blaine

I'm going to be doing some autocross and a track day in the coming months and would like to get some tires specifically for use on those days. Does anyone know of any tire options available in the correct sizes? I would like to stay as close to stock tire size as possible. Thanks!

B Stead

I don't see any autocross tires in 285/30-20. And getting wider wheels and stepping up to 305/30-20 (with 265/35-19) doesn't really help. There's a Pilot Sport Cup 2 (OEM for some Porsche) on back order. Also, it's 180 treadwear while many autocross rules are going to 200 treadwear.

I do see an R-S3 in 285/35-20 and 245/40-19. Those are tall at 27.9" diameter rear and 26.7" diameter front. And note the diameter stagger at 1.3".

There's a better selection of autocross tires for 19" rear wheels. Both the RE11 and the Ecsta XS are available in the OEM 18/19 sizes. Those were once top autocross tires but are now simply considered to be good enough. Getting wider wheels and stepping up to 305/30-19 (with 265/35-18) finds the RE11, R-S3, and AD08R.

But take a look at these sizes:

265/35-18
305/30-19

265/35-19
305/30-20

The first pair is about 1/2" less diameter than OEM while the second pair is about 1/2" more diameter than OEM. Both pairs call for 9.5"/11" wide wheels. The first pair finds a better selection of autocross tires. (Wheel offsets for a setup of average development should be 56mm to 51mm front and 79mm to 74mm rear.)


If the rules call for stock Z51 wheel sizes:

R-S3, designed for track and autocross
245/40-19, 26.7" diameter
285/35-20, 27.9" diameter
And those are tall at about 1" more diameter than OEM

Pilot Super Sport, non run-flats, okay for track and autocross
.

blaine

B Stead,

Thanks a lot for the reply! I'm planning to buy rim's as well and just assumed a used Z51 set from the forums would make the most sense, but if there is an aftermarket rim with a similar cost that will enable a wider tire selection without upsetting the balance of the car I'm very interested.

B Stead

Yeah, going to wider wheels and tires while staying within 5mm of OEM wheel offset is a mild mod (relative to suspension geometry). And probably, that's also staying within 1/2" of OEM tire diameter.

The proposed Cray 2014 line lists the 18 x 9.5 and 19 x 11 wheel sizes at the correct wheel offsets. Those are not expensive but also not dazzling.

The Oz Superleggera III has a 18 x 9.5 59mm (listed under the C5) and a 19 x 11 75mm (listed under the C6). Those are nice looking custom wheels at about $900 each. I think I would use a 3mm to 5mm spacer with the front wheel.

The BBS RS-GT weigh less but are only available in 18 x 9 and 19 x 10.5. The wheel offsets are correct at 54mm front and 74mm rear.

Or a 19/19 setup is possible but that's a situation of the front tire sidewall being shorter than the rear tire sidewall
.

gthal

Don't forget that the PSS on the Z51 have a special, and unique to the C7, rubber compound which is very sticky. It's quite possible that the Z51 PSS would as capable or better than tires like the RS-3, Direzza, etc. What comes on the car might be the best option outside of a pure R compound tire.

B Stead

Here's a link to the Pilot Sport Cup 2:

http://www.michelinman.com/tire-sele...2/tire-details

305/30-20, 27.2" diameter, 11" wide wheel, inventory available
275/35-19, 26.6" diameter, 9.5" wide wheel, limited availability


In the R-S3 I can find:

305/30-19, 26.3" diameter, 11" wide wheel
265/35-18, 25.3" diameter, 9.5" wide wheel
275/35-18, 25.6" diameter, 9.5" wide wheel

The R-S3 merchant is saying closeout but that's probably a mistake. They should be closing out the 140 treadwear but replacing with 200 treadwear.


