Why would a pedal spindle work loose?

[Sillyoldman]

This has never happened to me before. Today I was aware of a slight knock through my left shoe as I rode along. Thought it might be the cleat had come loose so I stopped and found the left peddle had come loose from the crank arm. Could only hand tighten it but it got me home ok.
I thought that the reason the left peddle (and right come to that) are threaded such as to prevent this from happening. The threads are not damaged and it’s tightened up properly. I’m just intrigued as to how it has happened.
I periodically remove the pedals and clean and grease the threads and they are usually right b*ggers to unscrew from the cranks.

 

[Ajax Bay]

I recommend removal of said pedal and a careful inspection of the thread in the crank. It's likely that it's now a bit damaged (because it was relying on decreasing numbers of threads as it loosened) and even if now/again secured tightly, the pedal will work loose / come off again. Some pedals have a hex socket in the axle and can be fully tightened 'on the road' using the allen key you'll be carrying.

 

[roubaixtuesday]

they are usually right b*ggers to unscrew from the cranks.

I have no answer for your question, but I did find the expensive but excellent solution to your other problem. Never looked back.

https://www.parktool.com/product/professional-pedal-wrench-pw-4?category=Pedal

 

[Sillyoldman]

Yes it’s a hex socket so the Park tool pedal spanner lovely though it looks, would be no use. Ajax Bay, it have looked at the crank and pedal axle threads and they don’t look damaged so fingers crossed on that. I don’t as routine carry a hex key that large but yes, for the next few rides I will be doing.

 

[Ajax Bay]

Think you'll have flats on the pedal as well as the hex socket in the pedal axle end. When travelling (and needing to take the pedals on and off to bag the bike) I've used an allen key without difficulty (for maybe 12 off and ons (OuiGo trains)), but for normal removal of pedals at home I would always use a spanner (and a mallet if necessary).

 

[Sillyoldman]

Think you'll have flats on the pedal as well as the hex socket in the pedal axle end. When travelling (and needing to take the pedals on and off to bag the bike) I've used an allen key without difficulty (for maybe 12 off and ons (OuiGo trains)), but for normal removal of pedals at home I would always use a spanner (and a mallet if necessary).

The pedals are Shimano Devore XT and the only flats are at the pedal end of the axle used to remove the axle from the pedal for bearing maintenance. Didn’t realise this until I bought them as I find the hex key harder to unscrew than flats and a spanner

 

[Drago]

This is why wise riders do pre ride checks.

 

[Sharky]

Does the peddle or pedal, spin freely on the pedal axle?

 

[Sillyoldman]

This is why wise riders do pre ride checks.

Tires and brakes yes. Do you seriously check every nut and bolt? Having never had a pedal work loose before it’s not something I would consider checking.

 

[Sillyoldman]

Does the peddle or pedal, spin freely on the pedal axle?

Yes all works well

 

[Specialeyes]

I thought that the reason the left peddle (and right come to that) are threaded such as to prevent this from happening.

I used to assume likewise, but then actually thought about it - pedals are threaded to loosen. I presume it's to stop them over-tightening themselves to the point of damage or siezing.

 

[figbat]

It appears counterintuitive but they are threaded to tighten, because the mechanism of this tightening is not one of friction but of precession. The spindle effectively rolls its way tighter, rather than being screwed out of the crank arm.

 

[Venod]

I periodically remove the pedals and clean and grease the threads

Could this be the problem, easy to get distracted and not tighten them.

 

[Ian H]

It appears counterintuitive but they are threaded to tighten, because the mechanism of this tightening is not one of friction but of precession. The spindle effectively rolls its way tighter, rather than being screwed out of the crank arm.

Tighten under normal use, but loosen if the bearings seize, so that you don't break an ankle (I have seen this happen).

 

[figbat]

Tighten under normal use, but loosen if the bearings seize, so that you don't break an ankle (I have seen this happen).

Yes, this is where the action of friction overcomes that of precession.