Let's talk tires for the Extreme ACR track rats

[VENOM V]

Hi all,

I've always run Hoosier R6s and now R7s at the track, 315s front and 345s rear on my 2013 GTS. I love their communicative feel, their Spiderman grip, the confidence that they inspire, and the lap times they give. However I've heard some great things about the new ACR Khumos. I am going to order a second set of wheels (probably Forgelines) and track tires, and am debating between a second set of Kuhmos or putting Hoosiers on.

Could this be the first time that an OEM tire is faster than Hoosiers? We've heard that the ACR is 1.5 seconds faster on the Kuhmos than on Hoosiers, and I've confirmed this is the case, at least at a specific track. Here are the choices that I'm contemplating:

Kuhmo Ecsta V720 ACR: (diameters are my recollection of what someone told me they measured, I may be wrong)
F: 295/25-19, 11.1" tread width, about 25.3" dia, 27 Lb
R: 355/30-19, 12.5" tread width, about 27.4" dia, 41 Lb

Hoosiers:
F: 315/30-19, 12.0" tread width, 26.1" dia, 27 Lb
R: 345/30-19, 13.0: tread width, 26.8" dia, 30 Lbf

Pros and cons to the two tires, the Hoosiers are much wider and the rears are lighter. The biggest drawback of the Hoosiers is that they are about 0.8" larger in diameter, which means the Hoosiers will raise the front of the car ~0.4", also shifting the roll center and creating push unless the suspension height is adjusted to compensate for it. You want the front to be at least 20mm lower than the rear. I'm not sure if you could lower the front to compensate, as I've heard the ACR's best track performance is with the front coil overs lowered close to their limit (not sure about this one, perhaps to prevent excessive rubbing). And you can't raise the back much, because then the air diffuser will lose downforce.

Plus, I've been told that the 30 series may rub.

And I would hate to have to adjust the coil over heights and need to re-align and corner-weight the car when switching between the Kuhmos and Hoosiers.

I had concluded that the Hoosiers were a bad idea, then I saw that Ralph just tracked with them. I'd hate to be the Guinea pig, but maybe I don't need to be. Any feedback on Ralph's lap times, R7s Vs. Kuhmos? Did he optimize height and alignment for the R7s?

Opinions are welcome, hoping to hear from my fellow track rats.

[Nambo]

Todd,

Did you see the pics of his Kumho's after only 3 sessions? Toast. The Hoosier may be a better alternative for durability. I would be curious what sizes Ralph was running and what his ride height was for sure.

[VENOM V]

Todd,

Did you see the pics of his Kumho's after only 3 sessions? Toast. The Hoosier may be a better alternative for durability. I would be curious what sizes Ralph was running and what his ride height was for sure.

No I haven't, can you point me to it?

Thanks

[TrackAire]

I would be amazed if the rear Kumho's weighed 41 lbs.....that sound way heavy.

[VENOM V]

I would be amazed if the rear Kumho's weighed 41 lbs.....that sound way heavy.

I know it. Perhaps for sidewall rigidity, and to withstand the tremendous downforce from that stupendous wing

I would worry about the Hoosiers holding up, but I see a lot of guys at the SCCA races with humungoid wings running R7s

[Rapidrezults]

There is a picture of Ralph corner-weighting with the Hoosiers before the NCM event. The car looks like it definitely sits unusually high, there is a ton of clearance up front even with the extended splitter, although it could just be the angle of the pictures. It's very interesting that Ralph ditched the Kuhmos for the Hoosiers and didn't think twice. Hoosiers just might still be the tire of choice.

I guess this is a bit premature to know for sure. I'd like to get Ralph's direct feedback on this.

[Nambo]

No I haven't, can you point me to it?

Thanks

Doris posted some pictures on Instagram, I'll see if I can find them.

[Bruce H.]

I'd want to get Ralph's feedback, Todd. I'm sure he tries lots of things to see what he likes, and just because he tries something doesn't mean he going to prefer it.

