Track Tire Size Data Point

[Pappy]

FWIW. For those experimenting with track tire sizes, here is a Yokohama data point. I went to the new Podium Club @ Attesa track (Casa Grande, AZ) this weekend and tried Yokohama A60s - the same tire Cable has been running on his 9L during Global Time Attack. 320/650X18 front, 330/710/18 rear. The 320s on front run wide and were the same width as the 335 A7s I had been running, and they were 1" taller than the factory Kumhos. I was running 2* negative camber and 7* caster, with the ride heights at 4"f and 5.5" rear. I also was light on front shock compression, and heavy on rebound. Got my eyes watered! Under hard braking the fronts would rub hard in the top inside of the wheel tub at the front. - so hard that it would cause a little side-to-side shimmy. Worse than that, with the nose down (heavy braking with lots of rebound stiffness) I could not turn without the inside edge of the tire hitting a protrusion in the front of the wheel tub (see photos). Cable got by with this because he had already worn big holes in the tubs - LOL. So, I couldn't brake and I couldn't turn, but the IATs were low and the car hauled like a banshee on the straights. I ran a few laps with light braking early and accelerated into the apexes to pick the nose up and minimize the binding, but that wasn't the quickest way around the track so I packed up. You could run these tires (they have great grip) with a little higher (maybe an inch) ride height, more compression stiffness, perhaps a different bump stop configuration on the shock, and holes in the wheel tubs - probably not worth it.

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[13COBRA]

I'd have a really hard time raising the car to run taller tires. Better off keeping it lower.

[Pappy]

I'd have a really hard time raising the car to run taller tires. Better off keeping it lower.

I agree. Cable and I discussed buying a new set of fender liners and cutting holes for clearance, keeping an unmolested set for replacement later. I could easily build CF "covers" for the holes that provide the necessary clearance while keeping debris out. It is really hard to find a good, wide slick, even in 18" sizes, that is as short as the factory Kumho with its "25" side wall. The Finspeed 12"-wide wheel seems to have the optimum back-spacing - I do not have room to move the tire in or out to improve the situation. In the photos above, the wheel/tire is jacked up to a point that puts the splitter 1" off the ground (anticipated full compression). I am looking at shock bump stop options that will control that height without an abrupt spring rate change when they bottom.

[jrubin80]

Does anyone have a photo of a slightly smaller mounted? Is there still a 300/650/18.

[TKO MOTORSPORTS TEAM]

I agree. Cable and I discussed buying a new set of fender liners and cutting holes for clearance, keeping an unmolested set for replacement later. I could easily build CF "covers" for the holes that provide the necessary clearance while keeping debris out. It is really hard to find a good, wide slick, even in 18" sizes, that is as short as the factory Kumho with its "25" side wall. The Finspeed 12"-wide wheel seems to have the optimum back-spacing - I do not have room to move the tire in or out to improve the situation. In the photos above, the wheel/tire is jacked up to a point that puts the splitter 1" off the ground (anticipated full compression). I am looking at shock bump stop options that will control that height without an abrupt spring rate change when they bottom.

If you car is fast and works well and lap times show it you would be much better off making the CF fender wells or finding a spare set of fender wells to cure the tire clearance problem. You never want to use shocks, springs , bump rubbers or adjust scrub radius to correct a body clearance problem.

[Pappy]

If you car is fast and works well and lap times show it you would be much better off making the CF fender wells or finding a spare set of fender wells to cure the tire clearance problem. You never want to use shocks, springs , bump rubbers or adjust scrub radius to correct a body clearance problem.

Agree. The bump stops I mentioned would be to keep the splitter off the track at full compression while hitting bumps - not to provide body clearance. That would be in lieu of a stiffer spring rate, increasing shock compression rates, or raising the ride height - or staying off the speed curbs LOL

[Pappy]

Does anyone have a photo of a slightly smaller mounted? Is there still a 300/650/18.

Don't have a photo, but the Michelin 30/65 is .5" narrower and .5" shorter and did not rub on my car. It looks like the offending area of the tub causing the rub is an indention for headlight assembly clearance. There is, however, an over 1" dead space between the tub and the assembly - plenty of space to prevent a rub.

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This eventually fixes the problem...

