Understeer, also known as a "front wheel skid", a "slide", or simply "Push", is the physical tendency of a car to be reluctant to turn into a corner, because the inertia is still pushing it forward (while the tires point sideways, into the corner). It is a natural occurance, in race cars like any other automobile or moving object. It is usually easy to control, but it can also be frustrating and, in severe cases, very frightening (Even more than "Oversteer"). Understeer is when the front tires slip and push you forward out of line, as if the car is "under-steered" (hence the name). Oversteer is the opposite effect, where the rear tires slip, and rotate the car "too much" into the corner, resulting in a spin.
Causes of a skid
The faster you go, the harder it is to turn the car. The sharper you have to turn the car, the harder it is. The more slippery the surface, the harder is the change of direction. Meaning, that all of these factors contribute to understeer. Therefore, you can have slight understeer caused due to speed, or you can have a very acute understeer, caused due to a combination of factors. The best solution for understeer (and oversteer) is to avoid or at least minimize it.
Understeer is a pure expression of the car's slip angle (often used as a synonym for understeer). When you turn the car in a physically perfect world, the car turns where you point it. In the real world, it will never turn as tightly. The front wheels will point into the corner, but the car will take a more shallow route. The differential is known as the "slip angle." The tires are elastic, and can twist. Because of this, when you turn the wheel, the tire twists so that the actual part that's touching the ground remains pointed forward. It can only bend so much, so it is turning into the turn, but not as tightly.
The slip angle creates a side force that pushes the front sideways into the corner. Since cars are obviously rigidly connected between their front and rear axles, this rotates the car and places the rear wheels in an angle relative to the road, which means that the rear is now experiencing a slip angle, that further rotates the car and it's front into the corner. Understeer is a situation where the front slip angle is larger than the rear slip angle, so the car is not turning as tightly.
It's important to understand that understeer is not a function of the car's setup, but of the very laws of physics. "An object in motion, remains in motion, unless acted upon by an external force." So does a forward-rolling car is reluctant to turn into the corner. For this to happen, it has to yaw into it, as you turn the wheel into the corner. We can in fact say that, from the point where the wheel is straight untill after you finished turning it into the corner, the car is experiencing a "transient" or a "change of directions" during which it's dynamic capabilites are tested. A car that turns by steering it's front wheels will ALWAYS understeer at this point. Any oversteer or neutral handling will only be experienced inside the corner, after turning the wheel.
That's not to say that you can't understeer past the turn-in. When inside the corner, every car that is being held on constant throttle to a point where it's not slowing down or speeding up, but is rather in it's 'natural' balance, only turning -- will experience understeer, starting from all road cars and off-road utility vehicles, to rally cars, drifting cars and even the most prominent single-seater race cars. In fact, proper race cars usually have a surprising amount of understeer!
It's important to acknowledge that understeer is much easier to detect and recover from than oversteer. To begin with, understeer is self-solving. After all, you made the car slide by making it take a turn at a speed which is too fast for the given radius. Now, the car is sliding forward and out of the turn, so it is increasing the radius and scrubbing off speed, so it will eventually re-grip. Oversteer, on the other hand, sharpens the radius from millisecond to millisecond, calling for an immediate response. Furthermore, understeer is easily felt through the steering wheel, and is quite intuitivelly recovered from by doing the most natural thing: Slowing down.
Speed is a direct and rather critical effect on understeer, oversteer, etc. As speed increases, the car's moment of inertia, and therefore the Centripetal force working on it in cornering, is increased squarely! The corner severity dictates the speed of engagement and this, in turn, dictates the whole driving style through the corner. Excessive speed, by the way, is a relative term, when considering driver's inputs, grip levels of the car and surface, suspension geometries, etc... As a rule of thumb, a race car's grip will not save you, from understeer in particular, should you enter a corner too fast. If you have done so, gently shift weight forward and make sure your inputs are more gentle rather than more firm. A great way of solving this situation is to adopt the "slow in, fast out" discipline for most corners. Otherwise it's simply "fast-in, wipe out". Usually, entry speed and exit speeds are rather contradicting subjects, and gaining a few tenths of a second in a corner or corner set, is going to weigh as nothing compared to eariler and stronger acceleration exiting the corner and along the whole, long, following straight. This is not a safety rule, but rather a rule of driving efficiency! A tight 180 degrees hairpin will have a radius of about 20 meters (made longer by use of the proper line). If you enter it fast, you gain a few neglegible tenths of a seconds in those few meters, but it you enter it slowly and exit it with mightly acceleration, you are going to be much faster down a straight that is likely to be in a length of a good few hundreds of meters. Simple math.
