Mountain Bike Reviews Forum banner
1 - 20 of 26 Posts

·
Registered
Joined
·
1,451 Posts
Discussion Starter · #1 ·
New Scott Ransom with a BB92 bottom bracket.

Shimano XTR M9020 crankset (previous 11 speed generation crankset)

I have measure the shell width with calipers at 92 mm which is spec. Using either a Shimano BB92 bottom bracket or a new Wheels Mfg. thread together bottom bracket, the overall width doesn't allow the left side crank to slide deep enough on the spindle. It works, but barely. The spindle is about 3mm too short such that the outer pinch bolt barely clasps on. About every 5 rides the crank gets loose and starts falling off and obviously I don't trust it for big landings. And this is with the left side lip seal removed. If I remove the right side lip seal the chainring bolts hit the frame it needs that little bit of clearance. These seals are very thin anyway.

I'm not really asking for a solution I just find it very strange that a bottom bracket spec'ed explicitly for Shimano cranks doesn't fit Shimano cranks.

I showed this to my expert LBS mechanic and he agreed there is no obvious problem with anything.

The bike came with GX cranks and a DUB BB, and I guess I will have to suck it up and put those parts back on even though I hate what SRAM has done with DUB. A 30mm spindle is even more problematic in this BB although it can be done albeit at a cost of $150 BB with non-replaceable bearings.

First picture is to show that the BB cups are very thin and not part of the problem, the 2nd photo shows how short the spindle ends up being in the crank.

Geez can we just go back to one simple BB standard !
 

Attachments

·
Keep on Rockin...
Joined
·
6,471 Posts
That is odd. I've got 2 sets of the 9020s and have had them on a few different bikes, including one's with BB92s. Never had a problem.

Over the next few days I'm tied up but might be able to pull one of them and get a precise measurement of the spindle. You could do the same and we could compare to see if that is your issue. Maybe post back with a precise spindle length?

Too bad because I've found that 9020 to be one of my favorite cranksets.
 

·
Keep on Rockin...
Joined
·
6,471 Posts
On second thought, I'm guessing your problem is the BB, not the frame nor the cranks.

That BB edge does not look like a Shimano BB cup flange, and I'd say its thicker than a Shimano cup flange IIRC.

Betcha that's your trouble.
 

·
Registered
Joined
·
1,451 Posts
Discussion Starter · #5 ·
Width of the shell is 3.614" ~= 91.8mm
width of the bearings 3.70 ~= 96.27mm
Spindle length from drive side crank bearing shoulder 4.5" ~= 114.3 mm

I had the same issue with a Shimano bottom bracket (forget the part number but it was actually a Dura Ace part for some reason). It was slightly better by like maybe 1mm (ie 30% better I guess), but that Shimano BB had visible lips on the bearing cups as well. It was far from ideal IMO.

The whole thing just seems very mysterious to me. Wish I had a BSA external cup BB installed around here somewhere to compare to. Even if some tolerances stacked up it shouldn't be this marginal. Maybe these cranks are only compatible with BB86 not 92 even though 92 is the mtb standard ?
 

·
Turns right, slides left
Joined
·
735 Posts
The spindle engagement in the second photo looks identical to my 9020's did on a BB92 frame and on my current threaded BB frame. Not sure why you are having this issue.

When installing did you use a torque wrench and alternate between the bolts as you tourqed them down?

You mentioned that if you remove the BB seals, the chainrings hit the frame. That has me very confused. Can you show some pictures of the drive side?
 

·
Registered
Joined
·
1,451 Posts
Discussion Starter · #11 ·
These are M9020 XTR cranks originally purchased in a 1*11. I then purchased the 2*11 caps so I could run the chainring on the inner bolts.

I have a late model Ultegra crankset on a BSA BB in a road frame and another mtb with the same 9020 XTR cranks in a PF30 BB. In both cases you can just barely see the crown of the splines, the mtb crank showing a tiny bit more, but both of them look correct and in spec whereas the BB92 looks very marginal. Or at least, more marginal than it should be. Its probably just a case of tolerances stacking up to the limit. The Shimano BB92 seems like it did have just marginally smaller lips than the Wheels Mfg improving that tolerance one more mm.

I exaggerated a little bit when I said the cranks were falling off all the time. But it has happened. I"m now checking them regularly and I also traded pre-load caps for one with deeper threads so I can pull it in good (the stock cap barely engages because the spindle is so deep). What I"m really chasing is a creaky crank problem one reason I changed BB's. It kept going away when I'd take the crank apart and tighten the chainring but I now realize it was probably because I was re-setting and re-torquing the cranks. I believe with that little spline engagement the outer bolt clamping area starts sliding outboard. Just a theory for now.

