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Old 10-26-12, 02:31 PM   #1
RubeRad
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Pedal and BB threading; why do they get tighter with use?

So everybody knows that pedals are opposite threaded, so that the rule for either pedal to put them on is, just hold the spindle and freewheel backwards; or to take them off, grab the spindle and pedal forwards.

And yet I've read that riding (pedaling forwards) doesn't cause pedals to loosen, but to tighten (such that they gradually even dig in to the metal of the crank, which I've seen the evidence of)

Similarly, an english BB fixed cup is reverse threaded, so it's rightie-loosie. Which would seem to indicate that pedaling forward would tend to loosen it -- but I've also read (and experienced!!!) that a fixed cup will get ridiculously tight with use.

Can somebody explain this to me?
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Old 10-26-12, 02:45 PM   #2
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It's called precession. Here's how to demonstrate it:

Take something like a thick, round pen/magic marker. Put it in a mug, leaning against the inside face of the mug. Now seewhat happens if you allow the pen to follow the inside face of the mug w/o sliding - the pen will begin to turn in the opposite direction of how its moving around the cup.

Now, for a pedal for instance, the fit is a lot tighter, but the movement is the same.
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Old 10-26-12, 03:03 PM   #3
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I don't see how the marker in the cup makes an analogy with the pedal, is the marker the pedal spindle, and the cup is the crank?

If my pedal seized on the spindle, and I somehow kept pedaling, wouldn't that unscrew the pedals as sure as grabbing the spindle with a wrench and pedaling forwards? And isn't a proper greased ball-bearing just making that seizure very very very gradual?
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Old 10-26-12, 03:17 PM   #4
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I don't see how the marker in the cup makes an analogy with the pedal, is the marker the pedal spindle, and the cup is the crank??
Yep.
If you look at the right crank, the point of highest pressure in the threads will wander around in a counter-clockwise direction, which will cause the pedal axle to turn in a clockwise direction, screwing itself in.

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If my pedal seized on the spindle, and I somehow kept pedaling, wouldn't that unscrew the pedals as sure as grabbing the spindle with a wrench and pedaling forwards?
If the pedal hadn't had time to torque itself in, or hadn't been mounted properly to start with - sure.

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And isn't a proper greased ball-bearing just making that seizure very very very gradual?
No. In this case, a lot of tiny nudges do not equal one big shove.

Last edited by dabac; 10-26-12 at 03:21 PM.
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Old 10-26-12, 03:28 PM   #5
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Guess that part of the SAT tests was dropped..


the balls in the bearings have an influence on this , turning axle CW the bearing turns CCW

but same axle on the opposite side turns CCW, as viewed from that opposite end..


BTW, the fixed cup has to be tight, thats why it's called that.

pedals have to be sufficiently tight too, otherwise It is an inadequate installation..

Last edited by fietsbob; 10-27-12 at 10:33 AM.
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Old 10-26-12, 07:49 PM   #6
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fietsbog, I don't think that works. On the drive side, the crank goes CW, and in order to stay flat, pedal is moving CCW, so bearings are going CW, so tiny rotational pressure on the axle would be CCW, which would loosen the pedals.

I like the idea of "the point of highest pressure in the threads will wander around in a counter-clockwise direction". I can almost wrap my mind around that.

So if you have threaded a screw into wood, and you have greasy fingers, and you hold the head of the screw and wiggle it in a small CCW, it will eventually screw in?
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Old 10-26-12, 07:55 PM   #7
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fietsbog, I don't think that works. On the drive side, the crank goes CW, and in order to stay flat, pedal is moving CCW, so bearings are going CW, so tiny rotational pressure on the axle would be CCW, which would loosen the pedals.

I like the idea of "the point of highest pressure in the threads will wander around in a counter-clockwise direction". I can almost wrap my mind around that.

So if you have threaded a screw into wood, and you have greasy fingers, and you hold the head of the screw and wiggle it in a small CCW, it will eventually screw in?
So are you saying that you don't believe pedals actually tighten with use, or that you don't believe the original explanation?
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Old 10-26-12, 08:04 PM   #8
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Dabac is 100% correct.

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So if you have threaded a screw into wood, and you have greasy fingers, and you hold the head of the screw and wiggle it in a small CCW, it will eventually screw in?
If it is a machine screw with smooth threads with a bit of play it absolutely would. You may have to press really hard or open a hole if you want to try it with a wood screw.
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Old 10-26-12, 08:16 PM   #9
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The precession occurs because, unlike what you are probably picturing, the pedal spindle rotates around the bearing exactly like the magic marker example. Although there's a circle of ball bearings the spindle only contacts one limited area at a time, just as it would without balls present - just with less friction. Great visual on Wikipedia.

http://en.wikipedia.org/wiki/Precession_(mechanical)
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Old 10-26-12, 08:44 PM   #10
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Deeper causality; the precession occurs because the forces on the pedal spindle (feet pushing down) are perpendicular to the force of the threads holding the pedal in place (only applies sideways along axis of spindle). Meaning, no matter how tight you crank them, the threads can never directly oppose the forces seen during operation; and some amount of wiggle is always guaranteed. Leads to precession.

