F355 rear toe scale factor

Hello guys from Fratelli 355 and Scuderia 355 (my choice )
Yesterday while installing a set of Michelin PS2 on the front wheels of my F355, I asked also for a check of the suspension geometry. Having no shim available, only the front toe has been tuned. All the four corners need to be modified, too many camber, probably due to the spacers ... and the fact that the car is still on the factory setting!

I need also to toe-in one of the rear wheel. This is the purpose of this thread.

On the rear, camber and toe trimming are almost independent. Add/remove the same quantity of shims on the lower A arm will adjust the camber only.
Add shims on one side on the arm, and remove the same quantity on the other side will adjust the toe only. Make a combination of the two and you tune camber and toe.
Camber scale factor (camber change per mm of shim) is rather easy to compute. It is purely geometrical.
Toe scale factor is my concern. Because of the bushings, twisting the lower arm will change the toe but not by the same amount. Half way if all the bushings had the same stiffness and probably something a bit different in real life.

My question ... finally : Does somebody know, by experience, this relationship between toe angle variation and the amount of shims installed.

My intention is to tune the geometry myself based on the actual measurements and the theoretical computations (some previous bad experiences made me a bit parano when it is question to work on my car) and to have it checked again at the time of rear tires exchange ... which will be soon :-(

Thank you for any data.

 

Man, you live in France where the true sexual revolution started and where both the felatio and the menage trois were invented! If I was in your shoe and I drove an F355 in such a beautiful country, the only toe I would be checking and working out would be the "Camel" kind!!!

 

Eric, it appears to me from the workshop drawing that the shims are inboard of the bushings; therefore exchanging shims will have no effect on the deflection of the bushing as both bushings are still relative to each other.

Toe change should still be a factor you can calculate mathematically.

Or am I missing something?
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Obviously, I miss some english vocabulary and expression even if I guess the meaning . Could you please PM me a description so that I can translate in french?

 

When you look at the drawing, there is no degree of freedom of the hub around the toe axis ... but the stiffness of the 8 bushings. If you put differential shimming under the lower arm, you will impose it a rotation around the toe axis. On the other hand, the upper arm I/F with the chassis has not moved. So the rotation of the hub will be somewhere in-between the rotation of the lower arm interface and the one of the upper arm interface.
To illustrate, imagine that the 4 bushings of the lower arm are very very stiff and the one of the upper arm are very very soft, then the hub rotation will follow the lower arm rotation. Now, if the 4 bushings of the lower arm are very very soft and the one of the upper arm are very very stiff, then the hub will not rotate at all because it is maintained by the upper arm which can not be shimmed. I think that in real life, with similar stiffness all around, the rotation of the hub should be close to half the shimming angle.
Does somebody has recorded the shimming effect knowing the angle before, the angle after and the installed/removed shims?

 

OK, Now I see what you mean.

 

this is from my website.

http://www.ground-control.com/toe-out.htm


eric355 has a good point, but it is what I would call a moot point. You shim the lower arm until the toe is correct, and any flex in the bushings is just the way it is. The front is more difficult in my opinion, as the wheel base, caster and camber are all tied to together, It is easy for an inexperienced (or experienced and lazy) mechanic to just get "the specs" right, instead of actually having the wheelbase(s) equal, caster correct, etc.

You need to do it yourself, or have a very trusted shop do it. Caveat: Many many "race mechanics" are NOT good at a true street car alignment and corner weight. PM me for references near you.

Jay

 

PM sent!

 

pied de chameau

This is the French Canadian visual translation.
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Nice! And I dont mean the city.

This is might help a bit:
http://youtube.com/watch?v=dHJA8M0diVw

 

Thanks guys to clarify ... !!!! Not sure it will help me to tune the suspension but was worth to learn ...

 

Thanks for the link ... but why are you making computations in "inches"

I made some computations based on bushing stiffness and arm levels. I got a sensitivity I will check during tuning. I would like to avoid the try ang guess method but I think I will probably have to make adjustements based on the theory, have the geometry measured, and finally make a fine tuning according to measured results. I definitively want to do it myself