HI,
I wonder if anyone can help.
I had my cranks changed at bike shop on saturday. They have come loose aprox 4 times since then (searches have suggested that this could be normal), the uni creaks a bit (may be related to when the cranks are coming loose) but also there is a slight difference between the cranks and the bearings on the left and right sides. Is this OK? Or is my uni about to collapse in a heap?
Help, thanks,
Cathy
Re: Crank problem
This may sound unconventional but it has worked for me in the past on cranks that just don’t want to stay tight.
Take the cranks off, remove the pedals too. and put the arms in the oven set to 400deg (f) for about 20 mins. install them while they are still hot and tighten down the nuts quickly, then cool them off with water before the heat soaks into the axle. Install one side. cool it off, then do the same on the other side.
I had a set of cranks that wouldnt stay tight for more than a 1/2 hour of riding before I tried this, after they stayed tight for a year when i decided to change them out they were still tight.
It will blister the paint on black cranks, but doesn’t effect chrome ones,
The left and right crank not having the same spacing could be where the chain ring would mount on a BMX bike’s right pedal. they sometimes use the same pedals on unis just without the chain sprocket.
Al
Put grease on the tapers and the cranks.
In my experience, don’t put grease on the tapers! Yes i know this is typical of asking any group of ‘experts’ a question, for unis with splines adding grease helps fill the tiny gap and stop movement, and the slip of the grease doesnt matter because the splines are not using any kind of taper. With square taper the friction between the taper and the crank is vital to keep the crank in place, grease will not help this.
The most obvious answer is, clean the threads of both the stud and nut or bolt and tapped hole (depending on which you have) with white spirit/meths/surgical spirit, allow to air dry and the apply a liberal dosing of locktite to the treads and tighten. There are various grades of locktite, use one of the higher ones, i think 262 is the one to use. This should stop the nuts ever coming loose, mayb this could be used in conjucting with the heating method described, i’ve never heard it used on cranks but it’s certainly standard practice for mini racing clutchs, which affix by a taper. If you do this let everything go cool before locktiting as heat will destroy it, maybe do the process as described, let cool and then remove the nut/bolt for locktiting.
Dqave
Hey, I’m having a similar problem with my old and beaten miyatta. Keep me posted on any progress.
Re: Crank problem
On Wed, 18 May 2005 16:09:06 -0500, “kington99” wrote:
>With square taper the friction between the taper and the
>crank is vital to keep the crank in place, grease will not help this.
I disagree. It’s not the friction that vital to keep the crank in
place, it’s the matching shape of the square taper and the square
hole. Therefore, having the crank very tight on the taper is the main
thing, and grease helps to achieve that. Also put grease on the thread
where the nut will go on.
There is a nice crank installation manual by John Childs, hidden
somewhere behind the search button. I couldn’t find it in the time I
alotted myself. And I didn’t spot a link on his megalist either.
Klaas Bil - Newsgroup Addict
wouldnt you be grumpy if somone just said you had PMS? - jagur
Here are two previous threads on installing cranks:
Cranks (yet more)
crank falls off
And one thread on what I think causes the cranks to get loose:
need some new cranks
The three things that make sure your cranks get on tight and stay tight
A little bit of grease on the tapers
Loctite on the threads
A torque wrench to tighten it all up
That’s the trick. No more loose cranks.
If your cranks get loose after doing all that then you’ve got either a damaged crank or a damaged hub.
I should link to one of those threads in my bookmark list. 
Re: Crank problem
It is possible that one of the bearings has slipped a little bit. The bearings are only press fit on the hub. There is nothing but friction keeping them in place. Sometimes that friction isn’t enough to hold them so they slip.
You can try pressing the bearing back on and see if it will stay. Pull the cranks off and then use a short piece of pipe with about a 18-20 mm inside diameter to press the bearing back on. Some seatpost like the Miyata seatposts have the perfect ID for that.
If the bearing slips again then Loctite the suckers in place with Loctite Sleeve Retainer. See this thread: moving my bearing.
