Wheel alignment shim thickness

jp928

Active Member
#21
Done some measuring, to be taken with some salt. I make the mounting points (bolts) of the bellcrank 280mm apart horizontally, the bellcrank 350mm from pivot to top ball joint, upright distance between ball joints 265mm. All should be within 5mm of true.
This means the following:-
-to change caster by packing BOTH mounts away from bulkead by 1deg would require 4.63mm, .182" or 3/16". Conversely packing here 1mm will change(reduce) caster by 0.215deg. All assuming nothing else changes. Based on my experience I would suggest starting with 1mm or 1/16" if trying to address heavy steering, but proceed at your own risk. Dont use alloy - steel plate only. Suggest one strip long enough to cover the complete mount, drilled and slotted so it can be slipped in sideways once the bolts have been loosened enough. Measure the bolt spacing under the glove box.

- packing 1 of the mounts by 1mm will move the top ball joint 1.25mm in or outwards, depending on which mount you pack - multiply the packing thickness by 350/280 (5/4) to get this.
- moving the top ball joint by this amount will change the camber by ~ 0.3 degrees.

Shows how sensitive caster is...our value is very low compared to some other cars whose values I know - 3-7 degrees is not uncommon. Stupid windows calulator doesnt have artan, so I have done some interpolation.
 

cobraboy

Well-Known Member
#22
All this to take off positive caster. What is your plan to gain positive caster ?

Lowering the car @ 2" sent camber from zero to 1 degree negative. Installing a pair of 3/8" form A washers (approx thickness 1.5mm ea) under the inner arm mounts brought the camber back to 0.5 degree negative.
 

jp928

Active Member
#23
Didnt have a plan for that direction! Those who dont have PAS dont want to think about that! Facing the bellcrank mount faces down?
Your 1.5mm shim giving a 0.5d camber change fits with my calcs I think - I get 0.4 deg. Was the -1d not good ? Some cars are sensitive to front end set up - MX5s corner better at -1.75d camber, in some hands anyway. Looked at offset bushes ?
 

cobraboy

Well-Known Member
#24
What may not figure in your equation is the fact camber gain increases with travel away from stock ride height. Lowering 2" is quite a bit. I was anxious about tyre wear, and having just forked out for four new 245 section tyres did not want to cut them out straight away, so went for 1/2 degree negative. Subsequent track testing and logging of tyre temps across the width of the tread has confirmed the choice to be good.
There is adequate material on the back of the arm mounts to mill off a good amount in order to gain positive caster. I think the car could do with more in order to improve straight line stability on rough roads at speed, it may also help reduce tramlining on motorways in the inside lane due to ruts caused by lorries.
 
#25
Done some measuring, to be taken with some salt. I make the mounting points (bolts) of the bellcrank 280mm apart horizontally, the bellcrank 350mm from pivot to top ball joint, upright distance between ball joints 265mm. All should be within 5mm of true.
This means the following:-
-to change caster by packing BOTH mounts away from bulkead by 1deg would require 4.63mm, .182" or 3/16". Conversely packing here 1mm will change(reduce) caster by 0.215deg. All assuming nothing else changes. Based on my experience I would suggest starting with 1mm or 1/16" if trying to address heavy steering, but proceed at your own risk. Dont use alloy - steel plate only. Suggest one strip long enough to cover the complete mount, drilled and slotted so it can be slipped in sideways once the bolts have been loosened enough. Measure the bolt spacing under the glove box.

- packing 1 of the mounts by 1mm will move the top ball joint 1.25mm in or outwards, depending on which mount you pack - multiply the packing thickness by 350/280 (5/4) to get this.
- moving the top ball joint by this amount will change the camber by ~ 0.3 degrees.

Shows how sensitive caster is...our value is very low compared to some other cars whose values I know - 3-7 degrees is not uncommon. Stupid windows calulator doesnt have artan, so I have done some interpolation.

Your calcs are a fairly good starting point. I've added 4mm of shim to the inboard top mount and my camber has gone from -1.4 to -0.45. Caster is - 0.35 and -0.45 degrees so I think I'll stop chasing perfection for now
 
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