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[TKO MOTORSPORTS TEAM]

Agree. The bump stops I mentioned would be to keep the splitter off the track at full compression while hitting bumps - not to provide body clearance. That would be in lieu of a stiffer spring rate, increasing shock compression rates, or raising the ride height - or staying off the speed curbs LOL

Thing with splitters ,wings and other aero parts they are more effective at lower ride heights/ ground clearance ( ground effect); You want to exploit that sweet, free performance gain "ground effect" as much as possible.; Its perfectly ok to run a splitter 1 inch or lower at full bump. A properly designed splitter will consistently make short contact with track surface.
When you start messing around bump stops its more for very fine tuning when your looking to squeeze out another 10th here and there.
EX: Corner entry car is good and takes a set, mid corner car is ok, beginning of corner exit part throttle car has a slight push that gets progressively worse adding throttle. Possible fix would be to pack the bump stop or maybe run a slightly taller bump stop. All tuning and messing around of course always hinges off everything else being in the ballpark.

[Pappy]

Thing with splitters ,wings and other aero parts they are more effective at lower ride heights/ ground clearance ( ground effect); You want to exploit that sweet, free performance gain "ground effect" as much as possible.; Its perfectly ok to run a splitter 1 inch or lower at full bump. A properly designed splitter will consistently make short contact with track surface.
When you start messing around bump stops its more for very fine tuning when your looking to squeeze out another 10th here and there.
EX: Corner entry car is good and takes a set, mid corner car is ok, beginning of corner exit part throttle car has a slight push that gets progressively worse adding throttle. Possible fix would be to pack the bump stop or maybe run a slightly taller bump stop. All tuning and messing around of course always hinges off everything else being in the ballpark.

Concur with your assessment. This crude construction shot shows the mold (under construction) for a front Kevlar and carbon fiber undertray that will accommodate one of your Viper extreme splitter extensions. The extension will attach exactly as it does on the Viper - flush with the undertray. It has four diffuser ramps, the back two of which will empty outside of frame rail barge boards behind the front tires. That will direct airflow down 14" wide diffuser tunnels that run between the frame rails and the outer side skirts. There will be strakes on the bottom of the undertray to help direct airflow toward the tunnels as well - much like NASCAR currently uses to direct airflow outside the rockers.

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NASCAR strakes. The C8 Z06 has them as well.

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[TKO MOTORSPORTS TEAM]

Concur with your assessment. This crude construction shot shows the mold (under construction) for a front Kevlar and carbon fiber undertray that will accommodate one of your Viper extreme splitter extensions. The extension will attach exactly as it does on the Viper - flush with the undertray. It has four diffuser ramps, the back two of which will empty outside of frame rail barge boards behind the front tires. That will direct airflow down 14" wide diffuser tunnels that run between the frame rails and the outer side skirts. There will be strakes on the bottom of the undertray to help direct airflow toward the tunnels as well - much like NASCAR currently uses to direct airflow outside the rockers.

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NASCAR strakes. The C8 Z06 has them as well.

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Are you running a rear diffuser?

[Pappy]

Are you running a rear diffuser?

Yes, it is a double layer diffuser with discharge air from the trans and diff coolers and the exhaust being discharged between the two elements (hopefully creating a slight low pressure area above the lower diffuser element), with the lower element looking very much like the diffuser on the Viper ACR-E (using your strake extensions). There is a small spoiler across the deck lid and a 72" APR complex wing as well. The diffuser pan starts forward of the rear axle, but because the engine/trans is so low in the chassis, the weak area will be getting semi-smooth airflow at the leading edge of the diffuser. There was no way to incorporate a full belly pan.

[TKO MOTORSPORTS TEAM]

Yes, it is a double layer diffuser with discharge air from the trans and diff coolers and the exhaust being discharged between the two elements (hopefully creating a slight low pressure area above the lower diffuser element), with the lower element looking very much like the diffuser on the Viper ACR-E (using your strake extensions). There is a small spoiler across the deck lid and a 72" APR complex wing as well. The diffuser pan starts forward of the rear axle, but because the engine/trans is so low in the chassis, the weak area will be getting semi-smooth airflow at the leading edge of the diffuser. There was no way to incorporate a full belly pan.

Sounds like a plan.

[Arizona Vipers]

Nice work Pappy! I'll have some Yoko 300 fronts for you soon, I have one set of the soft compounds and they don't make a 320.

[9.0L ACR]

Where do you guys buy your Yokohama track tires? I have been looking all over but can’t find new ones for sale.