XT BB with Shimano compact crank?

[zennmotion]

OK stupid question, I'm new to the newfangled external bottom brackets so be kind. I have an aluminum cross frame that for some reason has a 73mm BB BSC BB shell. I want to put a compact road crank on it for you know, cyclocross. I'm cheap and I have a good Ritchey WSC octalink crank and BB in good shape. So do I have the BB shell shaved down to 68 and use what I have, or bite the bullet and get a modern external BB crank? The tech question is, can I use something like an XT BB with a compact road crank, or would I have to shave down the BB anyway to accomodate an external shimano road BB? I see where some BBs can be used for 68 or 73mm shells? Or am I completely confused? I have no desire to use a mtb crank for cross racing. I'm also cheap- I have no issues with the Octalink stuff I have and I'm confident that it would last a few seasons at least.

[eddief]

your bb is 73 and road crank spindle length is built for 68. If your bb was 68 you could use a 73 external bb, but without the included spacers. It works in one direction, but not the other. That is my best guess.

[kramnnim]

Can you use an MTB Octalink BB meant for 73mm?

Edit: I guess the spindle might protrude too far away from center so the chainline would be off.

[oliver1850]

Shimano made an XTR V1 octalink BB in 112mm, but i don't know if they made it for a 73 mm shell or just the 68. If you could find a 73/112, your Q factor would only be 2.5 mm more than with the standard road BB. Be careful not to buy the XT 113 as it is the V2 long spine, and won't work with 105 - DA cranks.

Next longer size in an XTR is 116. It was surely made in 73 mm, if you can stand that much increase in Q.

http://www.bikerecyclery.com/shimano-bb-m950-xtr-octalink-v1-bottom-bracket-112-5

[eddief]

and go backward to square taper bb and the appropriate road crankset from yesteryear. It looks as though Shimano square taper bb's are available in 68 and 73. Get one and an old crank and then Bob's your uncle.

http://www.jensonusa.com/!hugir4bNeSkadgLVLyuCwQ!/Shimano-UN55-Bottom-Bracket?utm_source=FRGL&utm_medium=organic&gclid=CIz3pNfQwLwCFdKIfgod9xYAbA

[vqdriver]

Never mind. Reread the op

[eddief]

I checked ebay and there are two used ones. this one spendy.

http://www.ebay.com/itm/Shimano-XTR-BB-M952-73-x-112-5-1-37x24-MTB-Bottom-Bracket-FREE-S-H-/301089783948?pt=US_Bottom_Brackets&hash=item461a59808c

[CiclistiCliff]

External bottom brackets for Shimano mountain cranks are 1.5mm shorten than the road variants, iirc. You could use it but chainline would be horrid and the safety pin/plate would not engage on the NDS.

Find a Sugino 5x110 crankset for square taper and order a new sq. taper BB. Least expensive yet best solution.

[oldpotatoe]

your bb is 73 and road crank spindle length is built for 68. If your bb was 68 you could use a 73 external bb, but without the included spacers. It works in one direction, but not the other. That is my best guess.

Good guess and correct.

[zennmotion]

Thanks, this is what I thought. Looking at the BB shell it looks like I can use a facing tool ( or rather have a shop do it, I think this is beyond my comfort zone although I could borrow the Park tool from a friend) to remove 2-3mm of Aluminum from each side of the shell. Does this sound reasonable or are there issues I'm not thinking about? The square taper option might be another option, but I still really like the Ritchey + Octalink that I already have, they're both like new, and fairly light, and I'm sorry that Shimano moved on from what was a good system IMO. Plus any future cranks I would want would be based on a 68mm shell anyway, so cutting down the shell would need to happen. I just don't want to risk trashing the frame because of some bonehead factor that I'm not thinking of.

[eddief]

"stepping over quarters to save nickels." I think you might be doing that.

[zennmotion]

"stepping over quarters to save nickels." I think you might be doing that.

Thanks and maybe, but long term this is a cyclocross bike and there aren't many good options for MTB cranks with my preferred gearing 36X44. Life would be much easier with a 68mm shell that is found on nearly every CX frame that still uses a threaded shell. So cutting down the shell seems reasonable unless of course it ruins the frame- moving slowly on the decision because I don't know what I don't know. Y'know?

[cmbicycles]

I would say get a Phil wood BB, a square taper crankset, and then shell width isn't an issue and you can even adjust your chainline if needed.

Also if your preferred gearing is 44-36, you should be able to easily get that on a mtn triple crank, just leave off the inside gear.

As far as cutting the BB shell down, a facing tool is all some shops would have (if they even have one), but it isn't intended for removing the amount of material you are asking to have removed. If it isn't done correctly you will never get a good chainline, and may ruin the frame. If you decide to go that route, I would have someone with frame building experience do the work, but I kinda think it won't be as cheap as finding a crank and BB.

