Saved by thumb throttle

Today I just had the rather frustrating experience of losing the metric M8 allen bolt that secures right foot pedal to Bafang BBS02 crank arm. My pedal suddenly dropped to the ground and I had no luck finding the original bolt. However return trip was no problem riding on full throttle with just my limp right foot unable to pedal.

Hate to imagine a scenario where I'm miles from nowhere and this happens again. Not willing to use thread-lock on this, because IMHO this glue works too good and you may need a torch just to loosen the bolt again.

I'm curious what the hardcore MTB riders are using to secure this critical bolt in place. Safety wire and drilled head bolts as is done with aircraft propellers?

ngant17 said:

I'm curious what the hardcore MTB riders are using to secure this critical bolt in place. Safety wire and drilled head bolts as is done with aircraft propellers?

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You can use:

- an easier to remove grade of Loctite (they come in all sorts of grades of easiness)
- a nylock nut (if the arrangement allows it - it has to extend out past the end of the crank arm for this to work)
- a nylock bolt (harder to find but if you can find it they work great)

Since pedals don't attach with a bolt, I assume you're talking about the bolt that attaches the crank arm to the bottom bracket spindle. Once the arm has dislodged once, the square taper hole becomes distorted and will continue to loosen due to rotational play. You have to replace the arm.

Once you've replaced the arm, there are two measures to help prevent loosening of this bolt: First, make sure that the square taper interface is greased before installing the arm. Second, be sure to get the fastener very tight. Check it from time to time, but don't aggressively tighten it over and over; you can split the end of the crank that way.

Unfortunately, the freewheeled crank of a BBS02 is more vulnerable to such loosening than a normal crank. The late Jobst Brandt explains the principles at work:
https://www.sheldonbrown.com/brandt/loosening-cranks.html

There aren't many mountain bikers these days who still use square taper spindles.

Yes we're talking about the crank bolt. Actually now that I'm relaxed, I have to say that it was the left bolt not the right, I just came home partially frustrated and realized I got that mixed up. Can't change the square taper as it's Bafang.

As for Locktite, I understand the red requires heat to disassemble, and blue is more forgiving. Red Loctite 271 is for permanent assembly. only removable by heating to 500°F. I"m pretty sure that's accurate based on my previous auto fixes.

Blue Loctite 242 is gen purpose threadlocker designed for disassembly. So I'm thinking to go for the blue this time, and ordering extra 8M crank bolts just in case. I don't need this problem coming back to haunt me. If so, next time it's gonna be "'better red than dead."

My next frame will have to take in to consideration the bottom bracket/spindle design based on this experience. I'm still keeping 68mm bottom bracket. What is a better spindle design that might prevent crank arms from dropping off? Am I limited with the 68mm BB?

Don't forget the new crank arm. There's nothing you can do for the one that fell off. For what it's worth, you can feel the crank wiggling for a while before it goes off on its own. But if it wiggled around much, you'll still need to replace it.

Purple Loctite is the low strength kind.

When mine came loose (despite having been assembled with grease and tightened properly), I replaced the cranks with steel unicycle cranks. They were cheap, had a narrower tread width, and never came loose.

I don't know if anybody makes an integrated crank drive with another type of spindle interface, but note that ISIS cranks are just as likely to self-loosen as square tapers.

FWIW, I've disassembled a lot of (cheap) bikes, and a few middling-priced ones, that I've picked up over the years from thriftstores, garage sales, freecycle, etc., and a number of them have had blue loctite (or blue something) on the ends of various bolts, from the crank bolts to rim brake boss bolts to disc rotor bolts. Some have taken a bit of force to loosen, but none were impossible to get out.

But I have had a number of bolts with no lubrication or locker that were so hard to get out I ended up cutting them off instead....

Sounds like bso from department store.

amberwolf said:

bolts with no lubrication or locker that were so hard to get out I ended up cutting them off instead....

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Chalo said:

Don't forget the new crank arm. There's nothing you can do for the one that fell off. For what it's worth, you can feel the crank wiggling for a while before it goes off on its own. But if it wiggled around much, you'll still need to replace it.

Purple Loctite is the low strength kind.

When mine came loose (despite having been assembled with grease and tightened properly), I replaced the cranks with steel unicycle cranks. They were cheap, had a narrower tread width, and never came loose.

I don't know if anybody makes an integrated crank drive with another type of spindle interface, but note that ISIS cranks are just as likely to self-loosen as square tapers.

Click to expand...

Your advice for unicycle cranks is consistent with the positive reviews on Amazon for the Sunlite Unicycle Crank Arm Set, the majority of buyers that submitted reviews are BBS02 or BBSHD. So yes, replacing the aluminum stock Bafang crank is definitely on the to-do list now. Weight of steel isn't issue with +58v running my electric MTB. Dropping a Bafang crank arm w/pedal is a nightmare I don't want again.

I think I'll also go with the weaker glue as mating steel-to-steel male/female threads may hold their torques longer without the full-on holding power of red locktite.

ngant17 said:

Chalo said:

Don't forget the new crank arm. There's nothing you can do for the one that fell off. For what it's worth, you can feel the crank wiggling for a while before it goes off on its own. But if it wiggled around much, you'll still need to replace it.

Purple Loctite is the low strength kind.

When mine came loose (despite having been assembled with grease and tightened properly), I replaced the cranks with steel unicycle cranks. They were cheap, had a narrower tread width, and never came loose.

I don't know if anybody makes an integrated crank drive with another type of spindle interface, but note that ISIS cranks are just as likely to self-loosen as square tapers.

Click to expand...

Your advice for unicycle cranks is consistent with the positive reviews on Amazon for the Sunlite Unicycle Crank Arm Set, the majority of buyers that submitted reviews are BBS02 or BBSHD. So yes, replacing the aluminum stock Bafang crank is definitely on the to-do list now. Weight of steel isn't issue with +58v running my electric MTB. Dropping a Bafang crank arm w/pedal is a nightmare I don't want again.

I think I'll also go with the weaker glue as mating steel-to-steel male/female threads may hold their torques longer without the full-on holding power of red locktite.

Click to expand...

You can also put a electrical tape above the crank arm screw hole opening to hide the screw and not lose it if it does come loose.

If you do it with the same color, it will look better than without it.