[byndbad914]

Originally Posted by tyoneal

Why are 17 and 18 inch wheels considered the, "Best" choice for overall performance, (Road Race or Performance Street driving) while larger wheels (> 19 inches, and, < 17 inches) and smaller wheels are generally considered second rate when ONLY handling is considered?
unsprung weight and I would argue smaller than 17" is better in cases where the car is light enough to run smaller brakes

The new Vette and Camaro etc. all have moved to 19 and 20 inch wheels, and at least the Vette is considered one of the finest handling production cars in the world?

Is this just "Old" reasoning, that is now considered out of date or is there some factual reasoning for this that I have overlooked?

Does Staggering the size have any effect one way or the other on the handing of the car, (i.e.. 19 inch in the rear, and 18 inch in the front), or is it just strictly for looks?
you answered all three with the final - it is design IMO. You don't need 14" rotors for a streeter or a typical Joe like us at the race track. It is the typical bigger is better and posers want what real racers are using tho' they never will. You need the big wheels to fit over the brakes (brakes are driving the size v. actually wheel/tire performance). Only argument could be if you wanted an exact tire diam for some reason, say 26", then it is better for performance to have a shorter sidewall, driving a larger wheel choice. But designing the car to the chosen wheel diam instead means a smaller diam tire and different shaped wheelwells.

I have noticed that many great performing racing cars have a taller, wider tire on the rear and a slightly shorter and thinner tire in the front. (IRL, F1)
they have more rear weight and power is applied there is a reason for the big rears. The open wheel cars also started narrowing up the fronts years ago as they create drag, so a skinnier tire had less wind drag at high mph.

Is there any good reason for having the rear of the car higher in the air than the front for a road racing application?
slight downforce can be created on the car, but that is very shape dependent and how air generally flows over the car. Cars with heavy slant to the nose and so forth like it (Porsches for instance)

Is it a function of aerodynamics, while under heavy acceleration and "At Speed" running, that the down force will adjust the downward pitch of the car, so while running normally it will move into a nice level stable attitude?
No, it won't necessarily level out as you actually tend to push on the front more than the rear with rake (the roof tends to block the rear of the car from the flow), tho' again based on shape, the aero can change how it flows over the car with rake.

I had heard that depending on the front to back weight ratio of your car, that in some cases this weight differential helps dictate the width of your tires. (i.e.. A car that is heavier in the front than the rear will handle better with a wider tire up front as to help keep understeer to a minimum. While a car with more weight in the back will benefit from a wider rear tire to help reduce oversteer. Is this correct?

Before answering, the drive wheel/wheels should also favor a wide wheel as well.

For instance. A 68 Camaro with 55% of it's weight to the front/ 45% of it's weight to the rear, AND being a Rear wheel drive car. Would benefit from having wide tires in the rear, say 315's (Due to rear wheel drive) and would handle better with front tires being somewhat similar in size as the rear tires, say 295's because the front wheels are having to work harder at changing the direction of the car due to the front being heavier than the rear. (in other words a natural tendency to try and understeer)
so this is sorta yes and no. If you are front heavy, in theory you want more front tire. But you also don't want to spin the wheels out of the corner, so you run as wide or wider rears (unless you are so underpowered the rear wheels won't spin with narrower, "weight matched" tires. So one option is to run the same size all around with some sort of front downforce device to create more grip there when you would plow in the corners, and have the rear tire to reduce power wheelspin. Key to a good car can really come from tuning aero devices - even the race Vettes have a rear wing and splitter they can tune to balance the car v. the street cars.

Rear heavy, you will want wider in the rear period (assuming your suspension setup isn't all f'd up) i.e. look at every race 911-variant Porsche and you will see massive rears v. front widths.
This would be one of the reasons, other than weight and aerodynamics, why Drag racers run skinnys up front and as wide as possible in the rear. (Given of course it is rear wheel drive) Changing direction is not an issue so understeer is also a non issue.
unsprung weight and less friction with the ground that come to mind. Unsprung weight doesn't transfer well to the rear (unless you get the wheels completely off the ground) and the skinnier the tire, the less input it takes to roll it. Also, those cars use heavy rear brake bias or chutes and little front brakes strictly for the waterbox essentially.

Whereas, a rear wheel drive, rear engine car while having 315's in the rear, AND having a weight bias also in the rear could get away with wide tires in the rear and substantially thinner tires up front say 215's without having an undue challenge of dealing with car induced oversteer and understeer.
possibly - again all about how the suspension was set up. Bad suspension design and wrong spring choice, etc can induce understeer even with similar width tires. But, assuming all's good with the suspension, it is more or less true to say that. Again, look at Porsche racecars.

Is this a correct thought all other influences being static?

Do larger diameter tires as a rule run cooler than smaller diameter tires because the friction of the tread against the pavement is spread out over a larger surface?
I don't buy into that one completely, tho' I have no exact real world example to say yes or no. I know this much, the difference in contact patch between a 16"x10" tire and an 18"x10" tire is virtually nil according to the folks at Hoosier who test this stuff. Width is the key to contact patch, not diameter. So equal width wheels have nearly the same contact patch regardless of diam, so heat will get in or get out somewhat evenly I would think, tho' the concept of thermal mass would certainly say it is harder to heat the 18" than the 16". I suspect the difference is small tho'.

I'm sure this is blatantly obvious to most people, but I wanted to clear this up in my mind while giving some thoughts to some project issues.

Thanks for your patience.

Best Regards,

Ty O'Neal

That is all FWIW and my opinion based on a lot of similar questions over the years involved with racing and tire pros. Put it this way, on my race car I have 12" rotors, good pads (which make all the diff in the world), and 16" wheels to run the smallest diam tire possible on my car to get the least unsprung weight possible. I can drag that car from 140mph+ down to 80mph damn fast. I also built a mid-engine car because I road race, and have a 55% rear bias as I knew I wanted to run slightly wider rears (cuz I have room for it). I run a 25.5" diam rear right now as it works with the track I frequent (cuts down on my shifting on a couple stretches), but will switch back to 23.5" diam due to proposed track changes that make them longer so I have to shift anyway (hit rev limit right now thru the esses for instance).

A porsche site had similar threads (they do all the time) and I just spoke with the guys at Hoosier last week again about this and he assured me he will always run the smallest diam, widest tire he can on a race car.

Now if you are talking running with the big boys in serious racing like 24hr races, then you need large brakes to keep the heat out of them over a duration like that, and funky materials and so forth, so I understand why they run huge carbon ceramic rotors and therefore large wheels, because the unsprung weight tradeoff doesn't outweigh the braking benefit.

But for the street or a weekend track warrior, you are a fool to run anything more than a 17" wheel IMO unless the car is 3200lbs+ (because a 12" rotor with good pads and good 4 piston calipers is PLENTY of brake if you have any clue), then maybe 18" up to 4000lbs due to more weight to haul down and subsequent heat, etc. Anything beyond those based on car weight, you are just slowing yourself down. And with the popularity of 6spds especially, tire diam to help with street gear ratio isn't that important. I would run in the higher gears on the track with the smaller tires/wheels. Won't even get into mass moments of inertia...

And trust me, this discussion is far more in depth that I typed above, I just tried to hit some key points but there are many more. Suffice it to say, brake the car with only the maximum brake you will need for what you do and wrap the smallest tire and wheel package around it. Smaller on every component means less overall weight and less unsprung weight (key).