Recently, I purchased the Nimbus 36" Deluxe with AT7 touring handles from unicycle.com. Up until today, it’s been a dream to ride, and I’ve had no complaints about it. Unfortunately, today was the start of a HUGE problem:
I was descending a hill and noticed that the left crank was loose…wobbling back and forth at least a full centimeter. At this point, I walked it back to my friend’s house to find the problem. We were surprised to find the bolt that holds the crank arm onto the hub was extremely loose and the crank arm proceeded to come off VERY easily. We cleaned everything off and reattached everything tightly. After a ten minute ride back to my dorm, the left crank was loose again (exactly as serious as before). Upon further inspection, we found that the bolt that holds the crank onto the hub tightened clockwise, which was surprising, considering that it should, theoretically, tighten the other way, considering that the pedal tightens counterclockwise. I’ve not noticed the problem on the right side, but my left leg is the one that puts the most power into the cycle. Could it be that I am constantly loosening the cranks merely by riding?
No, the hub has right handed (clockwise to tighten) threads on both sides. It is only necessary to have left and right handed threads on the pedal/crankarm interface due to the fact that the pedal rotates at the end of the crankarm. Most likely, the retaining bolts at the ends of the hub were not properly torqued at assembly. Contact UDC and let them know about the problem in case the crankarm taper has become damaged
A hub cannot be put on backwards UNLESS it already has the cranks attached to it. The cranks AND pedals should somewhere be marked “L” and “R” for left and right respectively. These are the directional components, not the hub. Check those first.
I’ll venture a couple thoughts on this, although better advice may follow.
To your first question, yes it is definitely possible that unicycle.com assembled something incorrectly. It has been known to happen.
That said, if they indeed got it backwards, I think you would have seen a different problem, i.e. your pedals, rather than the crank coming loose.
Did the cranks come with an L or R on them, either etched into the metal or via a white sticker? My son’s Nimbus had them identified that way by UDC. I believe both of the nuts that hold your cranks on will tighten clockwise. Righty tighty, lefty loosie. I may be wrong, but I think it is only the pedals that have a different orientation depending on the side of the unicycle they belong on.
Do you have a torque wrench? That would be the best place to start, ensuring you have the crank nut tightened to the proper tightness (I use 40).
There have been threads in the past with specific instructions for mounting the crank arms properly, including making sure you grease up both the square axle bolt and the inside of the hole in the crank the bolt goes into. I think unless you mount it properly using a torque wrench, it will be difficult to isolate the problem. There may be a fit problem or an issue with the metal being compromised, but that will be difficult to determine if you don’t mount the crank to the proper torque. I have never had a crank come loose when mounting it to 40 (foot pounds?), even after 1,000 plus miles.
I’ll see if I can find those mounting instructions and post a link.
A loose crank with a square taper hub is a common problem. It’s something that you have to learn to watch for so you can catch the problem before it damages the hub or the crank.
Unicycles have more problems with cranks working loose that bicycles. This is because unicycles put both forwards and backwards pressure on the cranks regularly and cyclically. A crank tightening procedure that works fine for bikes does not necessarily work well for unicycles.
Anyways, here’s instructions on how to properly install square taper cranks on a unicycle: Square Taper Crank Installation and Maintenance. That procedure is the result of testing and experience under unicycling conditions. It’s reliable and proven. It has worked for me reliably, except I go to 35 foot-pounds instead of 40 foot-pounds.
If you or your friends don’t have the proper tools to do the work yourself then take the unicycle to a bike shop and have them do it. Print out those instructions and have them follow the procedure. They won’t quite understand why. Tell them that it’s because unicycles are unique and please humor you by following the procedure. After that the crank should never come loose. If it does it is probably because the crank tapers have been damaged or someone broke the Loctite bond by tightening or loosening the crank bolt.
Everyone, forgive my curt response earlier. My entire day has been sort of sub-par. In my angry state, I missed a few things in earlier comments. Anyhow, try not to hold it against me.
I wonder if it has something to do with the crank arm assembly itself. This friend of mine has the same unicycle, but with 125 mm cranks instead of 110 mm. Both of us have cotterless cranks, but for some reason, it seems that they are assembled differently. I can’t confirm this because we don’t have the right tools at the moment to remove his cranks. But something we do know is that the cap that fits around the bolt fits on his unicycle but does not fit on mine. Any ideas?
Dust caps on unicycle cranks can be a pain. Different brands and models of cranks fit on the hub to different depths. Some cranks fit so far on the hub that the crank bolt or crank nut sticks out and interferes with the dust cap.