Oh my goodness,
This is getting more and more complicated.
Have taken the uni back to the bike shop and given exact insturctions (as per this forum) for putting the cranks back on.
Sounds like I need to become a unicycle expert as well as riding the thing.
So if putting the cranks back on again still doesn’t sort out the problem, does this mean that I have a knackered hub and would neet to replace it?
I wonder if it’s time to get a new unicycle?
(Oh my god, how girly is that!)
Cathy
Howdy! I’m gaving a similar problem to Cathwood so have joined in.
I got a 24" with 90mm cranks that I commute on and the left crank keeps coming loose/off.
I put it on with a torque wrench and tightened it to 60lb/ft and it still comes loose!
I think its maybe cos the cranks are so wee, they’ll be taking loads of pressure.
Had to stop at bikeshop on way to work yesterday to tighten it so’s I could get to work on time, and then I had to walk home cos it was loose again!
Bikeshop recommended locktite.
I’m gonna try grease on the crank arms first, cos I’v got grease!
If that don’t work I’ll invest in some locktite.
T.
Actually shorter cranks take less torque than longer ones because your body force is the same but the effective distance between your point of force and the centre of rotation is shorter, that’s why its harder to climb hills or stop/start quickle on shorter cranks.
Dave
If you want your cranck to go on and off easy use grease.
If you want it to keep them tight, don’t (same for pedals).
Ever wondered why cranck nuts/bolts are having this extra grip on the inside?
Grease is not gonne help in applieng a stronger force.
Also, ever wondered why almost all proffesional bike mechanics use a controlled key? (answer: because too strong forces are demolishing the ax/hub).
So yes, the square shapes should fit together. If they don’t, they don’t.
When I first got my uni I took it to the bike shop and had the same problem. When I took it back loose again they used a pneumatic impact driver. DUH!! Fortunately they just ruined the threads on the bolts and not the hub. Some of them just don’t know. Grease allows the crank to move further onto the taper and anti seize on the bolt allows it to get much tighter without damaging threads. Some say loctite on the threads. Grease anti seize worked for me.
What evryone seens to agree on here is, that to get cranks to stay on the axle they need to pressed as far as possible onto the square taper. My suggestion of using heat is to expand the crank arm, making the hole larger in diameter than the shaft it is being pressed against, then when cooled it shrinks making the fit even tighter. This will get them much tighter than just using the torque applied to the nuts ever will.
Al
This sounds like there may be damage to the crank or axle. This can happen from riding with loose cranks, causing wear and tear. It also can happen in rare cases if you have a badly machined crank (or axle, but crank is more likely). 90mm is an odd size, and may be from a children’s bike not intended to get much torque on it. So you may need a new crank.
The other think all you square-taper riders can think about is bringing a wrench along. I have a Park Tool crank nut/bolt wrench (14mm, fits all common unicycles) that I bring with me on rides where I’m worried about loose cranks.
If I don’t bring it with me, the ride always starts with a tightness check, using same wrench. I did this before setting of last Sunday on the Strawberry Fields ride.
Cheers for the input. I thought wee-er cranks would take more but what Kington99 said makes sense. It’ll be my legs that are having to exert more force, rather than the cranks taking more pressure. I think…
I got them from UDC at BUC12, not even a month ago. They’re the same as all the solid-cast ones they sell, just shorter. I used to have 102’s on the wheel but wanted more speed. They stayed on fine and were the same type/make of crank.
I can’t be taking a wrench/socket with me to work every day, it’s only 1.5 miles.
The crank stayed on on the way to work tonight (and back!) and I put a wee bit o grease on it and tightened it to 70lb/ft. If it comes loose again, I’ll try locktite. If it still comes loose I’ll buy new cranks.
Cheers for the input again!
T.
And thanx from me too.
Bike shop replaced the cranks using the grease etc (although bike shop man was a bit dubious “I’ve never put grease on cranks before”. So I said, that was the expert advice.)
