EmpiricalGMan

I'd really appreciate it if someone could point me towards a wheel/tire bible that will explain the consequences of every change I make to wheels/tires, at least compared to my stock. Failing that, here's what I *THINK* I know, and maybe someone could correct me where I'm wrong:

Wheel weight: weight is always bad, decreasing every performance attribute except maybe traction (and there's better ways to gain traction).

Wheel diameter: more diameter has a negative effect on acceleration and braking. I think it allows higher maximum speed (if you bypass the governor). Does it improve handling at all speeds or just high speeds (or not at all)? Autocross types don't seem to go for big wheels, but there's alot of brief bursts of acceleration and hard braking in that so maybe they're compromising on the respective benefits.

Wheel width: increases traction, at least on good surfaces. The pictures of rally racing cars I've seen seem to have narrow front wheels, so I wonder whether there's a negative to width on loose surfaces. Desert warfare vehicles, however, seem to have wide wheels. Finally, I would think a car with wider wheels would be harder to turn, so perhaps this leads to understeer? But what if you have very wide wheels in back and narrow wheels in front, my instincts say that's understeering too (on a RWD car).

Wheel offset: the further towards the exterior of the car, the more stable, but harder to take tight turns? Yet if you had wheels outside the fenders up front, and inside the fenders in back (like snowmobile), I'd expect that to tend to oversteer.

Tire pressure: high pressure for precision handling, low pressure (in rear) for launches straight line traction (and a more comfy ride), but does it reduce acceleration at speeds where traction isn't an issue?

Tire sidewall: more sidewall is essentially like a combination of increasing wheel diameter and decreasing tire pressure, but does not have as dramatic effect as either alone.

Slicks vs. street tires: are slicks better at everything, and the main reason we don't use them is durability and bad poor weather performance?

And the final question, roughly how do these details interact? For example, if I wanted to put wider tires in the rear while maintaining the same under/oversteer, would reducing the amount of sidewall on the front wheels help accomplish that? If there's no increase in wheel weight, I'd have to assume the car would accelerate faster, but would it's handling be inferior despite doing things which typically increase handling in front and increase traction in back without creating "give" in the tires?

I'm intrigued by staggered setups in general and would like to hear more about the effects on performance. Much appreciation to the wheel gurus in advance.

ares

iTrader: (2)

hmmm ok, first tire width, this does increase traction, not quite as much as youd expect, you gain way more traction dropping 5psi in a tire, then you do increasing its width. dont think of the contact patch as a line, but instead of as a rectangle, make a really narrow rectangle 30mm wider, no big deal, but make it 1mm longer and youve increased it alot. not to say wide tires are all bad, they are good, in the case of rally racing, I think they are looking to cut into the rocks, if you have big fat tires, you can sort of drive over the rocks, 50 small rocks under a big fat tire will support the car, and the tire wont actually contact the solid ground. thinner tires will force the small rocks to hold more weight, and thus hold better, as well as the tire between the rocks will be pressed to the ground. same basic princible as snow. military vehicles are typicly driven in sand, where there is no solid ground to contact, so they want to do their best to float over the top.

weight, is always bad, yes it can increase traction, only in the way that a 5lb brick will slide better than a 10lb brick, it does not inherently make more traction, just puts more weight onto the tires, and thus, grabs more. not good. lower weight, is good, also needed is distance from center, this plays into rim width. the lip of a rim is one of the biggest heaviest portions of a tire combo. closer to the center, easier to turn, good.

overall diameter, changes your gearing, generally not good, gearing is set up in the proper way, altering this can affect performance. small changes are ok. <2% overall diameter.

offset, never heard that this could really do much, since typicly the changes would be measured in decimals, but I suppose for pin point handling, it would throw off the alignment, each rim inside and outside has to travel a different circle during a turn, and both must be angled to that circle, changing the offset throws this off. I donno that andretti could notice it tho.

sidewall=good ussually, it can offer more play in turns as the sidewall bends. but for acceleration, braking, and ride quality, it absorbs shocks from all directions. I dont think this means that .0002" of sidewall is the best at the short comings of the large sidewall, there comes a limit where its not helping. anything smaller than a 35 would be leaving performance oriented IMO. maybe a 30 if your going really wide like >300.

you have the slicks correct, they are great if the track quality is perfect. drag radials are more common I think, they keep a certain level of usability, tho they are not intended for street use.

FlyingToaster

To add couple of points to the discussion:

Wheel weight is important because it is 100% unsprung weight. The actual reason it is important is because of the ratio of sprung weight/unsprung weight. The higher this ratio, the better ride and handling you get. Obviously it is easy to increase this ration by making the car as heavy as you can, but the optimal point is minimum sprung and unsprung weights while obtaining highest ratio. The benefits are significantly measurable even by just saving 10 to 20 lbs from the wheels and brakes.

For example, the reason Porsche is adamant about ceramic brakes is because they save roughly 34 lbs from unsprung weight

In terms of sidewall: that term does not mean much without tire slip characteristics discussed in the same sentence. Generally however, larger the side wall, lesser resistance to tire slip, which means lesser lateral g's; so not too good. However, Some drivers, I am one of them, like larger side walls because you can sense easier when the tires are loosing their grip or are getting closer to the limit. But usually, it is hard to tell the difference between 35 ratio vs. 40 when driving.