andrewhake

(215/45/17 BS S-04 vs. 225/45/17 BS RE-11 on 17"x7" AP2 wheel)

Another thread got me thinking about wheel width vs. tire width and the positive and negative characteristics of a stretched or squeezed tire (not to any extremes). I am currently running 225/45/17 F 255/40/17 R tires on OEM AP2 wheels that are 7" wide in the front and 8.5" wide in the rear so that is why I wanted to discuss this. I imagine this is a such a minor change I wouldn't see many negatives in terms of grip and only suffer a slight loss in response and feel. Although this is more of a tire width question given the same OEM wheel width, the main thing I am interested in learning more about is the effect of a wider wheel width given the same tire size.

In the linked thread I asked why someone might opt for a 10" wheel over a 9" wheel given the same 255/40/17 tire size since there would be an increase in wheel weight. I assumed being in the middle of the range for recommended wheel widths would be the best compromise in most situations, a balance of grip and feel. But someone had mentioned that the 10" wheel would actually allow for more overall grip as well as the benefits in response. It was my understanding that a tire with a slightly larger sidewall will give more grip under acceleration and braking as well as more lateral grip, but also will decrease response. I was told that the wider 10" wheel will actually allow for more grip from the same 255 tire since less sidewall flex should occur under load. That is a perfectly reasonable explanation for why feel and response would increase, but it doesn't explain why grip would actually increase. Does anyone have any thoughts as to why you might notice an increase in lateral grip with the wider wheel? Or is the feeling of increased grip just a side effect of the improved response?

I found some good discussion of this (as well as many other wheel/tire related topics) here: http://www.motoiq.com/magazine_artic...and-tires.aspx

Something mentioned there (in the comments section) was given less overall flex in the tire with the wider wheel, the dampers should be able to do their job better as well and respond more effectively since tire flex is less of a factor.

Still reading through some of this but I was interested to see what some people here thought about this. Thanks! (Sorry if this is an obvious topic, I am no engineer)

andrewhake

That said, I am sure while I am on the OEM alignment I won't be making optimal use of the slightly wider tire setup, and even on stock tire sizes could use the tires more effectively with a increase in camber, but it really got me wondering if I might see better overall performance with 215/45/17 F and 245/40/17 R while taking my car to the track in it's stock form (I have some goals I want hit at the track before modifying my car). I pretty carefully manage tire pressures but I do want to try to hit my goals on the standard alignment setup even if it is a compromise. So far the 225/255 setup has been really positive, no major downsides in terms of response and feedback while at the limit and a definite increase in corner exit grip and grip under braking.

It is very encouraging for me if you can effectively improve grip as well as response with an increased wheel width. I plan doing just when I start changing the suspension and before moving to a square setup.

Since I do a lot of driving on the street as well, the slight increase in sidewall size should slightly decrease my likelihood to damage or bend a wheel should I come across debris, or even on the track when putting a wheel off.

andrewhake

Bullwings provided a good explanation.

"Not pro or even semi-pro, but diminishing returns for even miatas has yet to be reached. Emilio of 949 racing makes the 6UL in 15x7, 15x8, 15x9, 15x10 - the data and laptimes support that even for a low hp/tq momentum car like the miata (even more so than our s2k), wider is better. If you can fit and clear them, the 15x10s will turn better laptimes.

As far as 17x9 versus 17x10 on 255s, there's a range of rim sizes that a given tire will fit on (which you're well aware of). 255s are better on 17x10s since the lit bit of extra tire stretch (we're not talking hellaflush stretching) maximizes and widens the contact patch a little bit. It also stiffens up the sidewall, which prevents the tire from flexing and rolling, thus maintaining the extra contact patch through the corners (as mentioned by carbonblue)."

This makes sense. I still wonder though, the same amount of tire should remain in contact with the ground even under flex or slight rolls over right? But less flex and roll will allow the most effective part of the tire to remain in contact? Or does the actual size of the contact patch change as the tire flexes and begins to roll over? To me it seems like the more load on the tire, the more the contact patch would increase. But maybe as the tire rolls over part of the contact patch can begin to lift?

//steve\\

For what it's worth on my first s2k I ran both 17x9 and 17x9.5 with a 255. Even the marginal extra width on the 9.5 was noticeable. Unfortunately there just aren't a lot of great 9.5 or 10" wheels that fit our cars with ease or aren't really expensive.

Antonov

From my experience everything gets better with wheel width, given the same tire/tire width..... Go to tirerack.com and look up whatever tire it is you're interested in, click on specs, and look under "rim width range". What ever you see as the upper end of that range, is the wheel width that will make best use of that tire. I've tested 8.5J, 9J, 9.5J, and 10J on a very well setup car, and I'm very good at noticing alignment/setup changes, and every time I went up in wheel width (given the same tire) I felt a pretty significant improvement. Its not too often that you can just bolt on grip, wheel width IS one of those ways.

Next time you're watching pro road racing, pay particular attention to the tires (when in the pits) and the way they sit on the wheel. You will see that they all have that slight stretch to them.

robrob

This^ It's not sidewall flexing that a wider wheel helps with, it's the reduced tire "roll." When you turn the steering wheel the tire turns then the wheel (and car mass) want to continue straight and the wheel shifts outward in relation to the tire. A narrower wheel allows more shift which makes it more likely the tire will break traction when the wheel hits it's max shift (or deflection.) A tire that is "stretched" over a wide wheel doesn't get the spike in load when the tire reaches max roll.

psychoazn

Deflection like this?

boyguan

Now my question is would you rather do a 255 or 245 on a 9 rim?

Many non staggered run the 255 but does the 245 provide faster times?

andrewhake

Thanks everyone for the great answers!

Hahahaha. I see you moved away from the CR suspension? Or maybe just looks that way with all the load on the left?

oinojo

interesting topic guys. Here is some video I took while testing some tires and theories. 235/40R17 Toyo R1R vs 255/40R17 Dunlop Z1 Star Spec
http://www.youtube.com/watch?v=wdaRJ...BkQMxupo17OWOA
http://www.youtube.com/watch?v=HxJwc...BkQMxupo17OWOA

Keep in mind that this was a test done in an autocross environment. We reach peak lateral G a lot quicker and more frequent than on the track. Transient response and the time it takes to get to total lateral load is very important.