Hillhater
100 TW
..why are pedals & cranks threaded Left handed on the left crank ?
WHY ?
- Thread starter Hillhater
- Start date
I dunno that it matters, though. I used the cranks on CrazyBike2 reversed, left on right and right on left, so I could have the chainrings on the left side due to the way I had to setup the motor/chainline/etc. No problems with anything unscrewing even when I was just pedalling it, before installing a motor or after.
gtadmin
10 kW
The same reason the bottom bracket bearing cups are :wink:
Hillhater
100 TW
neptronix said:
Hmm ?.. think about that again ! :wink:
...which is ??? :?GTAdmin said:
gtadmin
10 kW
So that they don't unscrew during normal pedalling IF a bearing starts sticking. Ok, not likely, but that's the reason.
Cheers,
GT
Cheers,
GT
bobc
10 kW
Can be a funny one because at first sight it usually looks backwards (e.g. either pedal bearing seizing would cause the pedal to unscrew itself..!!)The answer is that the combination of sideways and rotational forces that the thread of the pedal sees sort of precesses the thing into trying to turn the other way. Mechanics can be just as esoteric as electronics sometimes
gtadmin
10 kW
That, of course, is correctbobc said:
A seized pedal bearing would tend to unscrew the pedal, not tighten it. Makes it easier to change I guess 
Edit: looking at the bike from the LHS, the crank normally rotates in an anti-clockwise direction, but the pedal rotates in a clockwise direction in relation to the crank.
dumbass
100 kW
Come on, you have to relize the idea is to simply always be tightening the peddle or crank bearing nut while peddling. When I was a kid I got a new bike for XMas and my brother assembled it. He didn't pay attention to what crank arm went where. So he installed them on the reverse sides. I spent my entire youth with my peddles falling off. Why? Because as I peddled I was slowly but surely unscrewing them. After several years of this crap I did eventually figure it out and reverse them. But hey, I was only a kid.
Bob
Bob
John in CR
100 TW
It's definitely to help prevent them from unscrewing, but it doesn't always work. On one of my ebikes one of the pedals kept coming unscrewed. That bike needed pedal assist up many hills, so those pedals got more use than on any of my ebikes. The threading was proper, so it baffled me for a while. I finally figured out that in flat road riding I backpedaled 180° as I set up for turns to avoid pedal strike, and those half turns added up eventually to a loose pedal. I finally cured it with the maximum strength Loctite.
northernmike
10 kW
Subject: Left Hand Threads
From: Jobst Brandt
Date: April 28, 2004
On bicycles, left hand threads are used mainly in three places, on left pedals, right bottom bracket (BB) bearing cups, and freewheel cones, to prevent unscrewing under operating loads. Unscrewing occurs from precession, in which a round object rolling in a circular ring in one direction will itself turn in the opposite direction.
For a pedal, a rotating load arises form downward pedaling force on a spindle rotating with its crank making the predominantly downward force effectively rotate about the pedal spindle. What may be less evident is that even tightly fitting parts have relative clearance due to their elasticity, metals not being rigid materials as is evident from steel springs. Under load, micro deformations, enough to cause motion, occur in such joints. This can be seen from wear marks where pedal spindles seat on crank faces.
Precession of right side BB cups is less obvious because the rotating load is only partial. The largest load being chain tension, that together with the moderately large downward force on the right crank and the smaller upward force from pushing down on the left crank, make 3/4 of a fully rotating load. For this reason some right BB cups have used right hand threads and some with left hand threads have loosened. The left BB cup with no significant rotating load has little tendency to turn.
Freewheel cones are more obvious candidates for precession, their load being mainly radial, and rotating continuously in the direction that would unscrew a right hand thread. There are other such but less common threads on bicycles.
Precession forces are large enough that no manner of thread locking glues, short of welding, will arrest them. Mechanical fretting, the micro-motion of tightly fitting parts moving against one another, is the mechanism of this motion. Motion in these joints causes visible fretting rouge, red iron oxide, on the shoulder of the BB cup and on the face of the pedal spindle.
Left hand threads would not be required on left pedals if a design common on cars were used. Before the advent of conical lug nuts, many cars used left hand threads on left side wheels. Today, stories of wheels rolling away from cars no longer make news, the conical seat having solved this problem on car wheels as it could on bicycle pedals.
However, unscrewing is not the main problem for pedals, but rather crank failure caused by fretting erosion of the pedal eye. Fretting initiates cracks that can cause sudden and unsuspected pedal separation when the eye of a crank breaks. Because this occurs equally with right and left cranks it is the more important reason for a conical spindle face and crank eye. This has been tested.
Jobst Brandt
From: Jobst Brandt
Date: April 28, 2004
On bicycles, left hand threads are used mainly in three places, on left pedals, right bottom bracket (BB) bearing cups, and freewheel cones, to prevent unscrewing under operating loads. Unscrewing occurs from precession, in which a round object rolling in a circular ring in one direction will itself turn in the opposite direction.
For a pedal, a rotating load arises form downward pedaling force on a spindle rotating with its crank making the predominantly downward force effectively rotate about the pedal spindle. What may be less evident is that even tightly fitting parts have relative clearance due to their elasticity, metals not being rigid materials as is evident from steel springs. Under load, micro deformations, enough to cause motion, occur in such joints. This can be seen from wear marks where pedal spindles seat on crank faces.
Precession of right side BB cups is less obvious because the rotating load is only partial. The largest load being chain tension, that together with the moderately large downward force on the right crank and the smaller upward force from pushing down on the left crank, make 3/4 of a fully rotating load. For this reason some right BB cups have used right hand threads and some with left hand threads have loosened. The left BB cup with no significant rotating load has little tendency to turn.
Freewheel cones are more obvious candidates for precession, their load being mainly radial, and rotating continuously in the direction that would unscrew a right hand thread. There are other such but less common threads on bicycles.
Precession forces are large enough that no manner of thread locking glues, short of welding, will arrest them. Mechanical fretting, the micro-motion of tightly fitting parts moving against one another, is the mechanism of this motion. Motion in these joints causes visible fretting rouge, red iron oxide, on the shoulder of the BB cup and on the face of the pedal spindle.
Left hand threads would not be required on left pedals if a design common on cars were used. Before the advent of conical lug nuts, many cars used left hand threads on left side wheels. Today, stories of wheels rolling away from cars no longer make news, the conical seat having solved this problem on car wheels as it could on bicycle pedals.
However, unscrewing is not the main problem for pedals, but rather crank failure caused by fretting erosion of the pedal eye. Fretting initiates cracks that can cause sudden and unsuspected pedal separation when the eye of a crank breaks. Because this occurs equally with right and left cranks it is the more important reason for a conical spindle face and crank eye. This has been tested.
Jobst Brandt
johnrobholmes
10 MW
Good read! I always wondered why they were threaded to undo themselves if the pedal spindle locked up . Precession, just like doing windmills. And knowing is half the battle!
Hillhater
100 TW
Thanks, bobc & Mike, good explanation of something very obscure ! 
Those pedals undergo a very unique load cycle.
Those pedals undergo a very unique load cycle.
bobc
10 kW
John's post made me laugh - with an ebike you only pedal backwards.......
