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?
why does the RHS of a BB have a left hand thread?
-
- Posts: 405
- Joined: Sat Sep 12, 2009 11:42 pm
- Location: Adelaide
why does the RHS of a BB have a left hand thread?
Postby cooperplace » Fri Jan 18, 2013 1:09 am
Please be nice to me, I'm not very bright.
- HappyHumber
- Posts: 5072
- Joined: Wed Nov 14, 2007 11:48 pm
- Location: Perth, (S.o.R.) W.A.
Re: why does the RHS of a BB have a left hand thread?
Postby HappyHumber » Fri Jan 18, 2013 1:25 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:
full article : herePrecession 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.
--
Hit me up via the BNA dm; I'll get an alert. If y'know, you know.
Hit me up via the BNA dm; I'll get an alert. If y'know, you know.
-
- Posts: 405
- Joined: Sat Sep 12, 2009 11:42 pm
- Location: Adelaide
Re: why does the RHS of a BB have a left hand thread?
Postby cooperplace » Mon Jan 21, 2013 2:10 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.
- HappyHumber
- Posts: 5072
- Joined: Wed Nov 14, 2007 11:48 pm
- Location: Perth, (S.o.R.) W.A.
Re: why does the RHS of a BB have a left hand thread?
Postby HappyHumber » Mon Jan 21, 2013 2:23 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
--
Hit me up via the BNA dm; I'll get an alert. If y'know, you know.
Hit me up via the BNA dm; I'll get an alert. If y'know, you know.
- fatherofmany
- Posts: 3494
- 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 fatherofmany » Mon Jan 21, 2013 9:53 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).
FoM
Newer lighter sealed bearings may put different pressures but the theory probably still holds weight (pun intended).
FoM
15 Bikes 2 adults 6 children, 2 dogs, 10 chooks and a heck of a lot of fish
- RonK
- Posts: 11508
- Joined: Mon Aug 24, 2009 2:08 pm
- Location: If you need to know, ask me
- Contact:
Re: why does the RHS of a BB have a left hand thread?
Postby RonK » Mon Jan 21, 2013 11:43 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...
-
- Posts: 405
- Joined: Sat Sep 12, 2009 11:42 pm
- Location: Adelaide
Re: why does the RHS of a BB have a left hand thread?
Postby cooperplace » Mon Jan 21, 2013 11:38 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.
- WyvernRH
- Posts: 3186
- Joined: Tue Nov 06, 2007 6:41 pm
- Location: Newcastle NSW
Re: why does the RHS of a BB have a left hand thread?
Postby WyvernRH » Wed Jan 23, 2013 8:55 am
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.RonK wrote:It's a typical piece of British over engineering. The Italians find it unnecessary and don't bother...
Cheers
Richard
Jump to
- General Australian Cycling Topics
- Info / announcements
- Buying a bike / parts
- General Cycling Discussion
- The Bike Shed
- Cycling Health
- Cycling Safety and Advocacy
- Women's Cycling
- Bike & Gear Reviews
- Cycling Trade
- Stolen Bikes
- Bicycle FAQs
- The Market Place
- Member to Member Bike and Gear Sales
- Want to Buy, Group Buy, Swap
- My Bikes or Gear Elsewhere
- Serious Biking
- Audax / Randonneuring
- Retro biking
- Commuting
- MTB
- Recumbents
- Fixed Gear/ Single Speed
- Track
- Electric Bicycles
- Cyclocross and Gravel Grinding
- Dragsters / Lowriders / Cruisers
- Children's Bikes
- Cargo Bikes and Utility Cycling
- Road Racing
- Road Biking
- Training
- Time Trial
- Triathlon
- International and National Tours and Events
- Cycle Touring
- Touring Australia
- Touring Overseas
- Touring Bikes and Equipment
- Australia
- Western Australia
- New South Wales
- Queensland
- South Australia
- Victoria
- ACT
- Tasmania
- Northern Territory
- Country & Regional
Who is online
Users browsing this forum: No registered users
- All times are UTC+10:00
- Top
- Delete cookies
About the Australian Cycling Forums
The Australian Cycling Forums is a welcoming community where you can ask questions and talk about the type of bikes and cycling topics you like.
Bicycles Network Australia
Forum Information
Connect with BNA
Brought to you by Bicycles Network Australia | © 1999 - 2024 | Powered by phpBB ®
This website uses affiliate links to retail platforms including ebay, amazon, proviz and ribble.
This website uses affiliate links to retail platforms including ebay, amazon, proviz and ribble.