My new Coker V2 is going back to the factory for repair : (
When I first started riding it, the left crank became loose from the hub. I took it to a friend who owns a bike shop and he tightened it up. It fairly quickly loosened up again so I called Coker and they recommended Loctite. I used the blue stuff - 242. I loctited, torqued, and let it sit for 24 hours. I went out to ride and it loosened up again. That is a total of 3 road ride attempts on my new Coker and 3 walks home. I am not happy.
I am posting this thread to ask if this is a common problem with Cokers. Would like to hear from Coker owners if you have seen this. Thanks, Jeff
It’s more of a problem with square taper cranks. From the way you describe it I would say that you rounded the flats of the crank taper when it loosened the first time. Once they are rounded, even less than you can see with your eyes, they will never seat correctly, and need to be replaced. If you haven’t sent the whole uni back yet, you may want to see if Coker can send you a crank arm for the side that is loose, and see if that doesn’t fix the problem.
You shouldn’t need loctite on the crank bolt threads, but if you do you would want to keep it on the threads only. There is some debate about whether or not to use grease on the spindle flats. Campagnolo has said in the past to clean the flats so there is no grease on them. I am generally in the camp of putting some grease on the flats, and then wiping it off. It may be nothing, but it seems like the thin, imperceptible, film left behind helps the cranks seat better.
Mine (Coker Bigone) used to do that on the first rides, I kept tighting it and I didn’t had any problem with this since about a year or so. If you wouldn’t have sent it already you could just bring a tool with you to tighten it when it gets loose.
The axle could be damaged, if the crank metal is as hard as the metal axle. The more common situation is that the crank metal is softer, so the axle is fine.
Once a crank has been ridden while it is loose, there’s a high probability that the crank will be unusable. It’s likely not a manufacturing defect, but the cranks may not have been installed to proper torque specs.
A prime uni maintenance issue is to make sure your cranks are tight, and to stop riding immediately if they’re not.
I have always advised to clean, grease, then beat on soundly all tapered fittings to a tapered shaft. Then tighten the holding nut or bolt. Same on all other engines or devices in my experiences.
The great "Danger Uni " himself (KH of KH) corrected me in a thread, advising against this, saying a mild torque up with the bolt was correct. I guess moment cranks may have magical properties.
As great as he is, he still seems a bit of a young bloke in mechanical knowledge to me. Greasing mechanical tapers, then seating it solidly with a few hammer blows is the way it is done with other devises. That is the way I put on all tapered devises, including my uni cranks. I have had 100 % success so far. None of my cranks move.
You want the metal clean and greased, then beat it into total contact with a hammer. Then snug up the retaining bolt. There should be zero movement or wear in from riding. Just like I put a fly wheel on a diesel engine. I clean, grease, and smack it on right . There is no “break in” where the parts wobble about and need to be tightened a second time. You would be a shit diesel mechanic if you did it that way. Uni cranks IMHO should go on and stay on. Movement after installation means you didn’t bang them on tight. If you need to ride a while, then tighten again, you are just stripping them to no good effect. Clean, grease, hammer, tighten the bolt. Metal to metal contact is your friend.
The issue with hammering the cranks on is that the action of hammering them may itself damage the cranks. No manufacturer specs their square-taper cranks for being hammered on, and most bike mechanics who express an opinion on the matter will tell you not to hammer them on. Install them snugly, with lots of grease, and tighten the bolt to torque ratings.
Just like KH doesn’t want to tell you to hammer them on. KH has the best warranty in the business. How do you warranty how hard a rider will hit a piece of metal with a hammer if you tell them to ? So of course, you tell them not to.
I am telling you to bang them on , clean, greased, using blocks of wood to avoid dents. I’m sure KH does it this way himself, he’s a smart guy, KH just doesn’t want to tell you to hit something with a hammer, promising he will warranted the result. He is a smart man and can see he cannot maintain his excellent warranty, if part of his installation advises includes a hammer.
The obviousness of the greater strength obtained by closer metal to metal contact, obtained by seating clean greased tapered parts together with hammer blows, is not really debatable. Clean, greased, banged together. Why are tapered uni cranks different from other such devices ?
