Where's the correlation in this...

FINNEY1973 · Sep 7, 2013

I saw this on an advert for a bottom bracket today - ebay.

"Axle Length 111mm is for double crankset however it may also be used with a triple crankset provided that the frame seat tube do not exceed 28.6mm in diameter"

For the life of me I cannot see how seat tube diameter has any bearing on bottom brackets. Anyone got any thoughts on why a seller would put this in the description?

GuyWood · Sep 7, 2013

maybe he means the granny ring would be too far inboard and would rub on a fatter seat tube?

Woz · Sep 7, 2013

The most achievable inner position of a front mech is typically limited by hitting the seat-tube, rather than the range of the adjustment screw. If for example it was a 34.9mm seat-tube, compared to a 28.6mm that is a difference of about 3mm for the front mechs most achievable inner position.

That 3mm can make the difference for the front mech to change from the middle to the inner position of a triple. Choice
of bottom bracket width allows you to compensate for that difference; a wider seat-tube would require a wider bottom bracket width.

In practice, it's a bit of trial and error since it also depends on which front mech, which crank, which rings on the crank, which chain width etc.

FINNEY1973 · Sep 8, 2013

Ah, ok - understand the principle - fairly straight forward. I always assumed that the complexities of a triple chain-ring set-up was dealt with purely by the length of the axle. I had always gone with the chain-stay as being the limiting factor, and not the seat-tube but yes it makes perfect sense that the a wider seat-tube could affect final set-up.

Luckily I don't envisage using a triple for quite some time but if & when I do i'll recall this thread before I jump in at the deep end. Cheers

Where's the correlation in this...

FINNEY1973

Senior Retro Guru

GuyWood

Old School Hero

Woz

Old School Grand Master

FINNEY1973

Senior Retro Guru

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