Retrobike Forum Index

It is currently Thu Dec 08, 2016 1:28 pm

* Login   * Register * Search  * FAQ



Post new topic Reply to topic  [ 8 posts ] 
Author Message
PostPosted: Mon Jul 29, 2013 1:22 pm 
Retro Guru

Joined: Sat Jun 26, 2010 11:07 pm
Posts: 1901
Location: brighton
Copied this from my post in the road section as I'm generally in this section and though it's a road bike I can't see that's relevant here.

I need to replace the BB on my 80s Dawes Sterling. The old one is cup and cone 68x 118 and I have a UN54 which is 68x122.5.

Will it work?

I guess it will affect chainline and the front mech but will 3.5mm really cause a problem?

All help much appreciated!


Top
 Profile  
 
PostPosted: Mon Jul 29, 2013 1:30 pm 
Retro Guru
User avatar

Joined: Sat Dec 17, 2011 11:11 pm
Posts: 874
If on a road bike, possibly as you're potentially going to be putting higher torque through the outer rings.
Depends on how you ride and how flexy the BB shell is, as you'll be putting more leverage through it. You might find it's fine to fit & align, but doesn't work when pedalling in a real world situation!

However.. You need to replace the BB, you have a BB - try it & find out?


Last edited by Osella on Mon Jul 29, 2013 2:22 pm, edited 1 time in total.

Top
 Profile  
 
PostPosted: Mon Jul 29, 2013 1:39 pm 
Old School Grand Master

Joined: Tue Oct 09, 2007 1:55 pm
Posts: 8216
Location: New Forest, UK
You should have no problem. The torque is the same (full weight on one pedal). Chain tension will actually be lower than MTB due to the larger chainring.

You might find that the chain wants to slip off in bottom gear when back pedalling - the only way is to try.


Top
 Profile  
 
PostPosted: Mon Jul 29, 2013 11:23 pm 
Retro Guru

Joined: Sat Jun 26, 2010 11:07 pm
Posts: 1901
Location: brighton
I'll give it a try! Thanks guys :-)


Top
 Profile  
 
PostPosted: Tue Jul 30, 2013 12:47 pm 
Old School Grand Master

Joined: Tue Jul 05, 2011 8:42 pm
Posts: 3193
give it a go but i think its worth getting the correct size

if you dont want to bung out £15 on a new one, you could always stick a wanted ad on here - i bet someone has one sitting in a box somewhere you can have for a couple of quid


Top
 Profile  
 
PostPosted: Tue Jul 30, 2013 1:05 pm 
Retro Guru
User avatar

Joined: Thu Jul 26, 2012 1:31 pm
Posts: 2366
Location: The Cave of Shame
Huh? some strange comments on this thread.

Bear in mind Torque is a vector unit - it needs to be qualified by a direction.

Where does the OP say he is using a bigger chain ring or cranks? If he isn't chain tension is unaffected.

He is also not putting more torque through the chain rings - Torque in this direction is unchanged as its nearly totally dependent on the chain ring diameter and the crank length.

Torque acting along a line of the BB spindle at 90 degree to the crank rotation is changed and is at its maximum with the cranks at the top/bottom of their rotation

T=f * r

1. Assuming f (pedaling force) is unchanged and r (BB spindle length) changes by 2.5mm.
2. Assuming for a ease of calculation that the original BB spindle is supported by the bearings/cups at 10mm along the spindle:
r = 0.01m for the original and r = 0.0125m for the new BB.

Which gives us a 25% increase in Torque along this line and thats without taking into account the length of the pedal spindle which would reduce it from this value.

In reality the calculations are more complex and I would guess a much lesser increase.


Top
 Profile  
 
PostPosted: Tue Jul 30, 2013 1:29 pm 
Old School Grand Master
User avatar

Joined: Sun Aug 21, 2011 11:57 pm
Posts: 4074
Location: Antwerp, Belgium
Too bad the postage costs are roughly the same as the price of the BB itself, otherwise I'd swap my (new in box) 68x118 UN-55 for your 122.5


Top
 Profile  
 
PostPosted: Tue Jul 30, 2013 2:45 pm 
Retro Guru
User avatar

Joined: Sat Dec 17, 2011 11:11 pm
Posts: 874
secret_squirrel wrote:

Where does the OP say he is using a bigger chain ring or cranks? If he isn't chain tension is unaffected.

He is also not putting more torque through the chain rings - Torque in this direction is unchanged as its nearly totally dependent on the chain ring diameter and the crank length.


This is hence why I referred to "If on a Road Bike" [as opposed to a mountain bike] - I should have made that clearer :facepalm: . I was therefore assuming more torque from my assumption that the chainring would be bigger, but my experiences of the same situation relates solely to MTBs.

secret_squirrel wrote:
Which gives us a 25% increase in Torque along this line and thats without taking into account the length of the pedal spindle which would reduce it from this value.

In reality the calculations are more complex and I would guess a much lesser increase.


Then it all comes down to whether this causes any problems or not in terms of transferring the torque into the frame at the BB shell, and whether it twists, and whether that causes any knock-on effect, hence trying it is almost always the best option.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 8 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: Bing [Bot], Hotwheels., james1985 and 42 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  

About Us

Follow Retrobike

Other cool stuff

All content © 2005-2015 Retrobike unless otherwise stated.
Cookies Policy.
bikedeals - the best bike deals in one place
FatCOGS - Fat Chance Owner's Group

Powered by phpBB® Forum Software © phpBB Group