heezy545i

I have some Michelin PS2s in the front and due to a pair of flat tires, now have Goodyear GS F1-D3s in the rear.

Prior to purchasing the Goodyears, I made a thread on this forum and did a search on other forums to see if mis-matching tires might pose a problem. From searching the boards, it was unanimous that unless you go to the track that it shouldn't post a problem. This was found to be true for the most part as I drove on them for a month and did about 2,000 miles without any problems. During this time I reached high speeds and took turns and circular on ramps aggressively without any problems.

Last saturday however, I went on a canyon run with some M5 guys and while taking some turns at speed, the DSC started to act up. During some long sweeping turns, I find the actual sensation difficult to describe, but the DSC would repeatedly turn on and off which made the car feel quite unstable. The thing is, when DSC started activating, I'm pretty sure the car wasn't losing traction, but rather looks to have been confused by the different tread patterns in the front and rear. I tried driving around with DTC on/DSC limited and the same thing occurred. It wasn't until I completely deactivated DSC that I could really drive and push the car.

On the drive home though, I took some circular on ramps quite aggressively with DSC on and things seemed to be fine, so not sure what to think about everything.

Even when DSC is working properly, I do think that it acts overzealous at times, but at the same time it is comforting to have that safety margin in the back of your mind. With that said, I'm surprised at how easy and stable this car was to drive with DSC completely off. I guess that with all the electronics that modern cars now have, you begin to use them as a crutch instead of really driving the car on your own.

I haven't driven this car in the canyons in some time, but in the past, I would drive with DSC limited and in this mode you can get a decent amount of wheel spin and even have the back end come around a bit before it activates.

Here are some pics of the drive I went on if anyone is interested http://www.m5board.com/vbulletin/showthrea...8005&page=8

dlevi67

Are you sure the tyres (or tires) were inflated properly and at the same pressure across each axle?

heezy545i

Yeah, I checked the night before. I actually don't remember off-hand, but I'm running something like 39psi around, or 39/40.

I realize that my original post might be too wordy.

So for cliff-notes: Different tires in the front and rear seems to have caused problems with DSC when driven very aggressively. For all other driving situations, things seem to be fine.

dlevi67

That may be part of the problem. BMW specifications say:

Normal load (up to 4 passengers)

Front: 29 -32 (depending on tire size)
Rear: 35 - 38 (depending on tire size)

Full load:

Front: 32 - 35
Rear: 42 - 45

So, if your front tires are inflated at 39, they will be too stiff. Also, for the suspension to work properly, a different pressure is required front/rear (and note that the difference increases with load, as front pressures only go up by 3, rear by 7 psi). Perhaps now that tires are different f/r pressure differences become more critical.

heezy545i

BMW recommendations are just that, recommendations. I've previously run 36/38, 37/38 with no ill affects. In fact, I've ran in the neighborhood of 39psi around when I had PS2s on all for corners and there were no ill affects. On other DSC equipped BMWs, I've even run greater PSI up front rather than in back to limit understeer some.

I feel that increased pressure to a limit will offer better turn-in and increased handling at the offset of some comfort. Also, as mentioned, less stagger in pressure between front and rear tires will decrease understeer a bit.

It was only until after the change in tires that I had any problems. Don't be mislead by the tire pressure as the differing tread patterns and level of grip of the tires is the culprit of the problems with DSC.

heezy545i

About the manual. There has been some discussion that the recommendations may be outdated as the door jam ratings of newer e60s show different recommendations than that of the manual or door jam's of older cars.

Although, it has also been noted that the ratings on the door jam may be just an average in a way, so as not to confuse the consumer. Even with that argument however, newer recommendations call for increased tire pressure in the rear tires.

dlevi67

Well, if the tyres have significantly different levels of grip, then DSC will have to intervene based on the lowest grip axle... Just wondering - would you consider swapping front/rear tyres and see what happens?

heezy545i

If I end up keeping the car, then I'd definitely switch out the front tires.

Michelin PS2s and goodyear eagle F1 GS-D3s are comparable tires, so I thought that things would be okay.

fraseman999

Since taking of the runflats i find my dsc comes on more. I think thats because the ride is softer. Posted earlier to ask abou tyre pressures. I am running fr 32 re 35. I am going to take it up to fr 36 re 38 to see if it stiffens slightly