Crank Nuts - are YOURS loose?

I know I could get myself into trouble with that subject line but here’s what I mean: a couple of weeks ago I had my first adventure with a loose crank. Luckily for me, everything turned out fine in the end with the original set of cranks re-installed and the hub still in good shape.
But this got me thinking about the other five uni’s in our household. So I popped off the ol dust caps and, armed with my trusty 14 mm socket wrench, started the assembly line of crank nut inspection. Lo and behold, every uni, save for my brand new MUni had loose crank nuts. Not loose cranks (yet) but every single one needed some tightening.
Hummm, this lead me to the conclusion that I obviously need a regularly scheduled ‘crank nut inspection’ session so that these loose cranks nuts don’t end up resulting in the crank itself becoming loose while out on a ride.

So, how often do you check your crank nuts?!


I tighten the crank nuts on my MUni before EVERY ride. They are modified, aluminum Shimano 175mm bicycle cranks and they ALWAYS need to be tightened. They get jumped on ALOT.

The aluminum cranks on Blue Shift get tested every 50 miles or so. They no longer need to be tightened.

Steel cranks I install, ride a few hundred feet, tighten, ride a mile, tighten, and then they’re good. I don’t do alot of heavy jumping on steel, square taper cranks.

(Get your mind out of the gutter, Erin.)

Aw, come on Harper, it was higher up than the gutter…:smiley:



After getting fed up with my loose crank arm (always the left side), I went to the local automtive store and bought a little bottle of high strength loc-tite (red, for this specific brand). It says that it’s good for up to 230/225 lbs of torque (I don’t know what that means). It says that’s for break/prevail force. All I know is that after putting that stuff on, I haven’t had that crank arm come loose again, even after the Moab trip. It seems to have worked.
Not a bad idea. I’m pretty sure most auto stores carry it, and if not, does.
Good luck!

I might be wrong (wouldn’t be the first time), but those dust caps probably lead to many riders neglecting to check the torque on their crank nuts, which probably leads to many damaged cranks. Dust and mud seem to be much less of a problem than crank nuts working loose. I trashed those caps immediately, and keep a crank wrench in my camelbak (which has come in handy on many rides). Every time I have to re-tighten the crank nuts during a ride, I remove the nut and re-apply lock-tite when I return to the van (and of course, I re-tighten the nuts using Yoopers’ tapping/tightening procedure).

So, according to some earlier posts when I was asking for suggestions related to my loose left crank, I thought that the crank nuts are actually coming loose because the crank is ‘settling in’ further along the hub and therefore exposing slightly more thread at the crank nut end.
So, if you use loc tight doesn’t it just secure the crank nuts in one spot? What happens if the crank moves in towards the hub then? There would be a slight gap between the loc-tightened crank nut and the crank arm itself, wouldn’t there? And won’t that be a problem?
So wouldn’t it just be better to check those crank nuts on a regular basis so that they can be tightened as needed as opposed to loc-tightening them in place? :thinking:


There’s been alot of discussion about loose cranks and nuts but I’ve never seen a description of what I think is really going on. I’ll give it a try …

When a crank is installed and the nut is tightened the crank if forced “up”, or farther on to the square taper. The square hole in the crank is being enlarged as the material around the square hole is stretched. But at the same time the end of the axle (the threaded part) is also stretching due to the force being applied with a wrench.

At some “tightness” or torque on the nut an equalibrium is reached where the square hole in the crank and the end of the axle have both stretched and won’t stretch any farther. Everything is tight, meaning that the tension in the stretched axle is pushing the back side of the nut hard into the crank. Then the unicycle is ridden …

Force on the end of the crank from riding and hopping has a prying affect on the square hole in the crank. The square hole enlarges momentarily but because the end of the axle is in tensionand and the back side of the nut is pushing hard on the crank, the crank moves slightly farther up the taper and the tension in the axle is relieved. There’s no problem with the crank moving up the taper except that now the nut isn’t being forced against the crank any more because the tension in the axle isn’t there any more. The nut is now loose and can unscrew over time. If the nut unscrews then the crank can move down the taper and become loose.

Usually after enough “ride/tighten” cycles even the force applied to the end of the crank from riding and hopping won’t enlarge the square hole any more and the tension in the axle isn’t relieved so the nut stays tight. But in the case of long cranks (especially aluminum) and large forces from hops and drops, the “ride/tighten” cycle won’t end until the square hole in the cranks splits out in the corners from being enlarged and stretched too much.

Grease on the taper is good because it helps the crank move farther up the taper (stretch more) when the crank is first installed. Pressing or pounding the crank on is good because it moves the crank farther up the taper than would otherwise be possible with just the nut alone. Locktite on the threads is important to keep the nut from backing completely away from the crank after the square hole has been pryed larger and the tension in the axle is gone.

I think that if the crank is installed per John Child’s instructions - greased tapers, pressed or pounded on with with Locktite on the threads - further tightening of the nut isn’t necessary. The nut won’t really be “tight” but it won’t back off because the Locktite will keep it in place. The square hole in the crank will temporarily enlarge from pedal force but it won’t move any farther up the taper because the tension in the axle will be gone but the crank won’t get loose because the nut is there to stop it from moving down the taper.

