LeMans05C6

For example i have been looking at the standard sizes for the C6. 245/40 18s and the 285/35 19s.

I am looking at going with the Hankooks Ventus V12 Evo K110 and they make them in 255/40 18s and 295/30 (they don't do make the 35s in the 295) 19s which would be a bit wider than stock on both front and rear.

Would this be an acceptable upgrade for stock 05' C6 wheels. Would they fit and also ride properly.

I am trying to understand obviously a wider tire would hopefully have a larger contact patch, but if the tire doesn't match the wheel exactly isn't that going to cause a problem.

Some education would be appreciated. I have done 2 HPDEs and plan to do a lot more so if a small adjustment like this is beneficial then I would be down for it.

gkmccready

245/40r18 is 25.7in tall. 285/35r19 is 26.9in tall.

255/40r18 is 26.0in tall. 295/30r19 is 26.0in tall.

With no front:rear tire size difference TC will be upset. I've been told you need at least 0.5in of tire diameter (ie. height) difference to keep the nannies happy.

I would try to fall within the manufacturers specified wheel widths for any given tire...

drivinhard

A lot of folks smarter than me (that's everybody will tell you the contact patch does not get bigger, it only changes it's shape.

There is probably no subject currently in my head more interesting to me than this.

This past weekend thanks to Z06cool, I tested some 08 date code (but much less worn than my cording 09 date code) A6's, T1 sizes on T1 wheels (315's on 10 and 11"). This was in contrast to my normal 275/35/18 A6 square set up I have to run for NASA TTA, on 10.5" OEM speedline wheels.

While I enjoyed getting the chance to do it, it was on a track that was very new to me, so I would probably be better results on a track where I'm a bunch more consistent. It was also hotter/later in the day, so the track obviously slows through the day.

In any event, at the end of the day, I ran slower (but in the ballpark) of what I had ran earlier. Possibly dead nuts max G loading in the corners, the car had a bit more stick, but the car was nowhere as nimble and lively. More numb to me, and I could light up the rears off the corners just as easy. Just scrubbing the tires on the warm up lap I could tell the car was different.

I think we are just so mentally programmed for "wider is better". Frankly, I think it may be one of the most over-hyped automotive subjects around.

I'm not saying a 315 on a 11" wheel isn't faster than a 275 on a 10.5" wheel, a nice overcast day with some instrumented testing with fresh tires for both would be needed, but I can tell you from running Khoi's tires/wheels, it was not some magical transformation to the car. I realize there are other issues (more thermal capacity during a race possibly, etc) but in terms of just all out performance, it may be faster, but it ain't much faster.

My .02 anyway

Jason

Mark is going to be winning TTA on bicycle tires soon.

WBHighwind

pizza cutters...

Bill Dearborn

The tire diameter is the big issue. You need to keep the ratio from front to rear the same or close to the same. If you don't then you will be wondering why your traction control or drag control is active when it never was active before. You aren't in Kansas anymore.

Bill

flink

Contact patch area = vertical load / tire pressure. So yup, a wider tire has a differently-shaped patch of the same area.

Advantages of wider tire:

- Better heat dissipation
- Slightly higher lateral traction due to longitudinally shorter contact patch

Disadvantages of wider tire:

- Heavier
- More wind resistance
- Takes longer to heat up
- Will attain lower peak temperature
- More tendency to wear inner shoulder under braking
- More expensive. Hence for a constant $ input, you're running older tires

So it's not an obvious decision!

The best tire-width performance comparison I've seen is http://www.ffrchallenge.com/tire_tes...test_2007.html. In that test, 255's were quicker than 275's.

argonaut

I've seen this equation and info posted before but I can't say I ever understood this concept. I've always just scratched my head and not worried about it. But today I looked it up on Wikipedia (which has almost no info) and that led me to this: Fact of Fiction: Tire contact patch size is determined mostly by weight and pressure.

I don't pretend to understand everything the guy is talking about but at the very least my take away is that its not cut and dried. At least according to the above link - we shouldn't be taking patch = weight/pressure to be a law of physics. Its a lot more complicated than a simple equation, even if the equation is close. However, I'll point out, the info in the above link talks strictly about measuring contact patch at various pressures and loads for a couple different tire sizes. It does not in any way talk about real world performance. As the link that Flink points out - Factory Five Testing proved (at least for their car and the specified tires) that the narrower tire produced lower lap times.

Interesting discussion.

Don Keefhardt

The fundamental misunderstanding that most people have is "air pressure holds up the car". That can be easily countered with "If air pressure holds the car up, why doesn't the pressure in tires change when I jack the car up and take the weight off the tires ?"

The sidewalls support the car. Air pressure defines the rigidity of the sidewalls and the rest of the carcass.

Tires are mechanically complex little buggers.

Good article at the link, btw.

Jason

read "the racing and high performance tire" by paul haney and you'll find out that not even god knows how a tire works and why.

drivinhard

On the scales tonight

CCW/ 315 Hoosier scrub – 48.1 lb
Speedline/275 Hoosier scrub – 41.2 lb

That makes a big difference, 28 more lbs total of unsprung and more weight for the drivetrain to rotate.

Solofast

The biggest thing about tires (pun intended) is that you need "enough" tire to make the car work. Bigger than "enough" doesn't buy you much performance, but if you don't have "enough" the grip isn't there and it doesn't seem to work.

The biggest question (sorry, pun intended again) is how much tire is "enough"

And the highly technical answer is “it depends”. What it depends on is the weight of the car, the construction of the tire, the tire compound, the suspension geometry, the setup of the suspension, the phase of the moon and whether or not the driver holds his mouth open when he is cornering… (Ok, I lied about the last one, but extra points to whoever knows what famous autocrosser used to go around corners with his mouth open and his hand on the inside top of the steering wheel on corner entry). If you get the idea that it’s a very complex set of variables that you have to solve for here you are just beginning to understand what works and how to get what you want the car to do.

For example, on a Corvette, for good grip it seems that you need, as a minimum, a 275 cross section front tire. When the C5 first came out it had 255's on the front and GM "benchmarked" the C5 against the 300ZX and the RX-7 and figured that, even though the C5 was heavier, that a 255 would be "enough".... As they say on Jeopardy ........ERRRRRRNT,!!!!!!, that's a wrong answer..... The racers figured out that you needed about a 275 on the front and that if you don't have that much tire performance will suffer. After they put wider rims on the front of the Z06, the C5 became the autocross weapon of choice.

Putting on 295 Hoosiers on the front doesn’t seem to give you any better performance than the 275’s, but that’s not to say that next year’s 295 won’t be a better tire than this year’s 295 and change the whole equation. On the other hand the guys running Solstici (sp?) in SCCA stock Solo II, found that putting a 285 Hoosier on an 8.5 inch rim really hooked up that car. Yes, the tire is a lot wider than you should probably put on that rim for best performance, and yes, it’s probably a lot bigger tire than you would think should work on that car, but it works, and if it works and hooks up, that’s what you do.

Pylons

only 16 of those lbs is tire though...according to the specs

(not saying the extra weight isn't bad...but it's not 28 lbs just from tire)