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So why does it seem like a lot of people use smaller rims when racing? Is it to do with lightness or some such? Surely you don't get better handling by going down in rim sizes?

small rims = less weight = less unsprung mass

Unsprung mass has 4 times the effect of sprung mass.

In other words, 2 kilos unsprung is having the same effect as 8 kilos in your passenger seat.


Hmm. im 90% sure its 4 times... anyone confirm?

Right, so it is to do with weight reduction. I suppose there mustn't be a hugely noticeable traction difference with bigger rims then?

It is weight in the wrong place. The rims are working against acceleration and braking, something quite important on the track.

Boags

its because the smaller ones are cheaper than the lightweight large ones. not everyone has $3k to spend on wheels alone. or if your getting serious, then its about $8k for magnesiums damn i want some though

but both boags and kitkat are right. but if you afford the lightweight wheels, then get them.

Price has nothing to do with racers using smaller wheels



Lighter wheel = less centripetal force. That means the suspension reacts easier, and the car accelerates and brakes quicker.

To answer to the OP's question, about racers using smaller wheels. Your average tyre has a flexy sidewall, so there is an advantage in steering response when running a larger wheel and lower profile. But for racers, or anyone who buys a decent tyre, there is no advantage to a lower profile, as good tyres have very stiff sidewalls which don't deflect.

Another consideration is tyre size, sometimes its hard to get a 195/40/17 in a sticky tyre, so you are forced to go for something that is brought into Australia.

For Targa Tas this year, I am told that a certain Lambo has wheels that cost $31K. And that is to go smaller (18s instead of 19s). All homologation issues aside (which have been done), this was so that they could get tyres. I noticed that 997 Turbo drivers have had to do the same.

rhysk wrote:Price has nothing to do with racers using smaller wheels

Yes it does...the tyres are cheaper!

Formula 1 wheels are just over 14\" diameter (360mm) so wheel size is much less important in racing - it is tyre construction that racers are concerned with - and a good 13\" slick will outgrip any diameter road tyre if working at optimum temperature on a dry track. Load carying capacity is also important - the heavier the car the stronger the wheel and tyre need to be, and this can dictate larger sizes. The current trend to 19, 20, 22\" wheels is crazy, as the wheels need to be much heavier to maintain strength, and you are effectively bolting on four dirty great flywheels - just stoopid!

rhysk wrote:Price has nothing to do with racers using smaller wheels

this depends on what racers he is referring to? if you mean professional, then no, price has nothing to do with it.

if he means, average racers like some of us on here (frequent the track but just for fun) then i would say price is a big factor. these are the ones i was referring to.

so it depends on what we are talking about here.

I'm not so much concerned about pricing. I was more curious just as to why it seems everyone says \"get a set of 14\" or 15\" for the track\", most notably in the MX5 vs Pulsar thread

Incidentally, how heavy are the stock NB8A rims anyway?

marcusus wrote:I'm not so much concerned about pricing. I was more curious just as to why it seems everyone says "get a set of 14" or 15" for the track", most notably in the MX5 vs Pulsar thread

Incidentally, how heavy are the stock NB8A rims anyway?

There is a lot of tyre choice in those 2 sizes also reasonably cheap.
16" is a shocker. All the tyre companys have decided to ignore that size wheel. You can get racing rubber or your standard crappy TR-1's & C drives (hoock is out).....but not a lot in between unless you start paying $300 a pop.

a lot more becomes avail at 17" but also at a added price.

15" seems to be a really good compromise to be able to get legal steet fitting & reasonably good for track use.

I personally reverted back to 15" rimms because of that reason. I had a gorgeous set of 16" OEM NB8B rimms on the weekend NA warrior but couln't get the tyres I wanted in that size.


so basically the same answer as Ted

marcusus - basically a combination of what everyone has said. Weight has a lot to do with it. 14s are now a pain to get tyres for too. If you look at rallying, dirt events will see all cars run 15s. But the same car at a bitumen event will run 18s as they can then run a big brake package and the tarmac tyre range is a lot larger. In circuit racing, there is often a control tyre in a lot of the CAMS categories. Add to that what is homologated for the car (size and load rating) for the GpN or production rules.

For most street punters, the choice of 17 inch rims doesn't often extend to lightweight rims. On the MX5, rim weight is so important. 15\" NB8A rim and R spec tyre is about 15-17kg from memory.

Ted wrote:For most street punters, the choice of 17 inch rims doesn't often extend to lightweight rims. On the MX5, rim weight is so important. 15" NB8A rim and R spec tyre is about 15-17kg from memory.

No doubt for the general person driving about with 17's or 18's, they aren't lightweight at all.

How do the NB rims compare in weight to other MX5 rims? Surely only the 14's would really be any lighter? But are they not as wide as the 15's?

For me it is price and set up...

I run 15s on the SE race car.

Firstly you can run the car lower.

Secondly the cost of the tyres is much less.

16\" slicks...$440 each.

15\" slicks...$350 each.

I don't worry too much about the weight of the wheels...15\" at 7kg are about $500 for a set...15\" at 4.5kg are about $2k per set.

If you can afford it then lighter wheels they are probably better but I don't think any of us are good enough to notice a 2.5kg. difference in the weight of the wheels.