So, I have a Middleburn Uno square taper crank and a Shimano UN55 square taper bb. When I start to tighten the crank bolt the crank presses against the casing of the bb before its anywhere near tight enough, so that you can’t turn the crank. Has anybody else experienced this?

BB spindle too short?

Seems to be I guess but wouldn’t the bit that the cranks fits to all be a standard length?

It did fit fine to a 73×113 bb but not to this 73×107 bb so that does sound like the case but I’m surprised that the fitting isn’t compatible all across the various size options…

Maybe there is a reason for the different size options and a reason to buy the right size. Otherwise why would there be size options.

Other than for frame differences of course.

This is when there is a little steel collar on the axle, yes?

I’ve seen a crank bottom out on that before being fully tight, but still tight enough to work. I’ve never known why the collar is there.

Middleburn do recommend a 113 mm axle. I’ve found that sometimes a 115mm is actually better for one of my pairs of RS7, them being 10+ years old and having been on so many bikes.

My Missus* says you have fitted a 6mm shorter axle that might be the problem.

*although she isn’t a engineer, doesn’t drive a A4 or own a filing cabinet.

Yes I had same problem with a 110mm on my RS7.. A 115mm sorted mine(couldn’t get hands on 113mm at time)

Take a hack saw and cut through the centre of the BB cartridge shall, but don’t saw through the spindle.

Then file roughly 3mm away from each of the inside surfaces of the cartridge shell and then glue it back together, so it now measures 67mm.

Then file the BB shell on the bike down to 68mm across, retap the threads, and refit the BB.

Put the cranks back on and they won’t hit the frame anymore.

Next problem is the chainline – anybody got any suggestions ?

I guess it’s back to the 113mm axle for me then! Pretty annoying to be honest because the chain line for my Alfine hub is way off. Oh well!

must admit that chainline is something I am not too sure about.

All mtb rear hubs are 135mm, so why do different bikes need different BB spindle sizes, surely a parallel line back to the centre of the rear cassette is always going to be from the same spot above the crank arm?

Yep, the hub width is fixed, hence the need for a shorter axle to bring my chainring I’m line with it. I’m running an internal hub gear so there is only one sprocket and one chainring. It runs fine but I’m presuming that it will all wear out quicker if it’s not aligned!

With a 113mm bb the chainline will probably be about 48mm I think. Which should be about 42mm for Alfine I think. I thought that alfine sprockets were dished and that it was possible to move the chain line over by flipping the sprocket? Could be wrong though. You could use chainring spacers to move the chainring overbuy it’s not the ideal solution.

3mm over 400mm won’t wear your chain out any quicker

Thanks Toppers- actually using Nexus sprocket, which I don’t think is dished (I’ll check)

Also thanks cynic-al- good to know!! 🙂

Ps toppers- all your number were correct, Shimano website recommends 42.7mm and I measured it at just shy of 49mm

An expensive option, but I used to run my RS7s on a 107mm Royce BB, the cups sit flush with the BB shell so chainline can be much narrower. Not sure if relevant to the UNO, but my old 5 arm spider has a narrower chainline than the newer 4arm jobby that i’m now running, if it’s any use then you’re welcome to have it.

Coatesy, if the op doesn’t want your spider, could I put in a request? Thanks!

You could do, almost an obsolete standard now, virtually no rings available from Shimano with their lovelylovely shifting, though other makes are still out there. Give the OP until tomorrow night, if he doesn’t need it then it’s yours.

Markie it’s all yours! 🙂

Ace! It’s to allow me to go singlespeed with a bigger ring than the UNOs, plenty of old DH rings that should bolt on and no shifting worries in sight!

My email is in my profile coatesy (plus I’ve mailed you :oops:). I’m sure we can make a deal work!

I have previously had exactly the same problem. Had a long chat with Matt at Middleburn and according to him there are two possible reasons.

The taper in the cranks is worn. Thus it “sits” further up the axle.

And/or

The chainset is for an older/narrower chainline and thus the lock ring protrudes towards the bb shell.

I solved it by getting a wider axle.

But you could get a different spider.

I measured the two spindles as I was puuting the old bb back in: the 107 is 2mm shorter than the 113. I guess that’s that!