Rather than design a better interface; we simply reverse thread one side so the precession causes overtightening over time instead of loosening.... less catastrophic to have your crank arm slowly grind away than it is to have a pedal drop off.

umh...
in summary: they get tighter with use because of stupid design
Somebody fix it.

p.s. what if the pedal eye; in addition to the current threading; also had a split and pinch bolt?....subject to same precession problems? or does the compression redirect things?

Last edited by xenologer; 10-26-12 at 08:49 PM.
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Old 10-26-12, 08:49 PM   #11
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Deeper causality; the precession occurs because the forces on the pedal spindle (feet pushing down) are perpendicular to the force of the threads holding the pedal in place (only applies sideways along axis of spindle). Meaning, no matter how tight you crank them, the threads can never directly oppose the forces seen during operation; and some amount of wiggle is always guaranteed. Leads to precession.

Rather than design a better interface; we simply reverse thread one side so the precession causes overtightening over time instead of loosening.... less catastrophic to have your crank arm slowly grind away than it is to have a pedal drop off.

umh...
in summary: they get tighter with use because of stupid design
Somebody fix it.
Cue Jobst Brandt: http://sheldonbrown.com/brandt/left.html
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Old 10-26-12, 08:57 PM   #12
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I don't think the bearings have much effect on the precession that occurs in bicycle pedals and bottom brackets.
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Old 10-26-12, 09:00 PM   #13
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So are you saying that you don't believe pedals actually tighten with use, or that you don't believe the original explanation?
My hands know for sure that old pedals are tighter than pedals I just put in (when I install pedals, I always only snug them a tiny bit beyond finger-tight), and it's not that don't believe the original explanation, it's that I'm trying to come to grips with it.
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Old 10-26-12, 09:04 PM   #14
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The precession occurs because, unlike what you are probably picturing, the pedal spindle rotates around the bearing exactly like the magic marker example. Although there's a circle of ball bearings the spindle only contacts one limited area at a time, just as it would without balls present - just with less friction.
OK, that is starting to click in my mind (even though I think it's the same as dabac said). So it's nothing to do with pedals rolling around the spindle transmitting tiny tightening pressure (as I am convinced that would be in the loosening direction anyways), but the pressure of the spindle in the crank rotating around.
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Old 10-26-12, 09:06 PM   #15
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If it is a machine screw with smooth threads with a bit of play it absolutely would. You may have to press really hard or open a hole if you want to try it with a wood screw.
But wouldn't a wood screw be a closer analogy, being tighter in the wood than a machine screw (like a pedal spindle is tight in a crank), but having precession gradually work it in?
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Old 10-26-12, 09:13 PM   #16
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So Jobst Brandt mentions something I've read elsewhere, that wear/failure at the pedal/crank interface would be eliminated if we used like the conical lug nuts on cars.

By this point, why has no manufacturer (that I've heard of) started making chamfered cranks/pedals? Or sold the service of converting existing equipment?
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Old 10-27-12, 03:49 AM   #17
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So Jobst Brandt mentions something I've read elsewhere, that wear/failure at the pedal/crank interface would be eliminated if we used like the conical lug nuts on cars.

By this point, why has no manufacturer (that I've heard of) started making chamfered cranks/pedals? Or sold the service of converting existing equipment?
Cost and risk of changing compatiblity standard outweights the benefit of apealing to the 3 cyclists in the world who even know or care about the problem.
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Old 10-27-12, 11:09 AM   #18
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By this point, why has no manufacturer (that I've heard of) started making chamfered cranks/pedals? Or sold the service of converting existing equipment?
Or simply reluctance to be incompatible with the rest of the market.
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Old 10-27-12, 06:25 PM   #19
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Or simply reluctance to be incompatible with the rest of the market.
Ding! Pedal threads are darn near universal the world over. It's been a long, long time since anyone attempted a proprietary pedal/crank interface: http://www.sheldonbrown.com/shimano1982/pages/15.html
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Old 10-27-12, 06:35 PM   #20
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Ding! Pedal threads are darn near universal the world over. It's been a long, long time since anyone attempted a proprietary pedal/crank interface: http://www.sheldonbrown.com/shimano1982/pages/15.html
I suspect many of those DynaDrive arms got reverted back to standard threading.

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Old 10-28-12, 12:28 AM   #21
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So Jobst Brandt mentions something I've read elsewhere, that wear/failure at the pedal/crank interface would be eliminated if we used like the conical lug nuts on cars.
Yes.
That design, and appropriate torque, would lock the pedal to the crank solidly enough to prevent precession.

