Narrowing a bottom-bracket shell for better chainline (BBS)

[joshseitz]

I have a 2006 Cannondale Prophet onto which I plan to install a BBS02 from Paul. It has a 73mm BB shell, so I'm concerned with the chain line being too far out there. I've read a bit about this on the giant BBS thread, but had some questions. I also stopped by a local bike shop to see if they'd do it or knew anything about the process, and it was a foreign concept to them. Their main concern was large chainring interference with the chainstay, but the Prophet has elevated chainstays, so that shouldn't be an issue.

My primary question is this: In order to convert my 73mm BB shell to 68, should I simply remove 2.5mm from each side, or 5mm from the drive side (or something in the middle?)?

Also, do I need to re-thread the shell, as I assume I'll be losing a fair amount of the internal threading if I remove up to 5mm of shell?

And who should I contact to do this? A custom frame builder (there are quite a few here in Portland)? A machinist? Can I create a jig and do it myself with a facing tool or something?

thanks all!

 

[amigafan2003]

Test fit the unit and measure chainline to centre of the seat tube - and machine off whatever to need to get to 47.5mm. So if its currently 50.5mm, machine off 3mm from that side and 2mm from the other.

No need to tap more threads, the bbs does use the threads.

 

[samsavvas]

I have a 2006 Cannondale Prophet onto which I plan to install a BBS02 from Paul. It has a 73mm BB shell, so I'm concerned with the chain line being too far out there. I've read a bit about this on the giant BBS thread, but had some questions. I also stopped by a local bike shop to see if they'd do it or knew anything about the process, and it was a foreign concept to them. Their main concern was large chainring interference with the chainstay, but the Prophet has elevated chainstays, so that shouldn't be an issue.

My primary question is this: In order to convert my 73mm BB shell to 68, should I simply remove 2.5mm from each side, or 5mm from the drive side (or something in the middle?)?

Also, do I need to re-thread the shell, as I assume I'll be losing a fair amount of the internal threading if I remove up to 5mm of shell?

And who should I contact to do this? A custom frame builder (there are quite a few here in Portland)? A machinist? Can I create a jig and do it myself with a facing tool or something?

thanks all!

Please be aware that the seat for the right side face of the BB is actually recessed a couple of mm into the rear of the gear case. This means that some clearance is required between the plane of the BB face and the outer edge of the right chain stay for the main gear case. Machining the BB face may reduce whatever clearance you may have here. You mention 'elevated stays which I've seen on other bikes but not on the Cannondale - this may make my concern irrelevant... I think kepler posted a nice diagram on the thread recently that illustrates all of this.

Personally, if you like the drive, I'd just try it first before worrying about modifying anything. I guess it depends on how many of the gears you plan to try to use. If you will just be riding on the road you will really only need 2 or 3 gears so you'll have to ask yourself how critical a straight chain line is to you.

Note that there are plenty of authoritative engineering types in the cycling world who reckon a straight chainline has far more to do with various, deep-seated (possibly UK :wink cultural obsessions from the pre-roller chain days than with practical modern realities! I'm using 8 speeds and a 135OLD on my TSR with no chain line or gear changing problems.

Saves.

 

[samsavvas]

I have a 2006 Cannondale Prophet onto which I plan to install a BBS02 from Paul. It has a 73mm BB shell, so I'm concerned with the chain line being too far out there. I've read a bit about this on the giant BBS thread, but had some questions. I also stopped by a local bike shop to see if they'd do it or knew anything about the process, and it was a foreign concept to them. Their main concern was large chainring interference with the chainstay, but the Prophet has elevated chainstays, so that shouldn't be an issue.

My primary question is this: In order to convert my 73mm BB shell to 68, should I simply remove 2.5mm from each side, or 5mm from the drive side (or something in the middle?)?

Also, do I need to re-thread the shell, as I assume I'll be losing a fair amount of the internal threading if I remove up to 5mm of shell?

And who should I contact to do this? A custom frame builder (there are quite a few here in Portland)? A machinist? Can I create a jig and do it myself with a facing tool or something?

thanks all!

