I had a bottom bracket cup loosen up on me today on a ride. It needs a somewhat specific spanner wrench to tighten which I didn't have on me, and which is probably why it came loose in the first place since I can't properly torque it. To make things worse this is an eccentric BB for a single speed conversion, which meant the chain kept tightening and the crank spindle would be out square.
Now my question is... I thought that BB reverse threading is done so that the cups self tighten as you pedal, am I wrong?
But if I look at it from the drive side for example, you tighten the BB cup CCW and you pedal CW. So doesn't pedaling actually loosen the BB? What am I missing here? Sorry if this is a dumb question

It's about precession, an effect that counter-intuitively acts in the opposite direction when you're talking about rotation and threads.

The BB threads are designed to unthread as you pedal forward in the event a bearing should seize up so that you won't break an ankle or other body part.
If the BB is designed for an eccentric insert to change from Ashtabula cups to three piece cranks, then there should be a way to lock the adaptor into place as in a pair of clamp bolts for each side of the eccentric insert.
If that is not present you will need to change the insert to one that can be fixed in place. The fixed ones centers the insert and the chain tension is adjusted by chain length. HTH, Smiles, MH

Eccentric bottom brackets rotate freely inside the bottom bracket shell. Then spindle is eccentric, that is, off-center, so you can position the unit for the correct chain line and chain tension. Then, when you find that point, you tighten the bolts, usually Allen-head, that hold it in place.

The cups self tighten because the ball bearings act like a sun and planetary gear system.



But we don't rely on this effect to set or maintain the bearing adjustment. It mainly helps keep the fixed cup from loosening.

The eccentric part is kind of irrelevant to my question, I just mentioned because it was an extra annoyance.
So you're saying the cups are designed to loosen as you pedal? This makes sense in my head as I look at it, but I always thought it was the opposite

Ok now I'm even more confused... Is the reverse threading done in order to self tighten, or self loosen? Conflicting info here

Don't feel bad, you're not alone in that.

Maybe this will help.

Can't you see the gif I posted? It shows how the driveside spindle tightens the reverse threaded DS fixed cup.

I wouldn't say it is 'designed' to work that way, but it is a happy coincidence that a seized bearing does the opposite of the self tightening tendency of a working bearing.

Yes but are you talking about a cup and cone bottom bracket? This a sealed cartridge bearing BB.
Also in that gif I'm assuming the orange gear is the spindle, the light green are the ball bearings and the dark green is the cup, correct? Except that in that illustration the green gears are spinning around fixed pins, while in a bearing the balls just roll forward

Sealed units replaced the older cup & cone technology.
They had to keep the same threading even though it became moot so the newer tech would fit would fit the older BB's.

A spindle, cup and cartridge bearing BB behaves the same way. The cartridge races essentially become fixed extensions of the spindle and cups that they are pressed into. The physics don't change, but the friction forces are different than steel on steel on steel. Like the large bearing seals.

The gears behave just like bearings - they are similarly trapped and turn in the same plane. The difference is that the outer gear ring - the cup - is not allowed to rotate, so the balls don't purely roll, but backslip. The gears illustrate what would happen if the cup had no friction and was threaded the wrong way, like Italian fixed cups.

If bearings have too much preload, that slipping becomes that much harder and the bearing have greater traction on the cups, increasing the transfer of motion to the cups.

Love that gif, it’s a great illustration of the effect, albeit the teeth intuitively make you think “well a smooth bearing wouldn’t do that”

As others have said, it's precession. But here is a way to better visualize it: Have you ever used a Spirograph? If you have, and drawn a pattern with a smaller geared wheel inside a larger geared ring, as the pen overall rotates the smaller gear clockwise, the gear itself rotates counter-clockwise:


The bottom bracket cup is the smaller gear, in the animation above, on the right (drive) side of the BB; As you rotate the spindle clockwise (under pedal force, a radial load that is constantly rotating clockwise), the cup precesses counter-clockwise, or to the left, so that's why you need left-hand (tightening) threads on the right side.

I don’t think that’s a better way to visualise it

Well, an actual animation of the BB cup in a slightly larger shell, would be better. HAH, here it is:


EDIT: This pictured example would represent the non-drive (left) side of the bottom bracket; The pedaling force forward is to the left, counter-clockwise, and thus same is the closest contact point between the cup (blue) and the shell (red) due to radial load at the bottom bracket, indicated by the green arrow. But then notice how the precession causes the cup to slowly rotate opposite, clockwise, to the right, thus the left side of the BB requires right hand thread to keep tight. The drive (right) side is opposite, precession to the left, so requires left hand thread.

Kind of yeah, but the bearings and the way they rotate backwards because they’re not frictionless are a key element of the effect and they’re missing.

Not really, because the bearings are not on individual spindles like planet gears. Precession in a bottom bracket is ALL about:
- the rotating radial load from pedaling, acting on...
- sufficient radial clearance between the threaded cup and threaded BB shell.

No-bearing example: Car wheels used to have left hand lug studs and nuts on left side due to precession. Tapered seat lug nuts eliminated the clearance required for precession and all cars suddenly had right hand lug threading on both sides. Heavy trucks that use flat-seat lugs still have left-left lugs, and even some with taper seats just to be on the safe side, in case non-taper wheels are mounted. Close-fit hub-piloting wheels also help.

Ok but when there are bearings, the force acting between the spiindle and the cup is transmitted through them so at least in my head the way they behave is a key element.

Yes, but the bearings are not pivoting in place (like fixed planet gears). The BB spindle rotates forward, the bearings roll along in the same direction as the spindle, only slower. Picture poor lubrication on the bearings and them beginning to freeze up; Which way is the force on them and the force they are exerting on the bearing cup? Same direction as the spindle, forward.

No they’re rotating in the opposite direction to the spindle, while they move in the same direction

although I think I get the thing about the position of the force applied shifting around the diameter of the cup as the crank moves.

OK, got it, yes, the balls are rotating the reverse of the spindle. But they're along for the ride with the spindle. Worst-case forces is if the balls start to seize up, and in that case, the cup will try to rotate in the same direction as the spindle, which would try to back out the cup, except, the precession forces, based on the rotating radial load and slack in the threads, wants to rotate the cup opposite the spindle direction, tightening it.

There another thread right now, suddenly spindle got tight; Lockring had loosened just enough that the cup promptly tightened on the bearings, due to precession.

Stud piloted dual truck wheels that used inner cones and outer nuts and ball seats had left hand threads on the left side of the vehicle.
Those gave way to hub piloted wheels in the late 80s. All right hand threads, no ball seats, and nuts with bell washers. One set of nuts secures both inner and outer wheel. No inner cone to secure the inside wheel first.
Of course, I refer to American units. Things may be done differently elsewhere.

So the crank grinding to a halt is going to break legs how, exactly?