Re: Bilstein PSS10 Damptronics: A Must For The Tur
Another quote from this interesting article http://www.autozine.org/technical_school/handling/tech_handling_5.htm. Previously I quoted the discussion on the importance of the concept of slip angle in understanding understeer/oversteer behavior.
- Understeer : Front Slip Angle > Rear Slip Angle
Oversteer : Front Slip Angle < Rear Slip Angle
Neutral steer : Front Slip Angle = Rear Slip Angle
Just as interesting is how a typical 4WD drive car is expected to behave. Although the discussion here is not specific to the Turbo, it allows me to understand why the first thing my tuner says about my car is that it doesn't "rotate" in corners, and why so many of us add negative front camber to reduce the Turbo's understeer.
Note also how controlled throttle oversteer (rotation per my tuner & NOT the same as lift-throttle oversteer!) could be a very good and fun thing. 
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Non-neutral steer due to Tractive Force
Car magazines often prefer the handling of rear-wheel-drive cars. They say FWD cars usually understeer while RWD is easier to provide power oversteer. Now, we use the concept of Slip Angle to explain this.
Consider a driving wheel, which is under cornering and has created slip angle. If tractive force (that is, the pulling force from the engine) is applied, the slip angle will increase (See Figure in below). This is because the tractive force applied between the tyre and ground will distort the tread on the contact patch further.
- Now the scene is clear.
FWD cars has the front wheel's slip angle > rear wheel's. This result in Understeer.
RWD cars has the front wheel's slip angle < rear wheel's. This result in Oversteer.
4WD cars, if the front / rear torque split is equal, has equal F/R slip angles, thus result in Neutral steer.
(Remind you, understeer, oversteer and neutral also depend on suspension design, weight distribution etc. So we cannot say all FWD cars must understeer or all RWD car must oversteer. In fact, car makers usually design the suspension geometry to compensate the non-neutral steering generated by FWD / RWD and weight distribution.)
Power Oversteer and Lift-off Oversteer
The more tractive force we apply, the larger slip angle is created in the driving wheel. Therefore, for the RWD cars, we can use the throttle to control the degree of oversteer. When the car is entering a corner too fast and seems likely to run wide, we can correct its direction by increasing the throttle (not to do this before reaching the mid corner !), then the car oversteers. If we find the correction is too much, we can ease the throttle and let the car returns to neutral steer or even mild understeer, depends on the suspension design and weight distribution.
Only RWD cars or rear-biased 4WD cars can do this ! In the same situation, the driver in a FWD car has nothing to do other than easing the throttle, slow down the car thus reduce the centrifugal force, and hope the car can overcome the corner. There are many disadvantages :
- You lose time during slow down.
- You lose engine rev during slow down, thus the engine takes longer to rise back to the useful power band once you exit the corner.
- Very often, if you miscalculate, you are unlikely to have sufficient road ahead for you to slow down, especially in tight corner.
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Therefore we always say RWD car is superior than FWD car in handling. There are, however, some well-sorted front-driver (especially some GTi) can play "lift-off oversteer", which is actually the reverse of "power oversteer" - a degree of permanent oversteer is built into the car but is only accessible when the car is pushing to the limit and with throttle disengaged. Step down the throttle again will reduce the oversteer and even back to understeer. Anyway, obviously this is still not as controllable as "power oversteer". While power oversteer can extract a lot of oversteer - actually depends on throttle - lift-off oversteer is rather limited, simply because it is impossible to build a lot of permanent oversteer to the chassis without deteriorating handling in lower speed or straight line.
Once again I have to emphasis that the power oversteer must be highly controllable by the driver, otherwise the car may lose control and spun. To make a good power oversteer car, the secret is to match the power and cornering limit perfectly at the speed concerned. If the cornering limit exceeded the power, the rear wheels will grip hard and refuse to slip. In contrast, if the cornering limit is too low or the engine torque is too high at the speed concerned, the rear end will slide severely once the throttle is pressed. Therefore, the cornering limit must be set at a level where the engine output, at the speed and road we normally want the car to power oversteer, has just sufficient power to exceed. To implement it , choose a suitable set of tyres, applying suitable amount of downforce and an adequate front / rear weight distribution is very crucial.
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Regards,
Can
997 Turbo + Bilstein Damptronic (Review) + Cargraphic Exhaust (Heavenly Race Car Noise 
Review)
)
My approximate ride height delta's were done using fender to ground -- before and after Bilstein; this is NOT the correct way.
, using mid-fender height to ground, shows the post drop heights to be 65.7 cm front, 68 cm rear. The front drop is therefore around 15 mm, rear around 10mm. 
)
). Of the 2 main ways to increase stiffness in a coilover:
. I believe raiyu's car is a Turbo with Bilstein suspension.
(Pics taken at past LA Auto Show.)
