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Fat-tired Roadie
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Discussion Starter · #1 ·
Due to parts availability, I just did this.

With my crappy calipers, I measured the inside diameters of the bottom bracket seals, the part where the crank spindle sits, as being the same.

The road cup housings were 1/32" wider, though. I noticed that the amount of housing above the level of the outside seal seamed a little higher, so I'm not sure if the part of the bottom bracket that the crank arms put pressure on is any further from the centerline of the bike. I suspect it is, but not by a full 1/32".

The spacers that come with the MTB bottom bracket measure 3/32", so I decided to install all three as normal - I figure this should result in my Q factor being a bit less than 1/16" wider, probably at the high end of acceptable for deviance from bottom bracket shell specified width.

Installation was no problem, and everything seems to work. I'm losing a minimal amount of splined interface, so I'm not nervous about damaging anything.

If anyone's curious in a week or a year about whether or not this works, ask me again. I might have a different opinion.
 

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Fat-tired Roadie
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18,453 Posts
Discussion Starter · #3 ·
I believe the Shimano road bottom brackets to be functionally identical to each other and the Shimano MTB bottom brackets to be as well. So if what I did works, the swap you propose should work too.

So far, my setup is working fine. If you try it, I'll be curious to know how it goes.
 

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Doesntplaywellwithmorons!
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10,939 Posts
The road BBs and mtb BBs use the same spindle shaft size so yes you can use either cups with either cranks as long as its the correct cup threading (obviously road italian external bottom bracket cups are not going to fit an english threaded mtb bb shell).
 

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Unfortunately, this is not enitrely true, as I had a displeasure of finding out.

ROAD cups extend 11 mm beyond the frame, while MTB Cups only 10,2 mm. This means that you can make it work, but have to use different than standard spacers.

For example, on an MTB with a 73 mm shell, normally you woulde use one 2,5 mm spacer. But with ROAD cups, you would need to reduce it to 0,9 mm (2,5-2x0,8), say 1 mm. However, your chain line moves by 0,8 mm to the left, as does th whole crankset, which makes it a little off center.

For most peaople it may not be noticeable, however note that assembly may be tricky.

With a Shimano crankset this may be relatively easy to achieve, as the left crank arm is pushed towards the frame with a platic screw and what really holds the arm in place is two other screws that you tighten later (i.e. you have some level of adjustment, as to how much you push the arm against the bearing).
In other cranskets, such as FSA, Aerozine, SRAM and probably some others - the torque applied to the tightening screw holds the arm in place AND squeezes it against the bearings. If you have too wide spacers, you will have friction when you tighten the screw too much, or your crank arm may fall off, if you tighten it too loose. If the spacers are too narrow, you may experience some level of play on the crankset, i.e. the crankset will not feel stiff and the chain line may move about a little bit, possibly making shifts less accurate.

Better use ROAD on road and MTB on mountain bike :)
 
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