[zennmotion]

Thanks all for the advice. I'm going to take about 1mm off the drive side and 2-3mm from the ND side where there's just a little more aluminum. Shop that I trust quoted about $80 and I already have the BB/crank so I'm not saving any money buying a Phil Wood and new square taper crank. I don't understand the comment about chainline- if the faces aren't square I agree that would be a problem but it wouldn't affect the chainline (1-2mm from driveside? On a geared bike?) I'm tempted to borrow the tool from a friend and do it myself, but I'll leave it to more experienced hands. I think getting it down to 70mm should be enough to work, and I'm cutting aluminum not steel, I'll buy the shop the proper cutting fluid and pay extra for them to go slowly, try to install the BB after just a few mm of material removed and do a little more a second time if necessary. I've build a couple of frames myself but this is a little out of my experience, but it should be fine. Thanks again.

[eddief]

I think you are the first person to pose this question of trimming down a bb in this way for this purpose. I wish you luck. If it was me I would just get a square taper bb and a square taper double crank and call it good...or good enough. You must love the Octalink stuff a lot. I sincerely hope your experiment works. Let us know if it does or doesn't. I am genuinely interested.

[CiclistiCliff]

I can see it now.

Thread title: "crank arms hit chainstays after shaving bottom bracket shell"

[oldpotatoe]

I can see it now.

Thread title: "crank arms hit chainstays after shaving bottom bracket shell"

Good call. A few frame builders, making the stays thick and wide, for tire clearance, spec. A 73 shell for clearance. Too bad almost all cranks nowadays cannot be spec'ed with a longer spindle for cases like this.

[CiclistiCliff]

Or you end up with issues where too big of a chainring in the inner (small) location will contact the chainstay because said bike was design with specific cranks in mind, hence, 73mm shell. Wouldn't be an issue with mtb double. Most likely would be an issue with a compact road crank.

[zennmotion]

Too bad almost all cranks nowadays cannot be spec'ed with a longer spindle for cases like this.

And this was my original question- I'm not sure whether this is sarcasm- that most modern cranks can actually be spec'd with a longer spindle, or whether you mean that it's too bad that they don't have this option. I'm operating under the assumption that my options for a road style crank (assuming a "standard" threaded BB shell, not some press-fit or other) are limited to 68mm shells

If I could get a longer spindle for outboard bearings on a Shimano compact road crank then that would be the solution. The clearance issue is not the crank arms, but the space between the small chainring and the chainstay (between 4-5mm by my caliper measure) On the non-drive side I have 5-6mm of space before there's any issue with the left crankarm- chainstay. So taking a couple of mms from each side should be OK.
But sorry,I'm still not clear- can I get a longer, modern external bearings, spindle for a 73mm shell with a compact road crank? Or is my only option for a modern crank a MTB crank with funky, separately purchased sized rings for cyclocross racing (36X44 is pretty much the standard size for amateur cross racers without the horsepower for bigger rings, and typical rings found with MTB cranks are too small for cross racing)

[pakora]

I don't know that this will be any help as I have a 68mm shell, but I have a weird frame designed around a mountain crank and a 135mm rear end, with the added tweak that my particular frame has a 130 rear end.

I ran xtr cranks with the 112.5 spindle (36 single ring, have also run a 38 single ring on a Ritchey crank and the same bb that baaaaarely cleared). chainline is a bit off but it's no big deal on my frame. I would never use the big ring racing in the mid-Atlantic anyway, but that's me of course.

Anyway what was here to post was: To my knowledge, all XTR 95x bbs are for a 73mm shell and came with spacers for 68mm shells. Not all of the used ones I've purchased came with spacers, but a couple did, and anyway you can just use the 2.5mm spacers that come with outboard bearing cranks.

You can NOT use an XT bb or any other Octalink v2 with your Ritchey cranks, assuming they're Octalink V1 as mine are. You will ruin them as the v2 splines are deeper. Ask me how I figured that one out. Also FS: ruined Ultegra crank, 130bcd, 175mm :p

PS getting extra crazy, just because I had the stuff around, I calculated what cassette I'd need to run with a mountain crank/ring setup - I think it was 32t with an 11-23. Worked fine, but felt weird.

[zennmotion]

Result: so I borrowed the Park tool from a friend with an enviable basement man-cave tool collection and used mineral spirits as cutting fluid and cut the bottom bracket shell from 73mm to just shy of 70mm, taking about twice as much from the ND side as from the drive side. Installed the crank, everything looks fine with adequate clearance, rode the bike hard for an hour this morning on a frozen hilly course to make sure the cranks didn't loosen up as sometimes happens if Octalink isn't properly installed. Problem solved, thanks for all the advice, even if I didn't take all of it, it helped me think through the potential problems and solutions. OldPotatoe for Czar or something!

[eddief]

if i am not careful I can learn something new almost every day.

inquiring minds would be interested to know what make of frame got the attention?

[zennmotion]

if i am not careful I can learn something new almost every day.

inquiring minds would be interested to know what make of frame got the attention?


It's a custom, but the communication gap regarding the desired BB shell width was mostly my responsibility for assuming and not checking that spec before approval, so I'll leave him out of a public forum. I otherwise love the frame, it's stiff and fast, lots of tire clearance, and has just the right weight distribution for me- it gets through sand and up slick hills where competitors are off the bike and running, and my skills are mediocre at best, the bike is an enabler. I raced it for several years with another Octalink-based crankset that for some reason wasn't a problem and I never even noticed the shell width until I went to change the crank. Eventually I'll join the 21st century and get a modern compact (and butt ugly IMO :butt:)Shimano once the BB starts to show wear or bearing play.