Recommended tire diameters

Looking at the various tires available for 15" rims, what is the recommended maximum tire diameter to be used (front and rear)?

Thanks
Bill D
 

Malcolm

Supporter
I did a paper on tyres for Avon. Must post it on the forum one day as it has quite a bit of info in. Someone pester me to do this!

Exact max size depends on wheelarch inner liner sizes. But from a driving position using a Renault gearbox 27.5 inch overall diamter rears is just ok with a 3.44 diff and standard 5th gear but too much for the taller 5th gear ratio. Car just doesn't pull. Ok for motorways though.

On the front the max size I have seen is a 245 50 15 tyre. But it sometimes fouled the inner arches plus also maybe the steering arm!
 
I'm looking at getting 10 by 17 inch rears anybody got a recomendation on tyre size for them. At a guess I'd say something around 275 / 40 /17 , anybody running these ?
 

Howard Jones

Supporter
I have 285/40/17 on the rear of mine. Standard rear bodywork with 5 inch rear ride height. Works very well. They are on 10" wide wheels.
 

Sandy

Gulf GT40
Lifetime Supporter
Bill -

What did you end up finding for tires in the 15"x14" wide sizes? I saw a few Avon's was that might fit. I have 17"x12"W for mine, but I was thinking about the 15" but might be a problem for tires...

Sandy
 
I talked with Fran to make sure the tirees would fit both the rim AND the car. For the front, Fran said to go with 8.5/23.5-15 on the 15x8 rims. It has a 8.3" tread width and a 23.8" diameter. For the rear, I can use either 13.0/25.0-15 (13.2" tread width and 24.8" dia) or 14.0/27.0-15 (14" tread width and 27.1" dia). I will probably go with the larger rear tire.

Cheers
Bill D
 

Sandy

Gulf GT40
Lifetime Supporter
Thanks Bill,

I saw those Avon sizes on the Sasco Sport tire site, good variety of sizes in matching sets. The Goodyear sizes are a bit harder to fit, and for the vintage look I was trying hard to find something that fits in their product line. The Goodyear Can Am series had nice rear 27.0x14.0-15" but the matching fronts are a tad too large. Trying to find something with the same compound and correct size and series seems a problem. I found one front from the goodyear site on the SHORT TRACK SPECIAL G19 27.0x8.0-15" that are close but don't know much about them vs. the vintage Can Am tires. Sizes look ok, but not sure how the tire is different, both were G19's and had overlapping compound availability, but hard to say. I might give a call up to the Goodyear racing distributor (I think it is Shelby down in Torrance) and see what they say.

I'm really trying to convince myself to get a set of 15" rims to get the really vintage look and the extra 4"'s of rubber on the road! I'll have to bug Fran and see if the brakes that I want will tuck under the 15" rims, if so, will likely have to get a one set in the 15" size.

Thanks for you tire info.

Sandy
 

Malcolm

Supporter
Below is the contents of the paper I wrote to Avon a couple of years ago. Enjoy but bear in mind I worte it from a GTD owners perspective! And it is my own opinion and not of any company or club.


There are 400 GTD chassis’ world-wide and the standard tyre sizes were 265 50 VR 15 on a 9 inch rim at the rear and 215 60 VR15 on a 7 inch rim at the front. Tyres made by Goodrich, the Comp TA 2. This tyre was just about ok until a driver got enthusiastic when it would quickly overheat and lose grip. Our cars could out perform the tyre, especially on a track day.

Other than these standard rim sizes a very popular rim size choice now is 10 inch at the rear and 8 inch at the front. I would suggest that the split is now 50/50 between these two pairs of rim sizes.

Since Goodrich stopped making the 265 50 15 rear tyre, owners have tried all types of tyre to get a good result. These have ranged from using HR rated tyres with a size of 305 50 15, Comp TA to Pirelli P7 275 55 Z 15 and Dunlop Post Historic 6.00 1200 15 or 5.25 1000 15. The latter are questionable over road legality.

The fronts are easier to get but to get a matched pair of sizes front and rear has not always been possible, with owners mixing makes, speed ratings or compounds! Not always a good idea.

There are two main types of usage namely road cars and those that get put on a track for track days or competition. I believe that the two types of use will have different size requirements although some owners will overlap into both categories.

