Thread: Time Attack BRZ
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Old 12-02-2018, 06:12 AM   #21
JDwhiteWRX
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Member#: 190502
Join Date: Sep 2008
Chapter/Region: International
Location: Sydney, Australia
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Decided to jump in the deep end and convert the McPherson strut front suspension to a double wishbone. It's something I have been thinking about since I bought the shell but the whole idea seemed very daunting and information specific to a conversion was not really available. The only way was to focus my research on designing a double wishbone suspension from scratch and then adapt it to my car.

So I picked up a pair of RX8 uprights for $200 complete with wheel bearings and tie rods.



It would have been nice to buy the entire RX8 front suspension including the subframe but unfortunately I could not find one for a reasonable price. luckily the guy I bought the uprights off was kind enough to let me measure all the factory control arms.

Next step was to make a jig to position an upright in the correct position in space that it would sit if it were mounted to a 18x11" wheel with a 295/35/18 tyre on it.



With the upright in the jig I was then able to accurately measure the ball joint and steering arm locations, these are your "fixed" points that you can not alter. The arm lengths, angles and points at which they attach to the chassis are all the points you have freedom the play with.



I spent the majority of Saturday night and most of today measuring everything and plugging the numbers into some software I got off the Locost forums. I have tried to stay as close to as possible the Mazda RX8 geometry as it is considered to be very good. The software takes a bit to get used to but basically it just plots everything in X,Y,Z coordinates and allows you to see what the camber and roll center is doing in bump and roll or both.

I then lowered the shell back to its planned "lowest" ride height and positioned the jig inside the front wheel well such that the face of the wheel bearing is in the same position as it would be on a stock BRZ.



Now I was able to take my arm lengths and angles and see where the inner pivot points would land on the chassis. The RX8 has longer arms than the BRZ since the engine bay is not as wide so the stock RX8 arm lengths would require major surgery to fit to the BRZ.

I then went back to the computer with my shorter or more appropriate arm lengths as measured to see what effect shorter arms would have on the geometry. The results were good, I am seeing a 1.4 degree gain in camber with 2 degrees of roll. With 1" of bump I am gaining 0.5 degree of neg camber and combing both I am seeing 1 degree positive camber. Roll center migration is very minimal at less than 1 degree under 2 degrees of roll. The results are actually very similar to the stock RX8 arm lengths once to change the angles around to suit the shorter arms.

I have not yet factored in the steering other than doing my "fixed point" measurements with 7 degrees of positive caster in the upright. Ideal positioning the steering rack is not too difficult to work out however there is the chance of something being in the way of the ideal position and then I have to get creative.
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