Chainline considerations when using triple chainset as double

[daddig]

If I use a triple chainset as a double. By removing the inner ring and swapping to the shortest Bottom Bracket (103mm) I can get, will the chainline be any different than I could get by using a double? Assuming the same bottom bracket (103mm) is used and there is sufficient clearance for the rings at the frame.

[NetworkMan]

Your question surely makes no sense without us knowing the length of the original BB axle. Assuming a chainring spacing of 5mm (pretty common) you would need an axle 5mm shorter so as to move the chainrings in by 2.5mm. That would align the midpoint of the outer rings with the original location of the middle ring. Hope that makes sense or have I completely missunderstood?

[Brucey]

depends on how long the triple BB is to start with.

The 'correct' chainset position is about 4mm different between a double and a triple (chainrings are usually on ~8mm centres), but the perfect chainline varies depending what width/speed the rear hub is, and whether you have an offroad framset or not.


But the question you asked is easy to answer; measure the clearance between the middle chainring and the chainstay. That is the amount you can shorten the RHS of the BB by before the chainrings will hit the chainstay. There may of course be other things that can limit the length of BB spindle instead.

What chainset is it? Any photos?

cheers

[Tompsk]

My preference is to have the chainrings slightly more 'inboard' than 'central'. Reason being is higher chain tension when on the smaller ring for a given pedal push so the chain losses / wear will be worse. This assumes of course that you don't plan to cross the chain on a regular basis!
As for bottom bracket length the limit for me wasn't the inner chainring hitting the chain stay but the crank arm (xd2) touching the splined bb fitting. I carefully removed about 1/2 mm from the outer edge of the bb end of the crank arm with a file.
Try to get several bb on sale or return, I swapped one of mine when it was too long, you can also use 1mm or 2mm bb spacers to fine tune position.

Edited for spelling error.

[Gattonero]

A 103mm bb axle seems way too short to me, even with a 28.6mm seat tube it's likely the FD linkages will touch the frame before bene able to effectively drop the chain.

[daddig]

Thanks everyone. The reason for my question is I removed inner ring from a Stronglight triple and swapped bb to the only one I had. The result is the rings are still well clear of the frame, however when I fitted a double changer the only way I could get the cage to clear the chain on the inner is to add tension to the cable as stop screw was as far in as possible but cage still fouled chain, too far in. So sti couldn’t pull sufficient cable to effect a change to the outer ring! Hope that makes sense?

[NetworkMan]

Thanks everyone. The reason for my question is I removed inner ring from a Stronglight triple and swapped bb to the only one I had. The result is the rings are still well clear of the frame, however when I fitted a double changer the only way I could get the cage to clear the chain on the inner is to add tension to the cable as stop screw was as far in as possible but cage still fouled chain, too far in. So sti couldn’t pull sufficient cable to effect a change to the outer ring! Hope that makes sense?

Have you measured the chainline? 45 mm for a triple and 43.5 for a double is common I believe.