crankholio

So it seems the 68x110 BB I got is asymmetric with the non-drive side being longer. I have a 2.1mm spacer I could put between the fixed cup and frame on the drive side to help even things out, but I'm wondering does it matter? Will 4mm cause any issues?

Bill Kapaun

Are you sure the DS chain stay isn't "pinched in" to make clearance for the rings?
How many rings & cogs?
IF both an odd number to can easily eyeball to see if the RINGS are where they are supposed to be.
Put on the center ring & cog.
Stand behind the bike and see if the chain runs parallel.
Then- check on EACH adjacent cog and see if the misalignment between those cogs is equal.

crankholio

Wouldn't that just make the spacing appear larger on the drive side? But the spacing is greater on the non-drive side.

79pmooney

What are you measuring to and from? What kind of frame, crankset and BB?

crankholio

Space between crank arm and chainstay at the center point of the pedal spindle (see orange arrow in picture):



Comparing to same measurement on the other side (not shown). I'm measuring about 12mm on the drive side and 16mm on the non-drive.

FMFXTR square taper single speed crank (it's a cheap China-special crank if you've never heard of it)
Shimano UN55 68x110 BB (it looked suspiciously asymmetric when I pulled it out of the box)
K2 Zed Sport (el-cheapo frame from Craigslist)

I replaced a UN55 68x118 as I'm trying to get the chainline correct and the chainring was sitting too far out. The chainline is actually good now with the 110, and if I use a spacer to try to fix the mismatched spacing between crank arms, then it's going to put the chainline out a bit again (although not as much as before).

Andrew R Stewart 

Back in the day having crank arm ends that were of different distances from the stays was the norm (although usually with the DS being further away from the stay). I never heard discussion of fit or body problems due to this slight offset.

Some BBs won't allow much offsetting to the drive side because the NDS cup has a lip that will dig into the shell's threads. (Shimano as a common example) Andy

nomadmax 

Ride that bike. If the chain line is right, the rest doesn't matter.

crankholio

Alright, thanks. I think I'll try riding it for a while with the offset and see how it works.

Well, if one crank arm was a foot further away from the chainstay than the other, it would definitely matter. So that asymmetry can definitely matter. It's just a question of whether 4mm will matter.

I'm posting because this is the second bike I've used this particular brand/model crankset on, and both show the non-drive side a little further out (the other bike is only about 2mm difference, though, and it's using a UN55 68x113). And after a bunch of measurements trying to get the chainlines perfect (which it seems I have), I have this other variable of unequal crank arm spacing, and I fear if I try to address it, then I'm going to mess up my chainlines again, and I'll be in this oubliette of dependent variables that will swarm and encompass me in a desolate pit of darkness and despair.

79pmooney

Shimano BBs are usually symmetric. Poor tolerances on the crank might make for a couple of mms difference in fit on the spindle. Maybe the right is a touch big and the left a touch small? (I replaced a Phil Wood that was a touch too close with a Phil spindle 3mm longer and gained 5mm. That's with very high quality stuff.

Symmetry makes very little difference to our perception of fit. My knees need the lowest Q-factor I can get but care zero which side that comes from.

Edit: While I am on Phil Wood, know that they will make any length and asymmetry you want. Shimano makes very high quality BBs so you can decide how much you want to alter each side, adjust the 110 symmetrical Shimano accordingly and order that from Phil Wood. Phil's BBs are adjustable so you can play with the symmetry to dial in the chainline. Expensive and not overnight, but fun. Good company and their stuff is beautiful!

Ben

nomadmax 

I said "Ride THAT bike" meaning the one with a 4mm difference, not a made up one with a foot difference.

4mm doesn't matter if the chainline is correct. Your time would be better spent riding and enjoying the bike.

crankholio

I did have a Phil Wood BB a long time ago. I got it because it was the only thing I could find with Raleigh threaded cups for an old Raleigh frame. It was a very nice BB, but probably the cost of 6-7 UN55's. I still have my Phil Wood BB tools as well, but I haven't used them since. Maybe one day once again

Will do. Gonna take it out tomorrow, I think.

IPassGas

OMG, thank you, someone still uses UN55 and the world is again ok. And, just perhaps, I need not yet hoard UN55's.

Andrew R Stewart 

[QUOTE=crankholio;21689748]I did have a Phil Wood BB a long time ago. I got it because it was the only thing I could find with Raleigh threaded cups for an old Raleigh frame. It was a very nice BB, but probably the cost of 6-7 UN55's. I still have my Phil Wood BB tools as well, but I haven't used them since. Maybe one day once again



Will do. Gonna take it out tomorrow, I think.[/QUOTEcrankholio-

I do this stuff 5 days a week, not including my personal stable of rides. During the riding (and thus work) season the last thing i want to do is to HAVE TO service my personal stuff. Thus (that word again) I have chosen to go Phil for most all my bearing needs. I can get (or could prior to the C-19 pandemic) UN-55s for very little and I have a number of 52s, 54s, 72s on my shelf nearly 5 feet below my desk top. I now use them to size what I need and then get a Phil.

O, too, like a narrow Q but I need low gears and thus (bingo) run a triple on all my frequently used bikes. The Phil BBs let me get the narrowest Q and also the best chainlines. Andy

delbiker1 

I have a couple of bikes with the UN55, including one that is brand new and just installed. Chance are good that I will never have to buy another one. Wonderful, durable BBs.

JohnDThompson 

Functionally, only the drive side offset makes any real difference. It has to be long enough for the chainrings to clear the frame, but short enough for the front derailleur to manage. The non-drive side only has to clear the frame. In most cases a few millimeters more or less is no issue. Some people may have idiosyncratic sensitivity to "Q-factor" (the distance between the right and left pedals) where non-drive side clearance may be a concern, but this is not common in my experience.