And so here's the AD08 R:

305/30-19, 26.4" diameter est., 11" wide wheel
265/35-18, 25.4" diameter, 9.5" wide wheel
265/30-19, 25.4" diameter, 9.5" wide wheel
275/30-19, 25.6" diameter, 9.5" wide wheel


I'm not looking at anything with a treadwear rating of less than 140 because those are usually R-comps (according to autocross rule books)
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VThokies

Has anyone looked at the tread widths on the OE Michelins compared to a larger tire? From what I have found the OE 285-35/20 is a very wide tire. So it may not be an advantage to go larger? Some larger tires even show a smaller tread width than the OE tires. Not sure what all this means, but would like to know what other folks think?

savage

B Stead, this is the first i heard someone say autocross rules are going to 200 treadwear, im in nccc and scca and about to buy tires, i run under 100 treadwear (hoosier a6 ) every where i go, parking lots, tracks, ect. where do you know of that has this rule in place? hoosier will not be happy, thank you, ps. i would not run more than 2 laps at a time on track day. those would be r6 days.

B Stead

The SCCA autocross classes that have a 140 treadwear minimum are said to be changing to 200 treadwear minimum. Of course that's really not necessary. A Star Spec at 200 treadwear is just as good as an R-S3 at 140 treadwear.

But I would require a 300 treadwear minimum as those the tires can hold consistency longer. Really ? Well, you don't know the trick about the top-time competition using new tires at every event but rubbing dirt on them ?

But R-comps are a completely different kind of tire. I wouldn't know how to define them except to say that they are usually less than 140 treadwear. Well, the true difference is the details of the compound. Most of the autocross tires that are not R-comps say that they are some type of silica compound. The R-comps just say r-comp compound, or competition compound, or not-intended-for-any-street-use.


Okay, I went into the SCCA autocross rule book (and SCCA news releases). The Corvette used to be in Super Stock which allowed R-comp tires. That class has become Super Street-R which allows R-comp tires. But the Corvette is also in Super Street which requires 140 treadwear tires in 2014 and 200 treadwear tires in 2015
.

B Stead

The autocross tires at 140 to 200 treadwear do happen to have more traction than tires at 300 treadwear. Both can have silica compounds but the tires claiming to be autocross tires seem to be more highly developed (and are proven). Of course the autocross tires don't stay sharp for very long.

If you want to get an autocross tire on the C7 there's a better selection for 19" rear wheels than for 20" rear wheels. Also, there's a better selection in the 305 sizes than in the 285 sizes.

As for width of the tire, a stiffer suspension does need a wider tire. That's because a stiffer suspension is faster load transfer and that works the tires harder. But this is important because faster load transfer is more traction on the turn-in
.

speedracer11

I was hoping to find a good R-Compound for the stock Z51 rims also. Looks like the Toyo R888 may have an option for us?

B Stead

I don't see a 285/30-20 in the R888. There is a tall 285/35-20 but not a 245/40-19 to match with it. (The 315/30-20 calls for a wheel width of 10.5" to 11.5" while the 265/35-19 calls for a wheel width of 9" to 10.5". However, front-to-rear stagger would be 1.3". Also, the 315 is said to touch the brake ducts.)

There's a Pirelli PZero Corsa in 255/35-19 and 295/30-20 but it looks like a closeout, back order, old production, or special order at TR. Also, it's expensive and not the more available of the R-comps.

But then if changing wheels, might as well go to 18 x 9.5 52mm with 19 x 11 74mm. Then tire sizes are 265/35-18 with 305/30-19.

Well, if this is an autocross subject then go look up the wheel rules for the new Super Street-R class and for the new Super Street class.

Super Street autocross class:
140 treadwear tires in 2014 and 200 treadwear tires in 2015
Wheel width must be the same as OEM
Wheel offset can vary by 1/4"
Wheel diameter can vary by 1"

Super Street-R autocross class:
R-comp tires allowed but not required
Wheel width must be the same as OEM
Wheel offset can vary by 1/4"
Wheel diameter must be the same as OEM


Okay, the best R-comp setups I can find for the OEM wheels:

Hoosier R6 (not A6)
255/40-18, 8.5" to 10" wheel width, 26.2" diameter
295/35-19, 10" to 12" wheel width, 27.3" diameter

Hoosier A6
255/35-18, 8.5" to 10" wheel width, 24.8" diameter
295/30-19, 10" to 12" wheel width, 26.1" diameter

Toyo R888
255/35-18, 8.5" to 10" wheel width, 25.1" diameter
295/30-19, 10" to 12" wheel width, 26" diameter

But this means that the best choice for a C7 in the Super Street-R class is the base C7 and not the Z51
.

speedracer11

As much as I don't want to- I think a set of race wheels are in my future.