They are different in that they wear and screw up because they aren’t banged tight like they would be if they were a tapered interface on a pro built machine. Bang bang, clean, greased and tight, is right. Snug that bolt up, and you are done.
avoid hammering when both putting on cranks and removing them…
my first 36(which was cotterless) came with many super loose spokes. it creaked like crazy and constantly loosened. when i gave the spokes a good tightening, the creaking stopped, as well as the loosening.
on my old cotterless nimbus 29er, the bolts loosened about once a week. then one day it just randomly stopped… and i never had to tighten them again, even when changing cranks.
I’ll give another vote for the 'clean, grease, hammer (with blocks of wood to protect) approach).
Every square taper unicycle I’ve owned I’ve used that approach on- not only do the cranks stay on with no loosening, I’ve also never experienced any hint of noticable damage resulting from the procedure.
I should say that I tend to err on the side of less rather than more when it comes to the hammering, using just a bit more than the momentum of the hammer and just a few taps.
I use a lot of force during the tightening with the spanner- don’t have a torque wrench so I don’t know how much, but pretty close to the most I can produce with arm strengh on a stanard length crank spanner (using my other hand to lock the tyre)
Another tip is to carry the crank spanner around for first couple of rides after installing the cranks (shouldn’t be needed, but, just in case) and, every several weeks, check to see if they need retightening.
This is the reason why cranks often come loose; most people don’t put nearly enough force into them. The specs are generally for 25-35 foot/pounds (35-45 Nm), which is a lot to get into a bolt with arm strength on a typical spanner(/wrench). If you don’t have a really long spanner or a torque wrench, you’re probably not getting the cranks up to spec.
What’s happening for the folks who say “install it, ride it, tighten it again” is that the crank is not installed tight enough, so in use it’s wobbling up the taper, which then makes the bolt loose, which makes the crank wobble more. The crank is being damaged by the process, though if it’s a steel crank it may not be enough to worry about.
The Coker rep told me to tighten the bolt to 20 lb-ft which did not seem like much to me (I did use a torque wrench). I could have applied a lot more. He didn’t mention anything about cleaning or grease or hammer. I am fairly certain the cranks got damaged in the wobbling process. If Coker had put it together properly in the first place, I would not be sending it back for a Nimbus Impulse. I am guessing the ISIS hub is more forgiving on mounting prodedure and less prone to loosening.
Doesn’t the Impulse have an aluminum spindle? If so, you are going to want to make sure that the cranks are on right. ISIS should help, but I would still make sure that you have the right torque. I’m not sure off hand what that will be for an Al/Al interface, but Josh at UDC should be able to tell you. You could also call them and ask Josh to set the cranks before it gets shipped.
A lot of bikes/uni’s are assembled at factories, but not adjusted, or setup correctly. When UDC sends a uni out in a box I don’t know if they are opening every box and doing the setup. My guess is that they aren’t by some of the posts about loose spokes that I’ve read.
Note that for unicycles, both right and left cranks behave like left cranks on a bike, because we don’t have chainrings. That’s one reason why these problems come up more often on unicycles than bikes. Also, we bash our pedals on the ground more often than bikes do, and we put more sideways forces on the pedals in use.
I have installed cranks a few times on my Nimbus Titan and have never had a problem with the cranks working loose. I cleaned the surfaces, lightly greased the cranks, tapped them together with a mallet using wood to buffer the blows on both sides of the hub. Finally I installed the bolts and tightened them to about 35 ft-lb using a torque wrench. (I backed off on subsequent installations and only used 30 ft-lb.) I have gone back and tightened them again and they did not move. I have followed this procedure a number of times switching cranks on this unicycle and have never had a problem. I do not know if there is some problem specific to Cokers. I do not see why you would need locktite if the cranks are properly installed. I would tighten the bolts again after the first ride to be sure the metal-metal contact is tight and it has not loosened up due to grease squeezing out of the joint. After that they should be good to go.
Jeff, If you have an extra $300 then the Impulse looks like a very nice 36er. I talked to Josh at UDC and he told me all about it. Maybe after a year or so of riding my (lesser) Titan I would consider investing in the Impulse. Too bad Coker didn’t just torque down the crank arms before shipping!
+1 on banging on the cranks using a piece of wood to protect the crank from damage. There is no way it it will get damaged if you put the wood between it and the hammer. I had to do this on my cheap 24" Avenir, and it is a known technique on square taper b*cycle cranks for decades. Then torque down the bolt pretty much as tight as you can. The Avenir hasn’t come loose since, and I have been learning how to hop on it. I weigh 175-180 so that is a lot of force.