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And you can get pretty abnormal wear. I ran a set of Dunlop FM901's, 225's on a 7" wheel and had to keep the air pressures up to avoid chewing up the shoulder due to the weak sidewalls during agressive driving. The Star Specs don't have that problem though (sidewalls are ridiculous on them thoguh)

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2012 MX-5 Sport SUV

Let me take a stab at this. R-compound sidewalls are far stiffer than any true street tire sidewall. The general rule of thumb is to match the actual section width to your wheel width for best performance. There is a major point of diminishing returns with street tires that you don't necessarily get with r-compounds. Ryan even commented on how much better his car worked when he switched from 8 inch wide wheels to 9 inch wide wheels using the same size tires.

I have run both large tires on small wheels and a tire that is perfectly fit to the wheel (ie. sidewalls are very square to the tread face) You lose a lot of steering feel and precision when squeezing a large tire on a small wheel. I am running the "small" tire setup on my S2000 because the car is extremely precise by nature and I found squeezing bigger tires on it just muddied up the steering feel and my ability to place the car as precisely. My example is using Hoosier A6's though.

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2010 Honda Fit Sport
Couple of bicycles

So would you not advise going any larger than 225mm (street tire) on a 7" wheel (assume a higher end street tire with stiffer sidewalls...say Star Specs or Ecsta XS)?

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2011/2012 Autox VP
2013/2014.5 President
2013 Top Gun

2015 Fit

22R-EC => 4G63 => D16Y7 + D16Y8 => EJ255 + K24Z2 => K20Z3 + K24Z2 => K24Z2 + M54 => L15B

I don't have any absolute answer to your question other than to point out that you can't just look at section width when asking that question. For example, something like a 225/70-15 versus a 225/35-19 would be profoundly different situations with respect to wheel width considerations. I realize I'm telling you something you already know -- just wanted to highlight the aspect ratio part of section width versus wheel width equation.

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Current stable:
2019 BMW M2 Competition slicktop 6MT
2011 BMW M3 sedan slicktop 6MT
2007 BMW 328i wagon (slushbox for now)
1975 CanAm 125MX2

Oops, my bad. Stock tire on the WRX is 225/47/17. Steve and/I are just curious what the pros/cons of putting say a 235/45 or 245/40 on our stock 17x7 would be assuming more performance oriented street tires (star specs for example).

I might be wanting to go to STU in the next year or so so I might just get some 17x8.5 or 17x9 anyway, but in the mean time, I'd like to get a bit more meat on my stock wheels if there are little to no drawbacks.

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2011/2012 Autox VP
2013/2014.5 President
2013 Top Gun

2015 Fit

22R-EC => 4G63 => D16Y7 + D16Y8 => EJ255 + K24Z2 => K20Z3 + K24Z2 => K24Z2 + M54 => L15B

I would wager that there is no clear cut answer to whether going from say a 225/45 to a 245/40 is "better" on a 7" wheel. What would work better on one car/suspension setup might be worse on a different one.

Two main things are at work on wheel widths/tire sizes: (1) tire sidewall spring rate, and (2) tire contact patch nature under load, dynamic and quasi-steady state.

Increasing section width for a fixed wheel width generally reduces sidewall spring rate hence the comments about such feeling gooey or less precise, etc. Reducing sidewall spring rate can influence the car's balance during transient conditions, slalom, turn-in, etc, typically creating more rotation as the tail is more sensitive, and it also makes the car less responsive to rapid inputs. Invert this behavior for wide wheels. It feels great to drive the same car/tires when you bump up wheel width -- responsive, better feel for the contact patch, etc.

The 2nd part, the contact patch nature, means what is the character of the tire to road contact patch during all conditions, from violent transient inputs all the way to almost steady state conditions like the middle of a long sweeper. This part is likely very important for maximum grip, and I have no clear answer since I think it would heavily depend on all the variables (specific tire carcass/sidewall design, air pressure, suspension design (i.e. great camber under load vs poor camber under load), etc, etc).

