Few things can cause a bigger headache for a bicycle mechanic than left-hand threads. The confusion and frustration of “loosening” a stuck pedal only to realize you’ve truly just been making it really, really tight is unparalleled, and replayed time after time in shops everywhere. Sometimes lefty isn’t loosey, and it’s a good idea to have a firm grasp on this concept before grabbing the cheater bar for that stuck bottom bracket.
Most everything in life is right-hand threaded, meaning that if you turn the head of a bolt clockwise the bolt will get tighter. As a de facto standard it works well for most applications of fasteners and machine parts. There are instances where the physics of mechanical movement dictate that particular items be threaded opposite this standard, hence left-hand threads that tighten when turned counterclockwise.
Parts of a Bicycle with Left-Hand Threads
Pedals – The left (non-driveside) pedal and crank arm
Bottom Brackets – The right (driveside) cup of English
threaded bottom brackets.
Fixed Hub – The lockring on a track hub.
Freewheel – The bearing cover. Don’t loosen it,
it’s a mess.
Mechanically speaking, pedals, bottom brackets and that freewheel bearing cover have left-hand threads where they do because of the principle of precession. Jobst Brandt explains, “[Precession], in which a round object rolling in a circular ring in one direction will itself turn in the opposite direction… Precession forces are large enough that no manner of thread locking glues, short of welding, will arrest them.” Newtonian physics at its best. Pedals are far more prone to precession than bottom bracket cups are, thus Italian and French thread bottom brackets which are right-