Square Taper Crank Maintenance Question

[skitlets]

I've ridden the VO square taper cranks for about 1000 miles. On a ride this morning, I felt the NDS crank loosen just a bit. Made it home without incident.
Should I simply tighten the crank bolt or should I remove and reinstall?

On initial installation, I lightly greased the spindle and torqued to about 45-50nm. I re-tightened to the same torque after the next two rides as well.

[joevers]

Grease the bolt, not the spindle. What's the bottom bracket? Are you sure they're the right taper? If they come loose once I'd just clean and reinstall, if they come loose again you have a problem and need to replace something.

[chismog]

You'll hear both sides I'm sure, but I'm a no-greaser with square taper spindles. Unsure why it would loosen, but you can probably just tighten it up.

[skitlets]

Is there a general rule of thumb on how frequently to retighten and/or reinstall?

[chismog]

Joevers' analysis is spot-on. Once it's set up and tight, it should be done and done. If they're loosening on the regular, you have a separate issue.

I pull mine apart *maybe* every couple years to clean. Square taper isn't a setup that needs much maintenance, imho.

[AngryScientist]

i use a dab of blue locktite on the crank bolts of ST cranks.

no grease on the tapers, install dry.

[skitlets]

Thanks all for the helpful info. If it loosens again I'll reinstall and revisit the grease/no grease debate. For now I took the lazy route of just tightening it back up to 45nm.

[NHAero]

Sheldon taught me to grease the flats lightly and lightly oil the bolt threads.
Jobst says this:
https://www.sheldonbrown.com/brandt/...ng-cranks.html

and Sheldon, revised by John Allen, says this:
Grease the threads of the fixing bolt or nut. Also grease the splines of a splined spindle. Much ink has been spilled about whether to grease square-tapered spindles. Jan Heine of Bicycle Quarterly magazine performed an experiment, voted for grease but indicated that some cranks will have problems, greased or not. The tide of opinion seems to be with light greasing.

I've ridden aluminum cranks on square taper BBs for almost 50 years, following the above advice, and never ruined a crankset. Currently have 11 sets between my wife's bikes and mine.

[jamesdak]

I'm in the lightly grease the tapers and use the blue loctite on the bolts. Never had a square taper come loose on my yet but also haven't had a problem removing when I needed to.

[Hindmost]

You shouldn't have to retighten a crank bolt once the cranks are installed. If you do recheck the crankbolt you might see that it has "loosened" slightly over time; I don't know how to explain this. It is said repeated retightening to full torque creates problems. The original schmanging of the crank onto the spindle is what holds it in place.

[Ralph]

I believe if you are going to tighten Campy ST crank bolts to about 30 ft lbs Tq, they probably should be dry. However...there is dry, and there is dry. I usually clean the tapers, and inside of the crank arms, as if they were new, but don't grease. If I did grease, might stop just short of 30 ft lbs....or tightn very carefully as I got close. And if movement onto the tapers stopped short of 30 ft lbs, that might be enough. With grease on tapers, they will go on tighter....be careful not too tight. I like ST cranks....they just work.

[Mark McM]

Originally Posted by Hindmost

You shouldn't have to retighten a crank bolt once the cranks are installed. If you do recheck the crankbolt you might see that it has "loosened" slightly over time; I don't know how to explain this. It is said repeated retightening to full torque creates problems. The original schmanging of the crank onto the spindle is what holds it in place.

Yes, never re-tighten crank bolts once installed. The reason bolts "loosen" is not because they unscrew, it is because cranks migrate up the taper - and in fact they migrate further with unlubricated tapers.

How do they migrate? Cranks are loaded in many directions during the pedal stroke, and experience a variety of bending and twisting torques. These torques create a "cocking" action on the taper, where one side of the joint is pushed up the taper while the other side is pushed down the taper. But the bolt provides a hard stop which prevents the crank on one side of the joint from moving down the taper, while on the other side of the taper the crank can slide a microscopic amount up the taper. During successive cycles of cocking torque, the crank slowly "walks" up the taper until it reaches an equalibrium between the bolt pre-load and the wedging action of the taper. Because the crank has moved slightly away from the bolt, the bolt looses some of its original preload. And this is why you shouldn't re-tighten the bolt - if you do that, it moves the hard stop platform further inward, causing the crank to find a new equalibrium even further up the taper. If the crank gets wedged too far up the taper, the tapered hole in the crank can be damaged.

Why do unlubricated cranks migrate further? With dry surfaces, there is more static friction between the crank and the spindle. So the crank won't slide as far up the taper for a given bolt static pre-load. However, the cocking action from crank loads can break the static friction, allowing the crank to migrate. Since the crank was less far up the taper to begin with, there is less wedging action of the taper to keep it from walking up the taper, and the crank will migrate further up the taper before reaching equalibrium.

[Ralph]

Originally Posted by Mark McM

Yes, never re-tighten crank bolts once installed. The reason bolts "loosen" is not because they unscrew, it is because cranks migrate up the taper - and in fact they migrate further with unlubricated tapers.