Hence my thought that on a given car/suspension setup, the 225/7" combo might time out quicker/slower than the 245/7" combo. I've always wanted to see carefully instrumented tests like this (and I mean wanted to for many years, as in 30+) where you would take the same car/tire and use the minimum wheel width and the maximum wheel width. Evaluate slalom, skidpad and road course performance (double blind tests). Do this for two significantly different tire designs (i.e. a Hoosier A6 might yield significantly different results than a Kumho V710 for example) and two completely different cars (perhaps stock suspension FWD versus a nicely setup RWD).

I've just blathered this all of the top of my head, so I know I've left a lot out -- this is (can be) a very complicated analysis/test (or at the very least I'm capable of making it very complicated, ).

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Current stable:
2019 BMW M2 Competition slicktop 6MT
2011 BMW M3 sedan slicktop 6MT
2007 BMW 328i wagon (slushbox for now)
1975 CanAm 125MX2

All I can say is that for stock class racing on r-comps, we ALWAYS found that more tire was faster. ALWAYS. It might not feel "right" or whatever, and sometimes it would just feel slower. But more meat always translated to faster times on the clock. *shrug* (Diameters and testing parameters being relatively equal, of course.)


--Donnie

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My Blog

Thought I post pics of my Avons. I actually did not think there was any cords in these. I thought that they must be rubber all through considering that they are 3 years old or so and have 3 years of AX and a bunch of track days on them. They are so hard that I think they are probably worse than old street tires. But I was wrong, there is cord in them.
Kevin and I had fun doing this one and Kevin managed to cord one of his V700s in 5 spots


[/img]http://www.shutterfly.com/lightbox/view.sfly?fid=db7b8f4c8a27a4e80031ecd658bfd733

A long time. If you follow the directions on our website http://thscc.com/general/directions/laurinburg.html and take Hill Creek Rd into Wagram, you will turn left on McKay St, follow that 3 blocks to US 401, then go STRAIGHT (past the gas station on your left) onto Riverton Road (and eventually turn left onto Airbase Rd, then left again when Airbase Rd. tees into.. Airbase Rd.

Maybe we need to back to bolting up orange plywood cones to road signs at the key intersections.

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Art McDonald
Premier Amateur #518
2008 Dishman Cup
Pivot Cone Snob

Rodney is a waxer (but in a good way)

My experience is the same for sure. My post up above was more geared toward street tires James was asking about which likely have softer sidewalls and less carcass stiffness, etc. All supposition on my part since I don't have any specific experience with cramming street tires onto narrow wheels, and I'm not confident in making guesses as to how such would perform on the street as I think there are too many variables depending strongly on the street tire in question (i.e. what might unseat a bead on one street tire doesn't affect a different one).

Without the benefit of testing, I guess I'd opt for the largest street tire I can fit in the fenders if I was just using it for autox only and not worried about street use. The assumption here being that an ultra performance street tire has enough carcass/contact patch stiffness that you gain from the increased width on same wheel similar to R-comps. I really don't know how good that assumption is.

I'd still like to see that double blind testing data using wheel width as the only variable.

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Current stable:
2019 BMW M2 Competition slicktop 6MT
2011 BMW M3 sedan slicktop 6MT
2007 BMW 328i wagon (slushbox for now)
1975 CanAm 125MX2

I'm not aware of anybody that's gone consistently faster by running oversized *street* tires on (relatively) narrow rims. In fact, most of the serious guys that have tried it wound up going slightly slower. This includes the current crop of top ST* tires.

James, I wouldn't bother trying to stuff too much tire on your car without first increasing the rim size to match. It's possible you might see a very slight gain on a gigantic sweeper but you'll wind up giving up so much transitionally that it isn't worth it for auto-x'ing.

R-comps are obviously an entirely different ball of wax, er, rubber...

Jim

Andy Hollis told me over on RRAX.com that in his testing you get the best results with the tread width pretty close to the wheel width. I believe him, my car felt the best it ever has on RE010R's last weekend on the 9" wheels (9.1" tread width) vs the 8" wheels I was on last year.

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02 Focus SVT
STF 9