I ordered the wrong offset chainring for my GX cranks so I'm going to leave the XTR's in there for another week long trip. If the creaking comes back I'll reset the crank and see if that's really it.
 

·
Hitching a ride
Joined
·
3,193 Posts
Width of the shell is 3.614" ~= 91.8mm
width of the bearings 3.70 ~= 96.27mm
Spindle length from drive side crank bearing shoulder 4.5" ~= 114.3 mm

I had the same issue with a Shimano bottom bracket (forget the part number but it was actually a Dura Ace part for some reason). It was slightly better by like maybe 1mm (ie 30% better I guess), but that Shimano BB had visible lips on the bearing cups as well. It was far from ideal IMO.

The whole thing just seems very mysterious to me. Wish I had a BSA external cup BB installed around here somewhere to compare to. Even if some tolerances stacked up it shouldn't be this marginal. Maybe these cranks are only compatible with BB86 not 92 even though 92 is the mtb standard ?
My BB92 bearing width is 94.55mm
Shell width is 92mm
SLX M7000 spindle length is 120.0mm
Rubber dust caps are 2.0mm each and I have two.
NDS crank arm is hanging 4.91mm off the end of the spindle.

 

·
Registered
Joined
·
314 Posts
I had the same issue with fitting 9020 cranks to a newer threaded BB Evil frame. It was just a hair too short like the OP was saying. The B.B. width was not the issue on this frame, it was getting the chain line correct as to get enough spindle length I had to ditch the drive side 2.5mm spacer. The 9020’s fit fine on certain frame for example I had them on a 2017 Kona Hei Hei using a standard press fit Shimano BB and the spindle length was just fine. Hell I have them on a 2016 Evil Following with a press fit B.B. and they fit fine.

I think your issue might be the alloy threaded press fit B.B. you are using. The Shimano press fit plastic B.B.’s do no extend as far out from the frame.
 

·
Registered
Joined
·
1,451 Posts
Discussion Starter · #17 ·
Yup that's what it comes down to I guess. Just disappointed that it all doesn't work out better on a BB designed for Shimano with Shimano cranks. Next time I'm at the bike store I'll take another look at the flanges on their BB92 BB and see how much smaller they are. I may go back to that if I ever decide what is causing the creaking. BB92 is a joke although I understand the stiffness argument. The bearings on the BB92 DUB are ridiculously tiny and obviously the 30mm would be worse (by 1.1mm ha ha). And to think I used to be annoyed by PF30. At least it leaves a little room for spacing the cranks around.
 

·
since 4/10/2009
Joined
·
34,347 Posts
Yup that's what it comes down to I guess. Just disappointed that it all doesn't work out better on a BB designed for Shimano with Shimano cranks. Next time I'm at the bike store I'll take another look at the flanges on their BB92 BB and see how much smaller they are. I may go back to that if I ever decide what is causing the creaking. BB92 is a joke although I understand the stiffness argument. The bearings on the BB92 DUB are ridiculously tiny and obviously the 30mm would be worse (by 1.1mm ha ha). And to think I used to be annoyed by PF30. At least it leaves a little room for spacing the cranks around.
Not exactly. At least, not always.

I use this bb on my fatbike right now.

https://www.endurobearingsonline.co...ntain-bike-bottom-brackets/products/bb86-4130

While I agree with you in general about BB92 (I don't like it much, either), this particular bb eliminates the cups from the equation (in traditional BB92, the bearings are pressed into cups, which are then pressed into the frame). By pressing the bearings directly into the frame and eliminating the cups, the bearings can be bigger and more supportive (the dual rows help, too). The sealing is pretty good on this bb, too.

I've not run a "traditional" BB92 with a shimano 24mm spindle or GXP crank/bb setup. I simply don't like BB92 and find it excessively limiting with component selection. After my fatbike, I went back to a frame with a threaded bb. It's just so easy to run either a 24mm spindle or a 30mm spindle on a threaded bb without compromising the bearings.
 

·
Registered
Joined
·
1,451 Posts
Discussion Starter · #20 ·
Yeah the 30mm flanged bearings you listed above are sold by Wheels mfg for $150 a pair. The ones you listed are sold out in stainless and $129 apiece in ceramic.

Given that I replace 2-3 PF30 bearings a year here in PNW, I'm a little loathe to spend $150 for BB bearings that will need to be replaced regularly. AT least the DUB BB is only $43 so I can replace those regularly. Not as nice as just popping $12 PF30 bearings in and out though.

That aluminum bolt is nice but I wish it was deeper as that is my primary problem.
 
1 - 20 of 26 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top