Busted
 

Hi everyone, I'm new here and I wish I'd found this forum before. As much as I use the internet, I never thought of it to learn about my old bike. (doh!)
The crank axle (correct terminology?) on my Motobecane Super Mirage that I bought new in '77 broke recently while pushing off from a stop sign. I fell maybe 20 miles before that, so I'm guessing the fall partially fractured the shaft, cuz I'm not that strong! :D
The catalog that came with the bike says it's a Tourney Luxe, alloy cotterless with alloy guard. That's about all I know about it. I've never needed anything but tires, brakes, etc. Do I stand any chance of finding a replacement?

Absolutely, I am assuming that you are talking about the bottom bracket spindle, you can buy replacements, just make sure it is the same length and that the ends are the same shape (probably a square taper)

Here is a link to what I am talking about:
http://sheldonbrown.com/harris/botto...ets.html#axles

while you are at it, it wouldn't hurt to inspect the bottom bracket cups and the bearings to make sure the impact did not affect them.

If you broke the crank arm, those are also abundant, just make sure the new crankset is compatible with your bike.

Matt

Probably french threaded, which is more difficult. Can start by taking the spindle (both pieces) to your local bike shop of choice. Some keep a box of random parts, you can get lucky. If you don't have the tools, you will need the bike shop to disassemble it for you. Keep all the parts! Those french threaded cups are hard to find too.

But before you do anything, take some pictures of what failed, so we can target the right part for you.

Visit the park tool site, they have excellent repair details. Even if you don't plan to do the work yourself, it is better if you understand what the shop needs to do.

Those spindles are pretty robust, so I am having trouble understanding how it broke/where it broke. And the spindle is shielded from any direct impact from a crash, whereas the crankset is directly exposed. Spindles commonly are carbon steel, and stronger than the alloy crank attached to it. Are you sure you didn't break a crank arm?? Take some pictures.

If it is a crank arm, you might be best served by picking up a vintage crankset and replacing the crank. They sell on ebay all the time. See the park tool site for details on how to do this job (its pretty simple, but takes a special tool).

It would be great if you took scans of the catalog for the bike and post.

Post a picture of what failed.

Motobecanes are SWISS thread. Just like frenc but one of the cups (the fixed one?) is left threaded. Keeps it from loosening itself.
Spindle should be the same.

I'd guess a 1977 would be French. At some point Moto switched over to Swiss threading, but I'm pretty sure that's later. My 1979 is French at any rate.

judging by your username i'm guessing you've got small block with a 4" stroke crank...
maybe some edelbrock or W2 heads or something...

My 76' Super Mirage is Swiss threaded.

Interesting. This question comes up periodically w/r/t Motos but there never seems to be enough data to really pin down when and if the switch was made for various models, or whether it was even that precise a switch. Anyway, mine's a 1979 Team Champion and it's French.

Appreciate the responses. I need time to look into it further, then I'll get back to you all. From what I'm reading here, what broke is called the spindle. It broke between the left side crank and the left side cup (external, not inside between the cups.). It seems to have been partially fractured for a while, because the surface of the break is partially covered w/ oil, whereas the rest of the surface is fresh. I agree. I never thought it could break! Sorry, no pics. This 'puter and me--grrrrr!

Not yet, but when the bucs are there... I have a '74 Dart Sport /6 that I plan on making into a Demon or early Duster. Street/strip car.

Given your description, does sound like old damage, perhaps before you even owned the bike. Your recent fall must have been the last straw. Time to measure up that spindle and maybe you will get lucky.

I think my 77 is Swiss thread..

He bought it new in 77. More likely it was a flaw in the metal, a stress fracture, or some combination of those two.

Got the reseet? maybe there is a lifetime waranty on that thing? :D

Threads
 

The only thing consistant about Motobecane BB threads is they are inconsistant. I have 3 Grand Records (72,73,74 ) one Le Champion 73, one Champion Team 81or 84 I know not which. All are French except the 73 Le Champion and it's Swiss. Why did France use the Swiss threads at all? Who knows.