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Custom rear camber shims to tune rear to match front agressive alignments, interested?

4442 Views 7 Replies 4 Participants Last post by  raamaudio
Custom rear camber shims to tune rear to match front agressive alignments, interested?
The ST factory alignment is engineered to allow the rear of the car to rotate more instead of plowing ahead, under steering, like most cars are setup as the average "driver" has no clue and is safer with under steer.

In the ST case less negative rear camber and more positive front than the regular Fiesta settings induce some over steering fun and can benefit fast good drivers.

When adding in serious front camber and we all need to if we want to really get the car to grip up front it can cause to much rotation in the rear at speed which can be harder to overcome, more dangerous for some, than under steering or neutral.

(Note: BALANCE, in all things, is all that really matters, to much negative front camber can cause a lose of grip out of the corners)

Optimally we want over steer at low speed and neutral or understeer at high speed which I was pretty easily able to do with my Vette but this car will take some effort and testing to get it right if we can at all.

I have not done any testing yet on the ST so looking for a low cost method like the rotating plastic shims I have seen and when finding a sweet spot having metal shims made from a pattern already worked out for these cars.

Looking at the numbers the ST is easily seen to be tuned to rotate and the regular cars to under steer like most cars are setup.

(Any math errors please point them out:)

Regular Fiesta stock front camber -0.70 ST-1.18 Proposed front camber for my application, -2.5
Stock rear camber -1.52 ST-0.64 Add same amount to rear to match front -1.32
Difference front to rear R-0.82 F 0.54

Quite a significant difference, to keep the same balance front to rear, if indeed that would be best, we would need to add -1.32 degrees more negative rear camber.

I might want as much as -3.0 degrees negative front camber which would mean adding -1.82 rear just to keep the factory front to rear angles on the same plane.

For reference I went back to my very fast SM class 250WHP turbo Matrix I built back in late 2002. It was not a performance model and thus had stock alignments to induce understeer, always, it was a twist beam rear axle car, tall, close to same weight, longer wheel base.

Stock front camber -0.77 Camber I setup, -2.50
Stock rear camber -1.45 Stock rear -1.45
Front to rear difference R-0.68 F-1.05

The Matrix had just enough over steer at slow speed and pushed(under steer) a bit on high speed corners, especially when throttle enhanced by having
-1.05 degrees more front camber than rear.

If you lower the ST and tune the front camber to -2.5 degrees and leave the rear at the stock setting you end up with a difference of
-1.86 degrees more negative than front camber which can lead to more rotation and higher speeds that you need, or want.

On the Scion TC, another fast car I built, IRS came into play, I found the best setup was -3.0 front and -2.0 rear
-1.0, a difference close to the Matrix that had
-1.05 more in the front than rear.

This does not give a final answer as to what is best but I imagine it is far from optimal with the stock rear setting and
-2.5 to -3.0 in the front on the ST.

Some have been using massive tire pressure changes, even trying different tires front to rear, etc....I think it is time to look at the root causes and fix them the best we can and then use spring rates, shock settings, sway bars and air pressures, in that order, to fine tune.

For now it seems a rounded up to
-1.0 shim would be a pretty good ball park change in the rear.

It is time to study what successful racers with twist beam rear axles have done in various cars and types of racing.

Anybody care to help?

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I don't know enough about this stuff I came from 4th gen Hondas which were multilink rear which made camber adjustment really easy. I also don't know if I'll need these ever since my FiST is a DD, but if shims can be made that seems better than permanently modifying the beam.
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