They stayed nice & tight tonight.
Fingers crossed…
Cathy
Grease or no grease on the tapers is a subject of debate amongst bicycle mechanics. They actually have debates and FAQs about the subject. There are points on both sides of the debate.
I recommend the grease because the grease allows the crank to slide on the spindle a consistent amount with a given amount of torque. It keeps the crank from getting hung up while installing them.
One concern for not putting grease on the tapers is they’re worried that the grease may allow an aluminum crank to slide too far onto the spindle and actually cause the crank to split. I don’t see that as a major concern. You’re much more likely to eventually ruin the cranks by having them get loose than to eventually ruin them by having them split.
The Loctite on the threads has two functions. It acts as a lubricant while tightening the bolt/nut. That allows you to get a consistent and more accurate torque reading with the torque wrench. When the Loctite dries it keeps the crank bolt/nut from vibrating loose during use. As long as that bolt/nut stays tight the crank will stay tight. It’s when the bolt/nut backs off that the crank gets loose. Keep it tight and you’ve won the battle.
The torque wrench is the quality control. It’s the only way to know if you get the crank bolt/nut tight enough without overtightening. Otherwise it’s all just guesswork.
The Loctite on the threads and the torque wrench for tightening it all up to 35-40 foot-pounds are the two most important bits. The grease on the tapers can go either way.
I have the same problem on my Qu-Ax. I have tried grease and locktight (or however it is spelt), but none worked for me. I think I’ll try heating them up, though, that’s also what is done when designing racingcars and other extreme machines so it is kinda cool to do that with my unicycle…
Anyways, thanks for all the advice. Now I may finally sort this out!
No heat; the system is not designed for that. Moreover, heating is not consistent because you cannot guarantee the temperatures when you finally get the crank to the axle. Heating (properly) works for bearings because they will always cool to the same size on the same shaft. With a tapered interface, when the crank finally cools, its position on the taper, and thus the resulting interface forces, are not controlled.
Grease on the taper is recommended by Jobst Brandt, who is the leading visible researcher in this area. When you torque on the crank if the surface is not lubricated, the material (especially aluminum) will form its own lubricant and “mush” along the surface, called galling. This is uncontrolled and also deforms the crank. Friction between the crank and axle surfaces performs no role in the strength and fit of the crank. It is the shapes of the two surfaces that is important. By greasing the tapers, the interface between the two is controlled as much as possible when the crank is installed. In addition, the grease helps protect the interface from both corrosive and erosive destruction during use.
If you are having trouble with the cranks after using the instructions that John Childs has listed, then
the problem is with the cranks. Don’t risk your entire wheel trying to get a bad set of cranks to stay on. Toss them and get another pair. 40 ft-lbs is about right; needing to go above that number means that there is a problem.
Finally, there is a popular myth that the proper way to handle new aluminum cranks is to carry a wrench and tighten them every once in a while (when they become loose) until they finally settle in and stop doing that. Although this is partly true, the truly proper way to do this is to remove the crank, clean, regrease, and retorque it on, not leave it on and keep tightening it. There is a “squirming action” during riding which moves the crank around, towards the middle of the axle slightly. This action results in a loose nut/bolt, but the crank itself is actually higher on the taper and tighter. After several successive applications of the wrench, the crank is on much tighter than it should be, and its life is greatly shortened because long-term cracks develop in the crank interface, especially in the corners.
So… the proper way to handle this is: after proper installation, go for your ride. During the ride, keep the nut snug, but don’t put any oomph into it. After your ride, go home, remove the crank, and re-install it, again with the proper torque. After a couple of rides or so, the crank interface should slightly deform in the right places and settle down.
Buying better quality cranks will make this whole process less obtrusive on your riding, since the surfaces will be much more to spec, and the material much stronger and more consistent.
For confirmation of the above, although with much less rider-friendly terminology, see http://sheldonbrown.com/brandt/installing-cranks.html.