Chainwheels and Bottom Bracket Axles.

[Andy in Germany]

I've got a couple of bikes in for repair with worn chainwheel (?) sets, and trying to replace them. Problem is that they're not all the same, and this area of bike maintenance is beyond my current knowledge.

Firstly, here's an example of the type of chainwheel in need of replacement (The item to be replaced is still on the bike):

And here's the most common type in stock:

One has the socket (?) for the square taper axle recessed, and one is flush with the granny gear.

So far so good, I understand that I need a different length axle (as opposed to a different length barrel) for these two, but then the axles come in a bewildering array of lengths: 107mm, 110mm, 113mm and then in leaps and bounds to 127mm.

How do I find out which axle is the correct length for a chainwheel, especially when I don't always have an old BB to hand? Sheldon Brown seems silent on the matter, but that may also be because I don't know the English terms for the items so any help there is also welcome.

 

[Big John]

Hi Andy. I hope all is well with you and the family. At our place we have an array of BBs, both new and used, so if we have an old BB we're removing we measure the axle and replace with one the same length or as close as poss within a mil or two. The idea is to make sure the granny ring doesn't foul the chainstay and not to push the chainset out so far that the front mech fails to reach the big ring. Is that any help?

 

[All uphill]

Hi Andy

I've had this situation; my approximate solution was to measure/ estimate the difference in distance between the inner rings and hub between the two cranksets. Probably 4 or 5 mm in your case.

Double this (8mm) and take that off the length of the existing axle. Double because the axle has two ends, and you need 4mm off just the drive side.

Maybe I got lucky but it worked for me.

Edit: as Big John says the critical thing is that the inner ring clears the chainstay.
Getting a perfect chain line is nice, but these older bikes are pretty tolerant.

 

[Ming the Merciless]

As above, the axle needs to be long enough that the granny ring does not foul the chain stay. A steel ruler and some calculations will get you there. If all else fails try a 110mm BB as a starting point but don’t do everything up really right. You want granny not fouling chain stay, able to shift into all three rings both ways. If you can do that then you can tighten it all up and fine tune gears.

 

[Andy in Germany]

Many thanks. From that I'm reading I need to make sure the axle is generally of the right size to avoid the granny/chainstay conflict situation, but there's a few mm tolerance on the mech so a small difference can be compensated by the mech. Is this correct?

 

[T4tomo]

Many thanks. From that I'm reading I need to make sure the axle is generally of the right size to avoid the granny/chainstay conflict situation, but there's a few mm tolerance on the mech so a small difference can be compensated by the mech. Is this correct?

indeed, especially a triple, if you can select all 3 rings and nothing is rubbing the frame then by default the chain line will be reasonably central. It doesnt matter too much if the middle ring isn't perfectly aligned on the middle sprocket (although that is clealy the goal for a perfectionist)

 

[Ajax Bay]

Many thanks. From that I'm reading I need to make sure the axle is generally of the right size to avoid the granny/chainstay conflict situation, but there's a few mm tolerance on the mech so a small difference can be compensated by the mech. Is this correct?

Just keep in mind that too long a spindle will mean the large ring may end up too far right for the FD to reach. Have a look at some triples and see, typically, how many mm the small ring is clear of the chainstay, and don't go more a couple of mm more than that. If the intended FD is already clamped to the seat tube you can unscrew its limit screws and check the FD cage range by hand.