15" vs. 17"

[big red]

I have 17" pin drive wheels but really like the look of the 15" wheels. I do not track. Why don't I want to trade out to 15"?

[sspano01]

i have just started this year doing track days and have been making various improvements to my cobra. I went back and forth between 15 and 17, but decided to stay with 15 inch. I am not trying to push everything to the limit on the track, so the 15 should still be fine.

I did just get the 15 Avon's - which are supposed to be "the" tire.
I'll find out 7/30 and 7/31 on the track.

From a "look" standpoint, you can't beat the 15's! I figure if I am not going super fast, i should at least be looking super fine :P


Untitled by sspano01, on Flickr

[Cashburn]

Because they won't clear your brakes.

[littlewave]

I had to put 11 3/4" rotors on the front because of the 15" wheels.

[tirod]

It's been the conventional thinking that to be competitive, you have to use the bigger rims to fit bigger rotors. Well, that all depends on what kind of track you're racing on.

On a drag strip, 15" have less rotational inertia, less overall diameter, and brake size matters a lot less. As tested by Car and Driver, a car on 15's can cut up to a tenth just by using 15" vs 17".

On a road course, the 17" typically use a shorter sidewall tire with more cornering ability - but that isn't an empirically tested standard answer. What went along with the shorter track times were those bigger brake rotors, too. Kit the 15" with better brakes, like 6 piston Wilwoods, and try to get comparable results. It's very, very difficult to make an verifiable comparison.

Inch for inch, a 15" rim will be lighter than a 17" when made identically, and so will the tires. If unsprung weight is a factor, or having less inertia to spin the tire up - or down - to speed, the smart builders pay attention. They compare what they are gaining and losing in the switch. So far, it's largely because of the idea that larger rotors offer more braking. I can see a smart car builder being able to get a 15" equipped car around just as fast - because he could corner at higher speeds and need less brakes. A 17" equipped car with no track time to balance the sway bars, set up caster and camber, or even get the tire pressure right, on street tires picked up because they were cheap won't run with a track car set up on 15" race rubber.

There is a lot more to consider than just the size of the tires, it's a working dynamic package. The end result is what counts - not what it "looks" like. That's what the less knowledgeable take away and copy, and it doesn't always work out so well.

I grew up on 15's, and the current fad to use the bigger rims on a street car seems like a major step backwards. There's more than one article in the press about the lower profile tires being horrible in winter weather, being damaged more easily on typical streets, and all that at a much much higher price. If you want to understand it better, just follow the money.

[Bobcat]

tirod ... interesting that you said what you did about the 17" wheels . I also track my ERA and was looking at going to the 17" wheels . Spent some time with a friend of mine who was a team manager Roush/IMSA and other teams and then talked at length with a race tire engineer from Hoosier.... who spends a lot of time at the track working with race teams . Long story short , they both talked me out of going to the 17" tire/wheel combo . Hoosier saw no advantage . His comment was that if the OD of the 15" combo was 26.5" and the same for the 17" combo , for the same tire size , the footprint was no bigger . It would also change my effective spring rates ( higher ) because the shorter sidewalls were stiffer .
So far , I`m spending my time and money on better brakes/pads ( I know a brake man with Roush ) , shocks and springs ( Hyper coil ) , suspension settings ..... and track time .
I think where some people feel that the 17" combo is faster/better is because they were able to go to a slightly bigger tire size .... or as you said , better brakes .... or maybe their springs were too soft .
Any way , that`s my story and I`m sticking with it .

[itstock]

The 17" will clear the brakes, and to add to that, you have a much better selection of performance tires.

Since you don't need the performance aspect, can you find someone with a much earlier build and trade out everything? I think up to 2007 or so, 17's were an option, and 15" were standard?

[undy]

I like the 17s because I run the M/T ET drag radials (315/35/17). They provide enough hook to lift the front tires but the 35 aspect ratio also lets them perform well in the twisties too. It's a totally different world when you can roll on the gas in your 600hp Cobra and it grabs, even in 1st gear, compared to most all street tires that make you feather the gas through 3rd gear. I actually find my car much "safer" with the drag radials. It surely seems to me that most Cobra accidents are the result of a sudden loss of traction.

[rpatton3]

What is DOT Treadware for the drag radials?
And price for 315 35 17's?

[undy]

Originally Posted by rpatton3

What is DOT Treadware for the drag radials?
And price for 315 35 17's?

They carry a 000 rating and will last (depending on driving) 3K-6K miles.

They run from $200 - $225 per tire. I'm on my 2nd pair and couldn't be happier.

[rpatton3]

Thanks!