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

Workshop tales, trials and disasters.
Maintenance tips, techniques and myths.
Technical discussion, description and outright lies

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

Postby cooperplace » Fri Jan 18, 2013 12:09 am

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?
Please be nice to me, I'm not very bright.
cooperplace
 
Posts: 288
Joined: Sat Sep 12, 2009 11:42 pm
Location: Adelaide

by BNA » Fri Jan 18, 2013 12:25 am

BNA
 

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

Postby HappyHumber » Fri Jan 18, 2013 12:25 am

"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
- Kym
Please don't assume I'm on Facebook.
User avatar
HappyHumber
 
Posts: 4178
Joined: Wed Nov 14, 2007 10:48 pm
Location: Perth, (S.o.R.) W.A.

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

Postby cooperplace » Mon Jan 21, 2013 1:10 am

so am I right in thinking that maybe the RHS should have a RH thread? I'm still a little confused here.
Please be nice to me, I'm not very bright.
cooperplace
 
Posts: 288
Joined: Sat Sep 12, 2009 11:42 pm
Location: Adelaide

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

Postby HappyHumber » Mon Jan 21, 2013 1:23 am

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 ;)
- Kym
Please don't assume I'm on Facebook.
User avatar
HappyHumber
 
Posts: 4178
Joined: Wed Nov 14, 2007 10:48 pm
Location: Perth, (S.o.R.) W.A.

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

Postby fatherofmany » Mon Jan 21, 2013 8:53 am

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). :lol:

FoM
15 Bikes 2 adults 6 children, 2 dogs, 10 chooks and a heck of a lot of fish
User avatar
fatherofmany
 
Posts: 2954
Joined: Thu Aug 14, 2008 3:13 pm
Location: Central Coast NSW Australia 33°26'16"S 151°20'39.50"E ish

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

Postby RonK » Mon Jan 21, 2013 10:43 am

It's a typical piece of British over engineering. The Italians find it unnecessary and don't bother...
Cycle touring blog and tour journals: whispering wheels...
User avatar
RonK
 
Posts: 5456
Joined: Mon Aug 24, 2009 2:08 pm
Location: Brisbane, Queensland

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

Postby cooperplace » Mon Jan 21, 2013 10:38 pm

HappyHumber wrote: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.
Please be nice to me, I'm not very bright.
cooperplace
 
Posts: 288
Joined: Sat Sep 12, 2009 11:42 pm
Location: Adelaide

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

Postby WyvernRH » Wed Jan 23, 2013 7:55 am

RonK wrote: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
WyvernRH
 
Posts: 1736
Joined: Tue Nov 06, 2007 5:41 pm
Location: Newcastle NSW


Return to The Shed

Who is online

Users browsing this forum: brawlo



Popular Bike Shops
Torpedo 7 Torpedo7 AU
Ground Effect Ground Effect NZ
Chain Reaction Cycles CRC UK
Wiggle Wiggle UK
Ebay Ebay AU

“Bicycles BNA Twitter
“Bicycles BNA Facebook
“Google+ BNA Google+
“Bicycles BNA Newsletter

> FREE BNA Stickers
> BNA Cycling Kit