TECHNICAL FEATURE:

Grip: The Basics

by David Finlay (2 July 2002)

Whatever the intended use of a car - Formula 1, the World Rally Championship, the BTCC, drag racing or simply everyday family transport - there are members of its design team dedicated to ensuring that it stays on the road. Or, in other words, to achieving a level of grip which is appropriate to the situations in which the car may find itself.

This is an enormous subject, and it would be as well to make clear at this stage that we are not attempting to cover all of it. There are people who dedicate the whole of their professional lives to that. However, it is possible within one article to discuss some of the basics.

The relationship between a car and the road it is travelling along must obviously be based on the point, or rather points, where the two meet, namely the tyres. Strictly speaking the important bit is actually the area of each tyre in contact with the ground, otherwise known as the "tyre footprint". However, since the footprint (a) constantly changes physically as the tyre revolves but (b) is essentially the same in effect no matter which part of the tyre is making contact at any given moment, we don't lose any significant meaning at this level of inquiry by considering the tyre as a whole.

The tyres are to some extent the easy bit, though there are many variables to consider. The rubber must be at a reasonable operating temperature - not usually a consideration in normal road cars, though competition tyres rarely work until they have warmed up through use, and commonly fail to work because they are too hot through over-use. The tyre must also be at its optimum pressure. Road car manufacturers do their own tests to determine this, bearing in mind the requirements of fuel economy and wear as well as grip, whereas competition teams are only really concerned about the grip.

To check whether they are getting as much of this as they can, they will measure the temperatures of a hot tyre in the centre of the tread and at the inner and outer edges. This information also tells them whether or not they have chosen the correct camber - or angle of lean - of the wheel, though that's not something we'll be exploring further here.

Let's assume that these and other variables such as road surface and conditions are taken care of. The main point is that a tyre will resist forces acting against it. The greater the force, the greater the resistance, until there comes a point where it can no longer cope. The resistance will suddenly fall away and the tyre will lose grip, only regaining it when the load falls back to an acceptable level.

So what loads might a tyre have to deal with? Traction and braking are two of them, but we're more concerned here with what happens in a corner.

Cars do not like corners. They are moving objects, and like all other moving objects, such as tennis balls or icebergs, what they want to do, as Isaac Newton pointed out in a more general context, is continue moving at their current speed (anything from zero upwards) and in the current direction. But we want them to accelerate, slow down and turn round corners, and we have to battle against the laws of physics to make them do this.

In terms of cornering, the physical problem is that the forces which make the car turn are matched by forces trying to straighten it up again. The result is a transfer of some of the car's weight towards the side opposite the direction of the turn. So, in a right-hand corner, more weight will be carried by the left-hand wheels than would be the case if the car were travelling in a straight line. This is the load that the left-hand tyres must resist.

If you fit larger and/or grippier tyres, the resistance to this load will be increased. But you might not be able to do that, for all sorts of reasons - expense, space under the wheelarches, tyre wear, regulations and so on. On the other hand, you can achieve a similar effect by reducing the load in a number of ways.

Imagine that in a certain corner a car reaches its absolute limit of grip at 70mph. You're not happy with this and you want to increase the potential. The only way of doing this without changing or adjusting the tyres is to reduce the weight transfer, which you can do in the following ways:

    * make the car lighter
    * make the car wider
    * lower the centre of gravity
    * increase aerodynamic downforce
    * make the suspension softer
    * drive more slowly
    * choose an easier corner next time

Clearly the last two suggestions are not the concern of the designer. Let's take the others in the order shown. The static load on a tyre is the amount of weight it has to support when the car is stationary and sitting on a level surface. If this can be reduced, then more weight transfer can be applied before the maximum amount of load the tyre can handle is reached. The tyre can therefore allow the car to corner more quickly, or - an important safety point - is less likely to lose grip at a given cornering speed.

A wide car is better able to support lateral weight transfer than a narrow one. You can make a good analogy here by considering a matchbox - it's much easier to topple a matchbox that's standing on one end than a matchbox which is lying on its side.

(Among the many compromises involved in this subject, though, is the fact that a car which is wide relative to its length is less stable than one which is long relative to its width. The long car is more controllable in fast, gentle corners whereas the narrow one can be persuaded to change direction more easily. This was demonstrated very well in the 1995 British Touring Car Championship, where Vauxhall's relatively narrow Cavalier was very successful on fast tracks - and dominant on the fastest of them all, Thruxton - but a midfield runner at Knockhill, where the corners are much slower and tighter. The wider Volvos demolished the Vauxhalls at Knockhill but couldn't approach their pace at Thruxton.)

The matchbox analogy is also useful in a discussion about centre of gravity. Apart from the obvious difference in height versus area, the matchbox on its end has a higher centre of gravity than the one on its side. Although you can measure where a car's centre of gravity is, it's not a specific object - it's just the sum of the weights and positions of all the car's individual components. But imagine it's the end of a handle stuck on a flat sheet of metal the same size as the area of the car. The longer the handle (i.e. the higher the centre of gravity), the easier it is to tip the sheet (i.e. the greater the weight transfer for the same force applied).

