Changing handling balance with wider rear tires

[dparm]

As I had previously written, my Michelin PS2s are at the wear bars. A popular "upgrade" is for guys to run the M3 GTS tires which are wider in the front and the rear. However, the front-to-rear width difference is even greater on the GTS and I'm wondering how that affects handling balance:

Stock:
245/35R19 & 265/35R19

M3 GTS:
255/35R19 & 285/30R19

As you can see, you have a 20mm difference versus 30mm difference front-to-rear. The other alternative is to run 255/275, which maintains the factory 20mm difference. Others have really pushed it by going to 275/295 but with spacers to avoid rubbing (yet still maintaining the 20mm difference).

For those wondering, the GTS has the exact same wheels as my car so there's no fitment issues. I don't intend to modify the car's suspension, nor do I want to run spacers or swap wheels. I just want a little extra meat.

 

[d00df00d]

What's important is not the absolute difference, but the front/rear ratio. Generally speaking, if you want neutral handling, you want that ratio to be in the neighborhood of the car's weight distribution -- although obviously suspension settings and tire pressure are important factors as well. In general, skewing that ratio toward the front will increase oversteer, and skewing it toward the rear will increase understeer. FWIW, my car's weight distro is exactly 50/50, I run the same tire size all around (245 in summer, 225 in winter), and my car still understeers for the most part; it just understeers less than it did with wider rears.

The M3 GTS's tire width ratio is skewed more rearward than the regular M3's, presumably to cope with the extra power and torque. I'd expect more understeer/less oversteer if you went to those sizes.

Whatever you do, keep in mind that wider tires on the front axle reduce steering feel, and wider tires on any axle reduce hydroplaning resistance and increase unsprung and rotating mass. Also, greater width doesn't necessarily translate to increased grip, as widening your contact patch also shortens it front-to-back unless you also drop your tire pressures. IMO, for a street car, it's best to treat tire width like oil viscosity: as little as possible, as much as necessary.

Tire type is MUCH more important. Whatever tire size you choose, switch to Michelin Pilot Super Sports and thank me later.

 

[dparm]

Yes, I'm already going for the PSS. I'm also aware of the reduced wet traction from a wider tire. The GTS only has 30 extra horse power (arguably the 240E integration leveled that playing field) but weighs 300 lbs less. It runs the same wheels as my car. My car's weight balance is 52.2/47.8%, if it matters.

I've just wondered why BMW uses 1/2" wider wheels all around on the Competition Package but leaves the tires exactly the same. Gotta admit, 245/35R19 and 265/35R19 aren't really that wide for 19x9 & 19x10 wheels, respectively.

255/35R19 and 275/35R19 will make the sidewalls slightly taller, yes. It will probably reduce steering feel a bit, but the PSS is probably a step up from the PS2 in that regard. 285/30R19 out back is actually shorter than stock.

 

[d00df00d]

Originally Posted By: dparm
I've just wondered why BMW uses 1/2" wider wheels all around on the Competition Package but leaves the tires exactly the same. Gotta admit, 245/35R19 and 265/35R19 aren't really that wide for 19x9 & 19x10 wheels, respectively.

Indeed.

Could it be to make the sidewalls more vertical?

 

[dparm]

If anything, I think it makes them look awkward. While I don't doubt BMW's judgment, I wonder if the same tires were used to keep costs down and simplify things. (no need for speedometer changes, parts logistics, etc)

 

[d00df00d]

I was thinking more vertical sidewalls might help the handling.

You might be right about the cost/simplicity thing.

I generally trust BMW, too -- although, given what they've been doing since MY2012, that might change...

 

[Quattro Pete]

Originally Posted By: d00df00d
I was thinking more vertical sidewalls might help the handling.

It should. The more the tire is stretched out on a rim, the less the sidewall flexes and therefore the quicker the steering response, although technically the rear wheels don't steer...

 

[dparm]

I understand the physics of what you're saying, but I thought sidewall flex was more a function of the tire's construction and the load rating?

 

[Quattro Pete]

Construction plays a part, but at the end of the day, it's still rubber. Granted, with very low profile tires like in your case, there isn't a whole lot of rubber there to flex so the effect is less pronounced...

http://www.slidewaysonline.com/?q=node/35

 

[dparm]

Originally Posted By: Quattro Pete
Construction plays a part, but at the end of the day, it's still rubber.


Still makes you wonder why the GTS, a more track-focused version, would actually run the bigger rubber. Perhaps they handle heat better? Maybe the rear wing provides more downforce and the wider tires assist with utilizing that?

But the Competition Package is of course aimed at...well, competition. So wouldn't better rubber (wider or stickier) seem to fall in line with that too?

 

[d00df00d]

Originally Posted By: Quattro Pete
Originally Posted By: d00df00d
I was thinking more vertical sidewalls might help the handling.

It should. The more the tire is stretched out on a rim, the less the sidewall flexes and therefore the quicker the steering response, although technically the rear wheels don't steer...


Quicker and more consistent response always helps, no matter what axle it's in.

 

[CATERHAM]

Originally Posted By: Quattro Pete
Originally Posted By: d00df00d
I was thinking more vertical sidewalls might help the handling.

It should. The more the tire is stretched out on a rim, the less the sidewall flexes and therefore the quicker the steering response, although technically the rear wheels don't steer...

Exactly, and you get more grip.

dparm, if the tires are down to the wear bars now they are ideal for dry track/summer use with zero tread squirm. I run my tires down to the cords on my non winter use cars.
Since you plan on driving the car in the winter now's the time to have fun to use up what's left of the rubber you have.

In choosing new tires, make note of your speedo accuracy now with a GPS. I choose the final diameter of the tires I choose (on the rear) to optimize speedo/odo' accuracy.
Most cars even on new OEM tires with full tread (maximum dia') overstate the speed as much as 10% which I hate.

 

[dparm]

Perhaps I will try to stretch it out a bit longer. It's not like I'm autocrossing every weekend anyways. Hopefully Discount Tire will have a good promo going for Labor Day on the PSS.

I've got one final autocross on 9/8 so maybe that will be their final day.