Please be aware that the seat for the right side face of the BB is actually recessed a couple of mm into the rear of the gear case. This means that some clearance is required between the plane of the BB face and the outer edge of the right chain stay for the main gear case. Machining the BB face may reduce whatever clearance you may have here. You mention 'elevated stays which I've seen on other bikes but not on the Cannondale - this may make my concern irrelevant... I think kepler posted a nice diagram on the thread recently that illustrates all of this.

Personally, if you like the drive, I'd just try it first before worrying about modifying anything. I guess it depends on how many of the gears you plan to try to use. If you will just be riding on the road you will really only need 2 or 3 gears so you'll have to ask yourself how critical a straight chain line is to you.

Note that there are plenty of authoritative engineering types in the cycling world who reckon a straight chainline has far more to do with various, deep-seated (possibly UK :wink cultural obsessions from the pre-roller chain days than with practical modern realities! I'm using 8 speeds and a 135OLD on my TSR with no chain line or gear changing problems.

Savvas.

 

[maxwell92036]

The motor will work with a 73 MM bottom bracket. Don't sweat it until you install it first.

 

[MitchJi]

Hi,

No need to tap more threads, the bbs does use the threads.

Justin posted, in his Patterson geared BB sales thread, that a 68 MM Patterson, will not work with a 73 MM BB, because there are not enough threads.

 

[tahustvedt]

Nothing wrong with a chainline that is a little out of whack, but the BBS chainline with a normal 68 mm bottom bracket is 51.5 mm. Not disastrous, but it makes audibly more noise than normal in first gear. But with a 73 mm bottom bracket it will be 54 mm, and that's pretty bad when using the larger cogs in the rear. Not only does it strain the drive train, but the chain will probably drop off the chainring quite often in lower gears.

I would remove 5mm from the right side only if there's enough room by the chainstay for the BBS gearbox to clear it. The pedal centerline is offset quite a lot to the right on a 68 mm frame, so this would move it 2,5 mm closer to centered in addition to bringing the chainline within specs for a 135 mm rear axle.

 

[joshseitz]

Thanks for all of your replies. Indeed I will likely install it first and see for myself, but I have a feeling that I may not be happy with the low gearing, particularly if I'm dropping chains a bunch.

I'm considering one of these 42T chainrings: http://www.alcedoitalia.it/shop/accessori-bbs/corona-42-denti-per-bafang-centrale-bbs01/. I've seen a number of builds on ES using this ring. Can anyone confirm whether it improves the BBS chainline, or simply matches the offset of the stock ring?

I may also end-up going to a 11-36 10SP cassette with Shimano Shadow Plus derailleur to help prevent dropped chains.

Or......I have an Alfine8 on another bike that I could move over, but it sounds like shifting under power is considerably better with a traditional cassette/derailleur system.

 

[maxwell92036]

Use a front derailleur to keep the chain from dropping or Paul components has a slick little device that probably would work even better.

 

[MitchJi]

Hi,

I may also end-up going to a 11-36 10SP cassette with Shimano Shadow Plus derailleur to help prevent dropped chains.

If you have a powerful system the 11t cog is a bad idea.

 

[pexio]

...I'm considering one of these 42T chainrings: http://www.alcedoitalia.it/shop/accessori-bbs/corona-42-denti-per-bafang-centrale-bbs01/. I've seen a number of builds on ES using this ring. Can anyone confirm whether it improves the BBS chainline, or simply matches the offset of the stock ring?....

I have one of the alcedoitalia 42t chainrings and it indeed improves the chainline over the stock 48t Bafang ring. I measure 47.5mm front chainline with the 42t on and it lines up perfectly with the middle of the cogs out back - absolutely no shifting or noise issues.

Bear in mind that it lowers your overall gearing and you may not be able to pedal as fast as you may want. With my 48v 750w BBS02, I top out at 33 mph with max throttle but that equates to 115 rpm pedal cadence - too fast for me! With the 48t, I could pedal along at 100 rpm at 33mph.

 

[pexio]

Hi,

If you have a powerful system the 11t cog is a bad idea.

Why?

 

[MitchJi]

Hi,

If you have a powerful system the 11t cog is a bad idea.

Why?

Not enough teeth engaged to handle the power. It will not last very long.