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By this point, why has no manufacturer (that I've heard of) started making chamfered cranks/pedals? Or sold the service of converting existing equipment?
Well, if the average buyer thinks that the current solution is good enough(and let's face it, crank/pedal failures aren't that common), why would he spend money on a fix that won't improve his ride in any way?

"it's better, but in a way you'll most likely never benefit from" isn't gonna look particularly compelling in the catalogue.

It's like with spoke lengths and hubs. Most rear wheels will use a 2 mm longer spoke on the non-drive side, which could have easily been avoided by designing a hub flange that's 2 mm larger. I'd rather have a wheelset that only took one spoke length than a chamfered pedal spindle.
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Old 10-28-12, 02:50 PM   #22
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why would he spend money on a fix that won't improve his ride in any way? "it's better, but in a way you'll most likely never benefit from" isn't gonna look particularly compelling in the catalogue.
Are you kidding me with this? It's not about actual improvement, it's about marketing. Would it improve my ride in any way to switch to titanium chainring bolts, or carbon brake levers, or...

This is a marketing opportunity. What makes you think cycling lemmings wouldn't fall over themselves to replace or retrofit cranksets and pedals if it were advertised the right way?

Besides, as we have discussed already, there is a scientifically accurate basis to claim that the design is improved.
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Old 10-28-12, 04:31 PM   #23
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Are you kidding me with this? It's not about actual improvement, it's about marketing. Would it improve my ride in any way to switch to titanium chainring bolts, or carbon brake levers, or...

This is a marketing opportunity. What makes you think cycling lemmings wouldn't fall over themselves to replace or retrofit cranksets and pedals if it were advertised the right way?

Besides, as we have discussed already, there is a scientifically accurate basis to claim that the design is improved.
It isn't purely a marketing problem - it is also an accounting problem: Every change made to any part costs $$, and every change needs to show sufficient net benefit appropriate to the corporation. The change must either reduce costs in the long term, make the product worth more to consumers to justify raising the price, or even reduce the cost of a product after the sale (stop lawsuits, for example).

Improving the pedal interface may reduce the cost of production slightly (no more LH threads), but it may be a wash (non-standard shape like convex shoulder on spindle may cost more than standard square shoulder). And to make it worth more in consumers eyes could be a tall challenge, and the design is not so flawed that it is causing problems on existing bikes.

I will wait and see if the free-market proves me wrong.
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Old 10-28-12, 04:40 PM   #24
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Yes.
That design, and appropriate torque, would lock the pedal to the crank solidly enough to prevent precession.



Well, if the average buyer thinks that the current solution is good enough(and let's face it, crank/pedal failures aren't that common), why would he spend money on a fix that won't improve his ride in any way?

"it's better, but in a way you'll most likely never benefit from" isn't gonna look particularly compelling in the catalogue.

It's like with spoke lengths and hubs. Most rear wheels will use a 2 mm longer spoke on the non-drive side, which could have easily been avoided by designing a hub flange that's 2 mm larger. I'd rather have a wheelset that only took one spoke length than a chamfered pedal spindle.
NOS 7spd freehub bodies transplanted to modern 135mm hub spacing FTW


regarding the pedal/crank redesign cost
what about cranks that used the same standard threading, but also had a split pedal eye and pinch bolt?
would the compression from that be enough to stop precession?
it would still accept normal pedals.......
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Old 10-29-12, 04:02 AM   #25
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Are you kidding me with this? It's not about actual improvement, it's about marketing. Would it improve my ride in any way to switch to titanium chainring bolts, or carbon brake levers, or...

This is a marketing opportunity. What makes you think cycling lemmings wouldn't fall over themselves to replace or retrofit cranksets and pedals if it were advertised the right way?
Because it doesn't offer any obvious (even if they are insignificant) rider advantages.

Making a bike lighter(even if it's by an entirely insignificant amount) will always be a good selling point.
Because regardless of how marginal, making a bike lighter is still a move in the right direction(unless, perhaps if you have to be UCI-compliant).
Making a bike more aero is also a move in the right direction(unless you collide with UCI rules again).
Claiming that CF bars/forks/stems/seat posts gets rid of road buzz, well, there's an easily understandable improvement there too, whether it's measurable or not.

Now when you mention them, I'd rather like to have CF brake levers on my winter bike. They wouldn't sap as much warmth from my fingers as the alloy levers do.

But if the current standard is good enough(ie, rarely fails), there's no advantage to the rider by improving it - unless with something that's lighter, more aero, stiffer, comfortable etc etc.

If you'd want to market it, you need to tap into something that would be perceived as a ride/performance improvement.

Maybe claiming that it makes the crankset stiffer would work.
When people started transitioning from square taper to the various versions of hollow axles and external bearings and all that, crank stiffness was a frequently quoted advantage. As precession is due to the pedal axle wobbling around a bit, so maybe one could claim that having a pedal axle that remains a little more perpendicular to the crank would make pedalling more efficient. There's a potential selling point.
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