Not being smooth and accurate with your inputs, especially the wheel. can result in a slide. Excessive or agressive steering coming into a corner is never the way to go. Contrary to what some drivers believe, steering agressively into the corner is not going to get the tail around (oversteer), but is rather going to make it understeer more sharply, by making the transient harder for the car. However, if you do this while decelerating in any way (so the weight is down on the front), it's going to create an interesting response: The front will experience a sharp transient that will create a strong, momentary understeer, but this very understeer will slow the front down more, shifting more of it's weight down on the front-outside tire, leading to a sharp transition for slight oversteer.
On the track (or road) our goal is linearity: Instead of this big shift from sharp understeer to oversteer. However, some corners might require an approach that is more similar to what we have mentioned above. The main application is in road rallying, where the driver is trying to get a better view of the road past the corner, thus being forced to keep to the outside of the corner for a longer time. This than forces the driver to turn into the corner very late, and than this technique comes in handy. Sometimes, it's used on the track, albeit more subtly, during tight hairpins or, alternativelly, in some fast sweepers where the car is not co-operating with the driver (typical in front-wheel drive cars over an uphill or banked corner).
In a straight line, braking and accelerating are operated opposite to steering. That's because, as speed increases, the car's moment of inertia grows greater, and therefore steering inputs must be carried out more smoothly. However, when we brake or accelerate, the faster the car is going, the harder it is to make it go faster or the more force is nessecary to slow it down so we need to compensate by braking or accelerating more sharply and with a greater leverage down on the pedals. Once we come into the corner, braking or accelerating complicates the equation. The car's tires offer 100% grip that is divided between steering, braking and acceleration, so in theory it's best to keep the car balanced on "constant throttle" so it's only cornering. However, this is true inside the corner. When we rotate the car into the corner, we can increase the grip by braking to shift the weight of the car forward. If done accurately enough, the grip reserve would increase rather than be reduced.
Easing off of the applications would normally require an even more smooth and quite gentle application: The steering lock has to come off smoothly and progressively, lifting-off of the gas mid-corner (if you are on the throttle) should usually be done quite gently (how much depends on the situation) and lifting off the brakes is similar: A decisive application, a smooth release.
Mismatched gears (shift-locking), downshifting without correct Heel & Toe application, sequental downshifting, agressive shifting or clutch control, improper shortshifting (shifting too early), would result in drag between the engine speed and the road speed, resulting in effects of forward weight transfer and often in a retardation of the rear wheels (in a RWD), or resulting in reversed drag (causing understeer due to shortshifting) possibly provoking a rather harsh skid. Push a front-wheel driven car hard into a corner while braking into it and you are practically on the limit. If you now downshift agressively, the engine braking would slow the car down even more and push it over the edge, locking the front wheels and causing understeer.
Passive factor: Car configuration
Suspension geometries, body roll, car weight, stiffness, chasis flexibility, center of gravity and above all -- tires and tire pressures, carry a great effect on the car's behavior in corners. We have mentioned that the "natural" handling characteristic of any car (when driven in constant throttle at the apex) is to understeer. This is dialed into the car by the engineers that plan it's chassis and suspension, but what about the tires? The tires are not an integral part of the car, so how can the engineers ensure the car will tend to understeer (and so remain safe to handle)? The answer is that tires by themselves will not change the car's basic handling!
Using worn, old or ungrippy tires on the rear will not make it slide and oversteer "naturally." It will understeer and, if the necessary mistakes are done by the driver, the car will oversteer more sharply than it would on good tires. There are exceptions to this: By using different tires on the rear, or in different inflation pressures -- this might have an effect on the suspension system (the tires' sidewalls are part of the suspension) as to cause as car to oversteer without being provoced.
Contributing factor: Road surface
The road surface (and the tires facing it) carry an indirect, however crucial, effect on the chance and manner in which the car would lose adhesion. First, let us distinguish the road surface (tarmac, bright asphalt, bitumen, etcetra) from road conditions (ice, snow, oil, mud, diesel, water, gravel, dirt, etcetra) and road contact (tires and dampening: rubber quality, thread, alignment, inflation).
