How to... Set up corner weights Ride heights and ballast are key to ensuring that your car corners consistently and you achieve faster lap times
W O R D S N AT H A N C H A D W I C K
IF YOUR CAR IS ACTING A BIT WEIRD THROUGH the apexes on track, setting corner weights could transform your confidence – and thus your lap times. So how do you go about doing just that?
PREPARATION The need to precisely hone the car’s ride height at each corner means you’ll require height-adjustable coil-overs. Whether it’s the spring base that’s tweakable, or the body, when corner weighting the spring-platform adjustment should be used. Variable drop links are also a must. Corner weighing means that ride heights will differ from side to side, thus straining the sway bar. This increases the car’s static spring rate, leading to unpredictable straight-line steering. By fitting adjustable drop links, the onus for compensating for the difference in ride heights is put on these components rather than the sway bar. Before starting a weighing session, make sure the scales are on a flat surface. The tires need to be at an even pressure all the way around, or how you’d actually like them to be on track. Incorrect pressures could raise or lower the car at a particular corner, leading to inaccurate weight readings. Next, disconnect one drop link at the front and at the rear, in order to stop the sway bar trying to compensate for the ride-height changes. You’ll also need the driver to sit in the car, or put an approximation of his or her weight in the driver’s seat, to understand how much you’ll need to compensate. 142
WEIGHT OF THE WORLD With the car jacked up, place the scales under all four wheels and allow the suspension to settle. Take a note of the current mass distribution; obviously it’ll show the mass at each wheel, plus the percentage difference fore and aft, and side to side – but the most important figure is the cross-weight percentage. On most scales this is displayed as CR, and it figures the difference between the diagonal weights of each vehicle (eg right rear and left front). You’ll use these percentage figures to fine-tune the car’s mass. You’re looking for a 50/50 percentage side to side in order to create a car that won’t surprise you apex to apex. Aiming for 50/50 front and rear is also a good idea, but you might want to change the weight for greater front or rear grip. The cross-weight percentage, however, has to be as close to 50 percent as possible so that the car exhibits the same cornering characteristics when turning either left or right. For rear-wheel-drive (RWD) models, more mass at the rear (say 45/55) would provide greater traction, while the opposite is true with FWD. For most RWD racing cars, though, 50/50 is preferred. Similarly, 50/50 front to rear is best for 4WD cars, so that traction is split evenly. Increasing a particular wheel’s mass necessitates raising the ride height, because this increases the leverage of the car’s body to the wheel, which in turn increases the mass on it. To decrease mass at a wheel, the same principle dictates that you lower the ride height. Perfecting the percentages takes time, M1 MOMENTUM
because adjusting one wheel will affect another, most often diagonally. Sometimes 50/50 perfection side to side, or your ideal front-rear percentage, is unobtainable; it’s better to accept this rather than have wildly different ride heights all around the car, because this in itself will have a knock-on effect on handling. A slightly uneven set-up and a more level car all round is far better.
ROLL WITH IT Changing the ride heights will also alter the roll-center location, leading to an increase or decrease in the roll moment at the end of the car, thus creating under- or oversteer. Different heights front to rear are therefore best avoided, unless the suspension geometry is designed to work in such a manner. The center-of-gravity position can be raised by jacking the car too highly, resulting in even more excessive roll. If changing the ride heights doesn’t achieve your mass-balance aims, adding ballast is an option. These plates bolt into the car in various locations, which obviously makes that section heavier and alters the car’s mass distribution. Once you have set the ride heights and achieved your distribution aims, you can lock the coil-overs off and adjust the drop links to connect the sway bar with the suspension without putting strain on the former. Ready to go? Not quite: all this fiddling with ride heights means wheel alignment will be considerably out. Once that’s done, you can hit the track, leave corner-to-corner concerns behind and focus on one thing – getting faster.
