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Author: Subject: Chosen tyres sizes
coyoteboy

posted on 24/6/12 at 10:37 PM Reply With Quote
Chosen tyres sizes

So any chance those with large-engine'd middys could post your tyre sizes and power/torque too, plus whether or not you feel you have traction issues? I am all for the lighter weight 13s and 15s but none of them come in a road-usable width that will hold down 300hp/lbft as far as I can see, without limiting my tyre choice massively. If I move up to 17s things get notably heavier (even with light forged rims I'd be looking at 20kg per wheel/tyre but then plenty of the reading I've done has hinted that on mid-rear cars sidewall stiffness is more important especially on the back, suggesting the shorter sidewall would be a bonus worth the weight.

I don't see the point in under-sizing tyres for some nominal weight weenieness without evidence. Narrower smaller radius tyres will give a longer contact patch which should help with launches but I think tall sidewalls and narrow CP will be a big penalty on the corners.

[Edited on 24/6/12 by coyoteboy]

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v8kid

posted on 25/6/12 at 07:19 AM Reply With Quote
Interesting point about the tyrewall stiffness being a desirable requirement for middies. I can't recollect reading that but then again I wasn't looking . I guess this requirement was for cornering rather than traction.

Do you think it may refer to supercars which tend to be on the porky side compared to us? (and with a higher CofG) If so is it relevent?

I was running 275x17 on the rear before I changed to track tyres and there were no issues with traction whatsoever - I could lift the nose if I was careless. But then again with a 70/30 weight distribution I was not expecting probs. Torque is 400 ftlb.

Cheers!





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coyoteboy

posted on 25/6/12 at 10:27 AM Reply With Quote
Well it certainly pertains to tin top cars, the various articles I've seen were about the likes of the elise and mr2, both of which are fairly heavy and do indeed have a reasonably high CofG but I'd venture to guess not massively high. So far none of the chassis/suspension design books I've read have gone down the route of recommending tyre sizes and reasonable USM weights and everything else I have to base it off is either joe bloggs random build with guessed weights or fll on track cars. What's worse is all the tyre related books I've glanced at assume it's a race tyre and has full performance data provided, none of which exists for road tyres. I'd planned to run 255/40/17s rear and 205/50/17s front but the rim and more importantly tyre weight is indeed very high and I do wonder if tyres that large in diameter over all look a bit odd on a flat spaceframe car. But step down to a 15" rim and you're looking at 225ish being the max width with reasonable choice of road tyres since I can happily spin up good quality 205s with the weight of a diesel lump on them and 1/3 of the torque I just can't see something in that region offering anything but spectacular black stripes at any set of traffic lights.

[Edited on 25/6/12 by coyoteboy]

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Fred W B

posted on 26/6/12 at 08:09 PM Reply With Quote
Hi CB

Have a look on www.gt40s.com, every possible performance road tyre in those sorts of sizes has been discussed on there. The jury is still out.

Cheers

Fred W B

[Edited on 26/6/12 by Fred W B]





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coyoteboy

posted on 26/6/12 at 08:34 PM Reply With Quote
I've been reading over on GT40s for a while now and it seems they follow the "wider is always better" route. It's prefectly normal in GT40 world to run 345s on the rear on 12" wide rims, and many of them claim the handling and lap times improve notably by going wider. This is totally contrary to what anyone on here recommends. They also are willing to fork out 300 a tyre, which I'm not really.

Their general response also seems to be if you want modern grippy tyres you need to go 17s as the choice is too low in 15s. This is the problem I've been finding too.

[Edited on 26/6/12 by coyoteboy]

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Doug68

posted on 27/6/12 at 01:28 PM Reply With Quote
Below is a random sampling of sizes taken from tirerack.com, I wasn't trying to make a particular point in choosing the cars, they were just ones that came to mind:

Original Equipment Sizes for 2011 Lotus Elise R
Original Equipment Front Size 1: 175/55-16
Original Equipment Rear Size 1: 225/45-17


Original Equipment Sizes for 2008 Ferrari F430 Scuderia
Original Equipment Front Size 1: 235/35-19
Original Equipment Rear Size 1: 285/35-19

Original Equipment Sizes for 2011 Lotus Evora S
Original Equipment Front Size 1: 235/35-19
Original Equipment Rear Size 1: 275/30-20

Original Equipment Sizes for 2010 Lamborghini Gallardo LP550-2
Original Equipment Front Size 1: 235/35-19
Original Equipment Rear Size 1: 295/30-19

Original Equipment Sizes for 2012 Porsche Cayman S Standard Brakes
Original Equipment Front Size 1: 235/40-18
Original Equipment Rear Size 1: 265/40-18

Original Equipment Sizes for 2005 Ford GT
Original Equipment Front Size 1: 235/45-18
Original Equipment Rear Size 1: 315/40-19

From this it looks pretty clear that somewhere between a 175 and a 235 front is what's needed with rears being 50mm or so wider, more if making silly hp.

The trouble is it's just conjecture without the tyre data which we're never going to get

I ended up with 245 fronts and 285 rears, I'd aimed at 225 fronts and 275 rears but it ended up different for some bizarre reasons, will it work? Someday I'll find out I guess.





Doug. 1TG
Sports Car Builders WA

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coyoteboy

posted on 27/6/12 at 01:37 PM Reply With Quote
Well in the absence of other reasoning I think I'm going to go with my plans of 255/40s and fronts around the 205 region. This falls in line, roughly, with the selection of similarly weighted cars with similar power levels as your search also confirmed. I've chatted to a tyre engineer who openly admitted that he'd follow the same process I had and would be in no better position due to the lack of data. In the hopes of keeping my machine viable from a pricing point of view I'll stick with affordable tyres and rims that will support something a little larger should I want it later. I'm just about to hit go on a set of 17" rims with 9 and 10J widths.

[Edited on 27/6/12 by coyoteboy]

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