BMW diff backlash

[sprocket]

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.

[The_ASH]

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.

[sprocket]

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

[ScottR400D]

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.

[The_ASH]

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.

[The_ASH]

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.

[sprocket]

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

[slap_ed]

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

[sprocket]

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

[slap_ed]

3mm past the start of the first zero.

[sprocket]

"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.

[Rattie]

Out of interest, if you jack the rear, hold one of the UJ cups stationary against the diff body, what play do you get in the wheel? The reason I ask is that 23 degrees you are seeing at the propshaft flange would only be 6.3 degrees of play in the halfshaft UJs if the diff had no play at all.

23 / 3.64 (replace with your final drive ratio if different)

Certainly ours have something like that and they dwarf any play in the rest of the system. Not saying yours do, but worth evaluating.

I for one dislike the tripod joints because of this play and would prefer at least the inners to use Rzeppa CV joints, like actual BMW halfshafts

[sprocket]

Hi Rattie, I finally got round to checking the drivesahft lash as you suggested. I didn't measure it but it is tiny compared to that in the diff, Thanks for your input.

[Tom_Arundel]

Re. backlash

the main source of backlash in an open diff is the sun and planet gears which are shimmed radially. Power/torque is transmitted through these gears and because of the way they work, moving one wheel against the other fixed wheel and/or propshaft tends to exaggerate the actual backlash. Any wear in the shimming or bearing/pins of these bevel gears produces quite a lot of backlash effect.....irritating but not terminal and unless better materiels are used it will recur.  Gear whine is `something else` which can be reduced.

[Rattie]

Interesting, sprocket. Sounds like we could do with replacing the half shaft joints with some better examples. Our lash is terrible. The diff on the other hand is fine. It has the Titan clutch diff though, so different beast I guess

[sprocket]

Aha! We may have a bit of an answer here and Rattie wins the cigar but maybe not why he thinks. I decided to take the diff out and strip it down to investigate what is going on. During the strip process having removed the halfshafts, I plugged in the original BMW stub shafts that were supplied with the diff to keep it clean during the build. Just out of interest I half heartedly checked what backlash I could detect across the diff and WOW! Where's it all gone? It now felt like the pinion might be close to the crown wheel after all. A quick change back to the tripod half shafts and guess what? clunk rattle clunk.

When I'd checked the lash in the halfshafts before I was only going along the length of each individual shaft.I now believe that the problem is that the end spline of the inboard tripod joint into the diff is undersize and contributes most of the free play in the whole driveline system. This is why I believed that the backlash existed within the diff.

As Rattie says above, changing the inboard joint to the BMW Rzeppa one could eliminate this source of backlash.

Does anyone have any experience of such a conversion?

Thanks to all who have contributed so far.

[sprocket]

If anyone is interested, I measured the backlash across the LSD comparing the BMW stub shafts and the tripod joints supplied by Caterham. Locking the outputs, the pinion rotation was - BMW 11.4 degrees and the tripods 18.3 degrees. ie 60% more. This is probably significant in the overall perception of torque reversal clunk and contributes to the kangarooing at low spreed. C'est la vie.

[Geoff Brown]

Changing to the original BMW Rzeppa inner joints would stop the diff output seals from leaking....no lateral play at the diff entry point.

[Mucus72]

I’ve read most of this thread and I only just understand it ;)

I have a question. I have a 2016 BMW LSD in mine and I’ve been noticing that as I pull away I get a clunk. I’ve looked everywhere for play and was wondering whether it was diff or half shafts. Is this a symptom of what you guys are discussing here?

[sprocket]

Hi Mucus72. Sorry to hear that you also suffer the dreaded CLUNK. There are many potential sources of backlash (free play) within any driveline but a Caterham with BMW LSD seems to have excessive backlash that results in an audible clunk either when you pull away or most noticeably during torque reversal (power on/off/on).

There is potentially noise due to the limited slip mechanism but this shouldn’t be noticeable in low torque, straight line conditions. I believe that in my car the biggest culprit is the half shaft spline that plugs into the diff which has at least 60% more backlash than the BMW equivalent stub shaft. I therefore get a huge mechanical clunk as the free play is taken up during torque reversal. My guess is that your car has the same problem.

The diff is very firmly mounted in a Seven so any noise is transmitted into the chassis and in the case of a Supersport with no carpets, straight into my ear. I’ve no idea what to do about this other than trying to change to BMW type Rzeppa half shaft joints so that I could use the BMW stub shafts but this would not be simple. Maybe the answer is to install very loud headphones connected to the exhaust and just enjoy the drive. :-)

Hope this makes sense

[Mucus72]

Hi Sprocket, thanks for explaining clearly enough for me, yes this is what I have. It wasn’t always there, so I was bothered something was coming loose. I am happiest driving with ear plugs in, so will continue to happily ignore yet another noise ;)