Dry tarmac, especially such used on race tracks allows for good grip. However, one should be carefull of several conditions, the most common of which is "first rain", which is a small rain shower after 3-4 dry days, resulting in dirt and foilage (with a stress on diesel) floating ontop the layer of water, making it quite more slick. A slightly less known case, however, is when similar dirt (this time with a stress on oil rather than diesel) on dry and hot days, would absorb some moist and than dry out again, generating slick points, often as slick as first rain.
Another problematic situation, is split-grip, where two wheels are on the different surface, normally the dirt, gravel or grass on the side of the track, or with each side dealing with a different amount of water on a wet road. This, along side with the speed and height differences, can quickly lead to an almost uncontrolable spin back across the road or away from it, even with ESP. This is often simulated on skid-pans with a material simulating ice (thrince as slick as oil!), but actually, a normal split grip situation is quite recoverable by stabilizing the car along the edge of the roadway and slowing down sensibly, and than gently getting back on the road or, alternatively, brake hard (Threshold braking) or even perform an emergency stop (locking up the wheels) on the shoulder. It will be mentioned, that some tracks have high-grip surfaces (colored in blue and red according to grip levels) at the edges of the track instead of dirt.
Split-grip can also occur in a longitudinal axle. One particular case is a splash of water or stain of oil (or diesel mixed with water), which creates a sudden reduction in grip levels before a corner or through it. Especially with water, the front tires dry out quickly when reintroduced to the dry surface, but the rear wheels -- being generally cooler and later to leave the slippery surface, will begin to oversteer slightly. If the condition of the tires or the change in grip level is more severe and occurs with the addition of lateral forces (mid corner), the situation can be much harder to control.
Bumpy roads also carry a similar effect. Rally drivers driving on gravel, for instance, have to look up and "read the road surface" ahead, looking in particular for gravel and little stones mid-corner (resulting in an instant decrease of grip) and are also forced to make constant steering corrections, even mid-corner. This is also the reason why ABS is not too efficient on gravel or dirt, as locked-wheels tend to dig into them and than wedge dirt in front of them to contribute to a stop, or grip the better surface underneath the gravel. Gravel and dirt also tend to wedge along the sidewall of the tire, scrubbing off speed while sliding (therefore, countersteering is avoided at all costs). Another maltreat of such roads, and many race tracks, are bumpy roads or uphill inclines, followed by a small jump. When the car is airborne, weight transfer still exist and the driver must use the power to soften the landing, and avoid any sideways forces, because in that case the resulted slide would be nearly impossible to catch.
Correction of understeer
Dealing with a skid is done is several stages:
The correction of understeer is quite intuitive: The car starts going wide, so you ease off of the gas, maybe brake a bit, and it comes back into line. But before you complain about this article being useless, there are some things to notice, because understeer is not that intuitive. When we slide and understeer or oversteer, we recieve too little or too much of a steering response. It's not that we understeered, it's the car that under-steers or under-reacts to our steering input. So the solution is not to steer out of it, but rather to use you feet: Ease off of the gas or brake. Most people will try to steer tighter into the corner. It sounds right, but it can lead to bad consequences.
When you steer more tightly, you have only increased the demands on the car rather than reduce them. We previously defined understeer as a slip angle: The front wheels are turned into the corner, but are actually turning in a line that is significantly wider than desired. So, by turning them deeper into the turn without removing the CAUSE of the slide, is going to increase the slip angle. Now, we also said that understeer is a naturally diminishing phenomenon, so it will cause the car to wipe off speed, eventually allowing it to turn again. This is why you can sometimes get away with this crime: You turn tighter into the corner, which increases the understeer, which slows you down more and eventually allows you to recover.
However, we do not want a correction that gets the job done. We want one that achieves it optimally. If you settle for a semi-efficient solution for skids, you are bound to one day meet a sliding situation which will be too extreme (or don in too tight an area) to be recovered this way, but the proper habit of how to recover from the slide efficiently will not be there to the rescue. Also, on the track we want to be faster by resolving any skid as quickly as possible.
In order to do this, we need to have a better understanding of a skidding car. Understeer is not about the car not turning, that's the symptom. The illness, however, is that the front tires are not gripping. The difference between the two stages is that the second one is seen by the naked eye, while the first is "felt" through the car. The same applies for the solution: It's not your job to get the back into line, that is what you did to begin with, now what you need to do is to regain adhesion to the front tires (by sense), and than let the car pull itself back into line (as seen through the eyes).