For road use the owners will want a tyre that looks the part of a sixties vintage tyre. This means a tall tyre. To fit our cars at the rear anything over 27.5 inches is very likely to rub the wheel arch on bump. Your thoughts of a 275 60 15 and a 255 65 15 come to 28 inches by my calculations and so would not fit. A range of road sizes would be between 275 50 15 to 305 50 15. To me a 295 50 15 is the ideal road size for the rear.

At the front the risk of tyres fouling the inner wheel arch linings is even worse. But it is known that a wider tyre really helps the car turn in. One owner used tyres up to 245 50 15 at the front. This was right at the limit of fitting. A 25 inch overall diameter would seem optimum for road use. This translates to 215 60 15, 225 55 15, 235 55 15 or 235 50 15. I think the 245 50 15 is too wide and would cause most owners problems with fouling. To match the 295 50 15 above I would think an ideal road size is 235 55 15.

In competition on the hills and in sprints an overall smaller diameter helps. Often Avon slicks are used and the sizes are, I think, 23.5 8.5 15 at the front and 25 11 15 at the rear. The small diameter rear really helps in acceleration off the line. You mentioned a 335 35 15. I would be concerned whether the width would fit in the arch without modification. Owners are generally reluctant to modify the body work on their 40 replicas. Especially once built. The 11 inch slick just fits. Just! The side wall is much squarer on the slick than on a road tyre so I am not sure how the two tyres would marry up in cross section. The 12 inch Dunlop post historic is 12 inches wide at its maximum i.e. the bulge of the side wall but has an approximate 10 inch tread foot print. The problem to me with this tyre is its weak side wall in that when the car is leaning round a corner, the roll of the tyre makes the tyre wall very close to suspension bits with the risk of slicing the tyre open. Not good!

If it is raining we would want a larger diameter tyre to reduce the effect of our powerful engines as the back end does break loose easily in the wet. The road sizes would be good then. If you were able to create a size that matched the slick sizes then that would be a good compromise for wet and dry usage. Not all competition guys have spare sets of rims due to the high cost for wheels.

KVA and GTD are no longer in production but Tornado is with CAV starting distribution in the UK this year. There are other manufacturers around the world with RF in Australia being the biggest of the names I know. ERA and Holman Moody do produce cars but in even lower numbers than the other manufacturers.

There has been lot s of discussion on the GT40s forum (www.gt40s.com) about tyres and a lot of owners are looking to 16 inch and 17 inch rims to get round the tyre size problems. It would be very good if you could set a new standard for 15 inch tyres as that is what the GT40 replicas need badly to look right. You may also wish to go to www.gtd40club.co.uk. Downloadable in car video and pictures that show what we get up to.

With most owners doing relatively low mileage per year say up to 5000 miles, often much less, tyre compounds can be quite soft. The softest road compound would need to last a track day without overheating.

I hope this information is of help and I would be very keen to see you produce some good sized tyres for our cars as I think that if you do, Avon may be the new GT40 replica industry standard
 

Sandy

Gulf GT40
Lifetime Supporter
Just got off the phone with the friendly folks at Shelby's Goodyear racing tire. For the rear's on 14" wide not a problem, the Can Am series (slick) is good, however the other tires (Short Track Asphalt) will not be a good match. He did mention that it would be 'No Problem' to fit the 23x10.5x15's to the 8" rim, and the tire will be a bit bulged. Still seems like it will have the same problem of a tire that is too larger (Fender clearance issues).

So for fitment, in 8x15 and 14x15 with correct fitting tires, has to be Avon's /ubbthreads/images/graemlins/frown.gif No flames, just would rather have the Goodyears. In those sizes. Now if not running the Gulf Flares and you want to run the 15"x10" rims the Vintage Blue Streaks will be nice fit, but he mentioned that they would not be really that suitable for agressive road racing... Not sure how agressive agressive is, but that is what I found out.

Sandy
 

Malcolm

Supporter
I have been chasing Avon each couple of months since and they have now said that it is a matter of when not if they make a 285 or 295 50 15 tyre, at least VR rated. This will be a fully road legal tyre world wide and not a cut slick or one marked for Racing Use Only.

