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Unread 12-21-2016, 07:58 AM   #1
Join Date: Aug 2015
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Default Controling a car at the limit

This isn't a response to some banter on the board, as I normally do. It's just a follow-up on my previous topic on how to keep the car off of the limit of adhesion as much as possible, which is very relevant in this winter. Again, this is under motorsport because it has a strong connection to that field, but it's also perfectly relevant to road drivers.

Sometimes, even a docile driver might reach the limits of the car's performance. One reason is a sudden maneuver forced on one by other drivers that may swerve into one's path, cut a corner or so forth.

Another reason is a sudden reduction in road grip because of a patch of black ice or a big puddle of spilled oil. Normally, it's something that can be seen and avoided but, depending on the lighting, it might be something of a surprise.

Coaching to control a sliding car can make a difference in those cases, even in cars with Stability Control. The first step is understanding the cause of the skid.

A car skids when one asks more of the car's tyres than they can provide. Because of that, usually only one of the car's axles tend to skid, and the type of slide depends on which one it is.

If the front tyres are sliding, that's understeer. The front tyres are the ones that are connected to the steering wheel so when they slide, they can't change direction as well and the car takes a wider arc than it should.

The main cause of understeer is excessive speed. Either from being too fast going into the corner or from too much acceleration in the corner, particularly if it's a long one. Turning the wheel too much and too sharply exacerbates the problem.

Another possible cause is braking too hard into the corner, so the tyres can't both slow down and turn well at the same time. Generally, if the braking is at a constant or rising rate through the corner - the car won't corner as well as it can.

Understeer is relatively common and thankfully is relatively easy to cure. The reason being that you feel it in the steering wheel, which goes "numb" and once the correction effects the car - it gets "solid" again. Using both hands and holding the wheel with ease helps feel that.

As with any slide, the correction is to remove the cause: If it's too much braking, reduce the braking. If it's too much speed, slow down with caution. The only thing to watch out for is not to add more steering because it would delay the correction having an effect.

When the rear tyres slide, that's oversteer. The rears slide out and instead of following in the tracks of the front in the corner, they over-rotate the car into an eventual spin. It's also the cause of jack-knifing in heavies.

Oversteer is normally caused by sudden deceleration mid-corner, usually without so much braking as to cause understeer. The rear tyres can't handle the deceleration and cornering simultaneously and break traction.

Another cause is sudden acceleration mid-bend in a powerful rear-wheel drive car, typically when it's far more sudden than what is needed to cause an understeer effect. The rear wheels spin and slide out. One other cause is aggressively jerking the car from one direction (say, a left turn) to the other.

Lastly, a sudden shift in road grip tends to result in oversteer because there is naturally going to be a point where the front tyres get back on the asphalt while the rear tyres are still on the black ice or diesel spill.

Down-hill slopes and off-camber corners are a contribution factor to oversteer. Knowing all those conditions is important because, wit oversteer, you don't get the "warning" at the steering wheel as with understeer, so you need to know in which circumstances it can transpire and be ready for it.

The correction here is more complex and depends on the car. Hence, the best hope for a driver without extensive specialist training is to brake as hard as possible. Because car brakes are heavily front-biased, that can stabilize the vehicle, or at least stop it.

The more professional correction is again to remove the cause. If it's too much power at the rear wheels, you ease off to a more neutral throttle position, and if it's hard braking, than the solution is to accelerate. This causes the car to squat on it's rear tyres, sticking them into the tarmac.

How much power depends on drivetrain. If it's front-wheel driven you can be quite liberal and use lots of power. Otherwise, you just need enough power to overcome the sliding that is dragging the car's speed down.

Since the car is rotating too much, there's steering to be done, too. If you are looking in the direction in which you want to go, you will do this out of instinct. You'll normally find that you had turned the wheel against the direction of the corner (counter-steering) and then, once the correction starts taking effect, you bring it back to straight.

How long you hold the steering before straightening depends on how much power you are using simultaneously. In a front-wheel drive, more power means you should straighten very quickly. Sometimes you might not even need to countersteer at all. Rear/all-wheel drive would normally require you to hold it in counter-steer for longer.

If you are driving on smooth ice, you're not likely to have enough traction to use power effectively at all, so you just declutch or flick an automatic into neutral and countersteer.

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