Wheel and tyre sizes?

[XKRacer]

I know this subject has probably been done a few times but trawling through the search is a nightmare, in typical forum search senarios it brings up a million (ok slight exaggeration but there was 92 pages) hits!

I have been looking into sorting the wheels for my track car. Looking at Image Wheels (billet) they will practicaly let me make up my own but the difference in price from 18's to larger is not that much and they offer Toyos T1-R or R888 tyres also in larger sizes not for that much more money.

Can anybody tell me the up and down sides to larger wheels, does anybody have experience with these tyres?

I am not sure which way to go! If a go to say a 20" it will make my life easier fitting larger discs and the calipers I have as I already have drawings for the caliper bracket to fit that size wheel.

Is 20" just too big for a track and not practical?

Any help will be appreciated.

[Al Weyman]

Funny enough I am using 18" Jag wheels (8" front 9" rear) on the IROC fitted and I like them fine and they look good as well and for the price I paid per set off eBay would probably not buy just one of those custom made billet wheels from Image. The first set were fully refurbed and came in at about £350 the 2nd set not quite so good but well good enough were a couple of hundred quid. These are fitted with medium compound Toyo 888's as per championship regs, different size front to rear and feel fine to me and I have tried Kumhos on 16" but they lasted about 4 races and were knackered. These seem better and the wear rate is better I was in a race with them at Croft and the heavens opened and I though thats it I'm off but amazingly as they were hot they were just as good dont ask me how but its a fact and I actually started pulling away from the smaller FWD cars that were harring me and all I could put it down to was they couldnt generate the heat in the tyres once it started tipping down.

[XKRacer]

I don't deny these wheel will cost me the best part of 3 grand .

But I need something very specific to miss the calipers and to go to the width I want at the back, probably a 12J, 9J front.... I need specialist wheels.

I am also debating on centre lock as I will be making new hubs.......maybe.... But thats more for BT factor than practical use.


Good to know the tyres work well.

And if you are interested I change Jag wheels all the time, if you fancy a few spare sets when I get them in I can send you a PM?

.

[Al Weyman]

Mine are the early bolt pattern 4.75" pcd(?) as fitted to the XK I believe? We cant use modular wheels so apart from saving bucks it was a good practical solution we also have a max wheel width of 9".

[XKRacer]

Originally Posted by Al Weyman

Mine are the early bolt pattern 4.75" pcd(?) as fitted to the XK I believe? We cant use modular wheels so apart from saving bucks it was a good practical solution we also have a max wheel width of 9".

Basically thats all the MK1 XK (96-05) wheel arrange bar the 20" BBS wheels.

The BBS wheels had a 10J rear but then again nothing stopping you using all fronts which were a 9J I have done that before.

The MK2 XK has a 108 PCD



Just as a side note BMW also use the 120.65 PCD (4.75) so I beleive.

[Al Weyman]

Yes I have BMW discs on the back of the car, the later ones are different I believe. I was going to use the 9" alround and use the 8's for the wet but I won 6 almost new 235 x 35 x 18" Toyos so are trying to use those up on the front (have you seen the price of the 275's?!)and to be honest I really need more grip on the back to get the power down as this car does not suffer from understeer too badly.

[XKRacer]

275's are over £150 retail but you can get cheaper if you shop around.

Ebay isn't much better.

And I'm thinking of going 315's

So any pros and cons to have large wide wheels at the back? Probably 245 at the front

[Al Weyman]

I think if as it sound you will have, you have a lot of grunt you need as much rubber at the back to stop those tyres spinning up and if its OK for the new generation of Ferrari front engine rear wheel drive cars (have you seen the tyre rim difference on those) and later Corvettes then its OK for me oh and it looks cool. :-)

[XKRacer]

I must admit that is my way of thinking, as big and wide as I can go, looks like I will be breaking the angle grinder out and giving myself some more clearance

[M30TGV]

I don't know the specifics of your particular car, but I can't see the need for 20" wheels. You'll gain more from spending more money on the tyres themselves and suspension set up.

As for caliper size - remember a McLaren F1 road car was only sold with 17" wheels... and the brakes on that were pretty good on a track as I recall!

[Notso Swift]

XK admits that some of his calls are made on a bling factor because the car also represents his business.
Which is good to hear because most people are not as honest about these things!

[XKRacer]

I do hear where you are coming from M30 but like Notso says I have to make allowances.


I think I would be happy with 18s but 20s have a bigger 'wow' factor and the tyres are not that much more expensive (Toyos T1Rs)


If someone said to me 20" would be detrimental to the way the car handled and performed, that would be a different matter but if the difference is negligable then the 20s hold a better 'pull' for me

[M30TGV]

OK - fair enough. I see you have other motivation! And if I were to guess, I'd say you have your heart set oon 20's anyway!

