why does the RHS of a BB have a left hand thread?

[cooperplace]

I was changing the BB yesterday, installing a Shimano bottom bracket, of the type where there are two outboard bearings and the crank has a hollow axle attached. The right hand side bearing has a left-hand thread.

Why? Any force from the crank transmitted through the bearing would be directed clockwise as you look at the bike from the right, which would tend to tighten up a conventional RH thread. So why do they use a LH thread here?

Or am I missing something?

[HappyHumber]

"Precession" was the word I was trying to think of before Googling Sheldon Brown's site. I knew I had read it somewhere before. To quote Jobst Brandt:

Precession of right side BB cups is less obvious because the rotating load is only partial. The largest load being chain tension, that together with the moderately large downward force on the right crank and the smaller upward force from pushing down on the left crank, make 3/4 of a fully rotating load. For this reason some right BB cups have used right hand threads and some with left hand threads have loosened. The left BB cup with no significant rotating load has little tendency to turn.

full article : here

[cooperplace]

so am I right in thinking that maybe the RHS should have a RH thread? I'm still a little confused here.

[HappyHumber]

Follow the link on the word "Precession" in my last post; the little animation sums up the effect quite well visually, if like me you sometimes find the wordy explanations still wanting

[fatherofmany]

Well it goes back to the original design where cups and bearings were used. The crank may rotate one way but it rolls the bearing the other way. The rotational force is now applied to the cup in the opposite direction to the rotation of the crank. So to counter that the cup is screwed in in a direction that would see it tighten rather than loosen. That's why the different screw threads on left and right sides.

Newer lighter sealed bearings may put different pressures but the theory probably still holds weight (pun intended).

FoM

[RonK]

It's a typical piece of British over engineering. The Italians find it unnecessary and don't bother...

[cooperplace]

Follow the link on the word "Precession" in my last post; the little animation sums up the effect quite well visually, if like me you sometimes find the wordy explanations still wanting

yes, the animation there is great, certainly tells a 1000 words, and makes it all clear. Thank you.

[WyvernRH]

It's a typical piece of British over engineering. The Italians find it unnecessary and don't bother...

And...unless you have a quality thread/b'bracket fit and are very careful with installation Italian and French fixed cups slowly unwind. Having dissembled a few over the years I have found 'production' Italian bikes tend to have a substance akin to soft Loctite in the threads of the fixed cup when removed.
Cheers
Richard