Maybe the Kumhos are designed like the Corsas where the 295 fronts are a different compound than the 355 rears.

[Darius]

I ran Hoosiers on my C6 z06 and they felt great. When I put them on my TA I wasn't impressed. The car pushed a lot and when I would get on the gas on exit it either felt like I was bogging it or it was getting sideways. It was hard to get a clean exit. Why not try running some Pirelli slicks or Michelins. That's what I'm going to try.

[VENOM V]

I ran Hoosiers on my C6 z06 and they felt great. When I put them on my TA I wasn't impressed. The car pushed a lot and when I would get on the gas on exit it either felt like I was bogging it or it was getting sideways. It was hard to get a clean exit. Why not try running some Pirelli slicks or Michelins. That's what I'm going to try.

The Michellin and Pirelli slicks are the best, but they are pricey. I wonder if your experience was from the Hoosiers raising the front and lowering the rear? Stock tires are 25" / 27.4" tall front/rear, compared to the Hoosier's 26.1" / 26.8". They seem to work well for me though.

Did you get a chance to align it after lowering, and try the Hoosiers again?

[VENOM V]

I hope Ralph's feedback materializes, I'm sure we'll hear something soon.

[SADVIPER]

I've got track pack wheels, do you guys recommend changing the size of the front tires as well for track days?

[Nambo]

I've got track pack wheels, do you guys recommend changing the size of the front tires as well for track days?

I ran a 315 Hoosier up front on my TA and was very happy with them.

[ACR Steve]

Pirelli DH slicks my vote. Last longer then Michelin and work much better then the R7's . You just have to set the car up for a slick and drive a little differently. The combination of Down Force and Slicks is the winning combo for lap times hands down.
Whats price difference on a DH vs R7 ?

[Nambo]

Pirelli DH slicks my vote. Last longer then Michelin and work much better then the R7's . You just have to set the car up for a slick and drive a little differently. The combination of Down Force and Slicks is the winning combo for lap times hands down.
Whats price difference on a DH vs R7 ?

I have been able to get a set of R7's mounted and balanced for $1,700. I have no idea what the Pirelli slicks would cost?!

[Rapidrezults]

Pirelli DH slicks my vote. Last longer then Michelin and work much better then the R7's . You just have to set the car up for a slick and drive a little differently. The combination of Down Force and Slicks is the winning combo for lap times hands down.
Whats price difference on a DH vs R7 ?

Steve,

What exactly is the difference in car setup between a Pirelli DH slick and a Hoosier R7?

Thanks.

[GTSilver]

Ralph lap

https://youtu.be/eZoSX02a_4I

[ACR]

Sorry Randy is simply a better and much faster driver than Ralph imo, to be expected anyway
https://youtu.be/vznuI4zfesU

[TrackAire]

Sorry Randy is simply a better and much faster driver than Ralph imo, to be expected anyway
https://youtu.be/vznuI4zfesU

In the upper right corner of Randy's video is a temperature reading......is this coolant temps? Doesn't look like he much went over 224 degrees on that lap.

Great driving by Randy....he probably could have gone faster if he wasn't so scared of the Viper, LOL.

[Bill Pemberton]

Ralph tries a lot of different setups , and it is not just his prerogative , but quite smart for one who has so much input in the design and execution of the Viper over the years. At Motorsports Park Hastings , this year, Mr. Gilles was blazingly fast on the Pirelli slicks. Enough so that many have considering using these rubber donuts, as Bob Woodhouse was running the same tires and he set the fastest time of all over the event weekend. Have run Hoosiers on the Viper and on another of my cars , and many in the past, but will admit after running some Pirelli slicks that they are a really fast set - up. Especially interesting since the rears are undersized at only 325! I am sure there will be tons of data coming out from the guys as they get their new ACRs on the track and always good for tire wars for racers - helps keep the prices down some, ha.