Bicycle pedals

I recently removed the pedals from my bike to make it easier to load into the back of the car. I was surprised to find that the right hand pedal has a right hand thread where it screws into the crank and the left hand a left hand thread. And this seems to be common across all bikes.

How could the cycle industry have made such a fundamental blunder? It clearly should be the other way around so that any friction drag in the pedal bearings will tend to tighten the pedals in the crank rather than loosen them.

Should I start a campaign?

Maybe that boat has sailed.
Dave W

I suggest you think again, Glyn ...

Maybe include in your analysis, the torque applied when pedalling.

MichaelG

The precession forces rotate reverse to the rotation. It's these forces that the LH thread is there to combat, not friction.

Mechanical precession

Having a pedal unscrew, particularly when pedalling hard would be dangerous, possibly even fatal.

If a left hand pedal were to seize to its spindle, the effect would be to unscrew a right hand thread. Hence the left hand thread. The same logic applies to the right hand pedal.

No, the industry got it correct a long time ago!

Howard

Not so Howard - when pedalling the LH pedal arm turns anti-clockwise but the pedal itself rotates clockwise on it's spindle. If the pedal was to seize it would serve to tighten a RH thread.

Hi Howard, Pete is correct, However, it would be more potentially dangerous if you have a fixed sprocket or bake peddle braking, if the peddles unscrewed, which may be more likely if the peddle seized. All the bicycles that I've ever had, have had the left on the left and the right on the right. I've only ever experienced a peddle unscrewing once and I think it was because I hadn't tightened it correctly, and although I wasn't peddling hard, it came more of a surprise than any ill will.

Regards Nick.

When I was a boy I stripped and rebuilt my bike and fitted a fixed wheel gear. When I rode the bike I could not understand why the pedals kept dropping off until I realised that I had fitted the crank and chain on the wrong side.

I have twice had a left hand pedal fracture at the root, with 40 years intervening. Fortunately never at that critical moment when half way getting on, but it is still a bit of a shock. First one was interesting as it had clear fatigue marks and I was a metallurgy student at the time.

I could never understand why dedicated cycle pedal spanners were so long until I asked at my local bike shop; the explanation is that over time they become very tightly screwed into the cranks and the leverage is needed to get them off.

If the OP is doing this regularly he could consider a pair of quick release pedals as used on folding bikes.

This topic has been discussed endlessly in multitudinous places over the last century and a half, and nearly everybody who has an opinion disagrees with everybody else because their opinions are different and therefore wrong.

My opinion, for what it's worth, is that the early bicycle makers found that one pedal repeatedly unscrewed and dropped off. Being practical folks, they tried using the opposite thread on the offending pedal and found that it cured the problem. And that's all.

George B.

The bottom bracket threads on British bikes also have the left and right hand threads. I once bought a Ballistic aluminium full suspension mountain bike frame which had been welded up with the bottom bracket the wrong way round. It did indeed come loose miles from home, just as well I had self extracting crank bolts. It got some weak threadlock on the XTR sealed bottom bracket and was ok after that. The funny thing is that Italian bike bottom brackets have right hand threads both ends. 

Pete Rimmer is correct in that it is down to precession forces are high because the shaft is overhanging from the fixing. Imagine having a housing with two ball races spaced 2" apart which have 1" bores. If you stick a 3/4" rod through the bearings and use a stirring motion, the bearings will start to rotate. this is an extreme example, but the best I can think of.

Edited By old mart on 15/02/2020 16:06:05

Hands up all those who were standing on the pedals when that happened.

Just me and Tiny Tim then.

It's a shame we're just past Valentine's Day because here's a handy sentiment to go with a box of petrol station chocolates: "Darling, your lips are like petals. Bicycle Petals."

Also, did you know St Valentine is the patron saint of Greeting Card Manufacturers.

And your skin... your skin is like peaches... football peaches.

Geo. B.

Never underestimate those old Engineers. They might not have had an electronic calculator but they knew how to use a slide rule - which I never did master.

I still have three slide rules: 10" Faber Castell, 10" Boots ringplan which is rather floppy now; and a 5" Helix. Every so often, when the urge gets me, I get one or other out and have a play.

Peter G. Shaw

As we appear to have drifted slightly off-topic. I used to use slide rules and still have a couple lying about in the darker reaches of the workshop. This is one of the less common variety, essentially a circular slide rule.

Apparently the advantage is in the length of scale. The red line at 12 o'clock is on the glass cover the black line at 2 o'clock is on a transparent (celluloid) cursor. The dial and cursor are rotated by the two knobs. Can't remember ever having used it in anger.

As far as bike pedals go, touch wood, mine have always stayed firmly screwed in!

John

If there was a problem with the threads on pedal cranks then pedals would be falling off all the time, like others here I have snapped a few spindles but never had one unscrew. Cycling with a snapped pedal does rather damage your shoes on the sharp stub.

Mike

Hi, never snapped a pedal spindle, but have had a pedal come off the spindle and it makes your foot ache after a while of peddling on the spindle. New pedals were not very cheap when I was only earning a total of £1.00 a week after school and on Saturdays. Oh yes! had to fund our own repairs once we started earning, at least we learnt how to respect things and to repair them also.

Peter G. Shaw, so you had a Boots 10" slide rule then I could only manage to get a Boots 6" Slide Rule but at least I could read it them days, need me glasses and a magnifier to see it properly now.

Regards Nick.