Roll bar stiffness

Theory

Q. What is a roll bar?
A. A tube running across the width of the car, attached to the suspension at either end. The bar is allowed to twist as the suspension moves in response to chassis roll, and in doing so, acts like a spring.

Q. How does a roll bar work?
A. By pulling up the inside wheel during cornering.

As the outside wheel is pushed up by cornering forces, the roll bar rotates up. The inside end of the roll bar then tries to pull up the inside wheel. The stiffer the roll bar, the harder it tries to lift the inside wheel. The force of the roll bar is split evenly between opposing the inside spring and helping the outside spring.

Q. Why do we have roll bars as well as springs? A. Because the car is longer than it is wide; the closer together two springs are, the less their roll resistance. It's a lever thing.

Thus, in a corner, the roll bar adds weight to the outside tire and subtracts weight from the inside tire. (That can reduce grip at that end of the car, because a tire with 1000 lbs on it does not grip twice as well as a tire with 500 lbs on it.) The degree to which the roll bar transfers weight for a given amount of body lean depends on how much the roll bar twists; the stiffer the roll bar, the more weight gets transferred. Manipulating the relationships among these corner weights is how cornering balance is tuned: the end of the car that shares the weight more evenly between the right and left tires is the end that gets more grip.

Roll coupling is the relationship of the roll resistance of the front of the car and the roll resistance of the rear.

If the front bar is (comparatively) too stiff, then (comparatively) too much weight will be transferred to the outside front tire, making it run at a larger slip angle, and so the front end will lose grip (compared to the rear end) and the car will understeer in the middle of a corner. You will probably feed in more steering to compensate for this. This is a Bad Thing, because when you start to come out of the corner and increase the throttle, the steering input may still be there and so the car will get power-on exit oversteer, just when you need it to be neutral.

If the rear bar is (comparatively) too stiff, then the rear will lose grip (compared to the front end) and the car will oversteer in the middle of a corner. This is also a Bad Thing, as it prevents you from applying the throttle early in the corner.

Therefore, you have to play with the difference between the front and rear roll bars, so that during long corners you are neither:

How do you measure understeer? Stick a piece of white tape on the top center of your steering wheel. Drive at a 'legally safe' speed around a long, constant corner. Note the position of the tape. Now drive quickly around the same corner. Technically, if the angle is larger at racing speed then the car is considered to be understeering, and if the angle is smaller (or even -ve!) then the car is considered to be oversteering.

If you squeeze the throttle a little harder, the car should just slide sideways more. You should be able to keep a nice, slip-angle-friendly slide all the way round, with the car yawed through about 10°-20°. At the exit of the corner, the steering should remain generally in the middle as you apply the throttle, letting you get the maximum power down.

The difference between the front and rear bar will control under and oversteer. The total of the two bars will control by how much the whole car will roll while cornering. Twistier bars = more body roll = more -ve camber required on the loaded tires, more time taken for the car to get into its cornering state. Stiffer bars = less body roll = less time required for car to get into its cornering state.

Application to GPL

Set the roll bars after you have set the differential, wheel rates and ride height right. These factors affect the setup more 'deeply' than roll bars.

There is a 'rule of thumb', repeated in the printed manual, that the front roll bar should be set to 10 lbs more than the front spring rate, and the rear bar should be about 70% of the front bar. Using this rule would result in relatively soft bars with a fairly random front/back distribution.

I use the following method to set the roll bars:

  1. Start by determining how much symmetrical static camber you're willing to use on the front tires. -0.75° is a popular choice (the idea being that with -0.75° the outside tire will hopefully be flat on the ground during cornering), but sometimes I prefer -0.50° (more stable when braking). (Forget about asymmetrical camber for now.) (BTW, in 1967 Jim Clark had -0.25° on his Lotus.)
  2. Add front roll bar until your desired tire temperature/camber setting goal is reached on the front tires. I generally find that you need quite a lot; 170-180 lbs on most cars. You're aiming to find the minimum amount of roll bar needed to do the job; I find that adding any more than the minimum just makes the car difficult over bumps and kerbs for no apparent gain.
  3. Add rear roll bar until the car handles how you like it. I can tell when I haven't added enough rear bar, because I aim for but understeer away from the kerbs of the fast right-handers at Spa/Cottage and Spa/La Carriére. I can tell when I've added too much rear bar when I regularly apply opposite lock in the corners, or when the car becomes unstable when on the grass.

Remember that it's the ratio of front to rear bar that defines the balance of the car, not any absolute values.

Three signs of not adding enough overall roll bar are (1) the car seems awkward through Spa/Masta because it takes too long to roll from one side to the other, (2) the car 'falls over' onto its outside front wheel when braking into the curved braking approaches to Rouen/Hairpin, Monaco/Gazometers and Mexico/Turn 1, and (3) the chassis can wobble and sway from side-to-side.

If you have too much overall roll bar, then the car will have no 'feel' in the corners, and be twitchy over bumps and under braking. I find a car with a lot of roll bar almost undriveable at the Nuerbergring.

I think that you always want the car to understeer a little. Firstly, this is so that you can apply the power sooner when coming out of a corner - the understeer gives you something to 'push against' with the throttle. How much of this effect you need depends on the circuit: very little for Monaco, the Nuerbergring and Rouen where precise handling is at a premium; more at Kyalami, Silverstone and Monza where corner exit speed is more important. Secondly, a little understeer creates some 'self aligning torque' into the handling, so that when you lose control of the car it will not become 'divergently unstable'.

Finaly, a quote from Allan Staniforth: "...an excellent and reliable guide to the handling performance of a car has, for some considerable time, been that one near it's best will be sensitive to small anti-roll bar adjustments. If it requires or is unresponsive to large adjustments, something else is badly wrong, needing to be located and corrected".

 
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