[djamgils]

I have received a new set of pedals and after measuring the thread diameter I found out that the left pedal was 14.1mm and the right pedal was 13.9mm. What are the tolerances on pedal thread because this is 0.2mm difference on a fine thread.

I am asking this because I used the pedals on 2 cranksets and both times the right side thread of the crank stripped. I think it is because of a production error but I would like your opinion on this before I return to the shop.

 

[w00dy]

That is nothing. Less than a thousandth of an inch. Don't pay any attention to the last digit on your digital calipers.

 

[JustMtnB44]

0.2 mm is .008 inches. Still, 8 thou is not that much and shouldn't really be a problem. From the picture there does appear to be a difference though. Given your problems, I would say that the pedal could be at fault, so you should probably talk to the shop about it.

 

[djamgils]

well apparently 0.2mm is enough. I tried to install them today but when I tried to torque it up the thread stripped. so back to the warranty department.

 

[sperkins]

If you feel that there is a problem, then you should contact the company that you received them from.

 

[kuksul08]

You shouldn't really torque the pedals to begin with. I put a very small amount of antisieze on my pedal threads and just snug them up. They never come loose because the pedaling motion tightens them...

 

[djamgils]

to begin with, this thread is old. problem is already resolved. I got my money back for the pedals and received replacement cranks.

I never torque my pedals very hard, just a little snug.
But pedals dont auto tighten, at least not with the atomlab pedals because those use bushings. Pedals with bearings do tighten themselves because of the balls that redirect the torque. But anyhow I dont think that those forces are big enought to tighten or loosen the pedal.

 

[alexchannell]

to begin with, this thread is old. problem is already resolved. I got my money back for the pedals and received replacement cranks.

I never torque my pedals very hard, just a little snug.
But pedals dont auto tighten, at least not with the atomlab pedals because those use bushings. Pedals with bearings do tighten themselves because of the balls that redirect the torque. But anyhow I dont think that those forces are big enought to tighten or loosen the pedal.

Thats false. The bearings don't transfer torque (that is exactly what they are there to avoid). Please don't confuse others with your mistaken ideas, check first. Bearings and bushings are no different in terms of forces on the spindle.

 

[djamgils]

my last sentence implied that the forces are very small. I was talking about fritctional forces from the grease and or friction between metals. I know that bearings/pivots cant take moments. I am a masters mech engineering student so I know some of the basics but sometimes you just think too complicated on a simple problem.

alexchannel, why is it then that the right side has "normal thread" and the left side has "reverse thread". everybody says it is to keep the pedals from unthreading. what is your opinion on that one?

 

[kuksul08]

my last sentence implied that the forces are very small. I was talking about fritctional forces from the grease and or friction between metals. I know that bearings/pivots cant take moments. I am a masters mech engineering student so I know some of the basics but sometimes you just think too complicated on a simple problem.

alexchannel, why is it then that the right side has "normal thread" and the left side has "reverse thread". everybody says it is to keep the pedals from unthreading. what is your opinion on that one?

Interesting, I was just about to say that is because the pedaling tightens them but the spindle takes an opposing torque compared to the pedal... I'm also an ME student and maybe I'm overthinking it as well. The drive side pedal is standard threads.

Pedaling forwards puts a counterclockwise moment on the spindle which technically should tend to unscrew the spindle. Unless the balls in the bearing actually reverse it like you are saying. A regular bushing would unscrew it.

 

[JustMtnB44]

Fellow master's mechanical engineer here...must resist urge to post...cannot resist joining in on nerdy conversations...

Ok, I think you guys are overthinking this too much.

The pedal body is rotating clockwise. There is basically zero moment on the axle because of the bearings or bushings, either way they accomplish the same purpose. However, if there were to be a significant torque on the spindle it would be because the bearings/bushings are not doing their job, and in this case the body would want to lock to the spindle in which case the spindle would rotate in the same direction as the pedal body. Hence why the left and right threads are different, so that the tightening force is always in the clockwise direction (looking at the bike from the drive side).

Have you ever tried to thread in a pedal by turning the body instead of the axle? If the bearings/bushings are snug or there is some friction, then you can rotate the pedal clockwise and the axle will thread in. If the axle was seeing an opposing torque then how could this happen?

 

[djamgils]

to tighten the pedal you lift the back wheel and then pedal backwards while holding the axle. So you are wrong with the rotational direction of the body while pedaling forward.

and you are right about the transfered torque being about zero to nothing so why is there the need for reverse threads on the other side?

 

[Wumpus]

Precession???

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.