X-Drive causes brake disk to explode with OE biscuit wheel
 

I was driving my X5 E70 40d back from vacation on the Motorway, and got a Tyre pressure warning on my iDrive. I pulled into the emergency shoulder and checked my tyres I could see no damage so reset my tyre monitor.

A short while later I heard a noise from my rear left tyre only to discover I had warn through the run-flat and got to the steel belt. At this point I decided to change to the OE spare wheel.

5km later at about 60km/hr, I heard an explosion, which made me immediately pull over (still on the Motorway). I discovered that the rear left disc brake was on fire and had completely exploded bit of molten disc had fallen into the grass beside the road and started small fires.

The car was collected by BMW on-call, and they have assessed the situation, they are saying that the damage was due to the stability control system (DSC) due to diameter differences of the spare wheel.

Wondering if anyone else has had a similar experience

Generally the same crowd here- albeit less active- as over on BF...

;)

So when you put the spare on are you suppose to reset something so the DSC is suppose to know?

I saw your post on the Facebook group. I still don't see how this could possible happen due to DSC.

1) Your spare was the correct diameter to prevent issues

2) If there was a mis-match that confused the DSC, it would simply shut itself off and give you a warning.

3) You must have been driving at high speed for a LONG time with other symptoms before the rotor simply exploded. You would feel the car pulling, you would feel firmness in the brake pedal, and if you slowed down in traffic you would smell the awful smell of burning brakes.

1. The spare is the spare supplied with the vehicle, it's width is thinner, but the rolling diameter is the same. The tire pressure was probably a bit lower then ideal, but it did not look completely flat. Again, we had no way of checking the tire, it was a point of there journey where garages are 100km apart.

2. I'm not sure, I would expect the DSC light to come on constantly and been flashing like mad, but there was no warnings I saw on the vehicle. BMW diagnostic said a single fault was logged, but for some reason I did not see that on the iDrive or centre cluster.

3. I was driving very slowly, the day in question was rainy and over cast, there was a moderate amount of traffic but it was flowing nicely. My speed once putting the biscuit was on was only 60km/hr as I did want to check it's pressure

Just because a fault was logged, does NOT mean there was a dash warning. Only specific codes will trigger a user warning- the BMW software determines this, so it depends on which code 'BMW diagnostic' claims to have found. Will they tell you? Will they tell you 'yes or no' that "this fault will always set a user warning". Might be important to pin down, perhaps. But dont assume you missed it. There are hundreds of codes, perhaps thousands, they dont set a user dash warning...They seem to be saying you damaged it by driving after the fault...if it isnt a fault that sets a user warning, how would you ever know?

Oh, that car has an event data recorder- FAR more data than just codes logged. Surely BMW has that data, and will not share it with you... But you can hire someone to download it and read it. Not sure what they might charge. Ive googled a few folks in the US that do it, guess there are some in SA....

Holy Molten Caliper Batman! Those pics are might impressive.
So they are saying supposedly the DSC thought that wheel was slipping cuz it may have been rotating slightly faster (underinflated?), so it applied the brake to that rotor to equalize it... Over a distance this overheated the rotor and it 'exploded'.

Holy crap.

There was something else going on to cause this. Even though the biscuit spare isn't the same width and possibly low pressure, that should never have activated DSC to keep that caliper engaged enough to cause this to happen.

The standard for the system with different diameter tires/wheels is +/- 3% diameter between the set. Even with low pressure in the tire (unless completely flat), it wouldn't be in excess of that variant.

Yes, a fault in the DSC (not because of the tires) could have initiated a braking on that one caliper or simply the caliper was not releasing properly. Either way, its needing to be found and resolved.

OP is probably just a two foot driver :p