Wheel rate

Theory

Why do cars have springs? Springs are used to hold the chassis up in the air, decoupled from the nasty, bumpy ground. If a car didn't have any springs, then it would lift into the air when it hit a bump, as the upward force of the bump would be transmitted directly to the chassis. The tires would come off the ground - or at least become very unloaded - and thus lose their 'stick'. With springs, (some of) the upward bump force is absorbed - stored in the spring - before this chassis lifting can take place, keeping the tires stuck to the ground.

Fitting harder or softer springs alters the amount by which the chassis moves up and down in response to load transfer. Softer front springs: the nose dips down further under braking. If you fit solid bars to the front instead of springs, the load still gets to the front wheels under braking, it's just that the nose won't dip. However, stiffer springs do transfer weight faster than softer springs.

Different springs can also affect where the weight transfers to, and this can be used to change the handling balance of the car. You're going to have to take this on trust, but suppose you were to take your road car and replace the front springs with solid bars. When in a corner, inertia would cause load to be transferred to the outside wheels. Instead of the load being transferred evenly to the two outside wheels, as would be the case with 'normal' suspension, all the extra load that would have been distributed between the two outside wheels would now be transferred onto the unsprung outside front wheel. The rear inside wheel would have no extra load on it. (Don't ask why this happens, it just does.) The unfortunate front wheel, because of this extra load, suffers a loss of coefficient of friction relative to the rear and as a result operates at a higher slip angle than the rear tire. The car will go into understeer as the outside front is hopelessly overtaxed.

This extreme example illustrates the point that a stiffer spring absorbs more of the load being transferred. Soaks it up, like a sponge. It lets softer springs elsewhere on the car get away with doing less work. The more load a spring absorbs, the harder time it will give its tire, in relation to the other tires. So: stiffer springs on the front (in relation to the rear) creates understeer. Softer springs on the front (in relation to the rear) creates oversteer.

Fitting harder or softer springs does not change the overall amount of load transferred during longitudinal or lateral weight transfer. The overall amount of load transfer is determined by three things:

  1. The height of the center of gravity - a higher CoG equates to more load transfer (it's a leverage thing);
  2. The wheelbase/track width of the car (a short wheelbase will transfer more load than a long wheelbase, and a narrow track width will transfer more than a wide track width. It's another leverage thing.);
  3. How hard you accelerate/brake/corner.

Softer springs create more grip, but only up to a point. The softer a car is sprung, the more it will roll. Each car will have a point at which the extra grip it gets from softer springs is lost by the inability of the suspension to deal with the increase in body roll. The body rolls so much that the suspension geometry can't maintain a decent tire contact patch, and grip is lost. Worse still, if the springs are so soft that the suspension components touch or the chassis hits the ground, then all grip will be momentarily lost as the spring rate effectively reaches infinity.

However, faster circuits will require stiffer springs. When a slow-moving car travels over a series of bumps, the suspension is presented with relatively soft and low frequency bumps - which can be dealt with best by softer springs. When a fast-moving car travels over a series of bumps, the suspension is required to absorb powerful and high frequency bumps - which demand stiffer springs.

Application to GPL

Front/back ratio

Since springs are there to hold the car up, it would seem reasonable to set the wheel rates at each end of the car in proportion to the weight at that end. For example, the Lotus 49 has 62% of its weight on its rear wheels, so why not set the rear wheels to have 62% of the overall wheel rate, and the front wheels to have 38%?

Well, you can do that, and use that approach to come up with a neutral-handling car. However, such a distribution produces a rather awkward and unstable car - one that doesn't willingly turn into a corner, and/or one that can be difficult to recover from a spin.

The Cooper, for example, has a 40/60% (2:3) weight distribution so you could set the wheel rates to be, for example, 80/120 lbs per inch. This produces an 'awkward' car. Shifting the distribution to 43/57% (3:4), for example 90/120 lbs per inch, produces a far more driveable Cooper.

Once you've established a front/back ratio for each car that produces stable handling, then select values for the wheel rate that conform to that ratio. In the Cooper example above, you could select any one of 60/80, 75/100 or 90/120 lbs per inch, depending on how stiff you wanted the car overall.

Overall wheel rate

In the real world, softer springs give more grip. Within the range of values you can select in GPL, that isn't so: a softly sprung car is no grippier than a stiffly sprung one. I'm guessing that this is because small bumps in the track aren't modelled. This allows you to choose wheel rates much higher than those used in 1967. I've found that you can use many hundreds of pounds of wheel rate before the car gets difficult to handle because bumps on the track send it into the air.

Therefore, the overall amount of wheel rate you apply to the cars in GPL tends to be subjective.

How do you choose the overall wheel rate? For the chosen ride height, make the springs as soft as possible without:

The harder you make the springs, the more feedback you get from the car, and the faster the car reacts both to steering input, pedals and road surface. Softer springs require slower and fewer steering inputs from the driver, and let the car ride the road surface more smoothly.

("Ah!" You might say. "Don't you need to increase the stiffness of the dampers as you increase the spring rates? So don't you need very stiff dampers with those very stiff wheel rates?" Well, yes, but GPL seems to feature magic dampers... see the section on dampers for more details.)

Finally, if the setup is still giving problems, you soften the end of the car that isn't working. For an extreme example, the wheelbase of the Honda is quite short and it's rear end very heavy. I've ended up softening the rear to the point where the front and rear wheel rates are the same.

 
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