How can I tell what BB I need?

[swee'pea99]

I'm starting to get rather ominous clunking from the BB area, and I think it's time for a change. I'm trying to figure out what axle length I need. 

The one that's in there now is 110mm, but I'm not sure it's ideal. My chainset is slightly bent, and when I first fitted the BB, the sprocket just brushed the frame at one point on its revolve. The only way to make it work was to screw it in tight on the other end, then leave the chainset end slightly proud of the frame - just a couple of mm, and that worked fine. But I'm wondering, if I go to, say, a 115mm, would that solve the problem? Is there any significant downside to having 'too long' an axle?

 

[MacB]

Can you look up the chainset and see what the manufacturer recommends?

 

[swee'pea99]

Unfortunately not. It's an antique, pulled off a bike probably 40+ years old. It's a Sugino single, but that's as much as I know.

 

[RecordAceFromNew]

IMHO a bb that is a couple of mm too long is not a big deal, although it does not help efficiency not least because the bike tips sideways more as one pedals. In fact modern cranks tend to stick much farther out (i.e. with higher Q factor) than vintage cranks, not because that is good for you because it is not, but because they have to accommodate modern, wider rear axle/stay etc. An interesting assessment has been written on this (pg1, pg2).

To minimise (alas probably won't eliminate) the possibility of mismatch, while the chainset contains little info,
a) what is the bb shell width? is there any marking on the current bb? is the spindle symmetric on each side? is it identifiable in Sheldon's bb database?
b) while the chainset was pulled from a 40+ year old bike, it might be younger? can you check it against Sheldon's bb database and see if it is there?
c) are the arms/pedals at equal distance from the centreline currently (assuming you would like the new one to improve left and right symmetry)?

 

[swee'pea99]

Thanks very much Ace - and sorry for not getting back earlier. It's been one of those days. I will do some checking tomorrow, but in the meantime, some of your queries are interesting because they touch on things that have intrigued me, like...

is the spindle symmetric on each side? - no. On the non-drive side, it seems 'too long'. There's a good 2-3cm of spindle before you get to the crank (whereas on the drive side, as I've said, there's none, give or take).

might the chainset be younger? Yes, I think it may very well be - not least because it's light. I don't know what it's made off, but def not steel, which I'm guessing it almost certainly would be if it was as old as the bike it came off.

are the arms/pedals at equal distance from the centreline currently (assuming you would like the new one to improve left and right symmetry)? As above, no. And yes, I guess other things being equal, I would prefer it (having said which, I haven't been aware of the difference while riding).

Thanks again. Like I say, I'll do some checking tomorrow (assuming I remember....I think it might be another of those days. *sigh*...)

 

[swee'pea99]

Ok, here's the shell

- it's a standard 68mm. As you can see, the left hand spindle is exposed 15mm or something; the drive side is snug. You can also see where I've had to leave that end a couple of mm sticking out from the shell (to prevent rubbing). It's perfectly tight, but not tight into the shell, if you see what I mean.

The chainset sprocket (if that's the right word) looks like this:

...the only markings being:

Hope this is some help. (I'm afraid I couldn't make head nor tail of Sheldon's listings.)

If you can help, it would be much appreciated.

 

[RecordAceFromNew]

Ok, the marking on the chainset is not a lot of information to go on, so guesses will have to be made...

From your photos, it appears it is an incarnation or variant of the Sugino Maxy, because of its similar appearance to this (apparently a late 70's double version) and this (the current single version).

If that is the case, there is evidence (at line 140) of this database that you need a 3SB bb of 121.5mm, which is said to be asymmetric, and is consistent with the corresponding 3S (B meant 73mm shell apparently) in Sheldon's database of Japanese BBs (Sugino is Japanese):

As Sheldon suggested, a 127.5mm symmetric one would preserve the chainline (but over extend the left crank). With your current 110mm bb, assuming it is symmetric it would be too short by (127.5-110)/2, or 8.75mm if the above assessment is correct. There are a lot of guesses here, but that might explain why you are having clearance problems on the drive side.

The next question is what to get to replace what you have got. Assuming a 3S is the one, and noting the right side of its spindle is 5.5mm longer than the left, it seems BRS23R from Harris is a close match (123mm, r is +5mm), BUT it costs $117!

The most common square tapered bb's are supplied by Shimano, if we look at page 10-16 of this document, we can see that the 122mm and 127mm ones are possible candidates. But because they are not symmetric (as indicated by their spindle length in conjunction with the relative chainline offsets provided by table 10.4), the 122mm NL model actually looks just right, and one can be bought for a very reasonable £8 or £12.

I have tried to provide whatever evidence I have, and shown the rationale that arrives at the above possiblity, but bb/chainline estimation is an incredibly tricky business, especially when vintage components are involved. I am afraid it will be hard to know how good it is until one tries in this case...

 

[swee'pea99]

What can I say, Ace? I'm overwhelmed by your expertise and even more by your kindness in taking so much trouble. I shall get the 122mm UN54 and see how it goes. As to the chainline, I've a fair bit of playing-around room at the back - I'm a dab hand at moving axles and re-dishing - so that should be sortable. As to symmetricality (is that a word?), as I said earlier, that would be nice, but it's really icing on the cake. It hasn't been symmetrical since I built it, and it hasn't worried me none, so...

Thanks again. 

 

[RecordAceFromNew]

The pleasure is all mine - it beats crossword puzzles, sudoku or even 3D chess! Hope it works.

 

[swee'pea99]

Ordered from Chain Reaction, came next day, fitted, perfick! No more clunking. Lovely. Thanks again.