Simplifying the science of how cars grip the road
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A fundamental requirement of any car is that it should be able to maintain grip on whatever surface it is being driven on. The more grip the car has, the higher its limits of both traction and cornering.
A car with a very high limit can be driven quickly, if that’s what you want to do. But this isn’t all about performance. You also want a high limit at normal speeds so you can retain control in emergency situations.
Most motorists don’t think much about grip unless they suddenly find they don’t have enough of it. It’s a fascinating subject, though, which we’ll try to explain here in simple but useful terms.
Tyres
Unless something has fallen off the car and is dragging along the ground, the only things connecting it to the road are the tyres.
They are therefore crucial in a grip context for two reasons. First, they have to provide grip themselves. Second, they are influenced by everything else on the car that affects grip. The key is to give them as little as possible to do so that they are not overloaded.
There has to be a balance. The best tyres you can buy still won’t grip well if other parts of the car don’t allow them to. At the same time, a car that is brilliantly set up in all other ways won’t have much grip if the tyres aren’t up to the job.
Tyre types
Different cars need different types of tyre. The ones fitted to everyday road cars are the most difficult to design, because they have to provide sufficient grip on both wet and dry roads, keep the fuel economy up and the CO2 emissions down and not make too much noise.
Off-roaders and many rally cars need tyres with specific tread patterns so they can find enough grip on loose surfaces. For snow, you want narrow tyres with high profiles. This goes against the general trend towards wide tyres with low profiles, which is why it’s now advisable to use different sets in summer and winter.
Racing cars have slick tyres (if the rules allow this) with no tread pattern for maximum contact area and minimum rubber movement. On a wet track, however, they are useless and have to be replaced with treaded ones.
Tyre compounds
Broadly speaking, the softer the rubber is, the more grip you get, in the same way that cooked pasta will stick to the wall of your kitchen while raw pasta won’t.
Softer rubber gets up to operating temperature more quickly but can also overheat. Dragsters have incredibly soft tyres, but the most powerful ones only run for about four seconds at a time, so this isn’t an issue.
Normal road tyres have to last for thousands of miles so they need to be much harder, though the lower performance of the cars they’re fitted to means they don’t need extremely high levels of grip.
Suspension
Selecting the correct springs and damper settings is crucial for grip as well as for comfort. What’s not often understood is that the softer the suspension is, the more grip you get, at least up to a point. A softly set-up car with a lot of body lean puts less strain on the tyres than one that hardly leans at all, even though it doesn’t look that way.
If that’s the case, why are modern high-performance cars and nearly all competition ones so stiff? Well, as you get further away from the car’s resting position the suspension geometry changes and becomes less effective.
For that reason you want as little movement as possible. The resulting extra strain on the tyres is solved by fitting better ones. The more grip the tyres provide, the stiffer the suspension can be.
There is also an issue with aerodynamics, which we’ll be covering in more detail shortly. The wings on F1 cars work best when they are mounted at a very specific angle. This angle is maintained by making the suspension incredibly stiff.
Aerodynamics: body shape
It used to be common for cars to have more or less vertical noses and long, flat bonnets. This was fine when speeds were relatively low, but as cars became capable of higher performance it turned into a problem.
Air would be pushed up when the nose hit it and hardly touch the car again until it landed on the windscreen. The large low-pressure area that resulted above the bonnet was an aerodynamic disaster because there wasn’t enough air pressing on the bonnet to prevent the front from lifting.
At higher speeds this meant that the tyres were exerting very little pressure on the road. In extreme cases they didn’t provide enough grip to steer the car.
The lower noses and more angled bonnets of modern cars have been developed mostly in the interests of fuel economy, but they also help front-end grip.
Aerodynamics: wings
In the 1960s, motor racing teams made big advances in aerodynamics by fitting wings to their cars. They were designed in such a way that air flowed more quickly under them than over them. Fast-moving air is less dense than slow-moving air, so there was more pressure on top than underneath and the wings were pushed down.
Since they were solidly attached to the chassis, the whole car was pushed down too. The tyres now provided more grip because you could work them harder before they started to slide.
Wings create a lot of aerodynamic drag, but not enough (if they’re well designed) to cancel out the extra grip. If you took the wings off a Formula 1 car, it would be enormously faster in a straight line but so much slower through the corners that lap times would suffer terribly.
