Bottom Bracket Spacer Puzzle

[fausto99]

I'm fitting a Hollowtech type chainset to a 60s F Moulton F frame.The Moulton frame shell width is 64.7-64.9mm (yes, really. I've measured it very carefully many times). I have a BB labelled 68-73 which came with 3 x 2.5mm thick spacers and a tube. The tube fits hard against a stop on the drive side but can slide in and out on the non-drive side. Measuring between the cup inner faces with the tube at both extremes gives 62 and 73mm.

If I fit the bb with no spacers, I can still fit the chainset, set the preload (fingertight) and all looks good. So, my question is what are the spacers for? Is it just to fine tune the chainline or am I overlooking something?

[Cyckelgalen]

Bottom bracket shells are (or used to be) either 68 or 73 mm.
If it is 73 mm, you fit one of those spacers on the drive side.
If it is 68 mm, one spacer on the non-drive side and two on the drive side.
The added thickness of the two extra spacers is 5 mm, that's the difference between the two standard BB shells.
I would fit it with the three spacers, and check the resulting Q factor. If it is too narrow, you may want to add extra spacers if there is enough thread to secure the cups firmly.

[Jdsk]

Do you have a diagram for the new bottom bracket that looks something like:

Jonathan

[fausto99]

Bottom bracket shells are (or used to be) either 68 or 73 mm.

I know but mine is just under 65mm

If it is 73 mm, you fit one of those spacers on the drive side.
If it is 68 mm, one spacer on the non-drive side and two on the drive side.
The added thickness of the two extra spacers is 5 mm, that's the difference between the two standard BB shells.
I would fit it with the three spacers, and check the resulting Q factor. If it is too narrow, you may want to add extra spacers if there is enough thread to secure the cups firmly.

Sorry but I don't see why I should increase the Q factor when it all goes together nicely without spacers (unless I decide to change the chainline later). In any case, the spacers are soft plastic which squirms sideways when I tried 2 on the right and one on the left.

[fausto99]

Do you have a diagram for the new bottom bracket that looks something like...
Jonathan

No, no paperwork at all with it.

[Jdsk]

What's the Shimano part number?

Jonathan

[fausto99]

What's the Shimano part number?

It's not Shimano. I do have some other BBs which are Shimano which I could use, but, in any event, the questions remain.
1) What do I do with a non standard narrower shell width
2) What goal should I be aiming for with spacer selection?

p.s. I'm after general principles here - not specific fitting instructions.

[Jdsk]

Thanks

The ones that I have fitted suggest spacer widths and sides for various set-ups. I've sometimes used that and sometimes done what looks right.

I don't know if you need at least one spacer in there to get the right force as you tighten it up.

Jonathan

[Cyckelgalen]

Your shell is 65 mm, that is why I suggested fitting all the 3 the spacers, as if it was a 68 mm shell. You fit fewer spacers if the shell is longer.
Q factor is not about how well the pieces fit together, it's not about mechanics. It is about biomechanics, the position of your hips when placing your feet on the pedals. If you have a shorter than standard BB shell and eliminate all spacers, you will end up with an unusually short Q factor. It may work mechanically, but t may not work well for your body. One way to know is riding the bike and see you how feel with that Q factor. I would also measure it and see how it compares to other bikes you find comfortable.

I reduced 10 mm the Q factor on my Dahon folder fitting a shorter square taper BB, because I wanted to tweak with the chain line and chain rubbing etc. It worked perfectly, better shitting to the outer and inner cogs, no rubbing etc. But the Q factor felt too narrow and uncomfortable and I had to go back to the original longer BB.

[rjb]

If you need unsquishable spacers Sturmey Archer steel hub sprocket spacers are widely available. 2mm thick I believe

[fausto99]

Your shell is 65 mm, that is why I suggested fitting all the 3 the spacers, as if it was a 68 mm shell. You fit fewer spacers if the shell is longer...
... to other bikes you find comfortable.

Thanks and understood. However, I was under the impression that all current Q factors (esp. MTB) were suboptimal compared to how close your feet are apart when walking or running. (A better Q factor requires a washing machine bearing according to Graham Obree)

[fausto99]

If you need unsquishable spacers Sturmey Archer steel hub sprocket spacers are widely available. 2mm thick I believe

Yup. That's what I had on before.

[Cyckelgalen]

I am no expert and have no idea if the optimal Q factors in walking, running or cycling are comparable. There is plenty of info online on Q factor and recommended values (150 to 170 mm generally). But at the end of the day, it is about how you feel on the bike.
As I told you, with my folder, I could not get used to a Q factor under 150 mm, and had to go back to the original BB, 10 mm longer. Being a square tapper BB, there are no spacers involved, but you can buy BB's of different lengths from 103 mm. That was the shortest, the one I tried.

[peetee]

I would not be happy adding in enough spacers to make it work. It will leave very little thread inside the frame to secure the cups. Better to wind the cups right in and source a couple of external spacers such as were used on the triple Hollowtech cranks. I don’t know how wide they were but I think it will get you most of the way and, if need be, chain line can be adjusted by removing the cups again and using finer spacers designed for use behind gear cassettes.

[slowster]

I've probably misunderstood, but I am surprised that the plastic spacers are compressing noticeably/significantly. Is the bottom bracket shell diameter smaller than the outside diameter of the spacers? Also, has the bottom bracket shell been faced?