B Stead

I'll try and put all the setups together:

Super Street autocross class
140 treadwear tires in 2014 and 200 treadwear tires in 2015
Wheel width must be the same as OEM
Wheel offset can vary by 1/4"
Wheel diameter can vary by 1"

Super Street-R autocross class
R-comp tires allowed but not required
Wheel width must be the same as OEM
Wheel offset can vary by 1/4"
Wheel diameter must be the same as OEM


Here's are setups for Super Street-R:

Hoosier R6 (A6 not available in the sizes) *
255/40-18, 8.5" to 10" wheel width, 26.2" diameter
295/35-19, 10" to 12" wheel width, 27.3" diameter

Pirelli PZero Corsa (limited availability) (tread pattern differs front to rear)
255/35-19
295/30-20

Or a shorter diameter for Super Street-R:

Toyo R888
255/35-18, 8.5" to 10" wheel width, 25.1" diameter
295/30-19, 10" to 12" wheel width, 26" diameter

Of if going out of wheel width spec in Super Street-R:

R888 *
265/35-18, 9" to 10.5" wheel width, but 8.5" wheel width must be used
305/30-19, 10.5" to 11.5" wheel width, 26.5" diameter, but 10" wheel width must be used


Then setups for Super Street:

R-S3
245/40-19, 26.7" diameter
285/35-20, 27.9" diameter

Ecsta XS or RE11 (but not the most recent of the autocross tires) (XS treadwear rating not allowed in 2015)
245/40-18, 25.7" diameter
285/35-19, 26.9" diameter

But then if going out of wheel width spec for Super Street:

R-S3 *
265/35-18, 9" to 10.5" wheel width, 25.6" diameter, but 8.5" wheel width must be used
305/30-19, 10.5" to 11.5" wheel width, 26.3" diameter, but 10" wheel width must be used

Pilot Sport Cup 2 (limited availability) (not allowed in 2015)
275/35-19, 9" to 10.5 wheel width, but 8.5" wheel width must be used
305/30-20, 10.5" to 11.5" wheel width, but 10" wheel width must be used



* These are the most likely choices however the Z51 can't change wheel diameters in Super Street-R but can change wheel diameters in Super Street

.

savage

one last question, what (scca) class will my non- z51 c7 be in with a6 hoosiers on 2002 z06 wheels, 17x9.5 front(295/35/17) yes i said 17's they fit great, 18x10.5 ( 315/30/18) on back . my nccc class is 2M, F.Y.I. i have 4 17's and 4 18's wheels and the 18x10.5 with the 315/30/18 fit great (no rubs) on front also! this is the same setup i used on my 2008 vert and before that my 2002 cp. ps. 315/35/18 dont fit on front, they rub. thank you for you time.

B Stead

That would be the obvious question, what is the lowest level of prepared or modified autocross class that doesn't limit wheels and tires ?

And both the C5 and the C6 are listed in Super Street Prepared. But there are two problems. The first problem is that Street Prepared allows certain club race cars to directly fit into Street Prepared. And then some of the regions don't like cars in Street Prepared if they are not race cars. The second problem is how many other entries are there in Super Street Prepared ?

But there is an autocross forum here under general Corvette topics
.

savage

http://forums.corvetteforum.com/imag...ies/salute.gif

80atez

Treadwear and grip are no longer so mutually exclusive: Michelin says their PSS is as fast as their PS2 and the treadwear is 300 vs 220 by using dual compound tread.. I'd say that may be a bit of marketing talk..

Also, I thought treadwear ratings are determined by the manufacturer to compare their tires to each other?? But of course the SCCA knows more than I do...

I would say, from my experience at HPDE, when you go to a "softer" tire, you will probably see it wear noticeably faster. The question is, will you be faster? Anything short of a PSC or DOT tire, you may not see much improvement, imho.

B Stead

Well, the R-S3 is going to 200 treadwear, the RE-11 as the RE-11A is going to 200 treadwear, the Star Spec ZII is already 200 treadwear, and the g-force Rival is already 200 treadwear.

I don't yet see the Ecsta XS, the R1R, or the AD08R going to 200 treadwear
.