Neutral / Understeer / Oversteer

We often hear these 3 terms in car magazines. I think few people would argue if I say they are the most important elements in the study of handling.

What is understeer ? Basically, if you turn the steering wheel and find the car steers less than you expect, the car is understeering. This is not because your subjective judgment goes wrong, in fact any car must have some degree of non-neutral steering due to the weight distribution, suspension design, tire used, lateral acceleration and road conditions. Further more, a car could understeer in this corner and then oversteer in that corner. The whole picture is very complicated, so I'll spend more paragraphs to discuss this topic.

What do we need ?

It seems that neutral steer must be more desirable than understeer and oversteer, but in fact it is not.

In fact, when running in straight line, we want a little bit understeer to make the car stable. When the car is subjected to side force, probably due to cross wind or the road's irregularities, understeer could resist the force and avoid the car to be steered automatically, therefore the driver need not to correct the steering frequently.

When the car is entering a corner, we also need a light understeer to provide the stability while the driver is easing off the brakes and building up cornering force. In mid corner, we need neutral steer. In the exit phase, a slight oversteer will be welcomed as it helps tightening the path. However, the degree of oversteer must be progressive and easily controllable by applying and easing throttle. We call this "Power Oversteer". Without power oversteer, we have to ease the throttle (thus loss time) or the car will run out of the corner.

However, I must make clear that what I say "slight understeer / oversteer" is usually deemed to be "near neutral steer" by most car magazines. This is because in reality there are too many cars running on severe understeer thus they used to them. In other words, if a car magazine said the Porsche 996 has mild understeer, it probably equals to "medium understeer" in our sense.

Basic Concept : Slip Angle

Before going on our study, we must understand the concept of slip angle first.

When a car enters a corner, all the tires are turned with respect to the ground. Due to the elasticity of the pneumatic tire, the tread in the contact patch will resist the turning action because there is friction generated between the rubber and the road surface. As a result, the treads on the contact patch will be distorted, whose direction always lags behind the direction of the wheel ( See figure in below ). We call the angular difference between the treads and the wheel's direction as Slip Angle.

Note : the car is turning left
In which direction the wheel is running ? It is the direction of the tread, not the direction of the wheel. I am not saying the tread has any ability to force the wheel to travel in its direction. On the contrary, the tread is only a sign showing how an arbitrary point on the tire surface travels. If the arbitrary point travels in that direction, so does the wheel which is the summation of thousands of those points.

Now you must think the existence of slip angle must reduce the car's steering angle thus leads to understeer. In fact, it is not so if everything else are perfect. Because both the front and rear tires have more or less the same slip angles, they counter each other thus the resulting steering angle remains unaltered.

However, if the front and rear wheels have different slip angles, then we get understeer and oversteer :
 

Understeer : Front Slip Angle > Rear Slip Angle

Oversteer   : Front Slip Angle < Rear Slip Angle

Neutral steer : Front Slip Angle = Rear Slip Angle



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 tire 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 :
 

1. You lose time during slow down.
2. 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.
3. 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 tires, applying suitable amount of downforce and an adequate front / rear weight distribution is very crucial.

RWD versus 4WD

Basically, 4WD does not introduce power oversteer. However, most people still prefer it simply because it provides superior cornering grip thus improve cornering speed. As I have promised earlier in the Cornering Grip section, here I'll explain how 4WD improve cornering grip :

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 tire 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 tire reaches its limit, it could not distort anymore, thus more speed will lead to the tire slide, and the car lose grip. This point is what we call "Cornering Limit".

A FWD or RWD car has already a lot of tire 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 tires 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 favorable 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 cannot have power oversteer. Bugatti EB110, with its 30/70 front-to-rear torque split, did that beautifully while providing tremendous grip. Even though a 50/50 4WD car like Mitsubishi Lancer Evo V could achieve slightly power oversteer by means of well-sorted suspension geometry. For example, if the suspension is setup such that to introduce rear outside wheel positively cambers when subjective to body roll, the contact patch area decreases thus slip angle increases, then power oversteer is also available. However, 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 stability.
 

New Trend for RWD cars

In the past 2 decades, we saw car makers gradually increases understeer in RWD cars, making them more "secure" to drive. Porsche 996 is a good example. Its predecessor 911 used to offer hell a lot of oversteer, now the 996 becomes a very civilized GT.

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 tires. In the past 2 decades, tires 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 :

Therefore, when apply the same power, the rear wheel slip angle increases in a lesser rate in wider tires. In other words, power oversteer is less obvious.

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 tires.

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 tires 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 trend is quite frustrating to the front-engined RWD cars. It makes them having 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 tires could actually improve the handling and driving fun.

Non-neutral steer due to front / rear weight distribution

Here we are going to discuss the theory behind front-heavy cars tend to understeer and rear-heavy cars tend to oversteer.

When a car is cornering, its CG is subjected to centrifugal force. The tyres generate slip angle thus frictional force to counter the centrifugal force, so the car keeps cornering without slide. (See figure in below)

If the car is heavier at the front, that is, the CG is near the front, obviously the front tyres shares most of the centrifugal force thus they have to generate larger slip angle thus larger frictional force to counter the centrifugal force. As a result, the front slip angles exceed the rear's, and understeer occurs.

On the contrary, rear-heavy car has larger slip angle at the rear, thus introduce oversteer. Similarly, we can find a 50/50 balanced car having neutral steer. This is our choice for optimum handling. We don't really need oversteer in this case, because such oversteer is not controllable, unlike power oversteer which we have found in RWD cars.

The result favours front-engined, RWD cars (FR), which is easiest to achieve 50/50 F/R weight distribution.

