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BMW Diffs
Sanzomat - 3/9/23 at 10:10 AM

Hi all, might be a blind alley someone else has been down already so before I start going down it I thought I'd see...

Background - I have a R1 4XV engined Locost with IRS. I'm currently running a 3.62 sierra viscous LSD diff with Lobro shafts. I run out of revs on the main straight at Castle Combe and bump off the rev limiter for several seconds before lifting off for Avon Rise so I'm pondering raising the gearing. From my sums a 3.14 would be too high and a 3.38 would be spot on. However both 3.38 and 3.14 sierra diffs with Lobro (with or without LSD) seem to be rarer than rocking horse poo.

Whilst searching for 3.38 sierra diffs the BMW ones often pop up in the search results and seem quite reasonably priced. In the images the output shafts look remarkably similar to the sierra Lobro connections. Anyone know if they are? I'm guessing the chances of the input flange bolt pattern matching are smaller than the chances of finding a good cheap sierra one but maybe!

Length from drive shaft plane to input flange??

Adapting the bracketry to fit it to the chassis is probably something I can fabricate.

Anyone done the conversion?

mcerd1 - 3/9/23 at 11:04 AM

quote:
Originally posted by Sanzomat
In the images the output shafts look remarkably similar to the sierra Lobro connections. Anyone know if they are?


Lobro is a GKN design, used by Ford, VW, BMW, porsche and loads of others too - but they come in a few different diameters as well as loads of options for teh range of movement etc...


the standard sierra & all 4x4 versions use the ones with 100mm outside diameter with 8mm bolts

but I believe most BMW ones use 108mm joints (I'm no expert on BMW's though) - same as the 2wd Sierra Cosworth (these also use larger 10mm bolts) - these also have larger splines to suit bigger driveshafts
saying all that the 108mm cosworth axle bits don't exactly grow on trees either - so if you go fofr the BMW diff then why not the rest of the axle too ?

a bit of googling suggests that some of the 1 series used 100mm CV's, but with the larger 10mm bolts - you can lookup the sizes on GKN's parts online catalog, but unfortunately you can't seem to do a reverse lookup






these is a little guide on the dimensions here: (these are a fairly typical motorsport parts so easily available in pretty much all sizes)
linky

[Edited on 3/9/2023 by mcerd1]

Schrodinger - 3/9/23 at 11:13 AM

I know this dosn't answer your question as such but have you tried Raceline for the ratio?
https://www.raceline.co.uk/products/part_section.asp?SectionID=86&CategoryID=6

Simon - 3/9/23 at 06:44 PM

If I hadn't sold toy car to help pay for extension I was going down the BMW diff route.
So many choices. I got a (iirc) 2.28 diff from Germany for about £150, and that was around 10 years ago