UndersteerFrom TrackpediaUndersteer, 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").This short article will teach you the difference between understeer and oversteer. We have several sorts of "skids":
Causes of a skidThe 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. In the bottom line, slip angles in tyres (which are the difference between the line in which the tire is pointing and line it's guiding the car through) are a constant and unavoidable physical phenomenon, to a certain degree it would actually optimize performance. The idea is to use suspension designs and driving style to avoid these angles to increase to a degree where the car is "sliding" and going out of line. It should be mentioned that any such tendency would not be considered as, and therefore no measured in, "sideways movement", but rather in the car's yaw rate. (Yaw is the car's natural orientation to keep on going straight). As a result, speed is being a crucial matter, far more substantial than the severity of the corner, and often more important than weight distribution or even sheer grip. As speed increases, the centripetal force (not centrifugal force) and moment of inertia working on the car would increase squarely! Excessive speedSpeed 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! Coarse handlingNot 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. However, it should be mentioned that there is also an element of "decisiveness" involved in the introduction of side forces coming into sharp corners or some corner sets. In this case, you leave your steering for a very late stage and than make a large and rather quick input from a slow entry speed. The stress is on smooth output, however important smooth inputs are as well. Remember, being too smooth is faster than too sharp! 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. In this respect, many road and race drivers are far too smooth and gentle when hard braking from a fast speed is concerned. In such cases, the driver hesitates longer due to a fear of a skid, and squeezes the brakes down too slowely. In fact, at a high speed, a greater force is nessecary to halt the car, and therefore the initial application needs to be very swift (however not instant) and powerfull and than ease off as speed decreases, rather than progressively increase the pressure. Once turning however, braking and accelerating would normally overload the car with longitudal forces and possibly unnessecary weight transfers. It would be advantagous to keep the throttle neatly balanced through the curve to keep a constant speed, however mostly the brakes are lightly pressed into the turn-in or deeper, with the intention to utilize the slightest of a forward weight transfer to increase overall grip for steering. In complex sorts of corners (like chikanes and hairpins, which have two parts) or corner sets, one may use this to draw the braking zone somewhat into the corners and delay braking on the straight. It all depends on the corner, and somewhat less on car design. 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. Being decisive with your applications is also important in order to minimize and avoid understeer: Here you can see Understeer due to steering which is too smooth and slow or too sharp. During slalom courses and evasive manouvering drills at Bondurant or performance driving on winding mountain roads in "Maslulim", "Driveart" and "Drivetech", the trainees are taught to steer later and more sharply to get the car to turn idealy. Further confirmation to this thesis can be found here and here. DragMismatched 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. In this case, the gears are the corrective approach. Otherwise, they are not. Passive factor: Car configurationSuspension 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. This depends on many factors and, unlike a common opinion, weight transfers and load transfers (body roll) would not nessecarily decrease handling effectivness. In this respect, we should mention that both understeer and oversteer can be caused by a change in one of the wheels toe setting during body roll ("Roll steer"), Dive and driving over bumps (Bump steer). These effects are not quite skids, but a result of a wheel moving in a arc while airborne, effectivelly steering the car in another direction. The correection would therefore rely on steering. Otherwise, the majority of the solution would rely on a correct application of the pedals, rather than on the steering corrections. Contributing factor: Road surfaceThe 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. Road contact is also important because it carries a crucial effect both on grip and balance. Most road drivers under-estimate the importance of tire inflation, and drive in a state of under-inflation, which can reduce the levels of grip amazingly and possibly envoke a spin during braking and/or cornering. Over-inflation is prefered in this case, because under-inflation is far worst in all terms: The car skids eariler and it is harder to recover, the ride is less comfortable and milleage is decreased and the chance of a blow-out is actually greater with under-inflated tires! Correction of understeerUndersteer (increased front-tire slip angle) can be caused in front-wheel drive cars (or any car really) all the way through the corner, typically between the turn-in and APEX. A late APEX helps in dealing with such sort of understeer. However, many rear-wheel drive and all-wheel drive cars also have a tendency of understeer, either through the turn-in or when pulling out of the corner. In fact, the greater portion of modern race cars are set to initial understeer, rather than oversteer (Commonly known as "fishtailing", Drifting or "loose"). In racing conditions, around tighter, sharper corners, oversteer is actually better than understeer. The key for successful road car designing is to ensure that the car understeers slightly, and does not easily oversteer. For a sports car, however, it would be better for a car to be more neutral, but still have an initial, slight, tendency of understeer. Understeer, in most of it's occurances, is easy to solve: either put in a bit of more steering, or put out more acceleration/put-in more braking. In more severe cases, more advanced techniques needs to be applied (see below). Power-understeer occurs in moderate-bends, and should be minimized by using brake turning or even trail braking. In sharp corners, understeer caused at the corner entry can be caused primarly by exccessive speed being carried into the corner, or by excessive braking into it, if the driver brake-turns. The driver should keep light brake pressure into the turn-in. In these particular corners, it's better to turn the wheel quite quickly. If understeer continues to appear around different corners, it might be a matter of tires and even of dampening. Dealing with a skid is done is several stages:
