Bottom Brackets, Cranks, and Chainline
I'm converting an old Puch frame into a fixed gear for my brother for christmas. I need to know what kind of bottom bracket and cranks to use. The rear wheel is the stock wheel off an old bianchi pista--so chainline should be around 42 mm. I know the frame takes a standard, english threaded BB. How do I determine what BB and cranket to use to produce a 42 mm chainline????
surly old man
You probably can adapt the current spindle and crank to acheive this. Some older spindles were asymetrical, so you can bring the crank in towards the centerline by flipping the spindle around. If the rings are held on with bolts, you can take them off and put the desired ring on the inside of the spider arms. If you still have room between the ring and the chainstay, you could put spacers between the spider and the ring. Get the ring as close as you can to the chainstay. Then sight down the cog and ring from behind the bike and see if you are in-line. If not, then you might have to look into a shorter spindle (check first to see that you have a gap between the base of the crankarm and the BB cups; if you then get a new spindle that is slightly less short than that gap is wide). Also, you could re-dish and respace the rear wheel to bring it out a bit.
The name of the game in these conversions is primarily to bring the chainring in as tight as you can. And secondarily to bring the cog out as far as is needed to meet it.
The first problem can be solved neatly with money (track spindle and track crank), or more messily by getting the ring in as tight as possible and seeing where that leaves you for re-dishing the rear.
The messy solution is not that hard, but there are a number of variables to work through.
Originally Posted by jgedwa
Thanks, the problem is that the original bottom bracket is probably useless, as my bro removed it with a screwdriver and a hammer. I would like to reuse the original cranks if possible, which take square a square tapered spindle, I believe. I know the BB shell is english threaded, so I need to get a 68 mm BB. The question is spindle length. Is there some formula that will let me calculate chainline from the spindle length?
I do not want to redish the wheel.
The mfg of the crank might have that data but I've never seen it available elsewhere. I'd just mount the crank with any old BB you have and measure the chainline of the crank. From there it's easy to determine the BB spindle length you need. Just make sure not to mix ISO and JIS taper BB's.
And if you don't have an old BB lying around, you could mount the crank to your spindle and measure the distance from the bearing race to the chainring center and go from there. That sounds error-prone to me but could be done. I've heard of thin vs thick BB cups, though, so watch out for that.
That's a toughie. I would guess that you could get an acceptable chainline by buying a road crankset and the appropriate bottom bracket to go with it. I've had luck doing so in the past. Go for a double crank rather than a triple, obviously. You will probably have to mess around with stuff and may end up buying another BB if you are unwilling to redish.