Richor

Hey guys,

Im planning on running hoosiers or r compound scrubs this season. I have a C6 Base Z51

My plan is to run 285's upfront with 305's out back. As far as wheels go I was planning to run 18x10.5's all the way around.

Does any one see any fitment issues with this or have any better suggestions?

Thanks for your time.

jasonberkeley

Run 315s on all four corners.

bobmoore2

I have a C6 Z51 also. I've tried a few different R-comp tire choices, since I started using the C6 on the track about 1 1/2 years now. My previous track car was a C5 Z51. I use scrubs, not brand new tires.

Hoosier doesn't make the R6 or A6 in a 305 x 18 size. So that option won't work.

I think you will find that a 315 tire will stick out too far on your front. A 305 (Kumho V710) on a 10" rim was as big as I dared to use up front. The 305 tire occasionally hit the fender lip in a hard bump, but not bad enough to damage the body or tire. So you probably don't want to use 315s all around either.

Let me tell you what I've learned in the last 1 1/2 years.

I started out running R6s with 285 fronts and 315 rears, and the car had serious understeer issues. It felt like the car was "pushing" in every corner. Believe me, you won't like the understeer/oversteer balance at all. So that's out also.

Then I tried Kumho V710s with 305/30-18 on 10" rims in front and 315/35-18 on 11" or 12" rims in back. I loved how they felt! Great grip and very good understeer/oversteer balance. Since that first set, I've worn out three sets of scrubs in this size combination (about 15 track days).

Unfortunately, it is getting very difficult to find scrub V710s in the 305 x 18 size anymore. So my next track outing (on Feb. 6th) will be with Hoosier A6s. I've decided I will be running the A6 295/30-18 on 10" rims in front, and the A6 315/30-18 on 11" rims in the back. I haven't actually tried this size combination on the car yet, but I think it will feel really good. If I don't like this combination, I might try Hoosiers with 285 in front and 295 in back, or with 295 all around.

One other thing you need to know - My wheels (one set of CCW C10s and 1 1/2 sets of CCW C140s) have the stock offset/back-spacing. You may be able to fit more or less rubber on the front or back with your wheels, because they probably have different offset numbers than mine. I decided to keep the stock offsets, because I think/hope my hub bearings will last longer.

Finally, here comes my advice for you.

If I had your wheels (four 10.5 by 18s), and I wanted Hoosiers, and assuming your wheel offsets are stock (probably not), I would put 295s on the front and 315s on the back.

If was buying the tires new (not saving money with scrubs), I would buy Kumho V710s with 305s in front and 315s in back (again assuming stock wheel offsets).

Bob

bobmoore2

Jason,

After thinking some more about the tire and wheel sizes I've tried on the front of my car, I'm not sure I've ever test-fitted a 315/30-18 with a 58 mm offset. I've tried a Kumho 305/30-18 with 58 mm offset, and a Kumho 315/35-18 with 79 mm offset. Besides the size and offset, one tire is 'square shoulder' design and the other is 'rounded shoulder'. Obviously I did an "apples to oranges" comparison, so I got a bad conclusion.

Are your tires the Hoosier A6/R6s or Kumho V710s or something else? Do your 315s touch the front fenders at all? What are your wheel offsets - front and rear?

I'm beginning to think the Hoosier A6 315/30-18 on a 58 mm offset wheel would work on the front of my C6. In other words, that a 315s all around setup might be the best one for me (and for richor)

My calculator says:

Hoosier 315/30-18
section width = 315 mm
section HEIGHT = 94.5 mm (315 * 0.30)

Kumho 305/30-18
section width = 305 mm
section Height = 91.5 mm (305 * 0.30)

That's a very small difference.

How does your car's computer deal with the tire circumferences being the same? I turn off the TC and AH on track, so that isn't an issue for me. Does the ABS still work perfectly with equal tire circumferences?

AU N EGL

Bob

have you physically measured those tires ?

What Hoosier puts on the sidewall is not really the true width. " Some ppl Call them T1 specs <koff koff> "

Hoosiers 315 is more like other manufactures 320 to 330 widths

Hoosier does make a 18x305/30 width. It is the VRL ( R6 compound but stiffer side walls )

I have also run same size tire on all four corners. ABS works just fine. but do need to turn off AH/TC which should be off anyway

jasonberkeley

Its all about wheel offset. Mine dont stick-out at all.

Everything works as normal. We've been running this set-up in T1 for years with no issues at all.

Running Hoosier A6's, which definitely offer the most grip of all of the DOT tires. Running R6s or another brand may be preffered if you are looking more for tire life for track days.

I bought my wheels so long ago I don't remember the offset. They are CCW Classics. Call John Purner at CCW and ask him for the offsets for T1 wheels. 386-258-0083.

