Track day wheels - the bigger the better?

Just a quick question for those of you regular track day goers regarding wheels.

I notice that not many opt for big wheels, i.e. 16s and above, when choosing track only wheels yet when watching BTCC etc they always seem to go for huuuge wheels.

Any reason for this other than cost? Are bigger wheels better?

 

Big wheels are (obviously) good at housing big brakes and (to a point) can help reduce wheelspin due to the greater leverage of the bigger diameter and also carry a very low profile tyre for less sidewall flex. BTCC type wheels are also light, so the weight issue is'nt so much of a problem, plus the whole package (i.e suspension etc) is designed to work together from the start.

Big wheels that are available to us are usually heavy, and when just bolted onto a road type car (be it for road or track use) can make the handling feel heavy footed and far less precise than with a smaller wheel. Plus not many have 300hp to try and put to the tarmac like a touring car ! (except the big power turbo cars of course)

 

I think the bigest problem with bigger diameter wheels is that you end up having to lower the car more to get the body back to the orignal ride height(wheel arch clearance permitting of cause). This has the adverse effect of the roll centre disappearing further and further below ground. If you could space the bottom ball joint back down by the same amount then this problem would be solved.

This is in effect what the BTCC boys do...they manufacture the uprights tall enough so that the bottom ball joint is as close to the ground as possible. Unfortunately we are stuck wth standard uprights so it all turns to rubbish.

So I would say the two key problems are wheel arch clearence and geometry.

 

Like Ian said the suspension/wheels and brakes are designed together as a package and have very different geometry to road cars. Most competitive mk1's and 2's don't go above 15" very often as its suits the geometry.
Also bare in mind than 15" racing tyres are far more plentiful than any other size...

 

bigger diameter wheels have a bigger contact patch = more grip (all other factors being equal)

 

I have to take issue with that....

a wheel/tyre combination with a larger overall size should have a greater contact patch. However - since the combo needs a smaller sidewall there's less flex - and so the contact patch could end up being reduced (but possibly a better shape)

For a given car weight, and the same tyre pressure the contact patch will be the same regardless - because it's simply the weight on the tyre divided by the tyre pressure...

 

That is true as long as you can get the other factors equal again after bolting on the bigger wheels....not always possible on a trackday budget.

 

of course you have to work inside the regs of the series you are in, and the budget that you have, and yes, on a trackday budget big wheels are probably not the solution - on a Mk2 golf size car a 15" wheel with a sticky tire is a great combo, plenty of choice of rubber including second hand stuff from like clio racers

one of biggest deciding factors is what tyres are available - I go hillclimbing and for the really soft rubber you need to stay below 16"

But given the budget and the freedom to operate, and sponsorship by tyre manufacturers, big wheels are the way to go (super tourers, WRC)

 

we are in agreement then

 

Just wanted to clarify it being the larger overall radius of the tyre that would affect the contact patch

 

Well, I now have 16s and 14s, just need to look for some 15s to make the set.

 

*personally* I would go for 15s as track wheels (except on my car, which runs 13s) as the tyre choice is excellent. The money spent getting big brakes under 15s is recouped from the money saved on tyres compared to larger sizes.

16" tyres in track compounds tend to be expensive - it's often cheaper to get track rubber in 17" - but that's quite a compromise to run on a mk2 due to the other setup changes needed to make it work.

 

i agree on the issue of 16" wheels. Either go one size up or down for choice

 

For a track day car there is little purpose in going over 15's as has been previously stated however it is a good idea to have a selection of sizes (as you do) providing you can get brake clearance as it is a simple and cost effective way to solve gearing issues you may have on certain circuits. Lots of club racers run different size rims from front to rear for all of the above reasons(from time to time).

 

OK, I've gone for 14x6 ET35 alloys (as they were free) with 185/60/14 R888s.

These should clear my 256mm brakes (I hope)....

 

Should do, I run those sizes with 270mm discs on the Mk1 (standard calipers)

 

We've been running 16" on the mk2 reasonably successfully for a year or two for the simple reason of initially fitting cheap wheels over big brakes... We've been using yoko A539s which personally we've got on very well with but if you start to look at proper tyres you find that they are sometimes more than 2x the price of similar 15" ones.

I think we're going down to 15" when money is available for that simple reason.

Gary's 888s thread made me cry as they were a very similar price as we have to spend on A539s...

 

i believe the vw cup runs standard size, 15's with some very sticky rubber!!!!

 

Exactly....its all about the bottom ball joint location

 

It doesn't matter what diameter or size(width) the wheel is, it's the tyre which is in contact with the road surface. And it doesn't matter what (outer) diameter the tyre is, or what width it is, the contact patch the area of tyre in contact with the road surface remains the same, for any given vehicle weight and tyre pressure. Increase the vehicle weight and the tyre will squash more, increasing the contact patch; increase the tyre pressure and it will reduce the area of the contact patch.

What changes with greater tyre width is the shape of the contact patch, and it is this which is important for high-performance driving rather then being long and thin, it becomes short and wide, which is better for lateral grip (albeit at the expense of aquaplaning resistance in the wet...) But the reasons for changing tyre pressures for track driving are a lot more to do with tyre stability than contact patch size...

but I could be wrong (?)