arid

I've been thinking about this for a few days and can't figure it out. I also can't find the answer anywhere at all. Why are (most) bottom brackets reverse threaded on the drive side. Everywhere I look simply says it's so that the cup won't be unscrewed during pedaling. I get how this is a good thing, but I don't get how it works. As you pedal, the crank moves in a clockwise direction, which is the direction that should unscrew the reverse threaded bottom bracket cup. And then on the non-drive side the crank is moving counterclockwise which should also be unscrewing that cup. It seems to me like the non-drive side of the bottom bracket should be the one that's reverse threaded. Just like the pedals. I'm sure there's a simple explanation that I'm just completely missing, but I just can't figure it out. I sucked at college physics.

Pocko

Excellent question arid!

I have often thought about this myself, but it's one of those things that one thinks of but never dares to pop the question and risk being seen as the village idiot who challenged the status quo. Too late now, looks like you and I are out of the closet.

The logic of right-hand and left-hand threads actually makes perfect sense to me when I contemplate on the rotational orientation of pedal spindles. However, the opposite seem to be true when the principle is applied to the bottom bracket axle. If anything else, the forward rotation of the axle would be in the same direction that would loosen the bearing race threaded to the BB shell.

I have often wondered if someone had made a mistake back in the early days, but by the time it was discovered, it had been already too far entrenched in manufacturing standards in order to be rectified (speculation). The first time I built a frame with a threaded BB shell got me worried that I could get the BB round the wrong way if I wasn't careful. In fact, because I was determined NOT to make this mistake was when I first realized this possible anomaly. I was so concerned that I checked, double checked, and triple checked this... and was really surprised by the threading orientation. It was the opposite of what I expected.

Perhaps someone here could give us the official (or historical) reason for this... I certainly would also like to know.

Takers anyone?

xenologer

...
Tough to describe.

It is similar in concept to a cycloidal drive or harmonic drive gear system.
Where the bottom bracket is analogous to the outer gear, and the bearing race the inner gear.

Basically, you think that since your pedal is rotating it will drag the race with it, but in fact there is very little friction since you have bearings.
Instead the major force on the race is going to be a push outwards at one point on the circumference from the cranks being pushed on, and this location will actually move around as you pedal.
This outwards force is very much like the 'wave generator plug' in a harmonic drive, and causes the race to jiggle slightly and rub against the bottom bracket threads causing it to 'walk' its way backwards agains them... anyway, look at the drawings of how a harmonic drive works, its makes it much more obvious why the race ends up trying to spin opposite your cranks.

https://en.wikipedia.org/wiki/Harmonic_drive

anyhow, obviously there won't be that much of a gap between your bottom bracket an race as compared to the gears in a harmonic drive, but no material is perfectly solid so the effect will still there slightly as things bend and flex from all the force of your weight pedaling...

Pocko

Thank you xenologer! That was the first logical explanation regarding the matter that made sense to me.

I often think of harmonics in high frequencies, but yeah low frequency harmonics would be present because lop-sided loads would be thrust upon the axle and bearing races - by two opposing cranks at either side of the axle. Come to think of it, I wouldn't be surprised if the group of ball bearings within the bearing races would revolve the opposite direction to that of the axle rotation (speculation)... not that it would matter, as I can envisage "counter-creep" occuring on the outer bearing race even if it didn't.

If even and constant torque was applied to the BB axle (like from an electric motor), then this oscillating counter-creep wouldn't occur...

I assume that this is the case in the pedal axles, which is why the thread orientation is more straight forward.

My son would appreciate this... because he asked me the very same question when one of his mates broke my pedal wrench because they got confused by this very same thing. (They re-build their BMX bikes all the time in my garage).

thanks!

dabac

The basic phenomena is that the crank axle will move in a pattern called precession, and due to the minute gap between the male thread of the cup and the female thread of the BB shell this generates torque at the cup.
An exagerrated but basically correct demonstration can be had by sticking a pen in a glass, tilting it against the side of the glass and then having the pen follow the circumference of the glass w/o sliding. You'll see that the pen will rotate inside the glass despite the contact surfaces being stationary WRT each other.
This gives you a few options, cut the thread the "right" way, and the precession-induced torque will keep trying to screw the cup in, use another kind of interface(self-centering interference fit) to kill the precession, ot torque the bugger to within an inch of its life at assembly so that precession torque won't be strong enough to unscrew it.

tatfiend 

I have certainly heard of Italian threaded bottom bracket cups having a loosening problem due to both sides being right hand threaded. This is the first time I have read a reasonable explanation of why the problem exists.

DaveSSS 

Precession is the cause of the (Italian) cup unscrewing, but the mistaken idea is that the forces are large enough to unscrew the cup, when properly tightened. Italian threaded cups have the same torque requirement as English threaded cups and are no more likely to loosen. If the cup loosens because the BB faces are not square to the threads, then it will keep unscrewing on it's own. I rode Italian threaded frames for many years and never once had a problem.

