Shifting Your Weight Distribution on a Race Car
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Shifting Your Weight Distribution on a race car Story Jamie Gibson There is a growing trend to find drivers using scales to help their racing. Walk around the pit at many meetings, especially F1's, and you will see some scales being used. For some this is quite confusing, "how is knowing what the car weighs going to make it any quicker?" Making it lighter can give a performance gain, but lightness is not always the best way to go, especially if there is a minimum weight limit in the particular formula. Making a car too light may also compromise its safety. The key is not what the car does or does not weigh overall, but it's where the weight is that counts. By changing the weight distribution on the car you affect the way the car will behave when cornering. The ultimate aim is to get the balance that suits both the driver and the car and that will eventually lead to faster lap times. Incorrect Distribution Let's look at what incorrect weight distribution can do to a car. If a wheel has too much weight the tyre will become overloaded. When the car corners, extra weight shifts to the outside through centrifugal forces. These combined forces overwork the tyre and it will no longer be able to grip. Too much weight at the front and it will get understeer, too much at the back results in overseer. A similar thing happens if it does not have enough weight in the right place. As weight moves to the outside the inside tyres are unloaded and again will not grip properly. In some cases (and quite often seen) the wheel will lift right off the ground. If all four wheels are not on the ground, the car is not at its best. Even if there is no way of measuring the weights, it can be seen to some degree if the weights are wrong. If the car pushes or is loose, look at the tyre wear. Lots of wear and that tyre is overloaded, it needs weight moved away from it. Not much wear and weight needs to be shifted onto it. How to When measuring or altering a car's weight characteristics, you need to be able to find out what the weight is at each wheel. The 'corner weights' are best measured using a proper set of computerised scales. An alternative method is to use a manual corner weight gauge, which is used to lift one wheel at a time. This is a much cheaper way of doing it, but has nowhere near the accuracy of a good set of scales. Maybe improvise, a set of four bathroom scales could probably cope with a light car. Taking readings accurately is a two-man job as the driver will be in the car when it is on the track, he needs to be in the car to get the proper readings, plus anything else as if the car was on track ready for the race. With a measurement for each wheel it is possible to work out the important figures. It does not matter which, kilos or pounds, or what the actual figures for each wheel are, as at this stage they need to be converted to percentages. The three main figures that are needed are the inside weight (total percentage on the two inside wheels), rear weight (total percentage on the rear wheels), and most importantly the crossweight. This is the combined percentage of the outside front wheel and the inner rear wheel. What to aim for Now the problems of incorrect weight distribution have been assessed, what is the setup to aim for. One of the most important things is to get as much inside weight as possible. This cannot be achieved by altering settings on the car. The only way to do this is to physically move things around the car. There are not many things to move on the modern race car, the battery and fuel tanks are the most common, as is putting all the plumbing and wiring down the inside. This is why many F2's now have the battery and fuel tank tucked under the inner nerf rails. In the extreme take a look at some of the American Modified formulas, these have engines mounted right out to the side of the car, with an offset chassis as well. You can add ballast to help (saloon car drivers having a couple of extra burgers regularly!?), but the power to weight ratio has to be considered, "will more weight slow you down?" Most oval race cars are front engined, so rear weight also needs to be as great as possible to even out the weight of the engine. Ways of achieving this are the same as for inside weight, move things around. One way to help this is to mount the engine as far back as possible, moving the weight more towards the middle of the car. This means it will be easier to balance the car. Wherever things are moved to, they must also be kept as low as possible. The higher up the weight is, the higher the centre of gravity. The higher this is above the roll centre (the axis about which the car rolls left to right) the more weight is transferred during cornering. Obviously the less weight shifts the better, as the car will remain more stable during cornering. Crossweight As mentioned before, the crossweight is the most important and this is the one that you can change by altering the car's settings. With a few simple changes to the crossweight of the car, it can radically change the way the car handles. To change the weight at a particular wheel, the ride height of the car needs to be changed at that corner. With coil-over shock absorbers, this is achieved by winding the lower spring platform up or down. Other cars may have height adjustable gas-shock absorbers, or multiple mounting points which can be used to achieve the same thing. Whatever the method the result is the same, if the ride height is increased at one corner, the weight on that wheel goes up. If the weight is decreased, the weight goes down. It is important to notice that as one is altered, all the other corners are affected at the same time. If weight is added to one wheel, the diagonally opposite wheel receives the same action and the other two wheels will have the inverse effect. If you do not know what you are doing, you may end up chasing the weight all around the car, never actually getting anywhere. Whilst in the workshop ball-park figures only can be aimed for. This is the best time to use the scales as it should have an almost guaranteed even surface, something not always found at the track. There is no magic number to go for, but the best area to start with is around 50-55 percent crossweight. If using anti-roll bars, disconnect them while setting up the car weights, as when making changes the bars may be working against you, resulting in false readings. Only with a lot of time and experience of both the car and the track it will race on, will you be able to be more accurate at this stage. Each type of car will be different. Differing wheel bases, track widths, roll centres, tyre widths. These are just some of the factors that affect the best settings for a car. Set Up - Nearly and Note it The car is now set up with the scales - unfortunately theory alone will not give the exact answers. Track testing is the best time to make the final settings, which will not be too far away if things went right in the workshop. First things first, find out what the car does on the track. If the car gets out of shape in the corner, note what it does first and for now ignore any other ill effects. If it pushes going into a corner, but is loose coming out, aim to fix the push first. When you have fixed that, then move onto the next problem, never try to fix more than one thing at a time. If finding the car does not have problems, make changes anyway, you may just make it even better. One of the most important things to do when making set up changes to the car is to note down what changes have been made. This way if it does not work, it is a lot easier to back- track, without having to guess and make it worse. Having found out what the car does on track, it is now time to change the weights to alter the handling. In nine out ten cases it will only need a change to the ride height at one corner, namely the inside rear. If the car pushes on into the corner, the most probable cause is the outside front tyre has too much weight on it. This means that your crossweight is too high and weight should be moved away from this wheel. To reduce the crossweight, lower the ride height on the inside rear, or to gain the same result, raise the outside rear. Never make too big a jump in the setting and only make one change at a time as it is easy to end up with more troubles than when you started. If the car tries to go sideways as you enter the corner there is likely to be too much weight on the outside rear.