[R59]

I moved my contraption onto 18's when I got it from 16's a Dunlop recommended to go that way as their 16's wouldn't be able to cope with the torque from my little V8. I run 9.5" on the front and 11" on the back (old Formula Saloons regs), but may well get the wheels rebuilt to take 12 or even 13" rims at the back.

It also allowed me to use their best GT spec rubber.

I'm also looking at doing a full godzilla conversion on my old Nova - and that will run with 18" boots too, though probably no wider than 9's.

[Al Weyman]

I also went up to 18's on the IROC as tyres low profile tyres were available giving it the same overall rolling diameter as the stock set up on 16" rims and taller tyres (16 inch sizes also not available in the sizes I need or even stock sizes from Toyo). !7's were an option but as XJK 18" were available cheaply on ebay and fitted the hubs and we have to run a one piece rim it made sense. I always thought the lower sidewall was an advantage is that incorrect? It would be interesting to see what F1 would run if it was free instead of silly 13" (why is that?).

[M30TGV]

The F1 13" rim is just a rule that's been there for so long, nobody feels the need to change it.

And I think that example in itself shows you that large rims are more style driven than performance driven. F1 cars have a decent amount of grip i think we all agree! And if they can do it on 13's, then it does amuse me when people cite "grip and handling" as the *need* for massive wheels. It might help, sure and of course can accommodate larger brake calipers - but as a be-all and end-all pre-requisite for grip, I can't agree. I've driven track cars with sublime grip and poise - and all running on 16's with an A/R of 50 or more.

People will argue that there's less sidewall deflection on a low profile rim. This may be true (note the use of the word "may"... depends on tyre construction and compound) - but what's really important is what's going on at the tyre/road interface - THAT's where your grip is coming from. Controling this is a matter of the suspension interaction with the tyre compound/construction/aspect ratio and track surface. Books have been written on this. Get the suspension/unsprung mass end of things wrong and it won't matter what size rims you have. Replacing 17's with 20's in such a case can just make things worse because of the disadvantage of larger rims being that grip breakaway tends to come much quicker and handling will feel a little more on the knife-edge and twitchy compared to a taller section which can accommodate larger slip angles. Confidence in your car can be worth more per lap than many so-called performance enhancements. I personally like a car that's a little softer sprung so when I drive someplace bumpy, I feel the car can stay more stable over them, and let me concentrate on placing the car where *I* want it rather than where the suspension and tyres tell me they're gonna land. Does that make sense? Obviously, I don't mean riding like a limo, but I hate that feeling when damping is so hard that you end up correcting (or over correcting) for small bumps that otherwise you could drive over and concentrate on your line and braking points.

EVO magazine (at least I think it was them... may have been CCC... anyway...) ran a test with the Mini Cooper S Mk2 (the turbocharged one) which as stock comes on 195/50R16 run flat rubber, and as an option you can have 205/45/R17's. Round a test track - best of 3 laps, there was "no discernable difference" in traction or cornering grip between the two. Laptimes so close and in one case the smaller rims were faster that you could not draw any conclusion from the addition of the larger wheels.

But yeah - I will concede that they usually fill the arches a bit better which on a road car is a matter of taste.

[Al Weyman]

So if 13" is so good why do they use large diameter wheels and low profile tyres on Sports GT's and Le Mans cars. I would suggest given freedom of choice F1 teams would have gone away from 13" years ago.

[XKRacer]

Originally Posted by Al Weyman

So if 13" is so good why do they use large diameter wheels and low profile tyres on Sports GT's and Le Mans cars. I would suggest given freedom of choice F1 teams would have gone away from 13" years ago.

I would agree with Al on this one, not that M30 isn't putting forward a good argument but I beleive given the choice F1 would of gone onto larger rims if they had the choice......just to give them more breaking advantages and ideas if not for anything else!

[Notso Swift]

Originally Posted by M30TGV

EVO magazine (at least I think it was them... may have been CCC... anyway...) ran a test with the Mini Cooper S Mk2 (the turbocharged one) which as stock comes on 195/50R16 run flat rubber, and as an option you can have 205/45/R17's. Round a test track - best of 3 laps, there was "no discernable difference" in traction or cornering grip between the two. Laptimes so close and in one case the smaller rims were faster that you could not draw any conclusion from the addition of the larger wheels.

Top Gear Australia did something similer with one of the latest HSV things
22" vs 20" there was a very decrenable difference in performance couple of seconds slower.

Couple of thing I can think of
Weight
Rolling weight, amplifies this effect (weight further away from axis point
Lower tyre may mean rubber that may not be as good, because need for puncture resistance
Stiffer sidewall can only be so good for so long, eventually on a wide section part of the main tread will loose contact with the ground