WNeal

I have done the same thing for the past 4 years with no issue.

Wheel offset will determine if you have any rub issues up front. I have stock C5 Z06 Rears all the way around and I had to do some inner fender well trimming, but not much.

The 285's are probably on the small size for that wheel so check that. I have run 295's with no issue though

With all the nannies off, no issues at all

bobmoore2

I have a pair of 315/30-18 A6s on 11" CCW wheels (79 mm offset) and I just bought a pair of 295/30-18 A6s that I will mount onto 10" CCW wheels (58 mm offset). When I get a chance to try them out (on Feb. 6 at Laguna Seca), I'm afraid they will be overly balanced toward understeer.

Oh well. I guess I'll put this day down as a learning experience. If so, then my next set of tires will be 315s all around.

What do you think about 295 in front and 315 in back? Will it be pushing really bad, or could I overcome the pushing by 'tossing it' into corner entry a bit?

CorvetteZ51Racer

Bob, if you have an adjustable front bar or adjustable shocks, you can tune those a bit to help with balance issues that may arise from the tire split. On my Z, I'm going up to 315s in the front mainly to help with braking. I plan on softening my front bar and shocks a bit to help the balance once I've got the extra grip up front.

bobmoore2

I went to Tire Rack a little later. They do measure the tires, or maybe they're using the measurements from the manufacturers. Anyway this is what Tire Rack says:

Hoosier 315/30-18
Section Width = 12.5"
When you convert inches to millimeters 12.5" = 317.5 mm.

Kumho 305/30-18
Section Width = 12.3"
When you convert inches to millimeters 12.5" = 312.4 mm.

Kumho 315/35-18 = 13.0"
When you convert inches to millimeters 13.0" = 330.2 mm.


So the Hoosier 315 is a little understated (actually between a 315 and 320). The Kumho 305 is more understated (actually between a 310 and 315). The Kumho 315 is Very understated (actually a 330). Of course the manufacturers could both be understating or overstating their measurements in inches too...

It's difficult for you and I to get an accurate Section Width measurement, because it is defined as "without any added width caused by the numbers and letters molded in to the sidewalls". Section width can also be changed by several factors that affect how much the sidewall bulges out, including tire pressure, wheel width, weight on the tire, etc.

Nevertheless, using Section width is still not a viable method to estimate Tread width anyway (what we really want to know most of the time), which depends more on the mold shape. For example, the Kumho "square shoulder" design has a wider tread per section, than Kumho's "rounded shoulder" design. Also worth considering is the fact that the rounded shoulder design keeps the tread in contact with the road better in a corner. The bottom line is that the specification numbers have some value when comparing the same molding design by the same manufacturer, but almost no value when comparing tires by different manufacturers or even different types of tires by the same manufacturer. Right?

redtopz

I ran a 1:38.0 at LS with 295 front and 315 rears (no aero). My car felt great. In fact, in NASA last year I ran 295 front and 335 rear and the car felt great. I also just ran this combo (295/315) at my first SCCA T1 regional. Next time I am going to try 315 all around however, mainly for more braking force.

redtopz

BTW, you will love A6's and even R6's. However, if you can find a set of hoosier slicks R80 or R100 get them. They are just as sticky as A6 when they warm up, but they last forever. Great hpde tire for somebody who really wants to get the most out of there car. I ran a set for months and they never wore out, although they gradually lost grip.

AU N EGL

That is for sure. My street tires, Mich PS 335s are almost an 1" less then the Hoosier A6s 335s

Richor

Bob/ Jason and other thank you very much for your insight. Bob please let me know how the 295 315 combo works out for you in terms of understeer. With the right offset wheels I think that may be the combo Im going to go for. Thanks once again

bobmoore2

Glad we could help, Richor.

I would have felt a little bad about taking over your thread, but I figured that you were sitting in the background and getting lots of good info from listening to us veterans talk.

I recommend you use the R6s, not A6s. They are a slightly harder compound, which means their maximum grip level is a little lower, but they are a little more forgiving (give more warning before starting an unrecoverable skid) and they don't wear out as fast.

I promise to report back in this thread after I've tried out the 295 & 315 A6s combo.

bobmoore2

I still have the stock suspension (nothing adjustable).

So you plan to add more grip up front with bigger tires. Then you plan to reduce your rear grip in the corners, by inducing more rear body roll. You could also reduce your rear grip by putting smaller tires there, or by changing air pressures (I forget which causes increased grip, more air or less), or a couple other changes which I can't remember right now.

Since I don't have any adjustability in my suspension, I am adjusting tire sizes and air pressures (slightly) to get proper balance in the corners. Unfortunately my changes also affect braking and accelleration grip. Your changes don't. Someday I may get adjustable shocks and sways.