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Got a BMW diff? - check your bolts


adz

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Greetings,

This is just an advisory.

At the weekend the carrier on my BMW diff failed, as in the metal broke in 2 places. Checking some other posts on the academy website and it appears this has happened to a few other folks, but mine may be the first road car that has failed. Caterham are going to send me a new version replacement carrier, which has been beefed up.

My roadsport was delivered april 2011 and so I had a fairly early version diff carrier.

When I took the car for its 1st annual service one of the bolts securing the diff into the carrier had fallen out. It was replaced. A couple of weeks later it was loose again.

 

So, check the torque of all the bolts on the diff assembly, especiallly the ones holding the diff to the carrier. Have a look at the carrier for cracks.

Check the dedion tube is not banging against the bolt heads that stick out the back of the carrier- you'll see marks on the tube if it is.

 

Cheers

adam

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Hope the stronger one is also truer! Mine was a couple of mm out corner to corner which made fitting the diff an absolute b**ch!

So to replace the carrier I guess the diff has to come out again? *confused*

Thanks for the heads up though *biggrin*

 

Edited by - Garry7 on 3 Oct 2012 17:22:59

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On the new design carrier, the bolts at the back that attach the carrier to the back plate of the diff are countersunk into the carrier. On the old one, they aren't countersunk. An old style one is here you can see the silver bolts on the upper back. On the new one, those aren't there.

 

The bolt loosening seems to be hastened by the de-dion hitting those mounting bolts and jarring the whole assembly. We have been told that the resulting "witness marks" (aka dents) in the de-dion aren't an issue. We shall see...

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My 2012 supersport delivered as a kit to me end of Feb looks to have this type of carrier. I will have to crawl under the car at the week end to see if I have any kisses on the de deon. From photos I took during the build it looks a long way from the tube to those bolts but on the road under suspension compression I guess it might get there.

 

 

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heres some photos of the cracks

 

bmw diff carrier fail

 

as an addendum, if you need to remove the bmw diff, you dont need to remove the watts link. after looking at other posts I took out the ARB, A frame and unbolted passanger hub from dedion ( to remove passanger drive shaft), then lowered diff with the aid of an engine hoist.

 

Adam

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Just checked my 2011 Roadsport SV 150 delivered in June 2011 - it has the old style carrier. Everything looks fine. Bolts tight. Agree with Pugwash above looks like bolts would have move a long way back to catch De Dion tube.....
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Just checked my Roadsport SV with superlite rear suspension, built March 2012. I appear to have the old diff carrier. But, I'm not sure that the carrier wouldn't touch the dedion tube, with or without recessed upper bolt heads, if the suspension was fully closed. As it is, statically with one driver on board, the clearance seems plenty, and there are no witness marks on the dedion so far.

 

But if I ever get air then land firmly (hump back bridge or similar) who knows....

 

Might try sticking a large piece of pastercine to the top of the dedion, to see how small the clearance becomes under normal road driving with a passenger on board...

 

(pmobbs - didn't really understand your last comment - were you looking at the lower bolt heads when you say "looks like bolts would have move a long way back to catch De Dion tube....." I think the issue is with the upper bolts, and these would overlap the plane of the dedion whether they are tight or partly loose on my car...)

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Parsley the Lion...was looking at upper bolts....will take another look. Good idea to get some measure with Plasticine. Have done 7500 miles in rural France mostly with 2 up and lots of gear stowed and no sign of any marks on De Dion so not too worried.....
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ParsleyTheLion and others with new diff and old cage - when I jacked up my car the other day, because I have to slew the trolley to one side, I was only able to inspect the passenger side of the De Dion tube - it's fine. Today I jacked up the car to look again and this time did things properly. I have posted a picture of the damage I found on the driver's side - not only is the tube damaged, it looks as if under certain circumstance the brake line can get clipped.....woe is me....and I feel a total tw*t for not spotting this the first time. Photo here: BMW diff

and here: BMW Diff

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Just jacked mine up and took wheel off and had a good look.

No visible sign what so ever of any contact with the bolt on either side. I can see how with enough compression the bolts could hit as they would be in the plane of travel of the de dion, but mine is clean.

 

For info, I think the supersport runs with a harder rear end, shocks, watts linkage and arb which may mean it compresses less than a Roadsport. Either that or I have not leaped over a big enough bridge yet! I have photos for reference if anyone is interested.

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Quoting 7 wonders of the world: 
If your unsure support the chassis on stands, disconnect the lower coil over bolts and with a jack under the DD tube to A frame mount move the axle through it's entire travel to check for potential collisions.

 

This is good advice, it just seems incredible that Caterham didn't try it in the first place!

 

Duncan

 

Edited by - DJ. on 6 Oct 2012 23:02:36

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Alan - have contacted CCs and sent them pictures like those above. It defies logic that they could claim that this is an issue limited to academy cars - the pictures show it isn't. It would appear that the design of the early carrier is flawed and for the safety of everyone who has a car with this design fault CCs must act to warn the owners and fix the problem. For my own and the safety of others I will not let this rest.
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