Need help eliminating oversteer

Originally Posted by BGTV8

Been busy .............. still traveling 2000km to work each week(by air - not driving) and trying to keep SWMBO happy leaves me with almost zero time.

In terms of rear suspension setup, I have deliberately used lower diameter tyres to drop the CoG rather than over-lowering on the spring perches which as several have surmised leads to excessive camber gain in the rear in bump and camber-reversal in pitch under braking being the cause of rear instability.

Over springing the car will kill rear grip - which some folks counter with a really fat tyre (needed perhaps for FI cars but not IMHO a NA car). I reckon that a 275 wide tyre in 35 profile on square setup to be ideal for me although others clearly prefer a bigger tyre.

I am happy with the balance on 265/35R18 (Yoki A050) or 275/35R18 (NT01) and will be running either Dunlop 280/650R18 slick or perhaps a Pirelli 285/645R18 and therefore following the pattern of lower diameter tyre to drop the car rather than lowering it on the spring which stuffs-up suspension angles and compromises rear camber control in bump as well as getting some weird angles in droop as the car pitches under braking which can affect your ability to rotate the car as you come off the brakes. Somewhere I have the SusProg details of the Z34 rear suspension and I was surprised at some of the changes in droop - which is something to remember - camber gain in bump is one factor to look at, but so is camber and toe change under droop as the car pitches under heavy braking. Remember that the rate of pitch is affected by rebound in the rear shocks and bump in the fronts and the recovery is governed by rebound at the front and bump at the rear so a hard turn-in can be affected by rear toe/camber recovery as you come out of the brake, depending on these settings

I have a 3.9:1 diff with Quaife LSD and a 4.08:1 with KAAZ LSD for fast and slow circuits respectively.

I go back to the settings that Doran Racing used in Grand Am and took my initial lead from them.

I've limited my drop on the springs and added 5mm of rake by dropping only 15mm at the rear and 20mm at the rear as my absolute lowering limit, which still does get played with from time to time.

I have just measured from bottom edge of the rim to the under-edge of the wheel arch which are 610mm all round on my DD wheels (245/45R18 fronts and 275/40R18 rears) - these tyres are Falken FK453 at 677.7mm dia front and 677.2mm rear. My track Enkei's RUN 275/35R18 NT01 at present and these are 650mm tall - so tyre height accounts for around 13mm CoG overall drop. I run KWv3 shocks (front rate is 425lb/in and rear is 450lb/in) but have Penske double adjustables to go into the car when I take it off the road. Spring rates will rise to ~550 front and ~650+ at the rear on proper coil-overs as a starting point but there will be a lot of testing to get it right.

I am running quite a soft setup (Whiteline front bar, full stiff and Whiteline rear bar fitted but currently disconnected - sometimes I run it on softest setting if I need to shift balance to the front depending on track and/or track grip).

I have the yaw-sensor DIY installed as the standard electronics are dangerous on the track - even with VDC off, there are sufficient electronic nannies left to cause problems which I reckon is dangerous.

I also have the SPL FUCA installed and the balance of SPL bits for both front and rear suspension/subframe but not yet installed as the car is still my sometime/weekend drive (I have a Ford Ranger ute as my actual DD) and not a dedicated track car.

As soon as the car comes off the road (it has 160K kms on it now) it will get the complete SPL suspension upgrade and Penske shocks installed as well as 4-litre forged engine, Motec M150 and QBE69G gearbox which is when suspension tuning really starts. I also have AP brakes to go onto it and if I can persuade one of my suppliers to part with his old-spec Bosch MotorSport ABS - I'll have that as well. The Motec can also support launch and tunable traction control but that will be a bit down the track I think