So, what to do with the steering? The ideal correction is to slightly reduce the steering input. The wheels are sliding so this will not alter the traejectory that the car is taking through the corner, but it will allow the sliding outside wheel to regain a more natural pace of rotation and reudce the slip angle and demands working on it. If we have understeer measured as a front slip angle of 10 degrees, if we reduce the steering input by 2 degrees we now have only 8 degrees to compensate for and can than, once the car is gripping again, turn back into the corner by two additional degrees. Overall, this would make the recovery faster and smoother. In practice, this is hard to achieve because the driver does not always detect the understeer quickly enough, so the car begins to plough out of the corner, and you cannot perform this steering manuever. The key is to realize the car is about to slide through the steering, apply the correction immediately and once the wheel regains it's "heavy" feeling, turn it back into the corner.
Another important note: The main corrective input is to slow down by easing the throttle, lifting off of it or even braking (depending on how serious the slide is). But this input should be applied with caution. Too strong or too rapid of application is likely to have desasterous effects: In most cars, jerking off of the throttle suddenly upon understeer would make the car shift it's weight forward as to cause sudden oversteer. This is perhaps the least controled sort of oversteer, because it's so unexpected and because it arrives after the understeer has already pushed you closer to the edge of the road. Alternativelly, braking too much will reduce the grip of the front wheels and make the understeer continue. Know your car and how gently it needs to be treated. Be especially gentle with rear-wheel driven cars. In such a car, a sudden lift off of the throttle is not just going to throw the weight forward, but also further reduce the rear grip because of the engine braking it applies to them.
In cases of serious understeer, the solution might involve straightening the wheel alltoghether while braking HARD for a moment or two, to reduce a seriously excessive amount of speed. Another case where this is effective is when you knwo you are going to run off of the road, and you want to it as straight as possible, rather than sideways. These cases are particularly relevant for professional racing. On the track, you will constantly drive the car to the limit. Once you start sliding at this pace, it's likely that you won't manage to recover neatly, if at all.
I remember Tiff Needel and Jeremy Clarkson complaining about what they called "cynical oversteer". Many people don't grasp this characteristic, which is so typical to many street cars. This term applies to any situation where the rear of the car kicks around, but just for a moment. It tightens the line that the car takes, and than the car immediately pitches back forward. Many people mistake this for actual oversteer, but it isn't. Oversteer and understeer are physical tendencies that depend on the setup of the car. Such "Cynical" oversteer is not a handling characteristic, but are rather a function of the rear suspension.
Some cars (the Renault Clio Sport RS being the most prominent example) have a rear suspension with "passive rear steering" that are designed to make the outside rear wheel toe-out under load. If the front begins to understeer slightly, further inputs of steering or even throttle, which will increase the load (and should therefore increase the understeer), will cause the outside-rear wheel to be turned towards the outside of the corner. This makes the rear go around the corner (the cynical oversteer) and rotate the front towards the desired direction, reducing understeer and than returning to normal. Other cars (Vulcan ZX, for instance) have this tendency turned backwards: When the car starts to oversteer slightly, the rear wheel will toe-in or be turned into the corner. This makes the rear wheel take an active part of the cornering effort and tighten the line and prevent the rear from oversteering, so the car will only "kick around" momentarily and return to line.
This exceprt from a comperative experience between ESP and none-ESP cars shows how important it is to remove the cause of the slide correctly, rather than manouver out of it:
In sharp cornering during a race, oversteer is better than understeer. Notice that in 1:32 and afterwards, the driver corrects oversteer without actually steering away from the corner (I.E. Without countersteering) but by slightly reducing the steering angle. This is done by firstly doing the right thing with your feet (which are more important) and will become more clear once we discuss oversteer.
Transitions to Snap oversteer
By putting in more locking, or less throttle/more braking, excessive and severe oversteer can occur. This sort of oversteer will be extremly hard to recover from. In order to avoid it to begin with, go off of the throttle quickly but smoothly. In less sharp and more level corners, you can almost lift-off at once, but you will probably be required to be a bit more smooth.
If oversteer does occur, reintroduce power moderatly but progressively to arrest the slide (regardless of drive). And continue the operation by one of two ways:
1) In a Front-wheel drive: Take off a generous amount of lock while increasing the pressure on the accelerator for a moment, and than release throttle to very light acceleration (not neutral throttle). There is a chance that this will require countersteering. In which case, point the wheel slightly away from the corner and immediatly straighten and power out as described.
