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I have a 2006 5 spot and the bottom bracket shell is offset 2mm to the driveside when measuring from the center of the downtube. Now when I put in a shimano non adjustable 73x113 BB (UN73) my chainline is 50mm instead of 47.5mm. this is old school square taper BB. This same crank and BB gave me a 47.5mm chainline on my other bike which the BB shell is centered with the downtube.
I measured another 5 spot in the store and the BB is offeset to the drive side by 2mm just like mine. The problem I'm having since the chainline is 50mm, the chain falls off when in the center chainring and 1st gear. It falls off just spining the cranks a few turns in the bike stand, without any load. The cranks are raceface next lp with raceface chainrings, cassette is XTR 8spd, chain is scram PC68 8 spd. Chain, chainrings and cassette are all brand new. Front deraileur in not touching the chain. I installed a 110mm BB and the chainline is now 47mm and the chain doesn't fall off anymore, but the chain is 2mm away from the bottom chainstay pivot torx bolt when in the small chainring. The drive side crank is a little closer to the chain stay than the left side crank using this 110mm BB.
What is the reason to have the BB offset to the driveside by 2mm? Tuner must have a reason to do this. The LBS didn't know why and just said to use the 110mm BB.
I measured another 5 spot in the store and the BB is offeset to the drive side by 2mm just like mine. The problem I'm having since the chainline is 50mm, the chain falls off when in the center chainring and 1st gear. It falls off just spining the cranks a few turns in the bike stand, without any load. The cranks are raceface next lp with raceface chainrings, cassette is XTR 8spd, chain is scram PC68 8 spd. Chain, chainrings and cassette are all brand new. Front deraileur in not touching the chain. I installed a 110mm BB and the chainline is now 47mm and the chain doesn't fall off anymore, but the chain is 2mm away from the bottom chainstay pivot torx bolt when in the small chainring. The drive side crank is a little closer to the chain stay than the left side crank using this 110mm BB.
What is the reason to have the BB offset to the driveside by 2mm? Tuner must have a reason to do this. The LBS didn't know why and just said to use the 110mm BB.