Alloy wheels/rims question

[ryans]

This may be a silly question, but do alloy wheels affect handling?
This isnt my main question, bear with me.
I was watching Trial bikes the other day - Motorbikes going over obstacles. I noticed that the tyre pressure was very low, I presumed for more grip and also maybe to soften the landings etc. Am I right in thinking that lowered tire pressure would also give you better grip in cars at the expense of top speed?
If I'm not then this whole question is a waste of time but what the hell.
I assume that the lowered type pressure gives more lateral grip and therefore the tyres also move laterally. Now my question is, with the new trend of super large alloys/rims does this then restrict the later movement of the tire - because only a small section can actually move, I would think due to the rims - and so do these super large rims affect lateral grip atall?

I love cars, but I am not clued up on much of the technical stuff.

Thanks.

 

[Emohawk]

Affect handling?: Yes, assuming they're lighter (good) or heavier (bad) then the wheels they replace. A lighter wheel is good in a number of ways:
-less rotating mass: easier to accelerate and decelerate.
-less unsprung weight: less inertia allows the suspention to react faster.
-less weight period

Trial bikes: Very cool, but you can't apply those principals to cars. The miniscule amount of air pressure they run allows their tyres to flatten out and provide a larger contact patch (rock crawlers do this too, it's essentially the same activity). They do this because a trial bike needs more traction then anything else, and anything else is just what they sacrifice. Acceleration, top speed, tire life, handling, trial bikes don't really need any of that, but cars do.
With a very low air pressure, the tire is constantly reforming itself as it rolls to form the big flat spot on the bottom. This takes a huge amount of energy, which is transformed into a huge amout of heat and nothing usefull. The faster you're going, the more energy it takes, which is why rock crawlers and trial bikes don't care. They never get over parking lot speeds.
It sucks for lateral grip as well, the sidewall (the bit of tire between the wheel and the tread) is just flopping around, it's just like having really weak spongy suspention. If you tried to turn, the front outside tire would fold over onto it's sidewall and probably get torn off the wheel entirely. (which is why rock crawlers (some of them) have things called 'bead locks' on their wheels. These are essentially big rings around the outside of the wheels that get screwed down to clamp the tire in place).

Part of the path to good handling, as far as tires are concerned, is to make the sidewall move as little as possible. Having enough air pressure obviously helps, but it's also good to have as short a sidewall as possible, so there's not as much material to move. That's what affects handling, it's not big wheels, it's skinny tires.
(the problem with short (and stiff) sidewalls is that they can't soak up bumps very well, and the ride quality suffers.)