Originally Posted by
allencb
I've only known one person who managed to quantify the amount of flex he was getting from his crank/bb/frame and that was because the cranks were hitting the chainstays.
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BTW, regarding the Iro kit, is it safe to assume their kit will result in a proper chainline if used with their wheels?
Chris
Regarding the flex and hitting of the chainstays, it was most likely his frame flexing. Did he have the problem with his previous setup? Did he only change the cranks? My thoughts here are that his newer setup (the RD's) put the cranks closer to the chainstay (lower q-factor), and he was getting the same amount of frame flex as before, just had less room for the frame to flex before the cranks brushed the chainstay. Think about this; in order for the crank to brush the chainstay, it's got to be flexing on the upstroke, which isn't nearly as powerful as the downstroke. So, the frame was actually flexing on the opposite pedal's downstroke, twisting the whole bb shell area such that the crank on the upstroke brushed the chainstay. Therefore, frame flex is the culprit here. Does that make any sense?
With IRO's cranks and their 110mm bottom bracket, the chainline will be 42mm. This is pretty typical of most track hubs (not Pauls though), including IRO's hubs, Formula, etc.
You need a 103mm bottom bracket with the RD's to get a 42mm chainline. Buy the correct size bottom bracket, no need for guess and check.
http://sheldonbrown.com/harris/cranks/130-single.html gives the chainlines for a bunch of cranks/bb combos.
It sounds a lot more complicated than it is.