Reverse-Threaded Pedals

The Reverse-Threaded Pedals

Cyclists who work on their bikes will eventually notice that removing both pedals is frustrating and weird. It turns out the left pedal on a bicycle has a reverse (left-hand) thread. This means you have to screw it “backwards”. WTF, WHY? Turns out, the pedals are designed to stay tight because of a phenomenon called mechanical precession.

Why One Pedal is Reverse-Threaded (Mechanical Precession)

Mechanical precession is the effect that keeps your pedals secured. In simple terms, when a round object (like a pedal spindle) is forced against the threads in a rotating manner, it will tend to slowly roll around in the opposite direction of that rotation. This means that even though the left pedal rotates forward (which would normally unscrew a right-hand thread), the precession effect makes it tighten instead. In fact, the torque from mechanical precession is much greater than any loosening force from the pedal’s bearings. In nearly all bikes since the 1930s, manufacturers solved loose pedals by using a left-hand thread on the left side.

Animation of mechanical precession in action
Animation of mechanical precession: The inner blue part (pedal spindle) rolls inside the outer red ring (crank arm) opposite to the green arrow (the rotating force). This causes the spindle to turn opposite to the force direction, effectively tightening a left-hand thread. Source: Chris Shannon (CC BY-SA 3.0) via Wikimedia Commons

You can even try a quick experiment to feel mechanical precession yourself. Hold a pen or pencil loosely between your fingers, and use your other hand to push the tip of the pen around in a circle. You’ll notice this causes the pen to start twisting in the opposite direction of your circle. A COMSOL Blog post walks through this with helpful visuals and a simple experiment. Similarly, each pedal spindle, under the downward force of pedaling, “rolls” a tiny bit against the crank thread in the opposite direction of your pedaling. That reverse rolling is what keeps the left pedal snugly tight.

What This Means for Cyclists

Thanks to mechanical precession, you dont have to worry about pedals falling off mid-ride. It’s the reason left pedals tighten themselves as you ride, rather than working loose. (On the flip side, the right pedal uses a normal thread because its precession goes the other way and also self-tightens.) When you do need to remove your pedals for maintenance, remember that the left pedal unscrews clockwise (the reverse of usual) while the right pedal unscrews counter-clockwise. In other words, the common phrase “righty-tighty, lefty-loosey” (such a weird phrase) only applies to the right side, the left side is the opposite!

By understanding this, you’ll not only avoid confusion during repairs, but also gain a new appreciation for the clever engineering built into your bike. A little knowledge of mechanical precession goes a long way in explaining what might have seemed like a mystery of the pedal that never falls off. And, if you didn’t know this - you’re not alone: many are frustrated with removing pedals. But now you know.