Picking the correct bottom bracket to get a 42.0 mm chainline

[petmcgratt]

Hi,
another technical question as I am now stuck as to what bottom bracket spindle length to go for.

I have got a single speed wheelset with a 16T cog set to a 42.0 mm chain line.

My bottom bracket shell width measures 68 mm.

I want to get a square tapered single speed chain set and therefore will need a square tapered bottom bracket also.

Is it as straight forward as adding 42.0 mm to 68 mm to get your bottom bracket spindle length which would work out at a 110 mm bottom bracket spindle length or is it more technical than that.

Thanks in advance,
Peter

 

[Gerry Attrick]

Personally I'd go for a "mid-length" size as a couple of millimeters either side of the true chain line will mean for nowt on a single speed bike. Just look at the relatively huge offset which a ten speed set up will accommodate.

 

[petmcgratt]

Thanks gerry attrick for the reply. What size of spindle do mean when you say a mid-length size.

I have got bogged down on reading different bits of info which led me to believe that if you didn't get this chainline set up correct it would be a disaster. I have also been reading that cranks and bottom brackets are made to two different standards (i.e. ISO and JIS) and that you will have problems in getting the chainline right if you do not have the same standard for both the bottom bracket and the chainset crank (applys to square tapered BBs and cranks i think).

Basically I can get a square tapered bottom bracket (JLS standard, 68mm shell and 110mm spindle) for less than 6 pounds but I am watching a vintage single speed chainset on e-bay with square tapered cranks for 12.99 but does not say whether it is ISO or JIS standard.

Has anyone else come across this before.

thanks,

Peter

 

[piedwagtail91]

i haven't had this problem because the pompino was staightforward. but a clubmate used a miche bb which is 107mm but has both l and r cups adjustable so he could get a few mm of adjustment to get the chainline right. i.e. the cup is slightly proud of the bb shell at one side and slightly inside the shell at the other.
not sure but think they're iso bb

 

[Gerry Attrick]

AFAIK, JIS is the standard that Shimano use along with most other far east supplied parts. Campy uses the ISO standard.

This link http://www.bikepro.c...ckets/shim.html

may help you decide which length to buy.

Take care that if you specify the 107mm length that the chainring has sufficient clearance from the chainstay.

Sorry to be a little vague, but even bike builders have to try different sizes for custom frames.

You don't say, but if you have the old BB or a spare one lurking in the shed, you could fit it and try the chainset, which even if it doesn't fit, will enable you to take measurements to ascertain the correct length. A good lbs may even be prepared to lend you one.

 

[RecordAceFromNew]

Unfortunately the calculation is not as straightforward as you have indicated, and there are some potential pitfalls. Chainsets have to be matched to bb's to deliver a specific chainline. You might want toread this and the associated links.

The easiest way is to get a knowledgeable shop to suggest both the chainset and the bb to deliver the chainline.

68mm shell and 42mm chainline are both common, however the chain stay angle also comes into play. If I were to source the chainset and bb myself, I would first check if it is likely that a 42mm chainline will result in clash between the chainring/crank and the frame, which happens occasionally. Fyi the radius of a 46T chainring is about 9.3cm, with every tooth increase/decrease changing the radius by 0.2cm, and if you are able to find the Q factor of the chainset then it might also help giving you some clue.

Since any particular model bb is usually available with different spindle lengths (while a particular chainset model won't come in different tapers), a chainset should first be chosen tentatively. I would then try to find out from themanufacturer (or from a knowledgeable seller) which bb will deliver the 42mmchainline for that chainset. For square tapers the chainset and bb shouldideally be of the same standard (i.e. both JIS or both ISO) even if mixing may deliver the right chainline - taper surfaces not coming in contact as designed may cause stress concentration potentially detrimental to the durability of the chainset.

Additionally, this database might help giving you some idea/indication, but you shouldn't rely on it solely to deliver your chainline.

Hope it helps.

 

[ttm]

I found this really useful:

http://www.lfgss.com/thread3372.html

Good luck.