8

Bike details:

  • Singlespeed (steel)
  • Square taper threaded BB
  • 58/19 ratio (hence huge chainring rubbing the frame)
  • 130mm bcd 5-bolt crank/chainring
  • Mileage on cranks: several thousand km in all weather (tropical climate)
  • Riding purpose: commuting, ~125km per week, with sporadic longer rides.

Background info:

I have this ridiculous tooth count due to component availability at the time of setup. Initially, there was a clear gap (around 1-2mm?) between the chainring teeth and the frame. All was good.

Almost two years and thousands of kilometres later, the cranks started to get loose (creaky on downstroke). After having to tighten the bolts twice over 2 weeks, I removed the cranks, cleaned and lightly greased the spindle (yes, I know, this is a point of contention too) and crank holes, and re-installed them (I do not have a torque wrench... let's just say they're tight enough but not too tight).

Now the chainring rubs at certain crank positions. Visual inspection of the crank arms shows no cracks/damage.

Possible solutions I came up with:

  1. Get a BB with a longer spindle (cheapest; sounds the most short-term; fit may be affected; chainline may be affected; crank arm wear and tear may bite me later).
  2. Change the crank arms (so far, impossible to find online. I tried CRC, Wiggle, bike24. The others have very expensive shipping to my place. Haven't asked LBS yet.).
  3. Change the chainring and cog (expensive since I'm running a White Ind ENO freewheel; crank arm wear and tear may bite me later).
  4. Change the chainset/crankset and cog (bloody expensive, and I haven't been able to find a singlespeed square taper 52t (for 52/17) chainset online, meaning the arms and ring will have to be bought $eparately. Haven't asked LBS yet.)

Suggestions? Any input is welcome.

  • 3
    On many bottom bracket assemblies both cups have lock nuts, allowing the axle to be shifted left or right by 5mm or so. Check yours to see if there are locknuts on both sides or only on one side. – Daniel R Hicks Mar 11 '17 at 19:41
  • 2
    But your statement that the chainring only rubs at some points in its rotation may suggest that the crank&ring are not squarely mounted on the axle. You may want to remove the crank and check for something stuck in there, preventing it from seating squarely. – Daniel R Hicks Mar 11 '17 at 19:44
  • 1
    What crank and chainring are you using? – Casey O'Neill Mar 11 '17 at 23:25
  • 1
    Have you pranged or biffed the bike recently? Could be a subtle bend in the chainring. I'd also examine the whole bottom bracket axle for play and replace it with a cartridge bearing if there's anything found wobbling. – Criggie Mar 11 '17 at 23:31
  • 2
    @ChrisH - What about if you're lying to them the first time? ;) – Daniel R Hicks Mar 12 '17 at 23:31
4

The first thing to do if you haven't yet is confirm that the taper bore on the drive side crank is still in good condition. Take it off, hold it up against a light, and look for any deformation. The squares formed by the two ends of the bore should be in perfect alignment. Because square taper cranks are a press fit, there is nearly no tolerance for deformation here before the crank has issues with coming loose or falling off.

What you're experiencing here is one reason, really the biggest, why I don't like to grease tapers. You get unpredictable things happening with how far the crank sinks on, and it seems like it makes it more likely to be an issue over time. Some hold the theory that the combination of greasing spindles and re-tightening the bolt can contribute to or cause the cranks failing outright over time, which is a rabbit hole of a topic, but google Jobst Brandt's writing on this if you want to understand that argument.

If the bores are still in good condition, and the chainline was right before and now it's too far in in front, then getting a new BB that puts the chainline correct again is a fine solution, barring any issues with tight clearance between the ring and the frame. Usually I don't really see anything less than around 3mm between the frame and any part of the crank as acceptable, although it's a line I'd be okay pushing on my own bikes.

