オーバーステア & アンダーステア
Oversteer in the “textbook” sense is the difference between cornering stiffness at front and rear, taking into account tyre characteristics, geometries, bush rates, etc, etc. By this definition, understeer and oversteer are independent of vehicle path or speed - they are purely vehicle properties.
Oversteer or its counterpart understeer is not the difference (or should it be the ratio of the two?) between cornering stiffness front and rear but it is a dynamic trajectory property caused by cornering stiffness. But cornering stiffness are not just static properties influenced by the factors given above. There are more factors that influence cornering stiffness: body roll, roll stiffness front and rear, load transfer, wheel camber change, roll steer (front and rear) and torque applied to the driving wheels. The tyre characteristics are speed and load dependent too. From this, it follows that driving characteristics of a car cannot be defined as being just over or understeering. At every speed and trajectory curvature (1/radius) one can say it is understeering, neutral or oversteering for those conditions. So the over/understeering character is a complex one.
In the everyday road car a compromise is sought to make the car responsive to the steering wheel and stable at the same time. From here follows the success story of the FWD concept (understeering). For a sports car that is quick (not the same as fast) and provides driver fun, a different concept is needed: mid-engined, rear wheel drive (initial slight understeer changing to oversteer by applying engine torque - power oversteer/steering with the throttle).
Perhaps it is useful to mention the definition of over and understeer as used by the scientists who perform research in vehicle behaviour? Suppose you are running a car at a constant speed with a constant curvature (the path is a circle then), if you then keep the steering wheel in the same position (angle) and increase the car speed gradually, then if the car wants to run on a circle with a smaller radius the car is said to be oversteering, if it remains on the same radius it is neutral and if the car goes to a wider circle it is understeering (for that speed and curvature!). The test can be done somewhat differently, again the car is running at a certain speed on a certain radius. Increase the speed and try to keep the car on the same radius. If you need to apply extra steering angle it is understeering, if you can keep the steering wheel in the same position it is neutral, if you can turn back the steering wheel somewhat it is oversteering.
Oversteer is when the rear wheels are carving a larger arc than the front wheels or the intended line of the turn. Rear “slip angles” exceed those of the front tyres. This is often described as a “loose” condition, as the car feels like it may swap ends, or be “twitchy.” This condition can be caused by “power oversteer”, where you need to reduce power in order to bring the back end back into line.
Understeer is when the front wheels are carving a larger arc than the rear wheels. This is often described as “push” or “pushing” - as the front end feels like it is ploughing off of a corner. Further acceleration only compounds the push, as weight shifts back to the rear drive wheels, off of the front turning wheels, leading to a further lessening of the car’s ability to turn in. Understeer can be remedied by slight modulation in throttle to transfer weight forward to the front wheels, aiding their traction and ability to carve the turn. Many cars are designed to have a tendency to understeer. If the driver gets uncomfortable and “lifts” off the gas, that will cause the front end to tighten the curve - a relatively safer, and more predictable condition.
Trail braking (a.k.a. ‘brake-turning’, braking while turning toward the apex of a corner) is another learning curve for you to climb sooner or later. However, learn trail braking slowly; if you’re used to road driving (where you’re taught to finish braking before turning into a corner) then you might find it tricky to learn the extra delicacy demanded by trail braking. In PU, the trade-off between brake pressure and steering input is hard to judge when you can’t feel the car turning and pitching through your body.
What is trail braking? In essence, it means continuing to brake after having turned in for a corner. The further you progress into the corner, the more you turn the steering wheel and the more pressure you release from the brake pedal. Typically, the procedure goes like this: You are hurtling in a straight line toward a corner; You apply the brakes - fully - while still travelling in a straight line; At some point, you release a little pressure from the brakes and start to turn in; As you bend into the corner and approach the throttle application point, you progressively release the rest of the pressure from the brake.
What’s the point of it? Trail braking helps you rotate the car into a corner by controlling the transfer of weight onto the front tyres, giving them more stick, and thus compensating for any understeering tendency the car would otherwise have. The alternative is: do all of your braking in a straight line, then release the brakes entirely, then turn in. The trouble with this technique is that when you release the brakes, weight - and therefore stick - will be removed from the front tyres, just when you need them to be loaded enough to turn the car into the corner. So - unless the car is set up to be driven like this - it will understeer away from the corner. This is typical behaviour for ‘street’ (aka massively understeering) cars that have been adapted for racing.
On the other hand, a ‘proper’ race car will probably oversteer if you don’t trail brake. If you turn into a corner with your feet off both brake and throttle, the front tyres will have all their traction budget available for turning while the back wheels will be doing some (engine) braking. Net result: oversteer. Application of the brakes settles down the oversteer by substituting a proportionately balanced loss of steering traction (because the brakes are biased towards the front). In fact, you use the brake pressure to control the rate at which the car rotates into the corner.
How much trail braking you do at a particular corner - i.e. what percentage of the corner is taken under braking - depends on the angle of the corner. For a 60° corner, you’d typically only trail for a few percent of the corner, for a 90° corner, you’d typically trail brake for maybe 25% of the corner, and for a bigger corner, you could do it for up to 50% of the corner. You are aiming to trail off the brakes until they are released completely at or before the throttle application point (which typically occurs somewhere before the geometric apex). - Thanks to the Virtual Racers Edge Site for this helpful info.
Think of the contact patch of your tyres. The few square inches that each tyre touches the road with may be upset not only by rapid steering input but by rapid brake input as well. ABS minimizes this, but there is a way that a good driver can shorten stopping distances even beyond ABS in some cases. How?
ABS works in different ways, but to illustrate this point, I’ll take an example of, say, a Subaru Outback with four people in the car on a snowy surface (this may be easier to visualize given a heavier car on a slick day): You begin a panic stop, depressing the brake. The car goes up to maximum braking, and then one wheel begins to slide or lock. As it does so, the car momentarily pulses the pedal back to you, which feels like a “burp” under your foot. In doing so, it allows that locked wheel to again rotate. This helps in two ways. First, a tyre brakes most efficiently when it is just short of locking up, exerting the maximum effort on the surface. When it goes beyond that point and slides, braking is diminished. Second, by keeping the wheels from locking up, ABS helps maintain directional control, especially if the driver is steering around an obstacle while braking. In effect, in a panic stop ABS allows the driver to pound the brakes as hard as he or she wishes, but still have effective braking and directional control. Remember that a tyre gets maximum braking just before the point of lockup. Without ABS a driver can get maximum braking effort by braking to the point of wheel lock, and then reducing the pressure ever so slightly to the point where the wheels are rotating again. This is known as “threshold braking.” The dangers of course are in backing off a bit too much and not getting maximum effort, and in maintaining directional control while braking.
Back now to an ABS equipped car: By using threshold braking, it is even possible in some cases to do better than ABS. Remember the pulsing action that some ABS systems use to allow the locked wheel to rotate again? While this is happening, the braking effort on the other three wheels is momentarily lessened also. On a slick snowy surface, if you continue extreme hard braking so that ABS is constantly activated, the lessened brake force is extremely evident. It almost feels as though the car starts sliding faster! If this happens to you, back off slightly to allow the ABS to disengage, and use threshold braking ... release ever so slightly as the ABS starts to “burp” back at your foot. A well executed stop will have you riding that edge, with ABS intermittently engaging as you modulate and keep feeling that threshold. On DRY pavement with good traction, you can stand on the pedal as much as you need to stop quickly and effectively.