Retrobike Forum Index

It is currently Tue Dec 06, 2016 7:20 am

* Login   * Register * Search  * FAQ



Post new topic Reply to topic  [ 4 posts ] 
Author Message
PostPosted: Sat Sep 07, 2013 5:38 pm 
Retro Guru

Joined: Tue Oct 16, 2012 4:16 pm
Posts: 1568
Location: NOTTINGHAM
I saw this on an advert for a bottom bracket today - ebay.

"Axle Length 111mm is for double crankset however it may also be used with a triple crankset provided that the frame seat tube do not exceed 28.6mm in diameter"

For the life of me I cannot see how seat tube diameter has any bearing on bottom brackets. Anyone got any thoughts on why a seller would put this in the description?


Top
 Profile  
 
PostPosted: Sat Sep 07, 2013 6:58 pm 
Old School Hero

Joined: Mon Jun 17, 2013 11:07 am
Posts: 165
maybe he means the granny ring would be too far inboard and would rub on a fatter seat tube?


Top
 Profile  
 
PostPosted: Sat Sep 07, 2013 8:03 pm 
Retro Guru
User avatar

Joined: Thu Jun 23, 2011 11:25 pm
Posts: 1785
Location: It's not easy being a dolphin.
The most achievable inner position of a front mech is typically limited by hitting the seat-tube, rather than the range of the adjustment screw. If for example it was a 34.9mm seat-tube, compared to a 28.6mm that is a difference of about 3mm for the front mechs most achievable inner position.

That 3mm can make the difference for the front mech to change from the middle to the inner position of a triple. Choice
of bottom bracket width allows you to compensate for that difference; a wider seat-tube would require a wider bottom bracket width.

In practice, it's a bit of trial and error since it also depends on which front mech, which crank, which rings on the crank, which chain width etc.


Top
 Profile  
 
PostPosted: Sun Sep 08, 2013 9:23 am 
Retro Guru

Joined: Tue Oct 16, 2012 4:16 pm
Posts: 1568
Location: NOTTINGHAM
Ah, ok - understand the principle - fairly straight forward. I always assumed that the complexities of a triple chain-ring set-up was dealt with purely by the length of the axle. I had always gone with the chain-stay as being the limiting factor, and not the seat-tube but yes it makes perfect sense that the a wider seat-tube could affect final set-up.

Luckily I don't envisage using a triple for quite some time but if & when I do i'll recall this thread before I jump in at the deep end. Cheers


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 4 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 16 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