Rear differential rubber mount problem

Thomas76

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I ordered w/n part # 33171121908 for the rear diff, problem is the new one is 3/8" shorter mount surface to mount surface. The blocks of wood in picture are mimicking the diff. Anyone had this happen? Seems like way too much variation.
IMG_20190818_163821568.jpg
 

pmansson

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I had the same problem with a new mount from W&N many years ago. Found another new one which was correct. Perhaps it is for another model, and got mixed up at the provider...?
Not much help, but a possible explanation
 

Thomas76

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Contacted W/N, they said it's a BMW sourced part that has changed a couple times over the years. One size fits all.
I can't believe 0.375" isn't going to affect things like driveshaft and half shaft angles. I'll try to update further into the project for Future readers. Might end up making a spacer.
 

Thomas76

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Dude -- Don is one of the best resources we have here. He is sometimes cryptic, but he doesn't steer people wrong.
Was thinking it could be sarcasm not criticism, I'm a little gullible.;) Would be interesting to hear more about Camber as it would make sense.
 

sfdon

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The sign of an erupted diff Mount is excess negative camber...
Your new diff Mount that is too small will push the top of the wheel outward creating positive camber
 

sfdon

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And a reminder that rear tires will get more toe in from less camber.
 

Thomas76

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Thanks for the help! Almost seems that a shorter diff mount could then reverse the effect of shorter springs?
My solution will be to create a spacer to make up the difference.
 

sfdon

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Took pics yesterday for you....
Old correct diff mounts and brand new W&N mount. Hope this helps!
00A0606B-2B65-48F3-8AF9-BC28D3B56576.jpeg
 

sfdon

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To be clear - the Walloth new one was exactly the same height as my NOS one.
2 3/8”
 

gwittman

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I know things go right over my head sometimes but would someone please explain how the differential position has anything to do with camber? Last time I looked it appears the trailing arms control camber and the drive axles flex to eliminate any angularity change from the differential. Is this a Gotcha?
 

Thomas76

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I played it safe and made a spacer to compensate. Unfortunately I did not measure ride height before disassembling so I can't document real data.
 

Honolulu

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Gwittman, I'm with you - the driveshafts have joints on both ends so can someone explain how diff position affects camber? I thought camber was a function of subframe construction angles. Trailing arm bushing eccentricity would primarily affect toe-in/out but could, I suppose, affect camber to a slight degree. I'm trying to visualize this and not getting it.

A spacer would be the "easy" way to compensate. I suppose a camber gage used before and after, spacer and without, would tell the story, though I don't understand how.
 

sfdon

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Lots of good reading out there

 

Bearmw

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Good read and I always likes Jlevie's observations. I guess the question comes back to is there enough loss of negative camber to justify adding a spacer to these 1/2" or so shorter mounts?

This might have a slight effect on driveshaft alignment too but is it significant?
 
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