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BMW diff backlash

I appreciate that there has been significant discussion regarding the BMW LSD but I’m afraid I can’t find a suitable answer so here we go again. Ignoring the horrendous whines, clunks and embarrassing kangaroo characteristics, (they all do that sir!) my 2012 differential having only travelled 9000 miles in a 140 Supersport, seems to have significantly more backlash than when it fell into existence. Before I take it apart to investigate what’s going on, I’d appreciate any wise words from the world of Blatchat.

I have measured the free rotation of the input flange against the handbrake and have approximately 23 degrees (20mm around the flange circumference). I know there is a small amount of free play in the driveshafts but this is minimal. 23 degrees in the diff seems colossal.

I have also tried to assess the free play by jacking up one wheel and rotating it against the gearbox in 4th gear (straight through). I marked the movement relative to “CR500” on the tyre. I would attach a photo but ain't clever enough. It goes from the start of the 'C' to nearly halfway through the first '0'.

My first question is whether anyone knows the specified backlash for this diff. Secondly, I’d really appreciate it if you also have this diff and can measure what backlash you have. I measured a friend’s and his circumferential measurement on the diff flange was 16mm which means mine is 25% greater. It would be good to get a number of measurements for comparison.

Thanks for your help.

Good question to which I have no direct answer but have a quick Google and it’ll throw up some results including some links to YouTube vids most if not all seem to be for LSD’s not open diffs but might provide some pointers. Perhaps the best thing to do would be visit your local BMW dealers but go armed with part numbers etc and be prepared for some belittlement when you tell them it’s fitted in a Caterham, I’m sure their technical system would have the figures you require recorded somewhere.

Thanks for your response 'The Ash'. I was hoping to get other owners figures for comparison before taking the diff out and stripping it. You are right that BMW would be best placed to specify the actual crown wheel to pinion backlash figure. I may even take the actual diff with me so that they can identify the type and advise any other problems .... if they'll talk to a mere Caterham owner Smile

I think you'd be lucky to find anyone at a dealership who would know anything about the diff to be honest. 

I'm away at the moment but my 7 is up on stands. As soon as I'm back I'll measure and post here. I do remember that when I built my car in 2015, I was amazed at the amount of backlash. I don't remember the details but I did jack up my 5 series to measure the backlash it has. It was much, much less than the 7.

Particularly dopey today, realised I have a BMW open diff’d car sat in the garage. I don’t have CR500’s or 13’s but have Avon ZV3’s on 14’s, as described by yourself I placed it in 4th with one wheel off the ground, with middle of the A of Avon as the start or one end of the lash I rotate the tyre and get to the middle of the V of Avon before it stops. Roughly 30mm at a radius of 240mm from the hub centre some of which though is lash in the gearbox.

Seeing Scott’s post above and thinking about it I kind of agree with him, the BMW stealers is worth a try but they may want the VIN number from your 1 Series before they can proceed and of course really they are just ‘fault diagnosers and parts fitters’. And of course if yours is an LSD it probably doesn’t have BMW internals, ISTR that they are Titan (someone correct me if I’m wrong please). However if there’s someone there who’s technical, an enthusiast and knows their stuff I’d bet the info on max allowable lash will be noted somewhere on their system for fault diagnosis purposes.

Hi The ASH, Thinking about it, BMW dealers probably haven't got a clue as they'll just replace the whole unit if it had a problem. Thanks for your measurement. Staring at mid A, mine goes to 2/3 through the O of AVON but as the tyres are different sizes this may not be a good comparison. However, comparing your 30mm at 240mm radius gives a direct angular comparison to mine which is 90mm at 240mm radius. Ouch! There will be differences in driveshaft and gearbox lash plus a minor difference due to final drive ratio but it starts to help me understand if I have a problem. Hopefully other LSD owners will chip in with some figures. Many thanks for your input. I'm looking forward to Scott's feedback when he can.

Cheers

I’ve just measure my BMW lsd diff which is on a 2011 Cat.

In 4th gear with CR500 tyres and just the one wheel jacked up I get play from the very start of the C to just making it past the first zero by about 3mm.

Hope this helps

Thanks slap_ed, Do you mean 3mm just passed the start of the first zero or passed the entire zero? Thanks for your input.

3mm past the start of the first zero.

"3mm past the start of the first zero" is similar to mine depending how vigorously I take up the lash. I'm not sure if this is a good or bad result. If they are all like this, there's nothing wrong with mine but then I can't improve it. Doh!. Let's see what others are like. Thanks.

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