the test was done on dunlop z1 star specs... which is definitely not a sloppy side walled tire. Actually its probably one of the stiffest side walled competitive tire available.

the reason the 225 hoosiers work better is that its sidewall is stiff enough to make the setup work. Street tires move quite a bit on the rim and it is really important to make sure that the sidewall stays happy. Hence stretch them and run the appropriate (most of the time lower) pressures for optimal grip. The tires should last longer and shouldn't overheat/heat cycle too quickly.

 

NMrado, thanks for taking the side by side pics. It was kinda shocking to see the 255s with a shorter diameter, but taking into account that its a well worn tire and that when comparing new tires it's only a .3 inch difference, I guess the pic is about right.

For those that run AP1s, it's very important that we try to get gearing back in our favor. A BS AP1 on Hoosier 245/45/16s IMO has the perfect gearing. The diameter is 24.7 inches. With the 255 street tires, the diameter is 25.0 inches, but we lose a lot of grip to the Hoosiers, so it works against us. Even if the 1 MPH, it's in the right direction. IMO gearing for nationals is perfect when we hit rev limiter at 63 MPH.

As for some of the opinions about the 245, here are my 2 cents. First, I thought that the tread width being wider than the 255s was too good to be true, and it was. Fortunately, it looks like they aren't that different. Tread width on tires generally increase slightly as they wear, and comparing new to worn, I'm thinking half an inch is ok. I think if you were to compare new to new, it'd be 1/4 inch of tread width difference.

Second, some are saying that you can stuff wider R-comps on a narrow wheel and it will be faster, but comparing R-comps to street tires isapples to oranges. R-comps have much much stiffer sidewalls, so you can get away with it. Not on street tires, and from what I've read about the Hankooks, they are pretty soft. If you look at the side by side pics that NMrado posted, notice that the 245s are actually slightly stretched on the wheel, which is a good thing. It makes it more responsive, doesn't require as much air, and as such, keeps the tread flatter to the surface. I think whatever small amount of grip the 245s lose to the 255s, it will make up for in faster transitional response. Hopefully I can get a set of 255s to test against.

James

 

Here's some pics of the tire:


The next two pics show how the tire is stretched a bit on the rim. With my wheels, the metal lip on the wheel sticks out a mm more than the tire, so I have to be careful not to lay it flat on the ground:



Here are some shots after 12 runs at El Toro, which is a high grip/highly abrasive surface. The suspension was JUST put together before the event, so no alignment was done, and I couldn't drop the car at all due to the damn driveway I pull into, so I kept the pressures high for concern that I would wear the outer edge too quickly, 36 psi front/35psi rear.:



God I love how beatiful these wheels are:

 

How did they feel compared to 255's?

 

If it was the Dunlop, and I think you're correct now that I think about it, then that really underscores the point -- even the Dunlop benefits from more effective lateral support at the expense of width. Yes, there is a slight gearing change in that test which could also have had an effect, but I don't think gearing alone could explain the difference in times that they measured.

You're right, the A6 and V710 have much stiffer sidewalls than any of the street tires, which allows them to work reasonably well even when pinched onto a narrow wheel.

 

I know of a few competitors that bring multiple tire sizes to an event so they have the right gearing for the job. To keep relevant the AP1 may benefit from shorter gearing where the AP2 may benefit from taller gearing. I question how much of a difference a mere .25" will make but 1" was definitely appreciable on some other platforms.

To add further fuel to the fire, the ST cars have used a wide variety of tires, often the narrower ones in favor over the wider ones. They are the most extreme case where every pound matters. The Miata example above might fall under this category. A local here ran 245/35 Toyos, went to 255/40 Hankooks, and is back to 245/35 Toyos. His level of competitiveness with our car has remained the same where our car has not changed since his tire switch.

Do you guys really feel like the transitional response is lacking on these cars with 255s? There is a difference between the stiffer Dunlop and the software Hankook. Both tires will run equal times but the Dunlop does so in a more precise manner where the Hankook is a little softer but more forgiving.

To address the contact patch width argument, sidewall height and stiffness also play into longitudinal grip. Less sidewall flex reduces the forgiveness both laterally and longitudinally. You might find that a 255 grips better under acceleration simply because the sidewall distorts a bit more and buffers the shock loading. Look at the Italian super cars, the JGTC cars, or drag racers: all with thicker sidewall rear tires.

You guys keep referring to using more air on these 255s but I've run as low as 32psi and as high as 40psi on both Hankooks and Dunlops. Unless 30psi and below is your threshold, I'm already "low". In my testing, 245 to 255 of the same tires (AD07s and Z1*s) liked ~2psi more for the 255s.

 

Also they used lightweight wheels with the smaller tire and heavy wheels with the larger tire.

BTW when I compared to the hoosiers, it wasn't to say that R comps and street tires are the same - I know they're not.

Anyway this is a good discussion.

 

^^^^^^^ I agree! I learn from you guys all the time, and appreciate the various point of views.

-Dave

 

I remember reading about the wheel differences. It wasn't a small weight difference in the wheels, as I recall... Enough to throw those results out the window in my opinion.

 

ok guys I got a idea...

heres what we can do to settle the contact patch arguement and this will give us hard data. It involves a few buck and a trip to wallwart. I'll do the 255/40 since I have a brand new fresh set, we just need someone to agree to doing the 245s.

who ever does the 245 use these steps

supples-
-poster board, the thin white one
-water based paint (so that you can clean off the tire afterwards)
-a small paint roller
- jack

jack the rear tire up just above the ground with the emergency brake on really well so theirs no chance of the tire rotating when its set back down.

jack up the tire only a few inches just enough to get the roller under there, we want as little movement as we can.

paint the bottom of tire tire about a foot long and the whole width of the tire. The reason for painting it so wide is to make sure the emprent is right.

put the poster under the tire

sit the car down on the paper, make sure you sit it all the way down with no pressure on the jack at all

lift the tire back up

pull out the paper and let it dry

then clean up everything


once the paper dries take messurments with a digital caliper and post them with picture.

Like I said if someone agrees to do the 245, I'll gladly do the 255. I'm about to leave for work and will get the weight of the 255 on my way.