Question about wider tires on roadstars? Ignoring manufacturer's recommendation?

[calimanusa]

I'm thinking about going from my stock setup on my 1999 M Coupe


225/45-17 front
245/40-17 rear


to r-comps


235/40-17 front
255/40-17 front


To maintain the balance, I don't really want to go wider on the rear if I can't go wider on the front. My main concern is that the approved rim on this tire size is min 8 inches. I've heard some tires fit ok, but r-comps usually run wider and might not fit. Any thoughts on going against the manufacturers recommendation? The price difference between 225/45 and 235/40 on tirebuyer.com is negligible.


Question: does anyone have experience with this tire size 235/40-17 on a 7.5" wide rim? The tire width is 9.49" according to Nitto's website.

Thanks!!

[fmcfad01]

Not sure about your specific tire, but many people go one size up to the 235/255's you're asking about. I've personally run that set up in the past on PS2's and Falkens.

[BMWBergy]

I ran 235/45 F and 255/40 rear (Dunlop Z1 StarSpec) on my Roadstars to keep the diameter delta between front and rear to a minimum, lest the ASC or ABS get a little thrown off. These were used on my car (lowered on TCK coilovers) with no rubbing at any point.

Fitment 1:
(F) 235/45 = 25.4" diameter
(R) 255/40 = 25"
Delta1 = .4", biased toward front

Fitment 2:
(F) 235/40 = 24.4"
(R) 255/40 = 25"
Delta2 = .6", biased toward rear

The delta is .4" for F1, and .6" for F2. Not a huge delta in the deltas, if you follow, but closer to the stock delta, which was .3", with a slightly larger front tire:

OEM:
(F) 225/45 = 25"
(R) 245/40 = 24.7"
DeltaO = .3", biased toward front.

[BenFenner]

I run 225s on a 7" wide wheel. I wouldn't hesitate to run 235s on a 7.5" wheel.

[Donlundm3]

Uniroyal 235/45-17 that I had rubbed on the inside on both left and right when turning at maximum.

[Z3speed4me]

I had 275's on my rears.....

[BMWBergy]

Uniroyal 235/45-17 that I had rubbed on the inside on both left and right when turning at maximum.

Hmm. I never had rubbing with my 235/45 17 Dunlops - this was with TC Kline D/A coilovers running a mild-to-moderate drop and minimal negative camber.

- - - Updated - - -

I had 275's on my rears.....

Ken, one member on here even runs 295s on his Roadstars...

[BenFenner]

Here's the thread everyone's talking about: What is the widest tires you can fit on stock M Coupe wheels?

[jamesembry]

I'm curious about changing the balance. Wouldn't going to an extremely wide tire on the back make the care push even more?

[BenFenner]

Yes it would, but is anyone talking about going wider in the rear and not appropriately wider up front in this thread?

[jamesembry]

Yes it would, but is anyone talking about going wider in the rear and not appropriately wider up front in this thread?

No, but what are the correct proportions....ie. 2 sizes in the back = x sizes in the front etc?

[BenFenner]

If you want to keep the same behavior as stock, you'll want to keep the front/rear width ratio the same as stock. Stock is 225/245 or a ratio of 0.918:1.0 for front:rear. Rear:front ratio is 1.089:1.0.

Take your desired/planned rear width and multiply by 0.918 to find out what you should run in the front.

Take your desired/planned front and multiply by 1.089 to find out what you should run in the front.


As an example, I plan on running 255/295 front/rear as soon as my wheel vendor decides to stop screwing around and send me some metal circles.
For a 295 rear, the front I'd want to keep stock behavior is a 270mm wide tire.
For a 255 front, the rear I'd want to keep stock behavior is a 275mm wide tire.

It looks like I should plan on having some extra understeer with my proposed setup.



What ends up happening with the math is that you want to keep a 20-25mm difference between the two axles to maintain stock behavior unless you go bigger than 330 out back.



This is obviously a bit simplistic, and doesn't take into account non-stock alignment settings, non-stock torsion bar solutions, sidewall heights and strengths, mismatched tire compounds, etc. etc. etc