Left Pedals, Reverse Threading, Precession, and What it All Means
There are few things worse than spending 5-minutes struggling with a bolt that won’t come loose, only to realize you’ve been tightening it the whole time. Besides the damage you can do to your components wrenching things the wrong way, it’s hard to get your security deposit back when your walls are full of holes from tools thrown in frustration. In today’s post, we’ll look at the most common tricky-bit on a bike, the left pedal, and exactly why it’s lefty-tighty.
Left Hand Thread
If you’ve ever assembled a bike or swapped out an old set of pedals, you know what many people learn too late – that left and right pedals are different. Most of the time they’ll be labeled with a small sticker or engraving indicating right and left, but even without explicit cues, a quick glance at the two side by side is all you need to tell the difference. Right pedals have a right-hand thread, so they follow the standard righty-tighty, lefty-loosey rules. Left pedals, however, have a left-hand thread, so they go righty-loosey, lefty-tighty. If you hold a pair of pedals next to each other with their axles pointing up, the right pedal will have threading that goes “up to the right”, the left pedal will have threads that go “up to the left” – easy. But why?
There are lots of theories as to why left pedals get unusual threading, but there’s one real, practical answer – so the pedal doesn’t fall off while you’re riding. Some say the Wright Brothers were the first to experiment with a left-hand threaded pedal but, whatever the source, prior to its implementation riders often had their trips cut short by runaway left pedals.
The culprit is an action known as precession. When the pedal axle is threaded into the crank arm and the rider pushes the pedal around to move, this causes the axle to “spin” the opposite way in the crank arm. Sheldon Brown suggests an easy way to visualize this, just hold a pen loosely in the end of a fist. Use your free hand to move the end of the pen in a circle and you’ll see the pen itself naturally rotates against your hand in the opposite direction. On a regularly-threaded left pedal, that action will cause the pedal to loosen, eat up the threads in the crank, and make its escape at the least opportune moment.
Along the way someone realized that by reversing the threading on the left side, those pedals would stay put – and the rest is history. A little bit of frustration for first-time pedal-techs in exchange for miles and miles of riding with both feet firmly on the bike.