F1 wings are now extremely complex and impractical for road use. Simpler versions, however, are common on high-performance road cars and even some hot hatches. Manufacturers claim they reduce lift, which is good, or in some cases provide downforce, which is better.
Aerodynamics: underbody
The next development in F1 aerodynamics was to turn the underside of the car itself into a wing and fit ‘skirts’ so that the air couldn’t escape out of the sides. This was soon banned, partly because good ideas in motorsport usually are and partly because cornering speeds became scarily high. Nowadays you can’t use skirts, and the bottom of the car has to be flat.
However, a flat bottom is still useful because it reduces drag under the car. It’s particularly effective if there is a diffuser at the rear. This increases underbody air speed, reduces pressure and allows the car to be pushed down as described earlier.
The influence of all this on road cars is minimal because it’s difficult to create a flat floor on one of those. You do see cars with things that look like (and are described by the manufacturers as) diffusers, but they’re generally mounted far too high to be effective and are really just there for show.
Weight
A light car puts less strain on its tyres than a heavy one, so it will grip better if all other things are equal.
Where the weight actually is can have a big effect. Ideally you want most of it to be as near the centre of the car as possible. It it’s all concentrated at one end, the tyres there have a lot more work to do, and you have to set up the suspension or choose your tyres with this in mind.
The ideal situation is to have the engine mounted ahead of the rear wheels. This is fine for supercars and some cheaper sports models but completely unacceptable for normal ones because it leaves no space for rear passengers and usually very little for luggage.
Centre of gravity
Another weight-related issue is the position of the centre of gravity. It’s useful to think of this in terms of a matchbox or, to an even greater extent, a pencil. Knocking either of them down when it’s standing on one end (high centre of gravity) is much easier than flipping one that’s already lying flat (low centre of gravity).
The lower the centre of gravity, the harder it is for weight to transfer from one side of a car to the other. Less weight transfer means less load on the outside tyres, which in turn allows the car to go faster round corners before the tyres lose grip.
Purpose-built racing cars are exceptionally low, and everything in them is mounted as near to the ground as possible, for exactly this reason. This can’t be achieved to the same extent in road cars, but sports models still have an inherent advantage over family cars, and even more so over pickups and SUVs.
Where the wheels are
The distance between the left and right wheels, known as the track, is important. The greater it is, the less weight transfer you get when cornering, so for example a car with a high centre of gravity might transfer an acceptable amount of weight if it’s wide enough.
A more familiar term than track is wheelbase, which is the distance between the front and rear wheels. Long-wheelbase cars are more stable in fast corners but less manoeuvrable in slow ones. Short-wheelbase cars are the other way round.
The relationship between the track and wheelbase can be as important as the actual dimensions. A short car may still act like a long-wheelbase one if it’s also very narrow, thought ideally you’d want it to be very low too.
Where the power goes
Tyres distort when power (or, more accurately, torque) is applied to them. The choice of which wheels are driven and which, if any, aren’t is therefore a major factor when it comes to grip.
Front-wheel drive cars tend to lose grip at the front as power is applied, in addition to the fact that the tyres at that end are already stressed because they’re carrying the weight of the engine and gearbox. Rear-wheel drive cars usually have better weight balance but are still more likely to lose grip at the rear.
The best option is four-wheel drive, not because it provides more grip but because it makes losing grip less likely. It involves carrying extra weight, but each tyre has to deal with only about a quarter of the engine’s output rather than half, so they are under much less stress.
Four-wheel drive is ideal for fast road cars, rally cars and SUVs, but not for F1 cars because it would create too many compromises elsewhere in the design.
The most important bit
No matter how hard a manufacturer tries to give a car good grip, the single most important factor is the driver, otherwise known as “the nut holding the steering wheel” or, to motorsport engineers, “the error exaggerator”.
Even if you don’t consider yourself to be an expert driver, you can maintain high levels of grip by feeling how the car behaves, steering smoothly, using no more power than necessary, choosing appropriate speeds for corners and observing the road and traffic conditions so that you can brake gently to avoid an emergency situation.
If you do all that, not only will you be a better driver, your car will be better too.