There are lots of ways of reducing the centre of gravity height, most of them involving either placing heavy objects as near to the ground as possible or, if they are mounted high up and you are allowed to do this, removing them altogether. But there are more fundamental solutions.

Unusually among modern cars, all Subarus use "flat-four" engines, with two pairs of pistons moving from side to side rather than a line of four moving up and down. An engine is a very heavy item whose own centre of gravity has a major effect on that of the car. The Subaru units have very low centres of gravity, which partly explains why the Impreza and Legacy models grip so well, and also why the Forester handles so much better (in my opinion if nobody else's) than any other semi-off-roader on the market. And although I'm no expert on the Jowett cars of the 1950s, I'm sure that their famously good handling had a lot to do with the fact that they also used flat-four engines.

(Brief aside: dragsters tend to have their engines mounted as high up as possible, but then they're not expected to go round corners. Despite incredibly grippy tyres, they have major traction problems as they are trying to put anything up to 5000bhp on the ground as efficiently as possible. Mounting the engines high up gives these cars a high centre of gravity, which transfers a lot of weight to the rear tyres during brutal acceleration, helping to bury the rubber into the tarmac. Ideally, a dragster will have enough rearward weight transfer to lift the front wheels slightly off the ground as it starts a run.)

Aerodynamic downforce is rarely an issue on road-going cars, and for competition vehicles it doesn't seem to have anything to do with suspension. But it does. Current F1 regulations mean that a sizeable proportion of the performance of a Grand Prix car depends on its aerodynamics (causing great damage to the spectacle).

The overall ride height, and indeed the difference in front and rear ride heights, have to be maintained with great precision to allow the aerodynamic designs to work, which is one reason why F1 suspensions are so stiff that most of the actual body movement is allowed by the compression of the tyre sidewalls. Another reason is that incredibly stiff springs are required to withstand levels of downforce which can exceed the weight of the car.

The dominance of aerodynamic effects in F1 means that the next point is of less relevance in that field, but it is still significant: soft suspension means better grip, because it creates less weight transfer. At first, this would seem to be contradicted by the evidence: after all, a softly-sprung car leans more obviously than a stiffly-sprung one. But this body roll is not because of weight transfer, it's instead of it.

Imagine a spring being compressed by the weight of a car. As the weight increases, the spring will compress further, absorbing the energy (which was created, you'll remember, by the opposition of the force that is turning the car into the corner). If it is a soft spring it will compress a lot, absorbing a lot of the energy. If it is a stiff spring, it will not compress so much.

But the energy still has to go somewhere. And indeed it does - it is passed on to the ground by the easiest route, the final point on which is the all-important tyre footprint. The tyre takes on the energy that the spring has not absorbed. In other words, it takes on more load, and therefore gets closer to the maximum amount of load it can accept before losing grip. The softer the spring, the longer it takes to reach this limit.

A competition car set up to run in the wet (or, in the case of rally cars, on gravel) will be more softly sprung than one intended for use on dry tarmac. That's because the surface on which the tyre is working is less grippy, and the teams have to compensate for this. Road cars are generally much softer than competition cars, partly of course because ride quality is an issue, but also partly because they need to have one suspension set-up that will work reasonably well on dry tarmac, wet tarmac, mud, snow and so on.

Electronically-adjustable shock absorbers on road cars are a relatively recent development, are only available on the more expensive ones, and give scope for tuning that a motorsport team would consider primitive.

Okay, so why are competition cars and high-performance road cars so stiff? Well, in fact there is a definite trend among the road car manufacturers to make their performance versions softer. Examples recently tested by CARkeys include the Subaru Impreza STi, the Jaguar S-Type R and the Audi S3.

As we've seen, aerodynamic considerations make up one reason why competition cars tend to be a lot stiffer than their road-going cousins. But there are others too. Remember that for weight transfer purposes, among other things, it's a good idea to have the car as low as possible to the ground, to keep the centre of gravity down.

That's why most racing classes include regulations stipulating minimum ground clearance (the whole point of most regulations being to make life as difficult as possible for designers). If your car is already nearly scraping along the tarmac, soft suspension will bottom out as soon as you hit the first bump, or even if you brake or accelerate hard. And since most suspension systems undergo an unwelcome change of geometry the nearer they get to their extremes of travel, it makes sense to limit body movement as much as possible to avoid this.

If the subject of vehicle dynamics is an ocean, we're still standing at the edge, having barely got our feet wet, and we're already staring at an incoming tidal wave of exceptions and compromises. If you think your road car handles well, or if you admire the cornering capabilities of a car you see during television coverage of a race or rally, just think how much technical expertise went into making it happen.

Grip: The Story Continues

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