Crank arms question

[Fordulike]

I thought that I was fairly clued up when it came to how things worked, but still can't seem to get my head around this:

" Loss of crank bolt preload is greater on left than right cranks, because left cranks transmit torque and bending simultaneously while right cranks transmit these forces separately. The left crank transmits driving torque through the spindle to the right crank and chainwheel while the right crank drives the chainwheel directly. Besides that, the right crank transmits torque to the spindle only when standing on both pedals. Doing this with the right foot forward (goofy footed) is the only time the spindle transmits reverse torque "

Taken from: http://sheldonbrown.com/brandt/installing-cranks.html

Can anyone explain in a simpler way, why a left crank arm will loosen under use, before a right crank arm.

Hopefully everyone's not thinking 'what a thicko' lol

 

[trex]

that's hair splitting a bit. I would have thought that if you are right-handed, your right foot would be stronger and the right crank is more likely to loosen first.

 

[Fordulike]

that's hair splitting a bit. I would have thought that if you are right-handed, your right foot would be stronger and the right crank is more likely to loosen first.

Yeah, me too, but I've read in a few places about the left crank problem and I just can't seem to fathom out why

 

[RobF]

Sheldon is great, but I think he's in danger of disappearing up his own backside on this one.

 

[mike killay]

The bottom bracket consists of an axle with the crank arms on each end.
Speaking theoretically, if the chainwheel was in the middle, then applying pressure to one of the cranks causes twist to be applied to the axle. This twist is transmitted along the axle to the chain wheel. Because the chain wheel is theoretically in the middle, then applying pressure to either crank will cause the same amount of twist in the axle.
But, the chainwheel is on one side, close to or even welded in one with the right hand crank.
So, apply pressure to the right hand crank and the amount of twist in the axle is negligible (or none in the case of welded together cranks and chainwheels.)
Apply pressure to the left hand crank though and the twisting force has to be transmitted the full length of the axle to the chainwheel.

 

[flecc]

As Mike says, the principle is the same as that when using a longer screwdriver to apply more torque than a shorter one.

This twisting force within the BB spindle is employed in the Panasonic crank units as the the torque sensing of pedal effort to control the motor power added.
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[Fordulike]

The bottom bracket consists of an axle with the crank arms on each end.
Speaking theoretically, if the chainwheel was in the middle, then applying pressure to one of the cranks causes twist to be applied to the axle. This twist is transmitted along the axle to the chain wheel. Because the chain wheel is theoretically in the middle, then applying pressure to either crank will cause the same amount of twist in the axle.
But, the chainwheel is on one side, close to or even welded in one with the right hand crank.
So, apply pressure to the right hand crank and the amount of twist in the axle is negligible (or none in the case of welded together cranks and chainwheels.)
Apply pressure to the left hand crank though and the twisting force has to be transmitted the full length of the axle to the chainwheel.

So, correct me if I'm wrong, to achieve the same amount of rotational torque to the chain ring, the left crank would require more force than the right crank to achieve the same amount of torque?

 

[flecc]

I'd say less force at any given moment, but for a fractionally longer distance/period. See my simultaneous reply above your last post for the longer screwdriver principle. The length of the BB spindle adds torque in the same way that a longer screwdriver does.
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[mike killay]

So, correct me if I'm wrong, to achieve the same amount of rotational torque to the chain ring, the left crank would require more force than the right crank to achieve the same amount of torque?

Not really. In practice we are talking about an axle stout enough to barely twist at all.
We must be careful about the word torque, but any suggestion that you have to pedal harder with the left crank seems fallacious

 

[Fordulike]

I'd say less force at any given moment, but for a fractionally longer distance/period. See my simultaneous reply above your last post for the longer screwdriver principle. The length of the BB spindle adds torque in the same way that a longer screwdriver does.
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Yeah sorry flecc, you must have posted that whilst I was typing.

I'm still not getting the link between this and the problems people have with the left crank loosening. A search on the net suggests that it occurs with non motorized bikes too.

I was thinking, that because the pedals are more often than not rotating forward, then any slight movement at the crank/spindle would start to unscrew the bolt on the left side.
The right side bolt would have a tendency to tighten up with forward pedal movement.

I could be talking utter b****x lol

 

[trex]

when you push down on the left crank, the twist causes a slight deformation of the spindle (which is often used by strain gauges for torque sensing). The deformation of the axle causes the loosening of the crank bolt.
This problem is much less on the right crank because much less twist is generated when you push down on the right crank.

 

[flecc]

I think this is rather theoretical. Because the left pedal force is transmitted through the length of the BB spindle, that spindle will twist very minutely*. Therefore the crank arm will travel further in distance and time to accumulate the additional torque. And it's that extra torque that causes the problem.

While typing this I've updated the page and see that Trex has replied similarly.

* This twist is real, surprising though it might seem. For example, the thick wall alloy sleeve on the right of this image below has the pedal force transmitted though it in the Panasonic crank motor units:

It might seem that such a rigid tube couldn't twist to a measurable degree, but here's the proof that it does. In the image below you'll see a coil winding on the right that sits around the arrowed roughened area of that alloy sleeve:

The coil is centre tapped and has a stable small current travelling through it, balanced on either side of the winding. When pedal force is transmitted through the alloy tube, the current in the two sides of the coil unbalances as the alloy tube minutely twists, and its this that the unit uses to assess the force applied. That in turn varies the help the motor gives.
.

 

[Fordulike]

The coil is centre tapped and has a stable small current travelling through it, balanced on either side of the winding. When pedal force is transmitted through the alloy tube, the current in the two sides of the coil unbalances as the alloy tube minutely twists, and its this that the unit uses to assess the force applied. That in turn varies the help the motor gives.
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Isn't technology like this fascinating. I'd love to meet the boffins who think this stuff up

 

[flecc]

Isn't technology like this fascinating. I'd love to meet the boffins who think this stuff up

You might need to learn Japanese though!
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[Emo Rider]

Excellent explanations about the transfer of power and torque whilst pedaling. Where else are you going to go to find information like this. Well done guys.

Having all that has been said, I have another take on the OP's question. When I first started repairing bikes I had the same question. So I started checking the crank bolt or nut tightness on all of the bikes I sold pre delivery. for some reason, I will leave you to speculate, the left hand fastener almost always required a bit or a lot of tightening right from the factory. More often than not the crank side was ok. So now I check them all and always.

If a left hand crank becomes noticeably loose, you have already damaged the surfaces of the crank. Tightening will work for a while but most will agree that it usually ends up with replacing the crank arm. If you purchase a bike I would suggest checking the tightness before you ride and not assume that they are tight from the factory.

 

[flecc]

I started checking the crank bolt or nut tightness on all of the bikes I sold pre delivery. for some reason, I will leave you to speculate, the left hand fastener almost always required a bit or a lot of tightening right from the factory. More often than not the crank side was ok.

That will probably be due to the absorbtion of applied torque. Given that the force applied by the operative will be the same both sides, that on the right will all be tightening.

That on the left will have some of the force soaked up by the twist in the BB spindle and lost as heat. To achieve the same tightening on the left, the applied tightening force on the left side would have to be held on for longer to take it past the initial twisting loss.

The difference is small but real.
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[Deleted member 4366]

Deleted post

 

[EddiePJ]

Sheldon is great, but I think he's in danger of disappearing up his own backside on this one.

I thought that he died ages ago, so I guess that is possible.