How do they migrate? Cranks are loaded in many directions during the pedal stroke, and experience a variety of bending and twisting torques. These torques create a "cocking" action on the taper, where one side of the joint is pushed up the taper while the other side is pushed down the taper. But the bolt provides a hard stop which prevents the crank on one side of the joint from moving down the taper, while on the other side of the taper the crank can slide a microscopic amount up the taper. During successive cycles of cocking torque, the crank slowly "walks" up the taper until it reaches an equalibrium between the bolt pre-load and the wedging action of the taper. Because the crank has moved slightly away from the bolt, the bolt looses some of its original preload. And this is why you shouldn't re-tighten the bolt - if you do that, it moves the hard stop platform further inward, causing the crank to find a new equalibrium even further up the taper. If the crank gets wedged too far up the taper, the tapered hole in the crank can be damaged.

Why do unlubricated cranks migrate further? With dry surfaces, there is more static friction between the crank and the spindle. So the crank won't slide as far up the taper for a given bolt static pre-load. However, the cocking action from crank loads can break the static friction, allowing the crank to migrate. Since the crank was less far up the taper to begin with, there is less wedging action of the taper to keep it from walking up the taper, and the crank will migrate further up the taper before reaching equalibrium.

Are these things someone learned as a bike mechanic thru experience.......or is this a basic engineering principle which applies to any arm with hole thru it being being tightened with bolt onto a shaft made kinda like a tapered BB axle? IE...would a mechanical engineering student likely run into this principle in a class text book? Just curious.

[jemdet]

Originally Posted by Mark McM

-cut-

I have one less thing to "thought experiment" while riding thanks to this post.

[Brian Smith]

Originally Posted by Mark McM

Yes, never re-tighten crank bolts once installed. The reason bolts "loosen" is not because they unscrew, it is because cranks migrate up the taper - and in fact they migrate further with unlubricated tapers.

How do they migrate? Cranks are loaded in many directions during the pedal stroke, and experience a variety of bending and twisting torques. These torques create a "cocking" action on the taper, where one side of the joint is pushed up the taper while the other side is pushed down the taper. But the bolt provides a hard stop which prevents the crank on one side of the joint from moving down the taper, while on the other side of the taper the crank can slide a microscopic amount up the taper. During successive cycles of cocking torque, the crank slowly "walks" up the taper until it reaches an equalibrium between the bolt pre-load and the wedging action of the taper. Because the crank has moved slightly away from the bolt, the bolt looses some of its original preload. And this is why you shouldn't re-tighten the bolt - if you do that, it moves the hard stop platform further inward, causing the crank to find a new equalibrium even further up the taper. If the crank gets wedged too far up the taper, the tapered hole in the crank can be damaged.

Why do unlubricated cranks migrate further? With dry surfaces, there is more static friction between the crank and the spindle. So the crank won't slide as far up the taper for a given bolt static pre-load. However, the cocking action from crank loads can break the static friction, allowing the crank to migrate. Since the crank was less far up the taper to begin with, there is less wedging action of the taper to keep it from walking up the taper, and the crank will migrate further up the taper before reaching equalibrium.

I concur with/like all of the above, nice post.

Originally Posted by Ralph

Are these things someone learned as a bike mechanic thru experience.......or is this a basic engineering principle which applies to any arm with hole thru it being being tightened with bolt onto a shaft made kinda like a tapered BB axle? IE...would a mechanical engineering student likely run into this principle in a class text book? Just curious.

No, this is not a textbook example, however it is well-understood (albeit tending toward deprecated) bike mechanic knowledge.

Originally Posted by skitlets

I've ridden the VO square taper cranks for about 1000 miles. On a ride this morning, I felt the NDS crank loosen just a bit. Made it home without incident.
Should I simply tighten the crank bolt or should I remove and reinstall?

On initial installation, I lightly greased the spindle and torqued to about 45-50nm. I re-tightened to the same torque after the next two rides as well.

Is it likely that your wrench is mis-calibrated? 50 n-m might be enough to destroy a cheap non-forged crank, but assuming it wasn't damaged and yet it still came loose in short order is not a good sign. Mechanic experience would suggest to me that if you're above, say, 220 lbs in weight and you're riding a properly-installed square taper non-forged crank loose after being over-torqued...well, you may need to choose a forged crank. It's much more common that either the crank coming loose in short order during use was installed with too little installation bolt torque than due to being "over-torqued" and damaged. If the crank bolt has a glop of that factory-installed thread lock on it, perhaps it was affecting your torque reading, but that is not super common either.

What follows is opinion. For a non-cold forged crank, choose 300-325 in-lb installation bolt torque, and live with the squirming up the taper until it's settled, and for cold-forged cranks you can use 350 in-lb. Alternatively, go with what the crank's marketer recommends, as if they, in 2020, actually know? Your warranty claim acceptance may be more important than your understanding.