The cheap and simple solution if it works is to put a 2mm or so spacer under the shoulder of the drive side bottom bracket cup. This will make the cranks asymmetrical by that much, but most people wouldn't notice. The reason this doesn't always work is that many left side cartridge BB cups have a lip on the end of the threaded section that will keep them from being able to sink into the BB shell to correctly secure the cartridge once the spacer is in place. (Usually the end of the threaded section pokes out just a little bit, with the spacer it will need to be below level of the shell face just a little bit.) Spacers such as this are a common bike shop item. Using a single 2.5mm Hollowtech 2 BB spacer is a likely thing to be able to find, or a freehub body spacer, or a cog spacer off a used cassette, or a 1.8mm 10->11 speed spacer, or a 1.8mm Shimano cassette spacer for Mavic road wheels, etc.

If the left cup does have that lip, you may be able to grind it off without weakening the cup too much.

  • Thanks for your answer, @Nathan ! I've read Jobst Brandt's article, it sounds pro-greased spindles though? Anyway, I degreased the spindles and bores (even wiped them with alcohol), and the ring still rubs after reinstallation. I think it could just be normal wear and tear, as they don't look distorted to me... what do you think? (see imgur link) imgur.com/a/WMD4e Though for my current situation, I might get a bb with a longer spindle, and/or ask my LBS if they have replacement crankarms. – pebkac Mar 13 '17 at 14:43
  • 1
    He is pro-greasing but he also takes the position that one shouldn't then do maintenance tightening/torque-checking on them because of the crank slightly migrating around on the spindle naturally. The theory is it won't actually be at risk of coming loose, but any movement lessens the thread preload some so the torque will seem low. I actually heed that advice on my bikes without issue, but at work I torque-check probably 40-50 square taper cranks a week. It's an unavoidable thing in service since you don't know the starting torque was right. Not being able to torque it to spec doesn't work. – Nathan Knutson Mar 14 '17 at 1:05
1

Well, you have learned that square taper cranks deform a bit when used. This should not be a reason to change them.

The sensible solution would be replacing the chainring and cog. For commuting there isn't any need to waste money on White Industries components. The additional benefit is that afterwards you can replace worn cogs with usual off the shelf components.

EDIT: Cogs for ENO freewheels are replaceable, and while they are not exactly cheap, replacing the cog costs far less than new similar freewheel.

Of course, if cheapness is the first requirement, you could make a shim between bottom bracket axle and crank. It is not a good solution, but not worse than mixing Italian and ISO bottom brackets.

  • have you actually had success shimming square taper bottom bracket? – Paul Mar 12 '17 at 7:05
  • Thanks for the reply ojs! I'd say my bike is a "high-performance commuter" and hence the White Ind freewheel won't be wasted. ;D I just worry that the cranks may need replacing soon, and if it's better to take care of that now than later. Also, aren't Italian and ISO BBs totally different things? Can they even be mixed? They won't fit right? – pebkac Mar 12 '17 at 7:32
  • The Italian and ISO BBs have same taper angle but slightly different dimensions. So, you don't get as much contact as with proper fit but people have been using mismatched BBs and getting away with it. No, I haven't tried the shim because I haven't needed one - and it's probably better to shift the entire bottom bracket right. – ojs Mar 12 '17 at 10:58
1

Summary: I should have changed the chainring, and maybe the bb.

Details: The cranks did indeed move closer to the frame when re-tightened. The key point was that the chainring teeth only rubbed at certain crank positions. At the rest of the positions, they were merely I'm-not-touching-you close. The damned thing was ever so slightly warped.

Anyway, I went to my LBS, where the mechanic assured me that my cranks were fine, and he'd try a longer-spindled BB (from 110.5 to 113), which solved the issue behind the original post - the chainring no longer rubbed the frame.

However, there was still a grinding feeling when pedalling, again at certain crank positions. Checked the chainline: nope, it was fine.

Lo and behold, it was from the chainring teeth rubbing... THE INSIDE OF THE CHAIN LINKS.

Thanks to everyone for the comments and answers!

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.