Back to business:
A common misconception about suspension (which took me some years to realise was wrong) is that harder springs prevent weight transfer to occur. They don't (at least we should think they don't). Weight transfer depends only on CG height, lateral (or longitudinal) acceleration, and how wide (or long) the car is. The CG position in a race car during maneuvers does not move significantly for roll (or pitch) to be considered important to weight transfer. They start to matter on off road stuff or Super Trucks with lots of suspension movement, but not on low rigid race cars.
Harder springs are actually a result of the primary intention to lower the car. With short travels you need them to keep the car where you want for aero purposes and not touching the ground. Their most noticeable impact is on how fast you will have to react to keep up with the car. Harder springs means higher frequencies, which means faster reactions. Sometimes it would indeed be better to have softer springs, so our (lack of) abilities are on par with the car's difficulty, but it's pretty much never possible.
Another very important contribution of springs is to the roll stiffness. And that's where I wanted to get since the start. I think the following is the MOST IMPORTANT MINDSET you can have in order to improve your setups, or even to improve your driving. To me everything in racing comes down to: Which will be the first to give up, the front or the rear? That's always the battle I try to fight while driving. Will I suffer from under or oversteer? In the end one will show up. Will I lose time with the car not turning, or will I lose time with the car spinning? And there are two things we need to know if we want to solve this mystery: (Oh boy, this is going far)
1 - A more loaded (with normal force) tyre won't gain as much grip as the less loaded one lost. (
Video from Niels explaining this)
That's also know as "the coefficient of friction decreases when normal force increases". All that means is that a pair of tyres will have their highest combined grip when loaded evenly. Whenever there is load transfer from one to another (weight transfer) their overall grip will be lower, because one lost more than the other gained. So weight transfer = always bad. (And remember harder springs don't prevent it!)
2 - A car is a hyper-static structure.
Do you know that chair with one leg slightly shorter than the others, that keeps rocking back and forth, side to side? That's a hyper-static structure, because it has more legs than what's needed for it to stay up - which would be 3 legs. The car is the same thing. Tricycles stay happily upright. Bicycles don't. Cars didn't need the fourth wheel, yet it's there and we have to deal with it. All of this to say that, during a maneuver, one wheel will tend to be forsaken by the physics Gods, just like that chair leg in mid air. That doesn't mean one wheel will always be in the air. Chair legs are rigid, car wheels are softer, so they settle better to the ground. But one wheel will have its load more transferred than the others (remember load transfer = bad). The thing is: Will that wheel be in the front or rear axle?
This brings us back to roll stiffness, finally. Think about it as the difficulty to roll the body of a car by pushing it from the side (which is basically what lateral acceleration does to the car). The overall roll stiffness doesn't matter much. What REALLY MATTERS is how the front roll stiffness is compared to the rear roll stiffness. This will help us see where that forsaken wheel is.
An imagination exercise in order to visualize it: Try to picture a car with the front shockers welded - no movement for the wheels at all - and regular soft rear suspension. Now start to push the side of the body with increasing force. As soon as the car starts to roll, one front wheel will lift off from the ground, because it's welded to the body. And what about the rear? It will still have both wheels to the ground, because the soft suspension let them sag and follow the ground shape. This means that we now have 100% load transfer at the front (hugely bad!), so the front axle overall grip is dramatically reduced, while at the rear things almost haven't changed regarding overall grip. This car is probably going to suffer from terrible understeer. The front may give up before the rear. I said "probably" and "may" because some other things (worn tyres, suspension geometry, driving style, etc) still can make the rear be as (or more) shitty as the front.
So that's the mindset I use. The axle with higher roll stiffness will be injured in this battle of axles for grip. Always one relative to the other. Both high or both low can have the same effect. Remembering that during a corner is very helpful in order to understand what's going on with the car. I have to say that roll stiffness is influenced by springs, anti roll bars, and suspension roll center (the latter can't be changed by setup parameters, so don't worry). But try to focus springs for bumps and maintaining ride height, and leave the anti roll bars to do the roll stiffness job.
The last thing is about caster and kerbs. Another caster function always overlooked is that it lowers the inside and raises the outside wheel when you steer. That is like growing one leg of a chair, forcing the chair to tilt. In a car (specially karts), that results in the rear inside losing load, because the car rolls, the front is now levelled with the road and the rear has to comply with that distortion. The exact same thing happens when you hit kerbs with the front wheel. It is like an instantaneous load transfer from the rear inside to the rear outside, decreasing rear axle overall grip, which induces oversteer and makes the car unstable.
I think this is all I wanted to say. Sorry for the long post. Now I'm in peace with myself.
