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Mtb crank on gravel bike...what do I need to know?

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Mtb crank on gravel bike...what do I need to know?

Old 03-04-18, 08:33 PM
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jzsoup
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Mtb crank on gravel bike...what do I need to know?

I bought a Nashbar gravel bike with FSA Gossamer 50/34 crank and bb86. I need lower gearing and was thinking of putting a double slx or xt crank (or similar). I tried putting my older xt triple (converted to a double) but the spider rubs the shell of the frame.


Is this the worst idea I've ever had (and I've had plenty)? How do I go about it?
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Old 03-04-18, 08:41 PM
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You need the right bb for the conversion... If it exists. Not knowing the particulars of frame and crank and bb, I'll guess you're trying to make a 73mm sheep standard mtb spindle crank work on a 86mm width bb

Got some links to your equipment or pics@
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Old 03-04-18, 08:59 PM
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Originally Posted by Marcus_Ti View Post
You need the right bb for the conversion... If it exists. Not knowing the particulars of frame and crank and bb, I'll guess you're trying to make a 73mm sheep standard mtb spindle crank work on a 86mm width bb

Got some links to your equipment or pics@



Nashbar Alloy 105 Cyclocross Bike - Bike Nashbar


This is what I bought. I'm not necessarily set on slx/xt, it's more important to get the gearing cause I'm old and fat. Thanks for any input you can give!
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Old 03-05-18, 07:28 AM
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Hmmm...Shimano makes an XT BB for BB86 frames:

https://www.competitivecyclist.com/s...NjQ2F0MTAwNDY2

Is it actually rubbing on the shell and not whatever BB that thing shipped with? BTW in doing this, a new FD is probably wanted presuming it can be made to work with brifters....IIRC most road FDs are not rated for lower than 30T.
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Old 03-05-18, 09:05 PM
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Originally Posted by Marcus_Ti View Post
Hmmm...Shimano makes an XT BB for BB86 frames:

https://www.competitivecyclist.com/s...NjQ2F0MTAwNDY2

Is it actually rubbing on the shell and not whatever BB that thing shipped with? BTW in doing this, a new FD is probably wanted presuming it can be made to work with brifters....IIRC most road FDs are not rated for lower than 30T.




It's rubbing the aluminum shell outside the BB. It's like the BB just needs a spacer installed and viola! But I don't see anyway you could add a spacer. That's a good link, thanks for sharing. Have you ever come across a good source for education on PF BB's? I just haven't dealt with them much and am ignorant.


I think I've got some old front derailleurs floating around the garage so I should be okay that way. I'll take all the education I can get, thanks!
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Old 03-05-18, 09:18 PM
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That FSA Gossamer should be a 110 BCD, so you'd be able to find some replacement chainrings to swap for the 50t if you just want to take a bit off the top end.
Are you trying to go lower than 34 in the front? If it were my problem, I'd replace the cassette with an 11-36t or something and fiddle with the b-screw to get clearance. Easier than messing around with getting the right crank for the right bottom bracket standard, imo, and less expensive.
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Old 03-05-18, 09:26 PM
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Originally Posted by jzsoup View Post
It's rubbing the aluminum shell outside the BB. It's like the BB just needs a spacer installed and viola! But I don't see anyway you could add a spacer. That's a good link, thanks for sharing. Have you ever come across a good source for education on PF BB's? I just haven't dealt with them much and am ignorant.


I think I've got some old front derailleurs floating around the garage so I should be okay that way. I'll take all the education I can get, thanks!
Best education, crosspost to the folks over in Bicycle Mechanics: https://www.bikeforums.net/bicycle-mechanics/

If you're on /r, the BikeWrench sub has a ton of knowledgeable people to ask as well.

I'll admit to never having tried to fit a Hollowtech crankset into a BB86 frame....There are spacers required for 68mm and 73mm shells....But AFAIK there shouldn't be any shims/spacers for BB86. Give the guys in Mechanics a ding with some pictures of where and how it is rubbing and hopefully they can set you right.


I was hoping some other folks in here might chime in...but fitment issues like these are Unique Snowflake problems, where every one is different.
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Old 03-05-18, 09:52 PM
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Originally Posted by Marcus_Ti View Post
Best education, crosspost to the folks over in Bicycle Mechanics: https://www.bikeforums.net/bicycle-mechanics/

If you're on /r, the BikeWrench sub has a ton of knowledgeable people to ask as well.

I'll admit to never having tried to fit a Hollowtech crankset into a BB86 frame....There are spacers required for 68mm and 73mm shells....But AFAIK there shouldn't be any shims/spacers for BB86. Give the guys in Mechanics a ding with some pictures of where and how it is rubbing and hopefully they can set you right.


I was hoping some other folks in here might chime in...but fitment issues like these are Unique Snowflake problems, where every one is different.



I hadn't thought to try ******. Good call. Yeah, the goal is to get lower, much lower. We have an event in Wyoming in June....last year I rode it on an old hardtail with a triple, and had to get off and walk several sections. The goal this year is to try to retain more dignity than last year.
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Old 03-05-18, 10:37 PM
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Why not go with a 46/30 ultra compact?

There are the Sugino OX91D, FSA Adventure and others.

Anyone using a 46/30T crank on a road bike?


-Tim-
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Old 03-05-18, 10:49 PM
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Originally Posted by TimothyH View Post
Why not go with a 46/30 ultra compact?

There are the Sugino OX91D, FSA Adventure and others.

Anyone using a 46/30T crank on a road bike?


-Tim-


That's what I'm after. I didn't know about these, thanks. I'd still like to see about fitting a mtn crank if possible.
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Old 03-06-18, 07:34 AM
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my gravel bike in stock form came with a Shimano 105 compact crank. I built mine from a frame, and I had a SRAM double MTB crankset, 28/42. You need 5mm of spacers to get from 68mm to 73mm, which is what most MTB bb are. I had a little bit of trouble getting the front derailleur to go out far enough because I had to put all 5mm on the drive side. I ended up with a Shimano CX70 FD, which shifts fine. It's possible I could have gotten the other FD to work if I had fiddled with it a bit more.
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Old 03-10-18, 11:43 PM
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Assuming you can work out the BB and FD issues, you'll also find that the Q-factor will be wider and the chainline will be further out from the center of the bike than with the road crank the frame was designed for.
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Old 03-11-18, 11:46 AM
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Hope this helps, a cross reference chart for various cranks to BBs.

https://wheelsmfg.com/complete-crank...-adapter-guide

If I am remembering correctly a BB86 uses the same width spindle on the crank as the external bearing english threaded BB ( Hollowtech, GXP) the BB86 is internal bearing for a wider BB to allow more room between the chainstays.
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