Chapter 8: Balance

Balance is without a doubt one of the most important requirements for a well handling car, you might say it is the very key to having a fast, confidence-inspiring car.
If I were to define 'a well-handling car', I'd say something like "A well-handling car is a car that even my grandmother could drive competitively.". What I'm trying to say is that a car should be as foolproof as possible. Granted, one of the key features of a good driver is his/her ability to compensate all of the car's ill handling effects, but driving, a balanced, predictable car will come in handy when it really counts, when the pressure and the adrenaline levels are up and the finish is near. Plus, a smooth car is always faster.

It's mainly because there are two suspension systems in a car: one in the front and one in the rear, and they are forced to work together. As you might expect, if they're not tuned to do more or less the same thing, 'working together' can become very difficult.

Consider the following example: a car with a perfectly symmetrical weight distribution, i.e. the same amount of weight on the four tires, and all the springs and dampers are of equal stiffness. But the rear roll center is much lower than the front. If this car were to enter a corner, it would turn in nicely, but after that, it would understeer horribly. Let's have a look at what actually happens. The front has a high roll center, so the roll moment will be very small; the front end of the chassis will not want to roll very far. In the rear, it's the opposite: the RC is very low, so the roll moment is very big; the rear end of the chassis will want to roll very far. Since the spring rates, or for that matter, wheel rates are equal at both ends, the force that tries to prevent the chassis from rolling will be the same, so the rear will want to 'outroll' the front. For instance, the front will want to roll 2 degrees, and the rear will want to roll 10 degrees. Obviously, if the chassis is a rigid structure, t h is will not happen, it will roll to the average of the two roll angles: 6 degrees. So neither the front, nor the rear is where it should be. The front would have transferred the right amount of weight from the inside front to the outside front tire if the chassis would have rolled 2 degrees, but it rolled 6 degrees, so a lot more weight has been transferred onto the outside front tire than expected. In the rear, the opposite has happened: if the chassis would have rolled 10 degrees, the right amount of weight would have been transferred onto the rear outside tire, but since the chassis only rolled 6 degrees, a lot more weight has remained on the inside rear tire. This is not a very healthy situation, and it doesn't allow the car to take a corner in a smooth, predictable, and fast manner. The excess weight on the inside rear tire, and the high amount of grip associated with it will push the car towards the outside of the corner; it will cause an understeer condition, aspecially under power. The excess weight, and grip, on the outside front tire will cause an unpredictable oversteer situation. So you see, the front and the rear are fighting each other, and this will only become worse when throttle or brake is applied. If more throttle is applied, more weight will be transferred to the rear, so there will be even more weight on the rear inside tire, causing an even bigger push. More brake means more weight on the outside front tire, which in normal circumstances is a good thing, but in this case there already was too much weight on it, so it will result in more oversteer. Needless to say, this is not the fastest way around the corner, and the car will require an incredible amount of driving skill. Having springs in the rear that are too soft would have caused a similar problem.

So, what does this example teach us? It teaches us that the front and the rear of the car should always be able to work together, but that's not all; we can also use the insight it provided us to tune a car to our specific handling needs, by purposely changing the balance.

To be able to judge if your car is balanced or not, you'll need some kind of baseline comparison. I use the following, imaginary set-up: a car with the CG right in the middle, and identical springs, dampers and roll centers front and rear. It's pretty obvious this car is balanced, but usually, front wheel drive cars will have more weight on the front axle, and rear wheel drive cars will have more weight on the rear axle. This is easy to compensate: just increase spring stiffness and damping by the same ratio as the CG moved. For example: if the rear has twice as much weight on it than the front, use springs that are twice as stiff in the front, and use a damper rate that's also twice as thick. If you try this, you'll find that it's very easy to drive, predictable and stable, and generally well suited to extreme track conditions. But you might not find it aggressive enough, or think that it's got a little too much steering for our taste,….

But there's a remedy for that; you can make the changes you like, as long as you don't disturb the balance too much. For instance, you can move the weight forward a little, but use a stiffer spring in front. This will give you more weight on the front tires, statically. So you'll get more turn-in, and probably a little more on-power steering, but you'll lose some rear traction. Or you can use an anti-roll bar in the rear, but use slightly softer rear springs. This will give you more steering in the middle part of the corner, and it will give you more forward traction. Another thing that's being used a lot is to use a higher RC in the rear than in the front, combined with stiffer springs (and damping) up front, and softer ones in the rear. This makes for a very stable car: it will turn in sharply at first, because of the stiff springs up front, but then, it will understeer a little, because with the stiff springs and heavy damping up front, it takes some time to transfer the weight onto the outside front t i re. This happens a lot faster in the rear. But eventually, when the weight is fully transferred, the car will steer very well. This setup can be very fast: the car can be 'thrown' into the corner, without losing a lot of speed because of the mild understeer. Then, at the apex of the turn, some braking will probably be needed, but after that the car will be very stable again, like in the entrance of the turn, which makes a high exit speed possible.