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Old 2005-05-19, 07:42 PM   #16
DarkTom
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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....

Quote:
Originally posted by johnfoss
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.
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.

Quote:
Originally posted by johnfoss
The other think all you square-taper riders can think about is bringing a wrench along.
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.
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Old 2005-05-19, 08:00 PM   #17
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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
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Old 2005-05-19, 10:38 PM   #18
john_childs
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Quote:
Originally posted by cathwood
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.
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Old 2005-05-20, 06:26 AM   #19
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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!
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Old 2005-05-20, 11:40 AM   #20
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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.
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Old 2005-05-20, 12:43 PM   #21
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Addendum

Another thing....

It may be tempting to conclude that the squirming action helps us out, because it helps tighten the crank without us stripping threads, etc. This would be similar to a procedure that was advocated a couple of years ago, which is to press the cranks on with a 20-ton press before the wheel is built.

However, this is deceptively incorrect. As Mr. Brandt points out, installing the cranks so that they are tighter simply leads to shorter crank life. Long-term stress resolution in the crank leads to cracks in the crank that do not appear upon installation.

If you install the cranks with the above procedure, you will not strip the threads, whether they are internal (crank bolt) or external (crank nut).

If you are following these instructions and using high-quality components, and still having troubles that replacing the cranks does not solve, then you should switch to a splined hub, where the interface is stronger, but the hub/crank choices are fewer and the cost is higher.

Note: these installation procedures do not apply to splined hubs in any way.
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