Peek the Geek

What determines the proper spindle length? Can a given crankset only be used with a specific spindle length? Or can you choose spindle length to achieve a particular chainline?

powers2b

Width of bb and 'dish' of crank.

moxfyre

Yes, you can choose spindle length to achieve a particular chainline. The limiting factor is that if the spindle is much too short, the crank may bottom out against the frame, and it won't be possible to attach it securely to the spindle.

Generally, the recommended BB length for a given crank is optimal, as long as you are using the recommended derailer as well. I suppose you might want to use a slightly different spindle if you're using a different front derailer.

In practice, I've been able to make basically any double crank work well with a spindle around 110 mm, and basically any triple crank will work well with a spindle around 118 mm. Getting the chainline nearly perfect is really only an issue for a fixed gear bike, since there are no derailers to guide the chain, and a dropped chain could be quite dangerous.

Peek the Geek

Okay, I'll just state my specific issue:

Bike is a single speed mountain bike with track-style dropouts, Paul hubs and and ACS freewheel (all SS-specific, so no tensioner being used). BB shell is 68mm. Current bottom bracket is a Race Face Evolve XC (ISIS) that has a 113mm spindle length.

I've run into the problem that, when the chain is taut, my chainline is off enough to keep the drivetrain from rotating smoothly. The chainring is about 3-4mm further inward than the freewheel.

I'm wondering if using a 118mm spindle length BB would be a reasonable solution.

[EDIT] Oh, and the crankset is a Bontrager Race Lite SS (rebadged Truvativ Stylo SS)

moxfyre

Oh, okay, it IS a single-speed. No derailers, none of that junk to worry about. By all means, go ahead and make that change. A 118 or 120 mm spindle ISIS BB should do the trick nicely. ISIS spindles are symmetric, so if you go from 113 to 118, you'll move each crank 2.5 mm further from the frame.

3-4 mm is probably enough to be noticeably noisy on a single speed setup, which will be whisper quiet with a perfect chainline. Though it's a bit surprising that it would be so rough that you can feel it... multi-geared bikes often have the chain bending 20 mm or more between chainring and cog.

Peek the Geek

I can't actually feel it when I'm pedaling. I really only notice it when I've got it in the stand and am turning the cranks or the rear wheel. Maybe I'm just a bit anal.


I actually took the bike to my LBS yesterday to have them cut the steerer tube of my fork down, and I mentioned the problem to them. Today when I picked up the bike they said they had "solved" the problem by putting a 5mm BB spacer on the drive side of the bottom bracket.

Umm... Doesn't that place the drive side crank 5mm outward while leaving the left side unchanged? I can't imagine that that is the most ideal solution for obvious (to me, at least) reasons. What say you?

moxfyre

Fair enough, if it bugs you and you don't mind paying for a new BB. Since you're presumably using this offroad, it might be a good precaution against dropped chain as well.

I don't even know how such a spacer would work. Is the spacer attached to the spindle or between the bottom bracket shell and flange of the right BB cup? I don't see how either would actually work, frankly, without seriously compromising the strength of the whole setup.

Possibly I'm not grasping what they did and it's perfectly strong... If that's the case, no problem with having the right side 5 mm outward of the left side. ISIS BBs are actually pretty unusual in that they're symmetric, whereas most square taper bottom brackets are definitely not symmetric.

Sheldon Brown

Generally, you can adjust chainline by using different length bottom brackets, but the choices available may be limited.

I have a couple of articles that explain this in more detail:

https://sheldonbrown.com/chainline

https://sheldonbrown.com/bbsize

Sheldon "Chainline" Brown

Peek the Geek

The spacer lies between the BB shell and the flange of the right cup, and as for compromising strength, that might be a valid concern. The Evolve BB already requires the use of a 2.5mm spacer. Add that to what the LBS installed and suddenly you're not using 7.5mm of the threads on the cup. Maybe I'll take a look-see when I get home tonight.

As for other types of BBs being asymetric, isn't that made up for in the design of the cranks? I have no idea, I'm just wondering.

And thanks for your help.

Peek the Geek

Thanks, Mr. Brown. I read your articles the other day, and after this thread and rereading said articles, I think I've got a much better handle on things.

It looks like, if I were to change the BB, Truvativ's Gigapipe series has 118mm BBs that should work. But...any idea what "E-type" means?

[EDIT] Nevermind. I think I figured out what E-Type refers to. Thanks.

moxfyre

Sounds scary to me... then again the way I'm picturing it seems so dangerous that it can't possibly be right. I haven't seen the spacer so shouldn't speculate.

It probably is made up for in the design of the cranks... I never thought about that. But if you use a longer square-taper spindle than recommended, you would still throw off the symmetry because square-taper BBs aren't symmetric. A few mm movement is likely imperceptible. Maybe if you use zero-float cleats it would hurt your knees and you'd need to adjust them.

I fiddled with spindle length on my old fixie, and actually flipped a BB spindle all the way around, which moved BOTH cranks about 5 mm to the left!! That got the chainline right, and felt fine to my legs. Then again, I'm told that I'm an exasperatingly un-finicky person in general... though I like to think that I make an exception when it comes to my exacting taste in bikes

Al1943

I had exactly the same thought, how much threading is now holding the BB in place. The spacer reduces the amount of threading. I used a 1.5mm spacer to correct the chainline on my wife's Ultegra triple and I've heard of 2mm spacers being used, but 5mm?, that sounds like a lot but then I'm not really familiar with your BB. I suppose it could be OK, but it caught me by surprise.

Al

moxfyre

That's what I was imagining too... 5 mm of right-side threads not engaged!?!?! Seems like a recipe for too much stress on the BB. Especially because the actual cartridge bearing only extends about 7 mm or so into a typical BB... and beyond that the shell of the BB is rather soft and doesn't support the axle.

Peek the Geek

Well, I took a look at things tonight, and the 5mm spacer took the threads from 16mm to 11mm. It didn't seem like excess stress would be an issue, but nevertheless, I removed the spacer. I just didn't want my cranks shifted off center that much (again, call me anal).

I think I'll make do for a while and consider replacing the BB eventually with a longer one (no big deal, since I got the current BB used, anyway).

Thanks for all the help, guys. This was yet another learning experience for me, and it's nice to know there are knowledgeable folks responding to these type of threads.

moxfyre

Ah, bikeforums You ask a relatively straightforward but well-thought question... and we all tell you how the situation is potentially much more dangerous than you thought, and that your bike might spontaneously combust and kill puppies...

Al1943

Actually 11mm of working threads doesn't sound as bad as what I was thinking, but better safe than sorry. The spacers are cheap and if you care to you could experiment with something less than 5mm. I know they are available in 1, 1.5, and 2mm thicknesses, no reason you couldn't stack a couple of them.

Al