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      02-13-2013, 03:14 PM   #17
DieselRocket
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United Kingdom
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Drives: 435d M Sport
Join Date: Sep 2012
Location: Bristol

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Hi Rich,

Sorry for the delay - had it in my head that the car was going in Tue, but it was today.

Not a good outcome, I'm afraid. I went up to the dealer and took one of the technicians out for a drive first thing this morning. He agreed completely about the noise - in fact, he heard it before we even got off the dealer premises when we went over a couple of quite abrupt speed bumps. We got onto some backgrounds and the guy could clearly hear that all four dampers were involved in the percussion party (nearside rear being by far the most easily provoked). I left the car with them, as agreed, and the head technician then spent some time with it, including back-to-back road test comparison with another car with adaptive.

Guess what? The head techie decided that my car was perfect, with no unusual noises. Needless to say, I wasn't delighted with the outcome, but I was in and out of meetings when the dealer rang to give me the news this afternoon. I've agreed on taking it back in a week or so for them to keep for a few days, but I think the fundamental problem is that some of these guys just don't have the driving experience and, perhaps, that they don't give the customer enough credibility (hence hurling the car round bends and running engine diagnostics, in spite of a very clear written brief about low speeds and straight lines).

To say the sound is obvious to anyone who is tuned into their car would be an understatement (my not-into-cars mate spotted it in seconds). Rear nearside sounds like a well worn ball joint on an old car, while the other three wheels sound like mildly worn ball joints. The noise is most apparent in a straight line at town speeds. I hear that the head techie put the car through lots of 'changes of direction' and, bizarrely, did an engine test well.

Nosy suspension isn't the ownership experience I expect at £42k, so I'll talk to my sales guy tomorrow and see what he thinks. I'll see if I can take him out for a spin this weekend and go from there.

This is a techie comment, but I've played with suspension on a few cars and bikes, and I think this is a case of the dampers being significantly under-damped on the rebound stroke (no problem on compression), resulting in them over-extending and hitting the end-stops at times.

I've never been called Sherlock, but there are several pointers toward this being true:

1. Driving over a speed pump at 5-10mph: no sound as the wheel hits and starts to climb the leading edge, but a distinct 'clonk' as it drops off the steep trailing edge (at which point the wheel is briefly airborne).

2. Driving in town at 20-30mph: if a wheel drops into quite a deep manhole depression at 20-30 mph, I get a clearly audible clonk. One for each wheel that travels over it. No rebound going on here of course: just a damper extending under drive of the spring until the tyre finds tera firma.

3. Cornering progressively at speed: Clonks are less frequent in this situation, but I get them on the inside wheels (i.e. the lightly loaded side of the car) if the inside wheels run over small potholes. In this situation, I get front and offside corners giving clonks (where it's generally limited to NS rear in town). So in my book, this is a case of lightly or unloaded inside dampers (i.e. poised somewhere around the middle of their travel range) which suddenly find a few centimetres of air under the tyre, provoking the over-extension again (driven simple by the springs wanting to extend fully, as there is little or no rebound going on from mid-stroke).

Another thing that I think is significant is that switching between comfort and sport has no impact on how much provocation is needed to cause clonking. This points even more strongly towards a damper fault, as I'd expect at least some rise in rebound damping rate in Sport, which ought to slow the speed of over-extension or stop it happening completely.

BTW my car has done this since mile one and the behaviour hasn't changed significantly since then, with nearly 2,700 miles on the clock now.

Anyway, this is just a theory and I could well be talking bollocks, being on my second beer. But ... does anybody fancy a wager on BMW UK acknowledging a problem with some batches of active dampers in a few weeks' time? Whether I succeed or fail in getting traction with my dealer, my wager is on the table :-) It will take quite a few tuned-in and persistent owners to get there, but it has to happen in time.

Rich, I'd be really interested to hear how you get on tomorrow. Not sure if you're experience mirrors mine but, if it does, tell them to drive the car at town speeds in a straight line and find a few bumps, potholes and manhole covers.

Good luck,

Dave
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