Ideal Rear Toe Curve?
 

So I will try a rear BSK for my otherwise stock AP1. Obviously the goal is to eliminate bump steer throughout the suspension travel. My question is, what are the best results I should expect?

I will build a bump steer gauge using lasers on graph board and track the changes in TOE. I will start with the OEM TOE curve as reference. If I start at ride height do I want the best TOE curve from 1" droop to 1" bump? Or from ride height to 2" of bump? How much good travel can I expect? Can you use both the spacers on the BSK and the OEM eccentric mount to adjust the toe curve? Once that is established, do you just use the TOE link length to get the static TOE back?

Last question, what can I expect as a benefit? I don't have any specific complaints, however I can't say its the most stable/predictable car I have driven in my life.

FYI this is for road and track...

I'd recommend you inspect your bushings to set an accurate baseline.

You want a little toe-in as the suspension compresses, what you want to limit is toe-out as the suspension unloads. Simply put, toe is wheel steering, if the loaded wheel is turning in the same direction as the front tires are steering you actually increase stability within a certain window. However what happens with our car is that the suspension toes out as it extends, and it happens more dramatically than the suspension toes-in under load. So the inside wheel toes-out and also turns in the direction of the steered wheels adding stability. But now what happens when the inside wheel hits a curb or a bump, well now it toes in and now turns opposite to the steered wheels, this is the instability you feel. The quick transition of toe from in to out. You want toe to change in the curve, how it changes is more important than the actual values.

With the shock removed I'd say check 3" of suspension travel, that's a good place to start. That will give you a good baseline.

*Edit rebound damper settings have a huge effect on your bump steer curve, incidentally the preferred characteristics of the dampers for our suspension is having a relatively soft but linear compression curve, and a relatively stiff progressive rebound curve with stiff high speed rebound but medium stiffness low speed rebound. Stiff springs help to limit suspension motion which helps limit toe changes, but clever use of packers and bump stops can be used with softer springs. Ideally you want ~1.5" of suspension travel in order to keep the toe curve from getting too crazy, it's not too easy to do with this car so there's some compromise that has to be done because ample suspension travel helps stability.

Wow, great post. The rear instability has never been explained to me before.


Ultimately I am just tinkering for fun, to see how well I can get the S to handle. What is the general feedback on creating a more linear TOE curve? Is there a real benefit to this?

You want the toe constrained within a certain window. Without toe out during extension you won't get the inside wheel to help stabilize at higher speeds. Ultimately it's a compromise, you give up a little steady state stability for dynamic/transient stability. The toe change because of the Ap1 subframe is greater than Ap2, which is partly why the rear sway bars are thicker than Ap2, it limits unloading of the inside wheel, keeping toe out in check.

It's funny you mention the shocks...

Last track day I forgot to stiffen up the konis. I usually run them 3/4 out from soft on the street and 1.25 out on the track. The suspension moved a lot faster as the car pitched and rolled more than usual. This was the first time I got the feeling the back end was doing things I didn't input. Especially when I hit the curbing. Maybe the quicker suspension movement allowed me to notice something I never did before.

[media]https://streamable.com/n78f[/media]

Extreme bump steer

Another question. Can the eccentric toe adjuster be used for vertical movement? Can it be used to fine tune bump steer adjustment?

You mean on the rear control arm or the tie rod?

Toe link