Asymmetry of bottom bracket or chainset

[therealkw15]

Recently I stuck an Alivio triple chainset on the recommended 68x123 cartridge bottom bracket, and was surprised at both the vast Q-factor and the fact that the right hand crank was far closer to the chainstay than the left hand one. The amount of axle sticking out of the BB shell of the bike was the same on both sides.

This was resolved by fitting a 68x113 BB to narrow things down. I still had to take a left hand crank from another set to make things symmetrical.

Today I fitted a Suntour chainset to a 68x123 BB (OK the same one as above) and again things were very unsymmetrical and bandy.

So are the cranks in the set tapered in an unsymmetrical way and must be paired with an unsymmetrical BB or what?

In the end I've bought this undoubtedly shabby piece of kit just to have the freedom to trim the chainline and/or symmetry of the cranks with respect to the chainstays.

Anyone else been bugged by this?

Cheers
Steven

 

[one-eyed_jim]

Recently I stuck an Alivio triple chainset on the recommended 68x123 cartridge bottom bracket, and was surprised at both the vast Q-factor and the fact that the right hand crank was far closer to the chainstay than the left hand one.

That doesn't sound right. I don't think Alivio existed before low-profile crank designs, which means that 123mm is far too long an axle. Who recommended it? 110mm would be about right.

Shimano's bottom brackets aren't all symmetrical.

So are the cranks in the set tapered in an unsymmetrical way and must be paired with an unsymmetrical BB or what?

Depends. First, don't assume that there's any general rule. Some manufacturers seem to pay more attention to symmentry than others. Many bottom bracket axles are deliberately asymmetric. Many cranks when fitted to their recommended axles are asymmetric.

When mixing and matching products from different manufacturers, you're on your own.

 

[therealkw15]

Recently I stuck an Alivio triple chainset on the recommended 68x123 cartridge bottom bracket, and was surprised at both the vast Q-factor and the fact that the right hand crank was far closer to the chainstay than the left hand one.

That doesn't sound right. I don't think Alivio existed before low-profile crank designs, which means that 123mm is far too long an axle. Who recommended it? 110mm would be about right.

Those crazy guys at Shimano recommend it in the datasheet for the chainset. And I can assure you that my 1989 Mountain LX chainset that preceded it is far lower profile (in terms of less splay of the cranks) than the 2009 Alivio one that replaced it.

When mixing and matching products from different manufacturers, you're on your own.

What, JIS square taper is not a standard !?

But a degree of mixing and matching is a lot of the fun in this, and hey, how much further wrong can I go if the "correct" parts give an asymmetric result? That is just WRONG! Aaaahhh, my beautiful knees...

Cheers
Steven

 

[one-eyed_jim]

Those crazy guys at Shimano recommend it in the datasheet for the chainset.

Do you have a reference to the data sheet?

And I can assure you that my 1989 Mountain LX chainset that preceded it is far lower profile (in terms of less splay of the cranks) than the 2009 Alivio one that replaced it.

I don't think you've understood what "low profile" means, in this context. Low profile cranks use short axles and therefore have considerable splay. Older (pre-low-profile) cranks like your Mountain LX use longer axles and less splay.

http://www.sheldonbrown.com/gloss_l.html#lowprofile

What, JIS square taper is not a standard !?

The taper dimensions are, theoretically, standardized, but that's not relevant to the problems you're having with axle length and asymmetry.

But a degree of mixing and matching is a lot of the fun in this

Precisely. So don't be surprised if you stumble on plenty of combinations that don't work. It's part of the learning process.

and hey, how much further wrong can I go if the "correct" parts give an asymmetric result?

There's still plenty of room, it would seem...

 

[therealkw15]

FFS and FWIW the data sheet is SI-6M10A, the chainwheel is FC-M411 (the 48-38-28 version), the recommended BB is BB-UN26 (-K) or BB-LP28 (-K), both of which are 68x123mm...

Those crazy guys at Shimano recommend it in the datasheet for the chainset.

Do you have a reference to the data sheet?

I expect you are trying to help, in which case I thank you. But have to say I'm feeling a tad patronised, especially by your last comment.

 

[one-eyed_jim]

I expect you are trying to help, in which case I thank you. But have to say I'm feeling a tad patronised, especially by your last comment.

I was certainly trying to help initially, but by the time I got to your winking emoticon I was probably a little less inclined to...

I'm not familiar with modern Alivio components, and details of their symmetry and compatibility. If I'd understood from the beginning that you were talking about contemporary parts, I wouldn't have attempted a reply.

I'll leave it there with, I hope, no hard feelings.

 

[FluffyChicken]

Are your chainstays symmetrical around this area. ?


Anyways...

Since these are 48 tooth chainrings I think shimano push them further out than the compact design to overcome the larger rings hitting the frame of the now designed for compact ring setup (the 10mm longer length). Also since they now line up for 50mm instead of 47.5 chainline that's another ~5mm length.
So since you frame can cope easily with the larger rings and your chainline will be based around 47.5 that would get you around 108mm length for your setup on an old bike..
This happens to be the length that Super Low Profile cranks used BItD when they where introduced. (107 to 113)
I assume that is why length seems wrong.
(just an idea)

 

[Anthony]

Recently I stuck an Alivio triple chainset on the recommended 68x123 cartridge bottom bracket, and was surprised at both the vast Q-factor and the fact that the right hand crank was far closer to the chainstay than the left hand one. The amount of axle sticking out of the BB shell of the bike was the same on both sides.
This was resolved by fitting a 68x113 BB to narrow things down. I still had to take a left hand crank from another set to make things symmetrical.
Today I fitted a Suntour chainset to a 68x123 BB (OK the same one as above) and again things were very unsymmetrical and bandy.
So are the cranks in the set tapered in an unsymmetrical way and must be paired with an unsymmetrical BB or what?

I don't see why it's a problem if the rh crank is further from the stay than the lh one. And I don't see either why the amount of axle protruding on the left has to be the same as the amount on the right. Provided the gears work and the chain is at a nice angle, you're likely to get less chainsuck damage if the right side is slightly out from the frame, so why knock it?