Condensed Answer: Bike pedals self-tighten as the rider pedals. This is accomplished via the strategic choice of thread direction on the pedal axles.
The drive-side pedal tightens clockwise while the non-drive-side pedal tightens anti-clockwise.
Despite the self-tightening function of bike pedals, if they’re not tightened sufficiently in the beginning, they will wobble and may even fall off.
Pedal Thread Directions
|Left Pedal||Right Pedal|
|Direction of Installation||Counterclockwise||Clockwise|
|Direction of Removal||Clockwise||Counterclockwise|
The left and right bike pedals have different thread directions. (The left pedal is on the non-drive side; the right pedal is on the drive side).
The left pedal has a left-hand thread. This means that the pedal is installed by rotating it counter-clockwise and can be removed by rotating it clockwise.
Meanwhile, the right pedal has a right-hand thread. To mount it, you have to screw it in clockwise.
The reason for this thread choice is to prevent the pedals from unscrewing during pedaling.
The Relation Between Crank Arms and Pedal Axles
To understand why bike pedals must be self-tightening for optimal performance and safety, one has to look at the motion of the crank arms and the pedal’s axles.
In order for the bike to move forward, the rider has to press the drive-side pedal down in a clockwise direction when looked at from the side of the chain.
As the crank arm is spinning, the axle of the pedal is supposed to be stationary. (The pedal itself is rotating around the axle thanks to a simple ball-bearing system.)
Since the rotation of the drive-side crank arm is clockwise, it’s necessary for the drive-side axle to tighten in that direction too. This is accomplished by using a standard right-hand thread on the axle.
It’s easy to comprehend what is happening by putting a finger on the drive-side crank arm and then spinning it to mimic forward pedaling.
You will see that the crank arm is rotating in a clockwise direction around your finger which in this case plays the role of the axle.
However, since the non-drive side crank arm is pointing in the opposite direction, it’s rotating anti-clockwise when the rider is pedaling forward. (You can do the same exercise to understand the process.)
To make the non-drive side pedal self-tightening, it’s necessary to use a left-hand thread so that the pedal doesn’t untighten during riding.
FAQ: Does this mean that it’s impossible for a bike pedal to untighten itself?
If the pedal has been properly tightened, to begin with, it’s highly unlikely that it will come loose.
Also, if the pedal is left loose ever so slightly (no threads should be visible), it will self-tighten until the axle no longer moves.
That said, it won’t be accurate to conclude that a bike pedal cannot come loose. It’s a very real scenario in the following cases:
- The pedal has been cross-threaded.
Cross-threading occurs when using the wrong pedals for the bike or when trying to install the pedals too quickly and with too much force.
Since the pedal axle is made of steel whereas the crank arms are usually aluminum, the axle damages the crank arm threads quite easily. (steel is harder than aluminum)
Eventually, this could lead to a weak connection and a wobbling pedal. The recommended solution is to place a threaded insert into the crank arm as shown in the video below:
- The pedal hasn’t been sufficiently tightened
If the pedal hasn’t been sufficiently tightened, the self-tightening direction of the thread won’t be enough to prevent the pedal from getting loose.
Ideally, the pedals will be tightened with a torque wrench with a pedal adapter to the specification of the manufacturer.
If you don’t have those tools, the recommended approach is to get the pedals snug but without exerting maximum force.
- The pedal is broken/destroyed.
It’s extremely rare for a pedal axle to break. Truth be told, you’re more likely to break the crank arm than the pedal axle.
That said, every part under stress can potentially crack. In some cases, the pedal’s bearings and caps can be completely destroyed to the point where the entire platform falls off the axle. Of course, this is a rare situation. And there will be plenty of warning signs before it occurs.
FAQ: What is the thread size of a bike pedal?
The size of most pedals is 9/16″ x 20.
9/16″ x 20 TPI stands for:
9/16″ (14.3mm) is the diameter of the pedal’s threaded part.
TPI is an acronym for Threads Per Inch and shows the number of threads per 1 inch/2.54cm. In this case, the threads’ density is 20 per inch or 20 per 25.4mm which amounts to 1.27mm between threads.
However, there are also 1/2″/12.70mm pedals that have a thinner axle. Those are usually found on the BMX market and are designed for single-piece cranks. If you have a 1/2″ pedal, you won’t be able to mount it to standard cranks.
FAQ: What wrench do I need to tighten and untighten bike pedals?
Most pedals require a 15mm spanner (In some cases, an 8mm Allen key can also be used.)
As long as the spanner is thin enough to get in the area and wrap around the axle, it’s gonna work.
That said, dedicated pedal wrenches are quite thin and have a long handle for additional leverage.
FAQ: How to tell the left pedal from the right pedal?
Most pedals have a single-letter indication near the threads. “L” stands for left and “R” stands for right.
If the pedals don’t have an indication on them, look at the threads. The threads of the left pedal tilt to the right whereas those of the right pedal tilt towards the left.
FAQ: Should I grease the threads of bike pedals?
This is a beneficial procedure that makes it easier to tighten the pedals and prevents corrosion. Too much grease will increase dirt accumulation, however (read more).