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There is an apparent conflict between setting the static ride height and setting the wheel rate. You often want to make the car as low as possible and as soft as possible: yet as you make the suspension softer, so you have to increase the ride height to keep off the bump stops and/or the ground. So, should you make the car high and soft, low and stiff, or somewhere in the middle?
As you lower the ride height, the CoG of the car descends toward the roll center(s). As this happens, the amount of leverage the CoG has over the roll center decreases, and so the car rolls and pitches less. Therefore: a car with a lower ride height is inherently stiffer than one with a high ride height.
This implies that - to keep body roll within a desired range - as you raise the car, you need to stiffen the springs. And as you lower the car, so you need to soften the springs.
There's a rare piece of synergy at work here. At the smoother circuits, you can run the car low and soft. As the track gets bumpier, you need stiffer springs (to absorb the energy from the bumps) and higher ride height (to give the suspension its full range of travel, and to stop the car from grounding).
You can quantify this. Playing around with the Lotus, I decided that I liked the handling of the car when the springs were set to 50/75 lbs (soft) and the ride height was 2.50" (low). Also, I liked the handling when the springs were 75/112 lbs (stiff) and the ride height was 3.75" (quite high). For some inexplicable reason, I noticed that if you divided the total wheel rate by the ride height of both these cars, you ended up with roughly the same number for each setup: 100 (lbs/inch/inch) (+/- 5). For example: ((70*2)+(105*2))/3.50 = 100, my Mosport wheel rate and height.
For all the combinations of ride height and wheel rate that produce a figure of 100 lbs/inch/inch on the Lotus, you get a table like this:
| Static ride height (inches) | Total wheel rate (lbs) | Wheel rate (front/rear) |
| 2.50 | 250 | 50/ 75 |
| 2.75 | 275 | 55/ 82 |
| 3.00 | 300 | 60/ 90 |
| 3.25 | 325 | 65/ 97 |
| 3.50 | 350 | 70/105 |
| 3.75 | 375 | 75/112 |
| 4.00 | 400 | 80/120 |
(The wheel rates are distributed 40/60% front/rear.) I've played around with rates and numbers outside these ranges, and have to admit that the handling quickly becomes pretty terrible - for me - below 95 and above 105. The Brabham likes a slighty softer overall wheel rate than the Lotus; a figure of around 90 seems to work well for me. The Coventry needs stiffer springs (all that extra weight), so I aim for a number above 100. You may have your own 'majic number' for each car.
Of course, there are exceptions. At Spa, for example, you need the car to be as low as possible (all those long fast corners) and yet unusually stiff (to absorb the forces produced by going over bumps at nearly 200 mph, and to stop bottoming at Eau Rouge). I tend to run the Lotus at 2.75" with wheel rates of 75/112, giving a 'stiffness number' of 136.
So, this table gives me a quick way of setting both static ride height and wheel rate at the same time. You will need to create a different table for each car, because of the different weight distribution and the different overall weight of the car (heavier cars will probably need stiffer springs).
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