2)In a rear-wheel drive: keep moderate-light throttle pressure while taking off the lock, countersteer slightly as needed, and than steer where you want to go twice, once into the direction to where the back end is skidding, and than -- once a pause in the skid occurs -- retract the lock to line it up again wit the direction of intended travel.
In this relation, one must have heard the old rule of "steer into the skid and if in doubt, both feet out, if you spin, both feet in". We dislike this rule and prefer another rule, used at the Bondurant racing school: "If the slide is in the ass, Hit the gas. If the front won’t go, You don’t need mo’".
Lets look at the mistakes of this driver: Fitting different tires on the front and rear will sharpen the car's tendency to understeer or oversteer, obviously not a good idea. A high hand position is not going to give the right amount of feedback (some drivers lose almost all steering feel due to bad hand positioning), and the attempt to steer tight into the corner while undsteering, or sharply against it while oversteering -- without doing the right thing with the feet - is the cause of failure of the driver to recover.
As one who understands Grip knows, driving on the limit, somewhat beyond it and often far beyond it, does not nessecarily means the car will lose control, while an unexperienced driver can lose control over the car before the car loses control. During high-speed racing, the car is constantly utilizing the majority of it's adhesion (100% and even somewhat beyond) constantly. Therefore, any small tumble beyond this thin line, can and probably will result in complete lost of control which no driver is likely to recover from and only locking-up everything helps.
Here's one story from Rally du-var (Group-N) in France:
If you wish to practice skid-control, you need to do it professionally with an instructor. One of the greatest misconceptions in driving is that practicing alone or gaining realtime experience is actually helpful. Such practice, with lack of professional input from a third party, carries random gain, with the risk of accidents or adoption of bad driving habits. The driver can completely misinterpret the situation his car undergoes, and do the wrong thing, or in the wrong order or do the right thing badly, and be inclined to believe his actions were correct and that the method that worked in that case is the correct means of action.
One of the archaic ways of practice is the skid-pan. The skid-pan is a large, circular piece of tarmac or bitumen, riddled with cones and often soaked in water and partially covered (usually in the center) in a material simulating ice (hidden under the water). This results in an instantaneous, drastic and unrealistic lost of grip, with zero feedback before the slide, and a very small ability to correct it or avoid it, and requires to use the clutch rather than play with weight transfers. Another form of such a skid-pan includes simulating split-grip by fitting the same hidden, icy surface on one side of the road and than having the unsuspecting driver (or spectator) turn the car or brake (without ABS) in these extreme split-grip conditions, resulting in a dramatic lost of grip. Another means often used is playing around with tire pressures. The latter two are used in ABS commercials, to present none-ABS cars as unstable and unsafe in terms of braking distances. However, the car always stop straight, even when they spin violently on split-grip conditions. Braking distances in these demonstrations appear to be random and not scientific.
A better way is a skid-car. This is an hydraulic instrument fitted on a standard car, allowing to loosen the rear or front axles independently or simultaneously in different measurements, as if the driver was riding over wet, greasy, snowy or icy roads and so on. Modern skid-cars do not intervene with the car's handling and dynamics, and therefore propose a physically real skidding situation over ice or oil, whereas the car is actually driving on tarmac or wet tarmac. The instructor is capable is simulating various sorts of skidding situations, "surprises" and transitions, as well as to prolong the duration of the slide to allow the driver to feel the slide going in and out and to recognize his actual corrective measurements with constant, professional input, before skidding in real-time. One should make sure the actual car fitted in the cradle is one similar to what he will be driving on real-time (Often, Skid-car courses offer Rear-wheel drive cars for road drivers who will drive on FWD cars, getting used to a different solution and handling!) and that it has standard tires fitted.
The problem of both means is that the speed is generally slow and the driver is not driving the car in real speeds. It is imperative to complete the training with a tuition on the driver's own car, driven through wet corners (formed by cones) or "traction circles" at higher speeds. A "traction circle" is a wide circular pattern cones, simulating the car's friction circle when driving around and applying different inputs of steering, braking, acceleration and gearshifting, with one section of the circle soaked in water, and with an instructor than can yank on the handbrake from time to time. This allows to practice sensing skids, avoiding them and correcting them, in higher speeds and in the driver's own car.