What effect does upping the pre load have? I always leave well alone as I don't understand.
I will try to answer as best I can.
It is the amount of time it takes for the power to kick in when you press the accelerator. A low pre-load is instant power. The higher you go with the pre-load the longer it takes for the power to kick in. We are talking about 10ths of a second here, maybe less. A high pre-load (say 5 in a GT1 car) will turn your accelerator movement into a smooth curve if you could see it on a graph. This translates to more stability under acceleration. However, on a track like Monza (long straights) it will mean that you get less potential acceleration force from the engine.
Pre-load is just one of a few settings that effect stability under power. The power in the diff and the rev limit also effect acceleration stability. I find that if the car spins under acceleration the instant you accelerate, then reduce pre-load. If the car begins to spin under acceleration after you have already starting turning and acceleration then reduce the power in the diff. If the car spins out under acceleration when you up shift (Zhuhai is a good example of were this could happen) then reduce the rev limit.
In short, set pre load as low as you can get away with. I always try to set it to 1 in any car and usually put it up to 2 in the wet or possibly more in GT1 cars in the wet.
Hope this helps. I suspect others may be able to give a better description.
