How does that left-handed thread on bottom brackets work?

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Anglian
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How does that left-handed thread on bottom brackets work?

Postby Anglian » 7 Jan 2013, 2:10pm

Hello,

This is probably the kind of question which I'll regret asking, once it's been explained to me.

So I've learnt that the drive side of a bottom bracket, on the right side of a bike, has a left-handed thread, and unscrews clockwise. This stops the component from unscrewing as one cycles along, turning the pedals.

But how does this work? Viewed from the right side, the cylist turns the spindle clockwise -- which is the same direction as the bottom bracket unscrews. How does the clockwise motion of the spindle make the bottom bracket tighter?

Clearly the same effect is happening on the left-hand pedal, which also has a left-hand thread. As one pedals forward, the left-hand pedal (when viewed from the left side), rotates clockwise with respect to its crank, which is the direction in which one unscrews the pedal.

Have I got my relative directions mixed up? Of course I have, but how?

Thanks for any clues,
Anglian.

jb
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Location: Clitheroe

Re: How does that left-handed thread on bottom brackets work

Postby jb » 7 Jan 2013, 2:16pm

Its all to do with the rolling motion of the cup threads in the shell. Imagine it was a very slack fit, the action of the rotating crank would unscrew it if it were a normal thread. In other words it would roll out.
Cheers
J Bro

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Mick F
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Re: How does that left-handed thread on bottom brackets work

Postby Mick F » 7 Jan 2013, 2:37pm

The balls in the bearings rotate with the crank spindle, but they do, they transfer the motion in the opposite direction.

This phenomenon would happen even with a plain bearing, so the ball rotation isn't the whole story.

Mechanical Precession is the main thing.
http://en.wikipedia.org/wiki/Precession_(mechanical)
Mick F. Cornwall

Brucey
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Re: How does that left-handed thread on bottom brackets work

Postby Brucey » 7 Jan 2013, 2:39pm

exactamundo...

nice animation

Image

from here;

http://en.wikipedia.org/wiki/Precession_(mechanical)

cheers
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Valbrona
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Re: How does that left-handed thread on bottom brackets work

Postby Valbrona » 7 Jan 2013, 5:05pm

You will note that in an Italian frame the bb threads are both normal right-hand thread.
I should coco.

Brucey
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Re: How does that left-handed thread on bottom brackets work

Postby Brucey » 7 Jan 2013, 5:06pm

Valbrona wrote:You will note that in an Italian frame the bb threads are both normal right-hand thread.


and on older French frames as well.

Both are more prone to BB loosening, too....

cheers
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Ayesha
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Re: How does that left-handed thread on bottom brackets work

Postby Ayesha » 7 Jan 2013, 5:25pm

Well done Birmingham Small Arms. Thank you for the Duplex Safety Bicycle Crank Axle Bearing.

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Trigger
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Re: How does that left-handed thread on bottom brackets work

Postby Trigger » 8 Jan 2013, 1:12am

I thought it was the other way around and the non-drive side was the left hand thread, shows how long it's been since I took one out!

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andrew_s
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Re: How does that left-handed thread on bottom brackets work

Postby andrew_s » 8 Jan 2013, 2:02pm

Trigger wrote:I thought it was the other way around and the non-drive side was the left hand thread, shows how long it's been since I took one out!

That's the pedal threads

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Mick F
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Re: How does that left-handed thread on bottom brackets work

Postby Mick F » 8 Jan 2013, 3:30pm

The nearside rear hub-nut on a Mini is LH thread, and it's a castelated nut secured by a spit-pin too.

The late Sir Alec Issigonis must've used a belt and braces approach to his design. :wink:
Mick F. Cornwall

Brucey
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Re: How does that left-handed thread on bottom brackets work

Postby Brucey » 8 Jan 2013, 6:07pm

Mick F wrote:The nearside rear hub-nut on a Mini is LH thread, and it's a castelated nut secured by a spit-pin too.

The late Sir Alec Issigonis must've used a belt and braces approach to his design. :wink:


yes; mind you it would be a bit off if your hub nuts unscrewed every time you drove backwards...!

:roll: :wink: :mrgreen:

cheers
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