|
Dennis,
It's not my field, so I may not be exactly correct on this, but my understanding is that (almost) everything comes down the the nature of the rubber. A particular rubber compound gets 'softer' (and therefore 'stickier') as it gets hotter. But at a certain temperature the rubber itself starts to break apart. What you are looking for, therefore, is to run as soft a rubber as you can get away with. Factors that influence this are:
- Surface pressure: If each square centimetre carries less load, you can use a softer, less durable rubber. Fitting wider or larger diameter tyres would achieve this (as in all cases here, assuming ideal, lab, like-for-like conditions). However, just fitting wider tyres will not give you more grip automatically - the benefit is in the ability to run a softer compound. Then you gain grip.
- Heat Cycles: Fresh rubber is partially cured (especially soft rubber). Each heat cycle increases the curing, and this creates a 'harder' compound. More significant with soft race tyres designed to last just a few miles, than more cured road tyres designed for many thousands.
- Carcass construction: In the tread, sidewall and shoulder, this is still about maintaining as much of the tread area at the optimum pressure (load/square centimetre, not air pressure) as possible. Some will be under-loaded, and therefore does not run in its optimum 'stickiness'. Some will be overloaded. You need to optimise this. This is also why instantaneous camber is so important (and tied to the construction of tyre you use), and why you might adjust camber for different circuits. Similarly, lower profile tyres potentially show a benefit by by reducing distortion, but only if you tune the suspension to suit. If the suspension has been tuned (with lots of camber change) to accommodate the profile distortion of a high profile tyre (e.g. historic), an ultra-low profile tyremight just stand up on its inner shoulder the moment you turn = tiny contact patch = rapid overheating = sliding plus a destroyed tyre.
- Hysterisis: Stretch and release a piece of rubber, and not all of the energy is released. It is retained as heat. Bend an eraser repeatedly, and it warms up. A tread block is repeatedly being stressed and released - heat builds up inside the block. You can use that to heat up the tyre, but the risk is that you overheat the rubber (losing grip). A tall (new) block can move a lot, and generates more heat as a result. Shave new tyres and you have squatter blocks = less sub-surface heat build-up = less risk of overheating. Wider blocks (e.g. Avon ACB) are stiffer in lateral movement, and therefore there is less heat build-up from hysterisis (assuming the same rubber compound). A slick is one huge block. You can therefore use a much softer compound.
So you can see that all of the tyre characteristics (and indeed suspension characteristics) seem to revolve around making as much of the contact patch work within its ideal temperature range (and this in turn also relates to the the circuit/road surface characteristics).
You can also see that you can bully a tyre (with aggressive driving or setup) to heat up quicker, which can be used to get more out of it quickly, but potentially at detriment to its race-life - think Webber in a Jaguar, or Bridgestone's 2005 F1 problems after a safety car period.
In a simialr vein, I remember hearing a story about F1 qualifying tyres from a few years back (possibly Goodyear). Drivers were having real trouble getting them up to temperature on the out lap, and having bullied them they would not last the lap. Eventually it came out that Goodyear used a mould releasing agent. The low friction surface prevented drivers from building up surface temperature, but their bullying built up sub-surface heat. They struggled to wear off the surface in time, and in bullying the tyre to achieve this, the exposed surface was too hot. Shaving off the contaminated surface solved the problem immediately. Then everyone found out that Senna had been doing this for the past six months...
So this is what I've picked up over the years. I think it is correct, but would welcome better-qualified opinion.
|