However Avon seem to have other priorities such as new racing formulas. Therefore I am not holding my breath for this but will be using these tyres as soon as they are availble. They already make a 245 60 15 and a 215 60 15 which some guys here already use and they get the thumbs up from those gents.

Some guys here are using the Goodyear Blue Streaks on track only as not road legal here and for track days they are good. Look the part too just like the Avons mentioned above.
 
Malcolm,

Next time you talk to the folks at Avon, you might mention that Factory Five has sold over 5,000 Cobra kits that use 235/60-15 and 295/50-15 tires. That is a lot of potential customers who are looking for high performance 15" tires. It is a much larger number than GT40's. Perhaps that would convince them that there is a larger market than they realize and get the project moved up in priority.

Kevin
 

Ian Clark

Supporter
Ah the tire question.... We've been using the Dunlop GT Qualifier quite successfully, both on the road and track. Decent in the wet too. 225/50/15 front 295/50/15 rear. Fronts are 23.93dia rears are 26.50 dia. just about ideal. There's a bunch of guys doing well on the 17" wheel and tire combos and I can see the day when I may have to go that way. I'm very interested in this issue.

There's no easy answer for the cars since big meats are the fashion today, oversize rims are all the bling and low sidewalls "equals handling". The tire manufacturers know this and there's gotta be ten thousand tuner Honda's for each GT40 replica so who's going to get the rubber they need?

I agree we should have a spec tire. The global consumption would be more impressive from a marketing than commercial point of view. The GT40 and Cobras could share 295/50/50 rears with 235/50/15 GT40 fronts, 225/60/15 on the Cobra front.

So who do you call? May be easier to get the attention of a
small tire company, looking for some high visability in timeless products. Any ideas?

Oh yeah, this is Angelo, still putting gas in:)
 

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Ian,

The first Small company with very high performance tires that comes to my mind is Nitto http://www.nittotire.com/

They already make the 555R in 275/50-15, 275/60-15 and 325/50-15 so they aren't afraid of 15" tires. (I know they call it a drag tire, but the editor of Grassroots Motorsports magazine mentioned a couple of years ago that he was using them on his autocross car with good success as were a number of other racers.) Many FF Cobra owners are using them on the rear on street driven cars (or front & rear with 17" tires) with great success because the low weight doesn't stress the sidewalls too much.

The Dunlop is a T rated tire. You find they are OK at high speeds on the track? Perhaps the lower weight raises the heat threshold sufficiently?

Kevin
 

Ian Clark

Supporter
Kevin,

Great suggestion about Nitto.

We've done limited track testing with no surprises. If I had to flog a set the tread depth would be shaved to reduce block flexing.

They are not a substitute purpose built tires, however it would be nice to see the engineering data that proves "x" z rated street tire is safer than "y" t rated street tire under race track conditions.

Keeping in mind also that we're way under the load range of these tires. Too much heat kills a tire and street tires should work predictably from cold.

I'm probably as worried the other way that running around on "DOT" street slicks is questionable given the weather and road conditions.

On the flipside, racing tires have no speed ratings and huge disclaimers... hmmmmmm
 
Ian - love the pump shot - that's not Regular is see Angelo filling in with (install a Gulp-Gauge?).

I still need rubber so I am glad to hear the Dunlops are decent. There isn't much to choose from.

Chris
 
Ian,

I agree about the DOT "R" tires not being good for the street, if for no other reason than that they heat cycle quite badly and in 6 months they are worse than ordinarry street tires.

That's one reason I like Nittos so much. They are a real street tire with a 15,000 mile projected wear rating, V-rated and a tread pattern to handle water, but still have a very sticky rubber compound.

Kevin
 

Tim Kay

Lifetime Supporter
Even though I love my Hoosiers, the Nitto's got my interest except for the fact they have a size for the rear but not for the front. The last I looked (6-8 months ago) they didn't offer a 245-##/15 or anything close. Has that changed?
 
Tim,

No it hasn't. The Question was what small tire company might be persuaded to make a suitable set of tires. Nitto being both small and already having a workable size for the rear was a good candidate.

Phone calls and e-mails to Nitto (and Avon) are in order. /ubbthreads/images/graemlins/wink.gif

Kevin
 
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