Plastic dust caps are also a pain. They don’t hold very well and will pop out. Plastic dust caps are also thicker around the edge so they can have additional interference problems with the crank nut/bolt.
The best dust caps are the metal ones that actually thread in the crank. Plastic ones don’t hold. UDC has metal ones with full threads. You can also find quality metal dust caps on eBay and at bike shops that carry old parts (shops that carry used bike gear are a good source).
An additional problem is that the style of dust caps we use on unicycles are old obsolete technology for the bikes. Bikes have moved on to integrated self-extracting crank bolts that have an integrated dust cap. It is getting harder and harder to find quality metal dust caps now. It’s not a part that is really manufactured any more. So most of what you find now is old stock that a bike shop still has on hand. I don’t know where UDC are getting their threaded metal dust caps, but I suspect they’re hunting around for old stock and buying it up.
I had one set of cranks on a particular hub that made it very difficult to get the dust cap to thread on. The crank nut/bolt stuck out so far that I couldn’t get the dust cap on. I ended up drilling out the dust cap to make clearance so the dust cap would fit on.
The main purpose of the dust cap is to protect the threads inside the crank. Those threads are used for the crank extractor. If those threads get damaged or get a lot of dust and grit in them it becomes impossible or difficult to get the crank extractor threaded on to pull the crank off.
I was going to 40 foot-pounds based on the test by Brian MacKenzie where he tightened up a crank nut on a Suzue hub and it failed at 85-90 foot-pounds. 40 foot-pounds became the standard after that. But then I stripped the threads on my widened Suzue hub even though I was using a torque wrench and going to 40 foot-pounds. But the threads were lubricated with Loctite and not dry as they were in Brian’s experiment. Since then I’ve toned it down to 35 foot-pounds considering that the threads are always lubricated with Loctite. Torque on lubricated threads should be less than the max torque with dry threads.
I haven’t had any problems with loose cranks at 35 foot-pounds along with Loctite. The cranks have stayed tight for me on my Coker and freestyle unis with both steel and aluminum cranks.
Park Tool gives suggested torques for cranks on bicycles. The suggested torque is different for different brands of cranks and bottom bracket. The max torque is in the 30 to 40 foot-pounds range for bikes. So 35 foot-pounds would seem to be a safe torque for unicycles.
Did you check and re-tighten the crank bolts when you got the uni?
Having had some cranks come loose I always adjust my crank bolts when I get a new uni.
The problem is that cranks are made to a standard and hubs are made to a standard and they should fit together (like all ISIS cranks and hubs should be compatible) however due to the number of different manufacturers and the differing tolerances the interface between the two might not be as good as it could be.
Riding as John Childs said puts a lot of force on the cranks and it is quite possible that they will “settle” onto the hub and this will leave the crank bolts loose.
Always check crank bolts after a couple of rides on a new uni as it can save you some wrecked cranks/hubs.
That’s true with hubs that have the thread attached to the axle and have a nut to hold the crank on, but with the (now more common) type where the axle is hollow and the crank is held on with a bolt, if the bolt head is poking out more than normal then the thread has bottomed out and it’ll never be tight (or of course somebody has put washers between the bolt and the crank).
EDIT: I use John’s suggested “40lbf torque and a dab of loctite on the bolt” and I’ve never had a crank come loose on my square-tapered muni. I’m not a big hopper or dropper, but it gets a lot of use on pretty rocky terrain. Square-taper axles aren’t as unreliable as some people will have you believe - if it’s coming loose (especially with road riding) it’s likely that either it wasn’t fitted properly (or tight enough) in the first place, or there’s something wrong with the crank or axle, or the bolt is too long and is bottoming out in the thread.
No, there is no way to put a hub on backwards.
Yes, it is possible that your cranks are on the wrong side of the hub.
Yes, it is possible that your frame is on backwards in relation to your crank configuration.
No, UDC connot be blamed for loose square taper crank arms (when I rode square tapers, I had one that would come loose almost every 15 minutes worth of riding, no matter how torqued the nut was)
Yes, we should always check pre-assembled Uni’s, or anything else for that matter, before using it. (when Nike bought Bauer, there hockey skate frequently were shipped with either two left or two right footed blades on both skates in a pair)
I’m thinking there may be something about the method these cranks attach. I’ve seen his unicycle and the 6 other cycles we all have (including my 36" nimbus DX with slightly longer cranks). His cranks bolt on with a large hex key type bolt and a plastic tapered washer. Not sure if that would cause any differences but its a pain nonetheless :p. I suggested to him initially that the hub or frame assembly may be backwards, but seeing as there’s no backwards to a hub and the crank arms are on the correct sides, it may have just come loose.