Mid-engined, RWD cars (MR), with its slight rearward weight bias at about 40/60, is slightly inferior in here. But remember, its superior steering response, steering feel and dynamic balance are probably more than enough to compensate.

Front-engined, FWD cars (FF) is the worst in here, and far worst. As all the heavy mechanical parts - engine, transmission, differential - hang over the front end, the front axle normally takes up to two-third of the weight. This tends to create heavy understeer. In addition to the understeer generated by the FWD configuration, the result is even worse. This require a lot of work to do in the suspension geometry and steering mechanism for compensation. And there must be some trade-off. Take an Alfa GTV as an example. It has to install an ultra-quick 2.2 turns steering to counter understeer, thus requires quite a lot steering effort. If power steering were increased, steering feel must be deteriorated. The multi-link rear suspension was also probably chosen for compensating the understeer because the geometry is more tunable than the original MacPherson strut.

There is another problem troubling the Alfa - the 3.0 V6 version, which is intended to be the range-topper, found its even heavier front end leads to inferior handling than the cheaper and slower 2.0 version. This is a headache to the marketing personnel.

However, once again I have to point out that everything must have exception, especially when all mass production cars are also limited by other factors such as packaging, requirements for refinement and cost etc. When both under these limitations, a well-sorted Alfa 156 could outhandle an ill-fated BMW 3-series. Although recently RWD luxurious / sports sedan / compact elegant sedan seems to be reviving, FF is still the main trend for the majority budget cars due to its lower cost and space-saving advantage.
 

Non-neutral steer due to Suspension Geometry

We've said a lot suspension geometry can alter the steering, and it is usually used to compensate the undesirable steering tendency due to uneven weight distribution and FWD / RWD. Now I'll briefly go through this.

Camber - Decisive to understeer and oversteer

As shown in below, if a wheel is not perpendicular to the road, then it is cambered. If it leans towards to the center of the car, then it is negative cambered. (or " toe-in"). If it leans outwards to the car, it is positive cambered (or " toe-out", as shown in the following picture.)

When a wheel has positive cambered, due to the elasticity of tyres, the wheel will be reshaped to something like the base of a cone. It will have a tendency to rotate about the peak of the cone, as shown in the picture. Now, you will see the wheel tries to steer away from the center of the car.

If both the right and left wheels are positive cambered (that means they leans to opposite direction), the steering tendency will be cancelled so that the car remains running in straight line. If the car is turning into a corner, weight transfer put more load on the outside wheels than the inside wheels, that means the outside wheel's steering tendency will have more influence to the car. As the positive-cambered outside wheel tries to steer the car to the outside of the corner, the car will be understeered.

On the contrary, if both wheels are negative cambered, the car will oversteer.
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For FF cars, we could introduce some negative camber to the front wheels to reduce the understeer. Similarly, more positive camber could be employed to the rear-heavy 911.

We may deliberately need positive / negative camber, but we don't want the camber to be changed when the wheel meets bump or when the car body rolls into a corner, otherwise the handling will be very unpredictable or even uncontrollable. Therefore we prefer a suspension geometry whose camber varies little under all conditions. As said many times in before, double wishbones, especially is non-equal length, non-parallel double wishbones, is generally regarded to do the job best. Therefore from sports car to Formula One, all the high performance cars use it. For other kinds of suspensions, you can read the previous chapter about Suspension.

 

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Steering Feedback and Torque Steer

The steering must offer enough "feel" to the driver so that he can sense what's happening as he approaches the cornering limit of the tires. It must also have some self-returning action, but it cannot be so heavy as to cause fatigue or loss of sensitivity. This feel, feedback and self-returning action is a function of kingpin inclination, steering offset and castor angle :



The more the steering offset D, the more self-returning effort generated. Similarly, the larger the castor angle, the more self returning action.

If the car is FWD, the steering offset D will introduce torque steer. This is because the tractive force will try to pull the center of contact patch of the front wheels forward, thus the wheel will rotate about the point the kingpin axle projected to the ground. The torque steer moment is the product of D and the tractive force. Therefore the amount of torque steer is proportional to D. The solution is to build more inclination to the kingpin so to reduce D. This is easy to be implemented in double wishbones suspension which is shown in the picture, but not MacPherson strut, whose kingpin also serves as spring and shock absorber. If we incline the kingpin too much, there will be too much lateral force transmit via the spring / shock absorber to the car body, thus causing shake and instability.

Therefore we say MacPherson strut is not very suitable for FWD cars having a powerful engine. Alfa Romeo 164 is one of the examples, whose torque steer ruined the otherwise brilliant handling. No wonder its successor, 166, has switched to double wishbones front suspensions.

 

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Chassis Rigidity

The last method to improve handling is to strengthen the chassis. Since the late 80s, we saw chassis rigidity of new cars have increased a lot. Whenever a new car is launched, the manufacturer must claim its torsional rigidity has been increased by at least 20%. This is partly due to the requirements for crash protection, partly in order to improve handling.

Consider a car with a very weak chassis which is easy to flex and twist under force. If it employ stiff springs and dampers to the suspension, the shock cause by road irregularity will be transferred to the chassis directly. The weak chassis will be twisted and bent, thus the suspension geometry will be reshaped, creating non-neutral steer and other side effects that is not the original suspension design intended to cope with. Therefore a weak chassis must ride on softer spring and dampers.

For the benefit of handling, we always want stiff spring and damper as long as ride comfort is acceptable. So we need a rigid chassis which could cope with the stiff suspensions without flex or twist.

Credits: Mark Wan, Bill Peirce

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