Technical FAQ: More on pedal thread direction
Feedback on pedal thread direction
Right pedals are right-hand thread and left pedals are left-hand thread so that they self-tighten as you ride, preventing them from falling off — IF your bearings are working.
The graphic below shows the situation for a right pedal. As viewed from the right side of the bike, the pedal rotates counterclockwise as you pedal, with the fixed-axis pedal spindle rotating clockwise. A bearing. therefore, rotates counterclockwise and serves to apply a teeny clockwise torque to the pedal spindle, tightening it. This obviously works as you can hand tighten a pedal and a week later the pedals can be quite tight in the crank arm.
I just wanted to submit a quick correction on your technical FAQ on why pedal threads are threaded the way they are. You’re right in that your foot will cause a seized pedal bearing to unscrew from the crank safely rather than wrenching your foot dangerously, but that’s not the reason they’re threaded opposite. Instead, due to an effect called precession, the bearings on the spindle will actually exert a torque opposite to the direction that the pedal spins relative to the bearing — that is, if the pedal is spinning counterclockwise relative to the spindle, as would be the case with the right-side crank pedaling forward, the spindle is actually being torqued clockwise. It may seem counterintuitive, but due to that effect, the spindle indeed self-tightens with normal pedaling — as long as the bearings aren’t seized.
While I agree with you and Alan and the many others who wrote to me about this to describe the self-tightening effect when the pedal bearings are working properly and the principle of precession, I am not sure I’m in agreement with you as to the reason that motivated the thread design in the first place. Pedals could be threaded the way they are in order to self-tighten them, as you suggest, or as a safety measure to prevent injury in the case of frozen bearings. I think a look at history would tend to favor the latter.
If you think of the days of penny-farthings, the high-wheeler bicycles that were common from 1869-1890, a frozen pedal that did not unscrew could be fatal.
High-wheelers and the boneshakers they replaced were the first pedal-driven bicycles; before that, bike riders scooted along with their feet like little kids on “strider” bikes.
Boneshakers and penny-farthings had pedals but no chain drive; the pedals were attached to the front wheel, like on a kid’s tricycle. The only way to increase the gear ratio on a direct-drive bike like that is to increase the diameter of the front wheel. This perched a fast rider who had the strength to push a high gear up high over a very tall front wheel (sans helmet, of course). Because of his body’s location, if the front wheel stopped suddenly, the only possible outcome was for the rider to rotate head-first over the front wheel. Such incidents often led to serious head injuries and death.
There were a number of features installed on penny-farthings to increase safety. If riders had the chance to prepare to stop, they could step back onto rear mounting pegs to keep their weight further back as they braked, pushing a metal shoe straight down on the top of the front tire. Penny-farthings were generally equipped with wire loops extending back from the top of the fork to prevent the rider’s pants from getting wedged between the fork and the tire; this occurrence would obviously have the same effect of slamming the rider’s head into the road. And the pedals, which sometimes had toeclips and other means of increasing pedaling efficiency, generally had minimal seals to keep water out and prevent seizing of the bearings or bushings. If a pedal seized up, especially if the foot were attached to it with a toeclip, if the pedal didn’t unscrew from the crank, the rider could easily be killed by something as simple as poor pedal maintenance or a couple of rides in inclement weather.
Of course, if you’re thinking more about this, you know that one of the braking methods used on penny farthings was back-pedaling, just like on modern track bikes and fixies. And any track rider or fixie rider knows that their pedals had better be screwed on tightly. If you only hand-tighten a pedal on a road bike, it will tighten up as you pedal. If you only hand-tighten a pedal on a track bike or fixie, it can unscrew when the rider backpedals.
The benefits of our threading directions on pedals is that they both tighten properly functioning pedals onto the cranks, and they unscrew and save the rider from injury if the pedal bearings or bushings seize up. It’s a win-win. I’m just not sure that these thread directions were chosen to tighten road pedals on freewheel-equipped bikes rather than to save the life of a penny-farthing rider.
Feedback on compact frames and pedal threads
I was reading your most recent column and it brought on some Déjà vu. Didn’t you answer a similar question several years ago regarding pedal threads? If I recall correctly, the answer to this current, “what is Theo missing” question is that precession will work in a manner opposite to a seized pedal bearing. While he is right that with a failing pedal bearing the current threading configuration could result in the loosening of the pedal from the crank, precession has the potential to loosen the threaded attachment of pedal and crank, even with an optimally functioning bearing. It appears that early bicycle “standards makers” recognized this and chose to prioritize the thread security of the many functional pedals over the few seized ones.
Also, regarding Steve’s standard vs. compact frame question, don’t forget that the compact frame will have a more exposed seatpost, and that seatpost deflection will also go up with the cube of its length. This means the compact frame will be stiffer, but with a more flexible seatpost. All else being equal, this is a combo that generally seems pretty good as it can provide solid standing efforts and secure handling, while keeping a little cush in the seatpost to reduce the harshness on the body.
Yes, we discussed pedal thread direction six years ago here. And you have a very good point about the added compliance of the seatpost on a compact-geometry bike.
Thanks to all of you who weighed in on the pedal threads issue. Here is a sampling of other letters on the subject, all of which are filled with wonderful pearls of wisdom and new ways to look at it.
I would like to respond to Theo regarding left-handed threads on left-side pedals and right-side BB cups. These left-handed threads were designed to counter mechanical precession, rather than bearing friction. Think of the pedal shaft as a circle inside of an ever-so-slightly larger circle — the threaded hole in the crank arm. If the pedal is loose, the act of pedaling forces these two circles into closer contact. As you pedal, this point of contact moves around the circle. Because the inner circle has a smaller circumference, it completes a rotation before it gets back to where it started. The counter-intuitive result is that the smaller circle (the pedal shaft) rotates backwards.
I read the question and your response regarding why left and right pedal threads are the opposite of what one might, at first glance, expect. The reason for this is due to the effects of precession, where one round object rolling inside another will turn in the opposite direction due to the clearance between the two. Except for a completely seized bearing, the forces due to this are considerably greater than friction in the bearing.
I just wanted to submit a quick correction on your technical FAQ on why pedal threads are threaded the way they are. You’re right in that your foot will cause a seized pedal bearing to unscrew from the crank safely rather than wrenching your foot dangerously, but that’s not the reason they’re threaded opposite. Instead, due to an effect called precession, the bearings on the spindle will actually exert a torque opposite to the direction that the pedal spins relative to the bearing. That is, if the pedal is spinning counterclockwise relative to the spindle, as would be the case with the right side crank pedaling forward, the spindle is actually being torqued clockwise. It may seem counterintuitive, but due to that effect the spindle indeed self-tightens with normal pedaling — as long as the bearings aren’t seized.
A long time ago, it was explained to me that pedals are threaded as they are to prevent a pedal that is not completely tightened from walking out of the crank arm from mechanical precession.