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Old 2005-05-18, 04:22 PM   #1
cathwood
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Crank problem

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
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Old 2005-05-18, 08:10 PM   #2
al_lieffring
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Re: Crank problem

Quote:
Originally posted by cathwood
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
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
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Old 2005-05-18, 08:57 PM   #3
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Put grease on the tapers and the cranks.
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Old 2005-05-18, 09:09 PM   #4
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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
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Old 2005-05-19, 03:07 AM   #5
unicyclemichael
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Hey, I'm having a similar problem with my old and beaten miyatta. Keep me posted on any progress.
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Old 2005-05-19, 07:03 AM   #6
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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.

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Old 2005-05-19, 07:33 AM   #7
john_childs
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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.
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Old 2005-05-19, 07:43 AM   #8
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Re: Crank problem

Quote:
Originally posted by cathwood
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
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.
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Old 2005-05-19, 10:32 AM   #9
cathwood
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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
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Old 2005-05-19, 03:07 PM   #10
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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.
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Old 2005-05-19, 03:38 PM   #11
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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
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Old 2005-05-19, 03:40 PM   #12
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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.
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Old 2005-05-19, 04:10 PM   #13
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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.
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Old 2005-05-19, 05:21 PM   #14
al_lieffring
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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
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Old 2005-05-19, 05:44 PM   #15
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Quote:
Originally posted by DarkTom
I put it on with a torque wrench and tightened it to 60lb/ft and it still comes loose!
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.
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