front tire width

[bryanearll]

Reading through an article I saw someone suggesting using a 245 tire on the front instead of the standard 225's for the 18" C2 996 wheel. Is that safe? Bigger tire in the front would help understeer but what's the downside? Better perhaps to use real 8.5" or 9"wheels on the front? What is the impact to handling?
bryan

http://www.rennlist.com/996_faq/b.htm
Todd says: "If replacing only the rears in a C2, the decision is whether to choose 265's or go to 285's. Just be aware that if you go to 285's, the car will understeer a bit more, since traction is increased in the rear relative to the front. If you're replacing all four tires, you can go bigger in the front - like 245's, to go with the 285's in the rear. Wheel size will dictate tire sizing. 8" width up front, rather than the 7.5"of the regular 996 18" wheels, allow 245's (which strictly speaking are recommended for 8.5" rim widths but seem to work fine)."

[Jad]

The article is correct, but a bit limited. Tire width is one measure, but again, it varies from brand to brand. For example an Azenis tire is wider than most other tires in the same 225 width (one reason they are faster). In addition, each car is slightly different and each has its own camber and ride height, so a tire that just fits on one car, won't fit another. Yet another problem, is tire height. You are talking width only, but some tires may come 245/45 which is 12.5 mm taller than a 245/40 tire. On my turbo, the 245/45/17 wouldn't fit, BUT 255/40/17's did as did 245/40/17's.

So, unfortunately, there is no simple answer, but a 245 probably JUST fits, whereas a 235 should be safe. The lowest profile you can find helps and of course alignment and rim offset all effect this. Trial and error is the only way to really know just how big you can go.

[David J Marguglio]

The lowest profile you can find helps

...but will also change your effective spring rates.

[Kim Crosser]

Jad said:

an Azenis tire is wider than most other tires in the same 225 width

Huh? Other than minor manufacturing variations, the section width (225) is the width of the tire in mm and should be consistent when mounted on the same rims (it will increase about 0.2" for every increase of 0.5" in the rim width). If Azenis is consistently wider than others when mounted on the same rims, then are they intentionally mismarking the tires?

[Jad]

It is 225 from exactly where to where That is the problem, as you can measure your 225 from a lot of different places and different tires range from square corners to rounded. How much of the tread block do you count as it rounds the corner and becomes sidewall? Like you said, changing rim changes contact patch as well, though they are still 225's....

I wouldn't necessarily say they are intentionally mismarking the tires, but it was created as a Japanese AX tire and I bet it does help them, so they may decide the tread ends and the sidewall begins a little different than on other tires Go to tirerack.com and check out the rev per miles number and you will see, not all tires of the exact same size have the same revs per mile.

[David J Marguglio]

Damn Jad! Were you part of that Ferrari protest of Michelin last year too? Perhaps, like F1, we should measure them after the race to see what there width is worn. But seriously, there is variation among manufactures on sizes, though Rev/mile will indicate the rolling circumference not the width, but I digress. I did want to clarify that it is the compound and construction that makes Azenis better performing tires and not the slightly larger width. Case in point, the boxster drivers downsized from a 255 to the largest Azenis (245) and went much faster.

[Kim Crosser]

The Section Width measurement is defined by the Tire and Rim Association (TRA). They specify how to measure it and on what mounting (generally a rim with 85% of the width of the section width. These aren't arbitrary numbers and supposedly all tire manufacturers are abiding by their standards. The section width really isn't affected by how much of the tread is wrapped around the sidewalls.

Falken could conceivably get a little more tread contact (legally) by stiffening the sidewalls (a LOT), thus achieving a slightly wider footprint relative to the sidewall flex (since the section width is measured at the "widest" point on the sidewalls. (E.g., solid rubber slicks with vertical sidewalls would have the maximum theoretical ground contact patch for a given section width. Of course, you wouldn't want to be driving over a rough road with them. )

If they are consistently wider, that would seem to be cheating - sort of like artificially marking a tire with a higher wear number to avoid points.

Regarding the rev per miles numbers - these are driven more by the tire circumference, which is affected by a lot of factors. When you factor out the rim size on which the tire was mounted, the section width numbers are pretty consistent (usually within 0.1-0.2 inches).

Interesting point on the wheel rims - I don't recall seeing anything in the rules about points for changing the wheel rim width, yet an inch of increased wheel rim width would conceivably give you almost a half-inch of increased section width for the same tire - i.e., increasing a 7.5 inch rim to an 8.5 inch rim (same diameter) could make a 245 tire act more like a 255 tire.

[ttweed]

Interesting point on the wheel rims - I don't recall seeing anything in the rules about points for changing the wheel rim width

The only rule that applies to wheel width changes is the 2 points you take if your overall track width increases more than 2". This should allow you to increase width by at least 1" (or more, depending on the offset of the wheel that is used) without penalty.

TT

[kary]

This is an interesting thread. There is much information yet nothing much factual to support the statements.

There are many tires, such as the Dunlop Super Sport Race tires that are marked a size that is smaller than the acutal width. Do not be duped though, section width is not the important measurement in performance, it is the tread width. The construction and section width that supports the tread width will also providing added performance as was pointed out by David.

Tires vary greatly in section and tread width. The section widths are measured fairly consistently however the tire manufactures are not necessarily labeling the tires the same. Case in point is a 235 Dunlop which measures out to a 245 in section and tread widths. The Dunlop 245 tire is actual smaller in section width than the 235 but has a greater tread width than the 235. A 265 dunlop measures out to about a 282 in section/tread width of comparable tires labeled 285. (See the numbers below)

Knowing these facts will provide a bit of an advantage in classes where tire sizes push a car out of a class. Wider rubber that performs better without points is a great advantage. That is if you know your tread widths in relation to your section widths in relation to the marked sizes


235/40-18 SW 10.2" TW 8.7"
245/35-18 SW 9.9" TW 9.1"
265/35-18 SW 11.3" TW 9.9"
285/35-18 SW 11.6" TW 10.8"

[Jad]

Not sure how Falken does it, but Ralph measured his 205 Azenis and they were slightly wider than the 225 Kumho? (may be wrong on what tires he took off) and my 245 Azenis seemed the same as my 255 Victoracers, but I just measured contact patch, not the official method you described. The Azenis does have a very vertical sidewall, but I don't think it would make that much difference. We should try measuring some other sizes to confirm.

As far as rev per mile, it does really mater as this topic is about whether a given tire will fit the wheel well and a taller tire of the same dimension may not fit if in fact it has a greater circumference.

I was not trying to say this is the only reason the Azenis are good, just one reason. You can tell the tread pattern is very aggressive with large blocks that don't squirm (until it rains ).