Steve Howard

Very helpful explanation Steve, thanks!


That is similar to how Jobst Brandt explains what happens in the rec.bicycles FAQ.

When the crank is under load and squirms up the taper it briefly relieves pressure on the retaining nut allowing the retaining nut to loosen and back-off. That’s why the Loctite on the retaining nut threads helps to keep things tight. As long as the retaining nut cannot back-off on its own the crank will stay tight. Or at least that’s how the theory goes.

Once I get a crank on tight with Loctite on the retaining nut threads, I don’t mess with it. Retightening it would just crack the Loctite bond.

I haven’t done muni on a square taper in a long time so I haven’t been testing this theory under heavy load. However, on my Coker and my freestyle unicycle I have not retightened the cranks since they have been installed (both unicycles have grease on the tapers and red Loctite on the threads). Once they’re tight and Loctited don’t mess with them.


what about overtightening the crank nuts? Are you not afraid of that?
About a month ago I installed new cranks on my son’s unicycle. I followed John Child’s advise and use grease and loc tite in the appropriate places. The cranks were protected by wood and pounded onto the tapers. Finally I tightened the cranks nuts. When I controlled them last week, I found that threading was gone on one side and the uni begged for a new hub :frowning:

So, the guy at the LBS told me not to overtighten the crank nuts.

From now on I will make it a habit to tighten the crank nuts carefully on a weekly base. Instead of aiming for a one time approach.

Hasta luego,

Thanks, John and Steve!

The dust caps protect the threads that you need to remove the cranks.

One way to prevent overtightening is to use a torque wrench set to about 35 ft-lbs.

Here’s one of the previous threads on loose crank arms. Seems relevant:

Is this the original “John Childs on loose crank arms” reference? This is what I came up with on my search.

This is why I’m still tightening periodically on Muni rides. The nuts are not loosening. The standard aluminum cranks on the Yuni seem to be pretty soft (flimsy excuse for the rider being too heavy), and are still stretching. After too many drops, I usually wind up with a loose crank. If it’s not tightened on the trail, it gets worse in a hurry, presenting the option of 1) tightening, 2) walking back, or 3) damaging the crank and possibly the axle by riding with it loose. I expect them to split soon. They will be replaced with steel cranks, and then probably Profiles (that is, if I find any good trails in Louisiana). This is one area where the weight savings of aluminum doesn’t seem to be very practical.

Why is a loose crank arm more likely to damage itself and the axle? Won’t the torque be the same whether it’s loose or not? I apologize for my lack of knowledge in physics-- I’m a Humanities major. :roll_eyes:

As you pedal a unicycle, you apply forward and reverse torque on the cranks. If they are loose, the tapered surfaces on the crank and axle can become deformed as they are worked back and forth with every rotation. Aluminum cranks are more likely to be deformed, but less likely to damage the axle. This has been explained in much greater detail in past threads. A search on ‘loose cranks’ should provide plenty of good reading from the experts.

Is the LocTight you use the ‘medium strength’, in other words the kind that says it can be ‘broken’ with handtools? (Color isn’t that useful an indicator as we have a red loctight (“LockThread” is the brand name I think) that is pretty heavy duty and the blue that is considered medium.)

Erin, just wanting to be sure she is using the right strength of Loctight so as not to end us gluing the cranks nuts permanently to the hub! :astonished:

I use the red high strength Loctite threadlocker part number 271. It’s what they consider to be permanent strength.

But don’t let the permanent strength designation scare you. You can easily break the bond with hand tools when the red stuff is used on larger nuts and bolts like the crank bolts. On a small bolts the red stuff can be close to permanent strength, but on larger nuts and bolts it’s easily removable with hand tools. I can easily remove the Loctited crank nuts with an 8" wrench.

When applying the Loctite try to make sure that the threads are clean and free of grease, oil, and dirt. Clean threads will bond much more strongly than dirty or oily threads.

And so, let’s say I was a chicken and started out by using the blue - medium strength Threadlocker, would it still be helpful or will the crank nut loosen given that this color Loctight is not that strong? :frowning:

Also, does it need 24 hours to cure - the package doesn’t say anthing about that… :thinking:


Loctite leads you to believe that using red is ‘permanent’.

I have yet to have any problem undoing anything with red loctite (high strength # 272)

This is the only flavour I use. every application, every size of bolt

I don’t know how the medium strength blue stuff will do. I haven’t used it on the crank nut. I suspect it would hold the retaining nut tight enough. The purpose of the Loctite is to keep the nut from loosening on its own due to vibration and other forces. The blue stuff is probably strong enough for that. If you have to take your crank off for some reason use the red high strength stuff next time.

The Loctite needs to cure for about 6 hours before use. You shouldn’t start riding on it immediately. It cures faster when it’s warm.

The threadlocker cures in the absence of air. That means that the stuff the oozes out and is still exposed to the air will not harden. The stuff under the nut and in the threads will harden. Just wipe away the stuff that oozes out since it’s not going to harden.