I have shredded the pedal threads on four crank arms since November. Can anyone think of a reason I might have developed this expensive habit?

I know how to fit pedals. They are fitted on the correct sides. I use Shimano anti seize. They are tightened correctly. Since the 2nd occurence I’ve been Mr Paranoid about tightness, and I check before every ride now. There is no (visible) damage to the pedal threads. The pedals spin freely in my hand, although I can sometimes make them squeak. Anyway, they just keep shearing off, the pedal stays attached to my foot but comes off the bike, and the result is a knackered crank. I get 1 or 2 rotations warning – it goes from normal to feeling loose, as if I’ve come out of the cleat…then bang.

Variables: It’s happened on both sides (1x left, 3x drive sides). It’s happened on three different models (all shimano: RS510, 105, Ultegra).

Constants: It’s me that fits the pedals. It’s me that rides the bike. It’s been the same set of pedals each time, quite old exustar look style. It has always been on the turbo.

Has anyone ever had this problem? What was the explanation?

Can anyone suggest a reason I’m suffering this persistently which I might have overlooked?

Have you tried a different set of pedals?

It’s been the same set of pedals each time

I would go with that given what you said, can only assume something up with the threads on the pedals – especially when you say you can’t see anything wrong with threads in the cranks.

Goosed threads on the pedals is my guess.

Thanks people. Keep other possibilities coming!

I am replacing these pedals. And no 4 has taught me not to risk them in the meantime. I guess I’m fishing for both non-pedal related possibilities in case that isn’t actually the issue OR pedal-related explanations I might nevertheless have missed for peace of mind.

I mainly suspect: pedal bearings and/or (invisibly/nonobviously) degraded/damaged pedal thread.

I’m just a bit uneasy about concluding that either could be the case, especially on both sides out of the blue though.

Well, the pedal thread could be ruined

Or the pedal bearing is tight so it’s precessing and ripping out the thread (I’m not sure this is possible, and surely you would notice this)

Or you have cross threaded each one of them 🤣

I sheared the (Lasco?) driveside crank on my Wazoo fatbike last July through the pedal threads, having had a set of Wellgo B144s fitted for over six months. I put it down to a bit of fatigue over 4.5 years on a cheap chainset, having done regular intervals up the local short ramps of up to ~20% until I got a turbo and for the last ~12 months before the shear, I was using the 17T sprocket with 24/38T rings on my commute that involves a few ramps.

It sheared on the way into work up the ~1min ramp of ~5% while in 38/17.

I suppose another possibility could be if you’re a pedal-masher. I’ve seen a couple of damaged bikes as a result of riders standing up and heaving when on a direct drive turbo, on the road the bike will rock side to side, on the turbo it won’t, and something ends up giving up.

Only time I’ve done this was with a pedal that had worn threads and had worked loose. If they are old and have been pulled out of 3 sets of cranks, it’s almost certain the threads will be goosed enough to not stay tight.

Admit it, your a crank destroying zwift power house 😃.

Maybe use light/medium threadlock instead of anti-seize ?

Nixie may have a point, if it’s yr trainer bike. Even on a rockers there’s less movement than you can generate on an outdoor bike so if you’re out of the saddle and sprinting often, I guess the forces on the pedal might be at least a bit different (can’t see it the same while sitting)

… and my entire bike, shoes etc get drenched in sweat (and tears) on a regular basis. Sometimes wonder how bad that is for parts, though I’ve never imagined it getting into crank threads

They are tightened correctly.

Would the defendant please elaborate for the benefit of the jury?

Admit it, your a crank destroying zwift power house 😃.

You didn’t have these problems until you started playing with the big boys up the front of the races! #massivepowerhumblebrag

As it’s the same pedals each time, I’d guess that the bushings or bearings in them are binding a bit when under load, which is then forcing the pedal axle to start unscrewing slightly and then as soon as that loosens a little the damage quickly happens.

The direction of the pedal threads normally stops them unscrewing when pedaling, but that is for the case when the bushings/bearings are working properly.

It could also be worth looking at having a Helicoil put in some of those knackered cranks if you still have them? Or was there too much damage? If properly fitted they can be stronger than before.

Very odd. I would use grease. If the pedal threads were damaged you might feel it when installing them. Pic of the pedal threads? It’s odd to think they would get worn/damaged by aluminium cranks.

I suspect not tight enough, and bushing bind in pedals. Burgtecs are notorious for the same issue as they have so much resistance.