Grip: The Story Continues
by David Finlay (25 July 2002)
In the first article of this two-part series we established that there are certain ways in which a road or competition car's ability to grip the surface it is travelling on can be improved. These included fitting better tyres, making the car lighter, making the car wider, lowering the centre of gravity, increasing aerodynamic downforce and - perhaps contrary to expectations - making the suspension softer.
As we saw, every item on this list is subject to compromise, and each can compromise at least one of the others. For example, making the suspension softer can ruin the car's aerodynamic efficiency as soon as the body starts moving around, to say nothing of creating geometric effects which can quickly overcome all other aspects of the handling.
But there's another issue which was deliberately missed out of that article. Most of the discussion was based on the effects of weight transfer across the car during cornering, from the inside wheels to the outside wheels. The whole business enters a new level of complexity when you consider that the weight transfer can be different at each end of the car, across what for convenience (and following industry convention) we'll call the front and rear axles.
It need not be different at all, of course, but that would be the case only if, among other things, each axle had exactly the same amount of weight acting on it, the centres of gravity were at the same height, the suspension geometries were identical and the aerodynamic downforce was equal at both ends and at all speeds. An unlikely scenario, to say the least.
In fact, in most cars all these elements are unequally divided between the front and rear axles, so different amounts of grip are available from the front and rear tyres. More specifically, each pair of tyres is subject to different amounts of force, so one or the other is nearer the limit of force the rubber can stand before losing grip. A car whose front tyres are nearer this limit will tend to understeer, and a car whose rear tyres are nearer it will tend to oversteer.
This explains a few standard handling characteristics. A front-wheel drive car carries its engine and gearbox, and therefore a high proportion of the overall weight, on the front axle. The front tyres are more heavily-loaded than the rears at any speed from zero upwards. The centre of gravity at the front is also quite high. When weight is transferred in a corner, the outside front tyre is by far the most heavily-loaded of the four, and its ability to cope with the extra force is the limiting factor in the car's ability to grip the road.
(In front-wheel drive cars with transverse engines, which is nearly all of them, the centre of gravity is not only high but offset to the same side of the car as the engine. This tends to be the right-hand side, so the limit of grip is more easily reached on left-hand bends than on right-hand ones.)
Rear-engined cars, of which the Porsche 911 is the only high-performance example these days, put most of the weight on the rear tyres, and in fact take weight away from the fronts, using the rear axle as a pivot. The rear tyres therefore take most of the load and are usually the first to lose grip. The effect is much smaller in mid-engined cars, including nearly all single-seat racers, where the engine is mounted ahead of the rear axle and applies some of its weight to the front tyres.
In each of these cases there is yet another complication. The wheels which carry the most weight are also the ones through which the engine's power is transferred to the road. This adds yet more load to the tyres, taking them still further towards the limit of grip. The effect is greatly reduced in four-wheel drive cars, or in cars where the driven wheels and the wheels carrying the most weight are at opposite ends (which in practice invariably means front-engined, rear-wheel drive cars).
All these compromises create a lot of trouble for suspension designers, but they can also use compromise to redress the balance. The obvious goal is to have the maximum possible grip at both ends of the car, and for this amount of grip to be as near equal as makes no difference. Where that isn't possible, for whatever reason, one solution is to reduce the amount of grip at the end which is not causing the problem.
A common example of this is, in the case of front-engined racing cars (particularly those which are simply modified versions of ordinary road cars), to fit much stiffer springs at the rear. Using stiffer springs at the lighter end of the car may not immediately make sense, but the point of it is to increase rear axle weight transfer so that it compensates for the extra weight, and the large amount of weight transfer caused by the high centre of gravity, at the front end.
Another way of doing it is to fit a stiff anti-roll bar. As their name suggests, these bars were originally designed to reduce body roll. But we saw in the previous article that if you reduce body roll you also increase weight transfer. In most motorsport applications this is the primary purpose of the anti-roll bar. Fitting adjustable bars at each end allows the team to fine-tune the handling balance of the car. This can be very effective, and if it isn't it means that there is some other problem which has to be attended to before there is any chance of making the car corner quickly.
The word "balance" is very important here. Reducing grip at one end suggests that you are reducing the car's cornering ability. It's more useful to think in terms of moving the balance from end to the other. A stiff anti-roll bar at the rear will certainly reduce the amount of grip from the rear tyres, but it will also improve the grip of the front tyres because they are now dominated to a lesser extent by what is happening behind them. The balance of grip has moved forward.
You'll hear a similar term being used in Formula 1 when drivers or technicians speak of the "aerodynamic balance" being moved forwards or backwards; what they mean is simply that the level of downforce has been improved at one end relative to the other.
It's easy to see when a high level of compromise on the part of the suspension people has been reached - the inside wheel at the non-driven axle lifts off the ground during cornering. In the ideal situation of the car with maximum and equal grip this would never happen, because once a wheel has left the ground its tyre is obviously not contributing anything to the overall level of grip.
But to cope with inherently high weight transfer at one end of the car, a team may have to apply maximum weight transfer to the other. When an inside wheel leaves the ground, that's the sign that maximum transfer has been reached. It's extremely common on race-modified road cars, and it can also been seen on Touring Cars, though in their case the wheel doesn't often lift by much, and the clearest evidence that it is no longer in contact with the ground is that it has stopped turning because the driver is still on the brakes.
Perhaps surprisingly, it is also very common to see F1 cars lift their inside front wheels. This is largely because of current regulations regarding wheel and tyre sizes. If you look at a Grand Prix car of, say, the mid-1970s, you'll notice that the rear tyres are massively larger than the fronts. In today's context this looks very strange, but it makes a lot of sense because as well as having their engines and gearboxes at the back, these cars were also putting several hundred horsepower through the rear wheels. Under power the rear tyres had far more forces to contend with than the fronts, so it made sense for them to be larger and therefore have a much bigger tyre footprint.
Modern F1 cars have much more power - over 800bhp in pretty well every case - but the regulations demand tyres of similar size all round. Their suspension is already very stiff (to ensure that the wings are in the same position relative to the ground), but the front suspension in particular is massively stiff, to the point where most of the movement is actually in the tyre sidewalls. Maximum front axle weight transfer is reached almost as soon as the driver turns the steering wheel, in order to keep the balance of mechanical grip (as opposed to aerodynamic grip) as far back as possible, where it is needed to cope with the substantial amounts of power going through the rear wheels.
There is a saying in motorsport that the most important component on any car is the nut holding the steering wheel, and it's certainly true that different drivers can make the same car handle in completely different ways. But that's a matter for our race driving expert John Stevens, and no longer within the remit of an article on suspension tuning.
