[albertw]

I just rotated my Pirelli runflat tires after 16000 km (10000 miles), and found the rears are nearly down to the wearbars at the centre but much less worn on the sides. Soon after I got the car I raised the fronts from the recommended 35 to the same 39 as the rear to improve steering response. Those are worn evenly across the tread. (My driving is 5% urban, 80% highway and 15% aggressive twisty road.)

This matches my experience with my C5 Z06, which came with non-runflat tires and a recommended 30 psi all around. The first two sets of tires wore the centres of the rears and the outsides of the fronts much more quickly than the rest of the tire. I found I got even wear front and rear by raising the fronts to 34 and lowering the rears to 27. On that car I had in-tire pressure and temperature monitors and found that that the change equalized the temperatures, whereas the fronts ran about 70 degrees F hotter than the rears at the factory pressures. That convinced me that it was reasonable to ignore the factory. (The car seemed a bit closer to balanced at the limit in moderate speed corners with these pressures, compared to slight understeer at 30 f & r.)

I don't have temperature monitors on the 240, but based on my past experience I've decided to run 39 front and 35 rear. The car seems slightly more prone to oversteer with the lower rear pressures, but that's great for my driving style. When these Pirellis are worn out I plan to switch to non-runflats (probably AS3+ because I really liked the AS3 on the Vette) and continue the same non-standard pressure difference.

1. Any opinions on downsides of running these pressures?

2. While a sample of 2 does not prove a trend, it does suggest to me that manufacturers of performance cars recommend pressures that are not optimal for wear, and in my hands do not improve handling and reduce steering responsiveness. Does anyone have a theory about why they do this?

[harma24]

Originally Posted by albertw

I don't have temperature monitors on the 240

You can code that on.

Originally Posted by albertw

The car seems slightly more prone to oversteer with the lower rear pressures

There's an optimal pressure for any given tire size which would maximize the contact patch, thereby optimizing grip. Over-pressurize a tire and you'll have increased tread wear in the middle of the tread and reduced grip. Underinflate a tire and you'll have increased treadwear at the shoulders and reduced grip. That being said, I'm sure I could find an infinite combination of front-higher or rear-higher tire pressures which would induce understeer (the majority of which would not optimize handling, fuel economy or safety).

If you're looking to maximize handling and induce oversteer, I'd determine the front pressure to maximize the contact patch on the front and then set the rears to higher pressure which results in a smaller contact patch on the rear tire than the front tire. The reason I'd go higher as opposed to lower is a higher pressure should result in a stiffer sidewall; a softer sidewall should result in worse handling.

Originally Posted by albertw

manufacturers of performance cars recommend pressures that are not optimal for wear, and in my hands do not improve handling and reduce steering responsiveness. Does anyone have a theory about why they do this?

Given the push to increase fuel economy, I'm willing to bet that recommended tire pressures optimize fuel economy as opposed to handling.



But, I am not an automotive engineer or a mechanic; I only know what I like, so take this for what it's worth (my two cents).

[keylime503]

I run 39 all around on my M240i

[MarcoZandrini]

I run 35psi in the front tires and 37psi in the rear tires.

[Maynard]

Optimal depends on specifics, what goals and audience you are optimizing for. BMW recc's are fairly generic, and even with that, my manual has about 3 pages of variations. Factory tires are as wide as possible for the rim, so they are already disadvantaged by the poochy sidewall (but a softer ride).

Your rear wear pattern is generally indicative of overinflation, try dropping back a pound or two. Runflats are rather insensitive or compromised in general, so if you are sensitive to small pressure changes my first suggestion is swapping to nonRFT (ideally on wheels that are .5 wider). This will dwarf any improvements from fine tuning what you have, well worth it. Then tune pressure for feel, wear - and temps if you want to get real fancy (side to center, not the dash-display).

[aerobod]

I have found 2.4bar / 35PSI all around for both summer and winter non-RFT tyres I use on the M240i, gives me the optimal wear pattern for general road use.

[RoundelM3]

I was going to ask - what's the recommendation for non-runflats on our 2-series cars? I've got M-sport staggered 225 F and 245 R on 18" wheels. I've been using the recommended pressures for the runflats, but 40 psi in the rear seems awfully high.

[Udayan]

Hi,

I'm using 36 front and 39 rear.

[Winemaker2]

Originally Posted by RoundelM3

I was going to ask - what's the recommendation for non-runflats on our 2-series cars? I've got M-sport staggered 225 F and 245 R on 18" wheels. I've been using the recommended pressures for the runflats, but 40 psi in the rear seems awfully high.

Guys, we just covered this in a thread with Oldbay, but maybe the Convid-19 crap is getting to me and I imagined it. My Indy performance shop recommends 36 psi all the way around on the staggered set up. For two years I have been running this on Summer tires, and the car has performed great. I even used this set up on a track day at Sonoma Raceway and the car impressed the instructors.

[RoundelM3]

Originally Posted by Winemaker2

Guys, we just covered this in a thread with Oldbay, but maybe the Convid-19 crap is getting to me and I imagined it. My Indy performance shop recommends 36 psi all the way around on the staggered set up. For two years I have been running this on Summer tires, and the car has performed great. I even used this set up on a track day at Sonoma Raceway and the car impressed the instructors.

I missed that post somehow - thanks for the update.

[albertw]

Originally Posted by Winemaker2

Guys, we just covered this in a thread with Oldbay,

I have not looked at any of the specialty forums because when I joined there was very little activity on them.

That said, the thread cited is mainly about front tire pressures, and I'm already happy with what I've chosen (39). My thread was mainly about rear tire pressures. I wondered whether others had excessive wear down the centre of the tire at the factory pressure. Also, I was hoping someone would have information on tire temperatures since my car can't do that, or some other reason for not going as low as 35. Based on the number of people happy with 36 in the rears I might raise mine to 36 and see how treadwear works out.

I am curious about why manufacturers recommend pressures that seem so far from optimal, and there were some good responses that support my own theory.

I suppose if the moderators were more rigorous in moving threads like this to a more appropriate forum, more people would follow and look at those forums. This thread already has almost twice as many views as the related one on the tire forum, so I'm not going to apologize for posting it here.

[Winemaker2]

I wasn't trying to scold anyone for posting on this thread, it was more for reference to gain some insight from another thread. I believe at 36 PSI in rear tires you'll be pleased with a little more grip and still optimal tread wear.

[IndyRed128i]

At least on mine, I have found that about 2 -4 additional PSI in each tire about the recommended makes for a more desirable driving experience.