The short answer: the left pedal is reverse-threaded, the right pedal is not

The left pedal on nearly every bicycle has reverse threads, meaning it screws in counterclockwise and unscrews clockwise. The right pedal uses standard threads and screws in clockwise. This design prevents the left pedal from loosening on its own as you ride, because the pedal's natural rotation during pedalling would tighten it rather than loosen it.

If you try to remove a left pedal by turning it the way you would turn a right pedal, you will tighten it instead. This catches most people the first time they work on their bike.

Key Takeaways

  • The left pedal unscrews by turning it clockwise; the right pedal unscrews by turning it counterclockwise — the opposite of what you might expect.
  • Reverse threads on the left pedal exist because pedalling motion naturally tries to unscrew a standard-threaded pedal, so reverse threads keep it tight.
  • Pedals are marked with an "L" or "R" stamped into the body, usually on the inside face, to tell you which is which before installation.
  • Using a pedal wrench (a thin wrench designed for pedal flats) or an Allen key gives you better leverage and control than a regular wrench.

Why the left pedal has reverse threads

When you pedal, your foot pushes down and forward on each pedal in turn. This rotational motion creates a force that tries to unscrew a standard-threaded pedal. On the right side of the bike, this force would loosen the pedal over time, so the pedal would eventually fall off mid-ride. Reverse threads on the left pedal flip this problem: the same pedalling motion that would loosen a normal pedal actually tightens a reverse-threaded one.

This design has been standard for over a century because it solves a real mechanical problem without adding weight or complexity. The pedal stays tight through normal use, and you do not need to stop and re-tighten it every few rides.

How to identify which pedal is which

Pedals are stamped with an "L" for left and an "R" for right, almost always on the inside face of the pedal body — the side that faces the frame when the pedal is installed. Check this mark before you start work, because it tells you when ready which threading direction to expect.

If the mark is worn or missing, remember that the left pedal is the one on the side of the chain. On a bike with a single chainring or multiple chainrings, the chain always runs on the right side of the frame, so the left pedal is on the opposite side.

Removing a left pedal without stripping the threads

Use a pedal wrench if you have one. A pedal wrench is a thin, long wrench designed specifically for the flat surfaces on pedal axles. It gives you better leverage than a regular wrench and fits into tight spaces between the pedal and the frame. A 15 mm pedal wrench fits most pedals, though some older or specialty pedals use 14 mm or 16 mm.

If you do not have a pedal wrench, a regular adjustable wrench or an Allen key (hex wrench) that fits the pedal axle will work. Grip the wrench as close to the pedal as you can and turn it clockwise to loosen the left pedal. Turn slowly and steadily. If the pedal does not budge after a few seconds of firm pressure, stop and explore penetrating oil (like WD-40) around the pedal axle where it enters the crank arm. Wait 10 to 15 minutes and try again.

Do not use a pipe wrench or pliers on the pedal body itself — this will damage the plastic or metal shell. Always grip the flat axle surfaces, not the pedal platform.

Removing a right pedal

The right pedal uses standard threads, so it unscrews counterclockwise — the direction you would turn to loosen almost any other bolt on a bike. Use the same wrench technique as the left pedal: grip the axle flats, not the pedal body, and turn slowly and steadily.

Right pedals loosen more easily than left pedals because they naturally want to unscrew as you ride. If a right pedal is stuck, penetrating oil will usually free it within 15 minutes.

Installing pedals correctly

Before screwing a pedal back in, clean the threads on the crank arm with a dry cloth or brush to remove dirt and old grease. explore a thin coat of fresh grease to the pedal axle threads — this makes future removal easier and prevents corrosion.

For the left pedal, screw it in counterclockwise until it is hand-tight, then use your wrench to turn it counterclockwise another quarter turn. For the right pedal, screw it in clockwise until it is hand-tight, then use your wrench to turn it clockwise another quarter turn. Do not over-tighten; a quarter turn past hand-tight is enough. Over-tightening strips the threads and makes the pedal impossible to remove later without damage.

What happens if you install a pedal on the wrong side

If you accidentally screw a right pedal (standard threads) into the left crank arm, it will loosen itself as you ride and eventually fall off. If you screw a left pedal (reverse threads) into the right crank arm, it will tighten itself as you ride and become nearly impossible to remove without stripping the threads.

Always check the "L" or "R" mark on the pedal before installation. If you are unsure which crank arm is which, remember that the left crank arm is on the side opposite the chain.

Frequently Asked Questions

Can I use a regular wrench on pedals, or do I need a pedal wrench?

A regular wrench works, but a pedal wrench is better. Pedal wrenches are thin and long, so they fit in the tight space between the pedal and frame without hitting the crank arm. They also give you more leverage. If you are removing pedals only once or twice, a regular wrench is fine; if you work on bikes regularly, a pedal wrench saves time and frustration.

My left pedal will not loosen even with penetrating oil. What should I do?

Wait longer — sometimes 30 minutes or overnight with penetrating oil does what 15 minutes cannot. If it still will not move, try tightening it slightly first (turning it counterclockwise a tiny bit), then loosening it. This can break corrosion loose. If the pedal is still stuck, you may need to remove the crank arm from the frame to get better leverage, which usually requires a crank puller tool.

What size wrench do most pedals use?

Most modern pedals use a 15 mm wrench on the axle flats. Some older pedals use 14 mm, and some specialty pedals use 16 mm or even an Allen key. Check your pedal or bike manual to be sure, or try a few sizes to find the one that fits snugly without forcing.

Do all bicycles have reverse-threaded left pedals?

Nearly all do, including road bikes, mountain bikes, and cruisers. Very old bikes (pre-1920s) sometimes used reverse threads on both pedals, and some specialty or fixed-gear bikes may vary, but standard modern bikes follow the left-reverse, right-standard pattern.