Chapter 9: Making adjustments

9.1 Before Making Any Adjustments

You can’t expect to be able to make your car handle better by making setup adjustments if the car’s not in perfect working order to begin with. Any small malfunction can mess up a car’s handling to the extent that it’s impossible to correct using setup adjustments. The following is a list of such malfunctions. So if your car is handling very badly, you might want to check this list before making any setup adjustments. Keep in mind that in oval racing some of these are purposely introduced to obtain an asymetrical handling pattern.

9.1.1. Unequal tire size

pulling to the side
Different diameter tires on one end of the car, like for example when the left front tire is bigger than the right front, will make it pull to the right, both under acceleration and deceleration. The biggest tire has a little more weight on it, because the spring at that side is compressed a little further. So it will have a little more traction, and on top of that, it will have a greater ‘rollout’. Those are two reasons why the biggest tire will want to travel a larger distance, pulling the car to the opposite side. So if your car pulls to one side while accelerating and to the other while braking, you might want to check the tires’ size first.

If the tires up front differ in diameter from the ones in the rear, other kinds of handling defects can occur. If your car is front or rear wheel drive only, and you have compensated the difference in ride height, the only difference will be a difference in grip. A larger diameter tire will have a slightly bigger contact patch than a smaller one, and hence it will provide just a little more grip.

If your car is full-time 4WD, tire diameters can influence the amount of steering your car has very dramatically. Suppose the front tires are larger than the ones in the rear. The front end of the car will tend to travel a greater distance than the rear. Around a corner, the paths covering the largest distance are located towards the outside, and the short ones are located on the inside. So, the front end will want to go around the outside, and the rear will want to go right on the inside, making the car point outwards. In other words, the car will understeer horribly. It will feel like a ‘sticky’ kind of understeer too, slowing down won’t help very much, and braking will only help for a very short time because the car will very quickly try to return to its understeering state. The opposite can also happen: bigger tires in the rear will make the car oversteer very badly.

And also, the difference in 'rollout' will cause extra friction, which can significantly reduce top speed. Equal diameter tires are definitely the way to go for 4WD cars.

9.1.2. Tweak

Tweak describes the difference in weight on two tires on the same end of the car. For example, if there's more weight on the left front than on the right front tire, the car is tweaked. The unequal tire load will make the car pull to the side when accelerating or braking, or even make it spin out for no apparent reason. The causes can be numerous: unequal tire size, difference in shock length or preload, tweaked (bent) chassis, ...
There are several telltale signs that your car is tweaked. For example: if it turns more sharply to one side, even though the left/right weight distribution is 50/50, if it hooks (oversteers) to one side, and understeers to the other side,...

9.1.3. Excessive Friction

Things like A-arms that aren't moving freely, faulty ball joints, bent or scratched shock shafts, tight steering mechanism, loads of crud in bearings or bushings,... all cause uneven tire loads and thus also erratic handling. The only remedy is regular maintenance; I suggest you have a good look at all the parts of your car after every race.

9.2 Making Adjustments

The purpose of making adjustments is to make the car go faster around the track, or to make it more controllable, or often both. A car that's easier to drive often causes lower, more consistent lap times, and more importantly, it will inspire the driver more confidence, which comes in handy when the going gets rough.

Before you even start thinking about changing your car's setup, consider these two things: firstly, is the car in perfect working order? Be sure that all of the suspension components operate freely, without excessive play, and that the car isn't tweaked. Things like that can really mess up a car's handling. Secondly: the fisrst things to consider are always the tires. Any time spent trying to get a car that's on the wrong tires to handle is wasted time; it won't be fast enough anyway.

The first, and most important step towards making an adjustment is determining the cause of the handling deficiency you wish to cure. Experienced drivers/mechanics or people with a lot of insight in vehicle dynamics will just know or feel this. In order to know what to change, you need to know what each element does, and does not do. For instance: changing the front toe angle does not change the balance of the car, it just changes the way the car reacts going into corners. From all the previous chapters, it should be clear, but in reality, it can be difficult to judge the difference between what the car is doing and what you'd like it to do.

Here are a few examples.

When you're racing on a big, flowing track that has a lot of shallow, rhythmic bumps in it, don't be tempted to use both a soft setting for springs and damping and a very low roll center: the chassis will roll from side to side in every small bump, resulting in a very unstable, unpredictable car with very little traction. In this case, stiffening both the damping and the springs will increase traction, but it won't be the best solution, that would be raising the RC, and making sure it stays high when the chassis rolls. Note that when the bumps are sort of rhythmic, soft settings for dampers and springs will make the chassis 'resonate' to the bumps, which causes a weird form of instability. In that case, damping and maybe springs that are a little harder would be better; it will make the car skip over the bumps instead of plunging into every one of them very deeply. And it's not just side-to-side movement that can be excessive on a bumpy track: if your car has a lot of negative suspension travel, its chassis can suffer from excessive pitch: it will kind of rock back and forth. The answer is simple: reduce the downtravel. Even though this will make the car bottom out on large jumps and bumps a little more, it will be a lot more stable in bumpy sections, aspecially sections where the car is accelerating or braking.