I installed this new bottom bracket and crankset. I placed two 2mm spacers on the drive side and I have a feeling I don't have enough spindle on the non-drive side crank arm. The little safety tab does drop in properly but I still think I don't have enough spline engagement. When I measure with calipers, the end of the spline is 9mm below the surface of the crank arm.

What does the forum think? Do I need to remove some spacers? Get a new crankset?

One problem might be that I'm using a mountain bike bottom bracket with a road crankset. Are the MTB crank spindles longer?

What bike do you have this in? What model crank is this?

I'm not sure removing the 2mm spacer will give you enough.

Do you see that stopper plate sticking out from between the gap where the pinch bolts are? That should be able to be pushed in when the crank arm is on the spindle far enough. But it's so far off the spindle right now, you might actually get a false positive.

Road crankset in a mountain bike is very likely your issue. Road bikes typically have a narrower BB shell and therefore a shorter spindle. And I think the mtn bike BB's also set the overall distance different in a road shell than would road bike BB in a road shell.

Spindle is too short here. Try eliminating the right side spacers. Otherwise, I'd pass.

I wouldn't be surprised if mtb cranks were made for 73mm BBs and included spacers for use on 68mm BBs.

End of the spindle should ideally be outboard of both pinch bolts.

If that Safety Tab was correctly engaged, it would be completely flush and not visible.

Barry

Do you need the spacers for crankarm/chainstay clearance?

Way, way too short. Plug the bolt/plug that threads on the inside of the spline may not reach the threads.

MTB BB shell width: 73mm
Road BB shell width: 68mm

Simple answer: Remove the spacers and the crank spindle will engage the splines as they should

I have to ask, are you sure the spindle is all the way in from the drive side? Sometimes it needs a little bump from a rubber mallet.

I just realized the OP said So 4mm might make that work. But still a road crank in a unknown frame with a unknown width BB shell we don't know anything about with a BB for a mtn bike. Knowing something about those and the number of speeds on the rear of the bike and OLD might let us guess if the chain line will ever be correct.

Did you try to install a road crank for a 68 mm BB shell in a mountain bike shell that's 73 mm wide? And then add 4 mm of shims so the crank would clear the chain stay?

Sorry for the thin information. Here’s the full story.

I’m trying to install a Hollowtech II bottom bracket and double chainring (50/34) crankset on a Brompton. My idea is to manually move the chain to the small chainring for climbs and big ring for flats.

The shell of the Brompton is 68mm. The challenge is that, without the spacers, the rear triangle of the Brompton interferes with the small ring when the bike is folded.

4mm of spacers gives me good clearance but not enough spline engagement. I switched to 1.5mm spacer and the engagement is better. (photo below).



The plastic safety tab does drop flush. But, I still can’t fold the bike if the chain is in the small ring.

Not sure if I’ll keep it like this or go back to square taper but it’s an interesting project.

Your outer pinch bolt is still outboard of the end of the spindle. I wouldn't trust that crank to not work itself loose.

Is this a Shimano road crank or Shimano mtn bike crank? Mtn bike crank will have a longer spindle/axle, IIRC. Measure the BB shell width between the cups.

This.

73mm is no go. 68mm is a go.

Actually not, A crank designed for a 73 mm bottom bracket shell might work, but the chainrings would probably be too small for this bike. The other thing is that the OP wants to shift the chain manually without using a front derailleur. I foresee many chain drops with this setup

But the OP appears to have a Claris road crank. If thew BB shell is 73mm on that Brompton, then the OP has the wrong crank.

In a quick look at the pictures of new Brompton folding bikes, I didn't see any with a readily identifiable Shimano crank. The OP really needs to lay out what they are trying to do. And with what as well as what the bike use to have on it.

Post #12 explains things pretty well. The issue is the folding function of the bike, hence the spacers on the drive side that are causing the spline engagement problem. My advice to the OP: go back to square taper on this bike so that you have more leeway with choosing spindle length to work with the folding function of the bike, and no issues with the crank being installed incorrectly, which is the case with the crank in the photo.

This was an experiment. I find the gear selection on a 6 speed Brompton to be inadequate when touring. Quite a few people have a 2X crankset on their Brompton and they seem to work well. I wanted to try it with off the shelf, cheap Shimano parts.

A 73mm MTB crankset would work but as the commenter said, the chainrings are too small.

You probably already know this, but in case someone else doesn't:

Assuming that the front derailleur is still in place and was adjusted correctly for the original chainrings, you can confirm that the spacer arrangement is correct by checking the front shifting. If only very minor tweaks to the limit screws are needed to get the shifting working right, the spacers are placed correctly.

But not in this specific case where there is no FD

I’ve seen a Dahon folding bike with a full Ultegra setup. That might be worth considering as an expensive alternative to hacking the Brompton?