[northX3]

Hello folks !

Just changed my summer tires for winter ones.

Unfortunately, I had to settle for non-run flats (with an emergency kit in the trunk...). That's fine with me, comfort is great and I don't drive that much during winter.

I just want to know if I should still pump the tires to 32 psi front, 35 psi rear ? They look (and feel) a bit "soft" and under-inflated... Maybe it's because of the tire's softer skeleton ? The tire footprint "looks" OK on the ground (eye-o-meter, about as good as my ass-o-meter for comfort...)

Do you guys push up more psi with non run-flats ?

[GenX3]

Should be the same pressure for non-RFT and RFT. Profile should be the same too... since you're never really riding on the sidewalls of a RFT unless you lose air.

Also, watch the tread wear... If the outside wears sooner than the middle of the tread, they might be under inflated. If the middle wears sooner, maybe a tad over inflated. Mine always seem to wear a dad over-inflated... I like 'em that way I suppose.

I'm running the 32/35 profile.

[xDrive35i]

The 32/35 rating is only there for the staggered summer tires. Equal width tires do not require a staggered PSI setup. For example, the all season 309s recommend 32/32 because they're equally sized all around. I'd recommend keeping all four tires the same for the winter (but if you must bias towards the rear, I recommend just 1PSI in variation).

[northX3]

Thank you

Maybe it's because I came from the 245/275 staggered summer setup with high-performance RFTs, but 32/35 with non RFTs does feel a bit too soft.

I'll try 35/38. Keep in mind, these are winter studded tires, so I do want maximum traction in difficult conditions (like Jeep off-roaders deflating tires before rocky passages

[paul386]

Originally Posted by Lotus7

For most equal front/back tires set up, BMW definately recommends keeping a 3 or more psi rear bias. The higher rear pressure is there for (2) reasons:

(1) Reduced slip angle at the rear which helps maintain a basic understeer condition at the front (which is the most stable cornering bias for most drivers under non-track conditions).

(2) Load capacity. Since additional cargo added to the car loading is almost entirely to the rear.

The 32/35 suggestion for a same-size set-up is a decent ride/handling compromise, but raising the pressures at both ends will result in better handling (slightly quicker turn-in and slightly higher ultimate cornering grip) and will still give predictable handling if you maintain the 3 psi pressure differential.

For normal street driving with the 245/50R18 tires, I find that 35 psi (F) and 38 psi (R) works best and with 7000 miles on the P7s the tread wear is very even and uniform across the tread width. Several people on the Forum who have been running at the factory recommended pressures of 32/35 have experienced slightly more edge wear, which means that pressure is toward the low-end of the acceptable range.

The issue of the optimum pressure for non-RFT construction vs,. RFTs is valid. The RFT super-stiff sidewalls are certainly supporting some of the load, and therefore reducing the load supported by air pressure alone. Going to non-RFTs should require an increase of air pressure to compensate for the lack of sidewall support. Try 35/38 and see if the handling and ride are to your liking. With non-RFTs, I'd certainly not go any lower, and a little higher might still give a decent ride on a 35i.

BMW Recommended pressures for the 35i (w/RFTs) under normal loads and less then 100 mph speeds:

I'm not sure I can agree with you. Certainly low tire pressure in a RFT will cause the very stiff sidewalls to support a larger portion of the vehicle's weight making the contact patch nearly non-existent. So low tire pressure is bad (on RFT).

However, high tire pressure will decrease the size of your contact patch and increase the average shearing stress. Based on the tire data I saw in my FSAE days (and everything I have ever learned about vehicle dynamics) lower average shear stress in a tire results in higher coefficient of friction (important at high speeds). Higher tire pressure should increase cornering stiffness so your car should feel more responsive and at low slip angles it should perform better. However, passenger car tires are never designed to operate at "low" slip angles so these conditions probably don't exist during spirited driving.

Front tire pressure also affects your steering effort... lower pressure should increase steering effort and higher tire pressure will certainly reduce it. Is this even noticeable on a passenger car? Doubtful. But I'm not a "driver". haha

I think most people are aware that higher tire pressure reduces rolling resistance but is also reduced induced cornering drag on a tire.

Tires are super complicated and not very well explained (the exact mechanisms of tire grip). I once attended a seminar about tires and the presenter claimed there are over 21,000 variables when designing a tire. RFT certainly add to the mix. I try to stick as close to the recommended tire pressure as possible... which by the way is interesting enough because my car is 32 / 32 and I have staggered setup Continentals 245 / 275's.

[renderfarmer]

I recently switched my wheels and tires out to staggered Non-RFTs and I noticed that the manual now explicitly identifies the specs being for RSC (Run-Flat) tires.

The specs have also changed to an even front/rear pressure for the staggered 19" setup, but is asymmetrical for the staggered 20" setup. What to you make of that?

[paul386]

Originally Posted by Lotus7

It's especially interesting that the recommended pressures for 19 inch staggered set up are the same F/R (32/32). while the 20 inch configuration still requires the additional pressure in the Rears (32/35).

The identification of "RSC" is also important as it clearly implies that these pressures may not be optimum for non-runflat tires.

Like all tire pressure recommendations, they are just a starting point and a wise driver will adjust according to load, expected speed, wear patterns, and personal ride vs. handling preferences.

For me, cornering balance and initial steering response are very important while ride comfort is much less important. Adjusting individual pressures at each axle is something I do for each car I own and whenever I change tire brands.

Our daily driver a few generations back was a Audi A4 2.8. It initially was delivered with soft Dunlop summer tires (forgot the model, but they were an extra-cost performance option), and handled quite well. When those wore thin I replaced them with Pirelli PZeros, which were a slight improvement over the Dunlops (which got better and better as they wore), but required less pressure at the front and more at the rear. The final set of tires for the A4 were a set of German made Goodyear F1 GSD3's that were a major improvement over the PZeros, but again required different pressures (more at both ends). On the Audi and on our X3 (and every other car I've driven in the last 20 years), the optimum pressure for my driving taste has not been the pressure on the door jam sticker. It's never lower, usually it winds up being as much as 3 to 6 psi higher, and is typically different on the front and rear axles.

On the other hand, there are plenty of people driving X3s, X5's 1, 3, 5, 6, and 7 Series tonight with tires that are at least 4 or 5 psi lower than the factory recommended pressure and with side to side differences that would drive me crazy. They are totally oblivious to how sloppy their car is handling, and would probably not notice any difference if the ghost of Mark Donohue set their tires to withing a tenth of a psi of perfect. Different strokes for different folks.

On my current (2) race cars, the optimum pressure is adjusted according to the track surface conditions and temperature as well as the age (number of heat cycles) of the tires. The ONLY way to get it right is to drive the car and find out what works as measured by a data logger on a particular track on a given day.

The ONLY way to get it right is to drive the car and find out what works as measured by a data logger on a particular track on a given day with a given driver.