Replacing crankset on early-'70s Motobecane 10-speed

Question

I need a replacement crankset for my Motobecane Nomade 10-speed road bike (early '70s model), but am not quite sure what I even have at the moment and, once I know what I can use to replace it with, I still don't know where I could find it. Can someone help me with either of these two tasks?

Bottom bracket

I know that when I bought the bike, the seller had replaced the bottom bracket because the one it previously had was bad, but I'm not sure with what sort of bottom bracket he replaced it (e.g. French vs. Swiss vs. modern).

Crankset specifications

I was told by a mechanic that's a double crankset with 52/39 teeth for a square bottom bracket and an 8-speed chain (I think so?-- could this be right?).

Answer 1

From this photo we can see you have cotterless cranks, probably square tapered, which you'll be pleased to know are still a very common interface. Yours look like they're held in place with a 14mm nut easily removed with a socket wrench and crank puller.

I don't think there's any modern, cartridge bottom bracket with this style of nut, so this will be a cup and race type. You can typically remove the non-drive-side cup with a large shifter wrench and assuming the drive-side is left-hand threaded you can remove the fixed, drive-side cup using this one neat trick.

Depending on the desired chainline, if the cups/races of the bottom bracket are smooth and without pitting or gouging you can continue to use this BB with another set of square taper cranks. If the bearings are pitted replace them (they're almost always 1/4").

The four important factors in choosing a bottom bracket are:

• Shell diameter
• Threading
• Spindle length
• Interface (i.e. square taper)

This crib sheet is useful.

Front chainline is determined by combination of the BB spindle length, BB offset (i.e. spacers, asymmetric spindle) and the crankset. The crankset manufacturer will typically recommend a spindle length (or see this database).

As for a replacement crankset, you can buy new (basically any road double crankset square taper is likely to fit; or there's a few manufacturer's making quality, vintage-esque cranksets, e.g. Velo Orange), however you can also find some great quality stuff at your local bicycle swapmeet/bike shed. Whatever you choose to do, I recommend getting something with replaceable chain rings, so next time you wear your chainrings down you can simply replace the chain ring and not the whole setup.

Be aware that when your chainring teeth are this worn you have likely accelerated the wear of your chain and rear sprockets. You quite likely need an entire new drive train.

On chain itself, this is dependent on the number of cogs on the rear. With 5 on the rear you're using 3/32" chain, but modern 8-speed chain fits just fine (this is often sold as 6/7/8 speed chain).

Answer 2

I am not going to offer a full answer - but I will offer what I know. In the time-frame that this bicycle was built, you had to make sure your bottom bracket set (the axle and the cups that threaded into the frame bottom bracket) matched the crankset. One mfr crankset might not match a different mfr bottom bracket. Because of this, I usually replaced both my bottom bracket AND my crankset at the same time. The angle of the taper varies, as does the length of the taper. The important things to consider were the bottom bracket (the frame) threading type, the BB axle taper length, the BB axle taper angle, and whether or not the crankset fit those measurements.

Therefore, you can replace your crankset with pretty much everything available today, IF you can find a BB (bottom bracket, or axle) set that has the same threading as your frame.