc rather than in a purely vertical straight line. That the upright doesn't rotate purely about either the half-shaft or the control arm is the definition of "Instant Center", actually. What the upright wants to rotate about for zero steer is the FVIC (which wanders laterally and vertically as the suspension moves). The knuckle at the toe link pickup lies somewhere in between and does not even follow a circular arc. But the knuckle end of the toe link has to by virtue of its single, physically fixed pivot.

That's why you usually DON'T want the toe link to be perfectly parallel to either the half-shaft or the control arm. When it is, it's more a matter of geometric "coincidence" (which may be "forced" by careful selection of the knuckle toe link pickup and either the upper or lower pivot points).


Norm