cam217 Posted July 21, 2009 Report Share Posted July 21, 2009 i was riding in leicester the other week and when i was on may way home pedaling along started to feel like the crank was mojorly loose and about to fall off, i was about 3 mins away from my house so i jus ignored it untill i got home.i took a look at it and i seemed like my pedal had snapped but not all the way through so it was sitting on an angle, i whipped the pedal of the find what had really happen was the force of me landing on the pedal from a gap had caused the pedal to go at an angle ripping all the thread out of my crank arm or somthing similarim not a big rider in anyway so i found this odd plus the crank were of here so i have to put some old ones on the cranks i were using are zoo cranks anyone see or heard of this happening before or was it just thread wear or my imense weight ? lol Quote Link to comment Share on other sites More sharing options...
Alex Dark Posted July 21, 2009 Report Share Posted July 21, 2009 i was riding in leicester the other week and when i was on may way home pedaling along started to feel like the crank was mojorly loose and about to fall off, i was about 3 mins away from my house so i jus ignored it untill i got home.i took a look at it and i seemed like my pedal had snapped but not all the way through so it was sitting on an angle, i whipped the pedal of the find what had really happen was the force of me landing on the pedal from a gap had caused the pedal to go at an angle ripping all the thread out of my crank arm or somthing similarim not a big rider in anyway so i found this odd plus the crank were of here so i have to put some old ones on the cranks i were using are zoo cranks anyone see or heard of this happening before or was it just thread wear or my imense weight ? lolsounds like it was already crossthreaded...pretty hard to actually rip the thread out Quote Link to comment Share on other sites More sharing options...
psycholist Posted July 22, 2009 Report Share Posted July 22, 2009 Since I changed to a new bike with front freewheel I've had 3 cranks go this way (No issues for years with Middleburns before that). I've changed my pedals as well as the crank this time as I reckon the threads on the pedal may have been undersized, allowing it to move too much in the crank causing an earlier failure and all is good so far (A couple of months on). Usually I only get crank failures at the pedal thread on high mileage cranks (I've had a few cranks burst around the pedal hole and more fail where the pedal threads come out of the crank, but these are almost all after years and thousands of miles of use rather than every 6 months with the trials cranks). The reason this happens is down to the bending the pedal axle carries causing small movements between the pedal and crank threads as you pedal. In trials this is exacerbated by the very high loads from landings and causes the threads in the crank to disintegrate over time until you actually can feel the movement. These same small movements are the reason the left pedal has a left hand thread, if it used a normal thread it would work loose all the time - as you pedal normally the pedals will tighten themselves (Part of the reason the end of a crank can burst rather than giving a thread failure in high mileage cranks). Google for 'Jobst Brandt' and 'pedal threads' or 'thread precession' and you'll find loads more information on what's going on.Greasing the life out of the pedal threads before assembling them will slow this failure, but not stop it. Quote Link to comment Share on other sites More sharing options...
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