Sponsored by The Tire Rack

[quiett]

I recently bought a 2008 135 convertible. Shortly before I bought it the run flats were replaced with high performance summer tires. When inflated to the 39psi that BMW specifies for the run flats the car rides like the tires are made of concrete. I have tried reducing the pressure two psi at a time, and at 30psi the ride is acceptable. Has anyone else come up with a better idea?

[MightyMouseTech]

What tires are on there? If you think it is bad now, it was 10X worse with the runflats.

[quiett]

The current tires are Hankook V12's. Also, the car has the sport package.

[rking117]

32 PSI in a normal style tire is acceptable. You can always goto the manufacturers website and determine the correct tire pressure for the style tire that you have. If the manu doesnt display it you can also ask a tire specialist.

Run Flats are meant to be harder and less flexible. The reason for this is so that the tire does not prematurely wear out around the rim lip or by flexing the plys of the tire to an extreme. A tire that has been over stress will likely have threads and beads of rbber inside the rim which s visible when dismounted (only)

That is why when you do have a flat tire on a run flat you are supposed to replace them after you drive up to 50 mles (@<50 miles per hour). If you have ever had a flat run flat you will certainly notice. The vehcle gets very squirrly compared to the fully inflated tires.

So with a regular style tire high performance or not 32 PSI will be fine. The V12's you have will wear just fine at 32 PSI. As a matter of fact at higher PSI the center of the tire will wear faster and give you what i call tire fade early on in the life of the tire. This gives you a very loose feeling in your butt under high cornering situations.

Unless you are trackng the car which is of course at your own risk I would not drive it with less that 30 PSI.

[quiett]

In lowering the pressure I have run afoul of the tire pressure monitor. Is it possible to reset the target pressure, or am I stuck with the factory 39psi?

[bmw1racer]

Originally Posted by quiett

In lowering the pressure I have run afoul of the tire pressure monitor. Is it possible to reset the target pressure, or am I stuck with the factory 39psi?

The TPMS monitors drastic changes in tire pressure, not the actual pressure itself.

You should fill/bleed your (cold) tires to the desired pressure, then reset the TPMS per the owner's guide instructions and you're good to go.

[gregthegr8]

Originally Posted by quiett

In lowering the pressure I have run afoul of the tire pressure monitor. Is it possible to reset the target pressure, or am I stuck with the factory 39psi?

The target will simply be wherever they are at when you last reset it. People have referred to it as a "stupid system." It only knows whatever spot you teach it and will merely alert you when it gets x amount below that or a certain percentage, which is escaping me. So, set 'em where you want, reset the TPMS system so it learns those numbers and know that apparently you have to do the reset while the car is on and you seemingly have to drive for a minute for it to learn, and then after a minute or two, the TPMS light will go off. So, know in advance that it doesn't go off immediately, which can be disconcerting, or at least was to me the first time I did it.

[horsepower_and_hounds]

I thought the recommended pressures were 32F 35R for either RF or conventional

[MightyMouseTech]

Originally Posted by ehagerty

I thought the recommended pressures were 32F 35R for either RF or conventional

On the 135, it is 36# front, 39# rear.

[VMRWheels]

Kind of surprised that on a set of Hankook Ventus V12s you found them to be too stiff. Those tires are known for being pretty soft too.

I wouldn't go much lower than 30PSI. Personally, I would just stick to the OEM pressure recommendations, because not running the correct pressure could affect your tire wear/life and your mileage, but that's up to you.

[wjk_glynn]

Originally Posted by MightyMouseTech

On the 135, it is 36# front, 39# rear.

Just a FYI that the door jam sticker and owner's manual of my '08 135i both say 36# front & rear.

Karl.

[horsepower_and_hounds]

Originally Posted by VMRWheels

Kind of surprised that on a set of Hankook Ventus V12s you found them to be too stiff. Those tires are known for being pretty soft too.

I wouldn't go much lower than 30PSI. Personally, I would just stick to the OEM pressure recommendations, because not running the correct pressure could affect your tire wear/life and your mileage, but that's up to you.

I just got the Hankooks and they are way softer than my runflats....

The 128 door jam actually quotes 2 pressures, one for speeds under 100mph which I do most of my driving and the other above 100

[wjk_glynn]

Originally Posted by ehagerty

...The 128 door jam actually quotes 2 pressures, one for speeds under 100mph which I do most of my driving and the other above 100

Ahh... I double checked and it's the same with the 135i in the owners manual (but not the door jam).

The higher tire pressures kick in if you're at 4x passangers & luggage above 100 mph...

Karl.

[gregthegr8]

Originally Posted by wjk_glynn

Ahh... I double checked and it's the same with the 135i in the owners manual (but not the door jam).

The higher tire pressures kick in if you're at 4x passangers & luggage above 100 mph...

Karl.

Weird, my 2013 says 36 front, 39 rear like others. I wonder at what point and why they changed the numbers. The convertibles are also higher at like 39 and 42 if I remember correctly.

[VMRWheels]

Originally Posted by ehagerty

I just got the Hankooks and they are way softer than my runflats....

The 128 door jam actually quotes 2 pressures, one for speeds under 100mph which I do most of my driving and the other above 100

They are known for being significantly softer than RFTs. Pretty much any tire that's not an RFT will be softer than an RFT.