If the precession forces were truly large, then you could lightly tighten an English BB and it would tighten itself further over the course of a few rides. I don't recommend that to anyone, because it won't happen.

bkaapcke

I lhad a car with knockoff hubs instead of lugnuts. It didn't make any sense to me either. Someone who would know expained that it was the braking force on the hubs that starts things coming loose. Then it made sense. However, this doesn't apply to a BB. Maybe it was a mistake that stuck. bk

Al1943

Makes me feel my age. If you hang around bike forums long enough you'll see this thread pop up many times. Same physical principals apply to pedals.

Al

Pocko

Not to worry Al, I've been feeling my age long before I stumbled upon any bike forum.

If the same principle applies to the pedals, how come the right pedal has a right-hand thread? If the precession rule applies there too, then the right pedal spindle should have a left-hand thread...

At any rate, I'm just happy there is some logic that applies to the quirk... my universe is in harmony again... it'll help me sleep better...

maddmaxx 

The precession direction is in the opposit direction on the pedals.

At the bottom bracket the "wobbling" force is moving clockwise around the bottom bracket on the drive sied of the bike.

At the pedals, the force is moving counterclockwise around the pedal thread on the drive side of the bike.

Thus the two threads are in opposit directions.

arid

Thanks a lot everyone! I hadn't ever heard of a harmonic drive, but it makes a bit of sense. As long as I know that somebody knows why it's this way and that we haven't all just accepted it without asking I'll be just fine.

Dan Burkhart 

Another place to better observe this phenomenon is the lock ring on a freewheel. They are left threaded to prevent loosening while freewheeling. I ran into a problem here while using a freewheel as a mid drive unit on a recumbent. Of course, the pawls had to be removed to make this work, but the forces are reversed, because rather than the hub rotating and the gearset remaining stationary, the mid drive has the gearset rotating around the stationary hub. I started off with blue loctite on the threads, but I had to move up to the red to keep it from unscrewing. This indicated there is a fair bit of force at work.

jesselyman76

The axle/spindle spins in the direction you pedal, however it rests on bearings and just think about which direction the bearings are rotating inside the cup/bracket. Since the bearings are spinning in an opposite direction to the spindle the force on the cup/threads is opposite to the direction of pedal rotation.

Homebrew01

Cars used to have left-hand threaded lugnuts on the driver side wheels.

Jeff Wills

Oh, this has been discussed for longer than Bike Forums has been around. Here are some snippets from the rec.bicycles.tech Usenet group regarding bottom brackets, a couple from nearly 20 years ago:
Bottom bracket (Jobst Brandt)

a77impala

A light bulb just flashed in my head! The crank goes clockwise, driving the bearing clockwise, hence we actually putting a tightening force on th BB threads! The 12 o clock rotation of the bearing and the 6 o clock force of the bearing are actually opposite.

habilis

As a kid, i crashed my bike and damaged a pedal so that it still spun, but with some drag. The bearing drag overcame precession and caused the pedal to unscrew sometimes. My father would wrench it down with a lot of muscle, but eventually it came loose again.

Bikerdave222

I had the same thoughts myself. The threading on a bottom bracket is backwards and I too wondered if a mistake happened early on.

xlbs

There is a simple and good reason for the two threads on the British BB. I had a retired bicycle factory machinist tell me this, so this information came from someone who did it daily for many years. When the machinists cutting the threads in the factory used a lathe to cut the threads they could keep the frame mounted and turn the threads for both sides of the BB because the lathe shaft only had to turn clockwise to cut both threads. This simple tactic meant a much faster operation (no dismounting of the frame from whatever jig held it). It also meant that the left-hand threaded cup (on the drive side) would remain tight under pedalling because of the rotational direction of the bearings against the bearing race, as described above. No errors at all, but rather a smart way to enhance the automation of bicycle manufacturing and to solve a chronic problem of loosening BB cups at one shot.

Plimogz

edit:nevermind

HillRider

This is nonsense. If you cut threads completely through a cylinder, say a bottom bracket shell, in one pass, the threads will be right or left handed on BOTH ends.

xlbs

Please don't comment on a process that you don't understand. I didn't say one pass cuts both threads. Clearly you've never seen a lathe set up to perform this operation. The lathe shaft is smaller diameter than the shell. The left hand threaded die is fixed to the end of the lathe shaft after it is inserted through the shell after the first cut. Then the second cut proceeds, the die threading in from the outside of the frame. The Brits were the world's foremost experts at the mass production of bicycles for many years, and with good reason. They thought through each and every operation thoroughly, and are still copied today in many factories. And, yes I have seen this operation, having spent some time working in a bicycle factory.

HillRider

I do understand the process. What didn't make sense was your original description of it.

xlbs

Thanks! It is quite the thing to see. If ever anyone has the opportunity to see a modern bicycle factory in production it is a most interesting experience well worth the time and effort to get there. The few times I have had the privilege, including my brief stint working in the Miele factory in Toronto many years ago, I learned more therein than I did over many years of wrenching (about 20).