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.
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.)
Only RWD cars or
4WD cars can do this ! In the same situation, the driver in a FWD car
nothing to do other than easing the throttle, slow down the car thus
the centrifugal force, and hope the car can overcome the corner. There
are many disadvantages :
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.
Consider a driving wheel running in a corner. Due to the frictional force applied from the road surface, the tread in the contact patch distorts and creates slip angle. The faster the car corner, the more centrifugal force generates thus the larger the slip angle becomes. You can interpret this as the elastic distortion of the tyre generates a counter force to keep the car fighting with the centrifugal force. When the car is accelerated fast to the extent that the elasticity of the tyre reaches its limit, it could not distort anymore, thus more speed will lead to the tyre slide, and the car lose grip. This point is what we call "Cornering Limit".
A FWD or RWD car has already a lot of tyre distortion (slip angle) in the driving wheel because the tractive force is shared by only two wheels. Therefore there is not too much space left before the tyres running into their cornering limits. On the contrary, 4WD cars distribute tractive force to all wheels, thus each wheel shares considerably less tractive force thus create smaller slip angle in cornering. The car can corner at higher speed before the slip angle reach the cornering limit.
Grip aside, we concentrate back to our current topic - steering tendency.
There is always argument that whether the neutral steer of 4WD is better than RWD's oversteer. Although neutral is more favourable in the entry phase and mid corner phase during cornering, it doesn't provide the "correctability" of power oversteer in the exit phase. Remember, no driver could avoid miscalculation, no matter Mrs. Robinson or Michael Schumacher. Normally we need to feel the car's attitude and the road condition every moment before deciding how to control the car in the next moment. In this sense, RWD's controllable power oversteer is what we want.
Moreover, power oversteer of RWD ask the driver to intervene the throttle during cornering. This let him feel more involving and that he is mastering the car. In contrast, 4WD cars let the tremendous grip, the limited-slip differential and even the computer to rule the car's cornering. Therefore we always hear road testers said RWD is more fun to drive.
I am not saying 4WD
have power oversteer. Bugatti EB110, with its 30/70 front-to-rear
split, did that beautifully while providing tremendous grip. Even
a 50/50 4WD car like Mitsubishi Lancer Evo V could achieve slightly
oversteer by means of well-sorted suspension geometry. For example, if
the suspension is setup such that to introduce rear outside wheel
cambers when subjective to body roll, the contact patch area decreases
thus slip angle increases, then power oversteer is also available.
you cannot set the suspension to provide power oversteer as much as RWD
car since there is a trade-off in total grip and straight line
This is partly due to the market orientation ( it seems the wealthy customers tend to love secure rather than excitement), partly due to the use of wider tyres. In the past 2 decades, tyres of sports cars had been widened for about 50%, in addition to the growth in diameter, the contact patch area had been largely increased. Of course this is intended to increase the grip. However, increased contact patch area means every square inches of the contact patch carries less cornering force, so the tread distort less and the slip angle is reduced.
It is known that for the range of slip angle we concern (normally less than 20°), tractive force has less influence to the narrow slip angle than the wide slip angle, as illustrated in below :
This explain why the 115 hp version BMW Z3 1.9 has virtually no power oversteer ability. Its engine lacks the power to generate sufficient slip angle to the wide 205 rear tyres.
If it get considerable more power, like the M Roadster, power oversteer would have come back. But then again the car maker is very likely to install even wider rear tyres in order to cope with the increased performance, as did in the M Roadster. So once again the power oversteer is quite limited.
In my opinion, this
is quite frustrating to the front-engined RWD cars. It makes them
less and less fun to drive, although the increased grip will ultimately
improve cornering time. To mid-engined cars, whose rearward weight bias
used to create some undesirable oversteer, the adoption of wider tyres
could actually improve the handling and driving fun.