Understeer is safer than oversteer: First off, in understeer, the car is pushing wider, making the Centripetal force acting upon it smaller and effectivelly solving itself, instead of growing worst by tightening up the circle in the event of oversteer. Additionally, this phenomenon will instinctivelly cause drivers to do actions that will usually be helpfull: tighten up the steering, taking off the acceleration or braking. In general, understeer can be solved by many ways:
The "Apel contra Apel" can be implied according to the situation and the car: In some rear-wheel-drive, and even more so in an all-wheel drive, you can lift-off of the throttle almost at once. In sharp cornering, or when cornering while driving uphill or on a turn with a side slope. The ideal correction of understeer, like any skid, begins with avoiding it. Understeer is something you just don't want. It's a useless pain. That's where the "Slow in, fast out" rule really shines. However, since understeer is a constant physical phenomenon, it can't be fully overcome, it should be minimized to a level where it is controlled and can't push the car completly out of line. In order to do this, we need to have a better understanding of a skidding car. Because, 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). This is actually the most important bit. You need to feel the slide to immediatly remove the cause of the slide (normally by using the pedals to do the exact opposite of what caused the slide) and than make little corrections to get the car back on the right line and than quickly return to normal driving state (get back on the power while taking off lock) to avoid any other slide or further reduction in lap times. Without removing the cause of the slide, trying to steer along carries a great potential of decreasing chances of recovery. Furthermore, most of the corrective measures you take, require additions tractive demands, and can momentarily decrease the chances of recovery. This is another reason to avoid the slide to begin with, because -- unlike what you may think -- even the pros find it difficult, and in many times impossible, to contro a sliding car when driving on the limits of grip. By coarse handling or slight excessive speeds you can create a "spike" of tractive demands momentarily excedding the limits of grip of one axle. In professional racing, however, you are constantly using every bit of available traction. This means you have no reserve to work with if you slide and all that is left to do is to brake hard to a stop with near-straight steering, without any bad attempts of controling the car. You do not have to drive on the limit to lose control of your car, neither do you have to spin to lose control, nor to lose control by driving beyond the limit. You need to match your control boundries to those of the car, and that requires training. A note about "cynical" oversteer I remember Tiff Needel and Jeremy Clarckson complaining about what they called "cynical oversteer". This is a situation when the car stepped-out of line laterally for a moment, without "kicking out the tail" in particular. It is important to state that "cynical" oversteer and oversteer caused by bumpsteer are not real cases of oversteer. 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:
Here we have the point that makes understeer so easy to control: It has a very unique, although quite misleading, sense to it. It feels, as though if the wheel is lighter and less responsive. It does not mean that the car is less resistant to turn, it means that the front is losing traction. What you need to do, in order to regain this traction is simply to slow down. This will shift weight to the front of the car and surely enough, minimize understeer. Braking into the turn-in point ("trail braking") is a technique that exploits this point. Additionaly, turning the wheel smoothly, but quickly if nessecary, will enable the car to make an easier turn-in. While feeling the skid going in and out is crucial, eyesight still has it's role. You need to avoid locking you vision at the edge of the roadway to which you are heading, and look to where you want to go (into the corner). Once the technical skill of solving this situation is acqquired, it can be refined by keeping the eyes focused ahead at the intended direction of travel to begin with. If you do find yourself staring at the edge of the roadway, you will need to shake your whole head in order to pull your eyes away from the danger zone. It might also help, to focus on a far visual target, or a specific streach of road, in the direction of intended travel. If understeer is terminal, brake as hard as you can, and keep wiping off speed untill the last possible second, upon which you begin regressive braking to release the locked tires. You don't want to reach Threshold braking but actually something a bit short of that, so release a nice bit of brake pressure. If it does not seem to help (or if you are very inexperienced), mesh it back down again, keep everything locked to a stop. 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. Summary: Understeer is the tendecy of the car to run wide at a corner, normally because of speed, or power. It is felt through the steering wheel. The idea is to avoid it by entering a corner slowely while lightly braking. If understeer still appears, you are probably ploughing or powering the car. If on the gas, ease off while taking off a bit of steering lock, and than tighten the steering into the corner while braking ever so slightly. Get off of the power gradually but decisively. In an AWD (or even RWD) and when going uphill, one can go off of the throttle faster than in a front-wheel drive or when driving in a straight line. If you are on the brakes when you get understeer, chances are you are either braking too hard or ploughing too fast. If you are braking too hard, you need to let off most of the steering angle while quickely (albeit smoothly) peeling away from the brake untill it's only lightley threaded and than tighten the steering back into the corner. If you are lightly on the brakes and you understeer you have ploughed into the corner too fast. Therefore, you will need to take off most of the lock while braking harder (to safely wipe off speed) and than steer back into the corner while trailing off of the braking (staying only lightly on them). Transitions to Snap oversteerBy 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’". This driver makes several mistakes To begin with, he had different tires fitted on one car. Second, he turned the wheel excessively when understeer occured. Third, his 10 to-2 grip is not giving him the nessecary leverage and feedback. Forth, he spun due to a common mistake of countersteering in a FWD. 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:
Skidding simulationsIf 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. Trails:
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