dbwoi

Converted a Motobecane Grand Record but the chainline is way off, no matter which side of the spider I put the chainring on. There's no room on the hub to put spacers on to move the cog outward, so I figured I'd just go buy a shorter BB spindle to pull the chainring closer. My question is, how can I figure out what length to get? Is there a generally standard size that will fit? Or is there a way I can actually calculate it?

illdthedj

do you know the spindle length of the current bottom bracket?
you PROBABLY need a 107 length. dont quote me on that.

dbwoi

I have no idea of the current spindle length, its just the stock component that came with the bike. IIRC a guy in a LBS said 110 was probably a good idea, I just came here because it seems the people on these forums know more than any of my LBS's.

illdthedj

are you able to remove the bottom bracket? once you do get a ruler and measure it.

dbwoi

Measuring from end to end, I gathered it was somewhere around 144 - 145mm.

nuhtowel

This is what I did, get a ruler or a caliper and take these measurements

1. Measure the length from the center of the bike to the chainring

2. Take out the BB and measure the spindle length (or see if its stamped on there somewhere), then divide that number by 2

3. Measure the distance from the center of the wheel to the cog

With the first two measurements you can figure out how much offset your crankset puts in your chainline, with the difference known, you can mess figure out what spindle size would give you the same distance found in measurement 3.

mihlbach

Did you convert a new faux Grand Record (from Bikes direct) or an original vintage Motobecane Grand Record? Motobecanes (the real ones) are old French bikes, which used many standards that are now obsolete. It is very unlikely that a modern BB will be compatible with the frame (wrong threads) or the crankset (wrong taper).
You might be better off adjusting the chainline by moving spacers around on the rear axle, then redishing the wheel to center the rim.

https://www.sheldonbrown.com/velos.html

tashi

if it is an old Moto, french BB's are now available from Velo-Orange at fairly reasonable prices.

If you're adjusting chainline at the hub you don't need "room" for spacers, you need to swap them from one side of the axle to the other to move the cog relative to the dropouts. If there isn't room on the side you move them to, you just thread the cones, locknut and spacers a little further onto the axle. For example, if you want to move your cog left (inboard) 2 mm, you'll take a 2mm spacer off of the left side an install it on the right. If there isn't room on the right, you just thread the right cone a little further to the left, the left cone and locknut a little further left, and install the right-side spacer and locknut. You should have relatively equal amounts of axle sticking out of the locknut on both sides once you're done.

Redishing the wheel will probably have to follow. This can often be done on the bike quite easily by tightening the spokes on the appropriate side evenly.

Bat56

perhaps i do not understand. but can't you use chainring spacers between the spider and the chainring to move the chainring inboard?

it's what i do.

like this:
https://www.bikeman.com/Wheels_Manufa...g_Spacers.html