Took on a repair job for a broken bicycle bottom bracket (ww2 Swiss army bike)
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  1. #1
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    Default Took on a repair job for a broken bicycle bottom bracket (ww2 Swiss army bike)

    Hi folks,

    While not at all inline with my regular gig, a customer/friend brought in a broken part from a rare old bike. Basically it is a simple shaft that connects the two peddle cranks and is supported by ball bearings that are pre-loadable in the bottom of the frame. The bike is old and strong. The bikes owner uses it in WWII reenactments. The shaft that sheared, clean half in two... is about 3/4” diameter and feels like heat treated medium carbon steel. The part is in good shape except for the issue of being sheared in half. No telling how it broke, my guess is it has to do with metal fatigue.

    I could remake the part but for the weird left hand pitch thread. I don’t even have the thread pitch gauge that matches. The owner can’t find the part and will accept a repair that makes the bike a static display piece if necessary. I’d like to do better.

    Unsurprisingly, the shaft sheared at the end of the threads which were finished without a relief. I have been noodleing on how to fix this. The “nut” includes the hardened and ground bearing race and that is not a part I am capable of making either. I don’t feel like there is room to go up a thread size although it isn’t out of the question as it is a very fine pitch thread. I suspect the nut is hard also. I am thinking of milling a pocket in the core of the threaded section and a coresponding slot in the other half of the shaft and make a 4140 or 1045 steel “key” to fit the shafts together before attempting a TIG weld or hard solder join.

    I thought I’d share the problem here since most of you are far more knowledgeable than I. To me it is an interesting problem and I want to fix it because all the “machine shops” around have declined to deal with it. If I do fix it, I’m not going to charge for it, just do it with the intent of keeping an old bike alive and to give myself a little challenge that will push the envelope of my limited skills.

    I hope others will se the project as something interesting and perhaps share experience and ideas with me. Thank you in advance, Rick.

  2. #2
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    Pictures please, and a dimensioned sketch to the best of your ability. I've had to reverse engineer a lot of stuff like this, but I've had the part in hand. If you only have inch thread pitch gages, give the two tpi that are closest above and below.

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    The pitch of the thread is very close to 24tpi. It’s Swiss I expected metric... the bike was built in the early 1940s.
    87368be0-5b28-4d9f-80bd-c7a29d0efac5.jpg0f6b3613-7fb7-4088-beff-2f19286734b0.jpg

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    I would bet it is 1mm pitch. If your lathe is not capable of cutting metric, find someone that has the capability. Machine the threads so the nut fits. If the pitch is a little bit off an undersized thread may allow the nut to fit. For material I suggest 1141 (Stressproof) or 4140HT.

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    I believe it may be a British thread. Check to see if it is 55 or 60 degrees.
    Bill D.

    Bottom Bracket threading and other specifications for bikes with traditional three-piece cranksets by BikeRaceInfo
    Last edited by Bill D; 10-30-2019 at 02:03 AM.

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    I think i misunderstood. The weird thread is on the rotating crank? It is not the bottom bracket cup threads?

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    I echo that it is likely to be 1mm pitch. 55 deg is about as likely on a bike from that era as 60 deg.

    A photo would really help. Most European bikes had no threads on the crankshaft. The shaft had outward-pointing conical inner races forged and ground directly on it, and the inward-facing bearing cups screwed into the threaded bottom bracket. The cranks or pedal arms were attached to the flatted ends with transverse draw keys.

    Many American (Schwinn) bikes had what were called "Ashtabula" cranks, where the pedal arms and crankshaft were a single forging. The bearing cups faced outward, and pressed into the bottom bracket. The inward-pointing bearing cones screwed onto threads on the crank.

    The arrangement described in the OP sounds like a hybrid of these two styles

    But once the issue of Metric threading is solved, making a new crankshaft should not be too difficult. But it is a fairly highly-stressed part...carrying cyclic combined loading of 150 pound-feet of torque plus around 40 pound-feet moment, on a 5/8" diameter. I would feel more comfortable with a part heat-treated after machining to 40 HRC or so, but PHT 4140 or 4150 may do.

    Do pay plenty of attention to minimizing stress-raisers.

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    Now I see the photo. Looks like you might be able to make a new one out of a single 12.9 grade bolt, if you can find one of the right diameter and pitch combination. A bolt would have the advantage of rolled threads But I think your threads are finer than standard for the diameter.

    I would not attempt to repair this shaft.

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    The part you are asked to repair is referred to as a spindle (that is threaded for one of the two bearing cones.) This is not the way these parts are constructed in any current bicycle I am aware of but I would strongly suspect is very familiar to folks on vintage bike collecting forums. You are likely to get very authoritative info from such a forum. But before I went that route I would walk into any bike shop in a reasonable-sized metro area and ask about the part and anyone the folks there know who is interested in vintage bikes. I'd be amazed if they can't point you to someone in the area who is a vintage bike "nut."

    Once I figured out the thread pitch (which could also be easily and reliably done with an optical comparator like my vintage Scherr Tumico), I would not hesitate to make a replacement from A2 or O1 tool steel. It is very easy to heat treat that steel and make an excellent part. This part was made during or post WWII from who knows what---whatever was at hand likely and lasted 75 or more years.

    No need to settle for some kludge replacement. Get the data, make the part and get it heat treated. (In desperation, send the machined part to me and I will heat treat it though I have no certifications, I would help out a fellow rider and PMer.) This is a very doable project. That part is subjected to human muscle loads only----not that great. Though it might more likely have failed from a crash....

    Denis
    Last edited by dgfoster; 10-30-2019 at 11:56 AM. Reason: Typos

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    FWIW, the photo of the end of the failed shaft looks like a fatigue failure to me. Likely lots of hard starts on that bike and many cycles on that, um, cycle.. And maybe reversals if it used a pedal brake??

    Avoiding stress risers at the end of the thread would be a good idea if it's meant to last in hard use.

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    Does the cup that's on the shaft screw off? There are a couple for sale here on our little version of Ebay, and I'm sure many more available. The seller in the link below says it's a 17mm x 24tpi left hand thread.

    Achse zu Tretkurbel acheter sur ricardo.ch

    This one seems a little longer, and has the cups.
    originale Tretachse Militarvelo, 140mm acheter sur ricardo.ch

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    Just a thought. Could the thread be 26 TPI, which is a pitch used by every cycle manufacturer back in the day as it was a British Standard Cycle thread. Even continental makers used it. For some strange reason, it has a 60 degree thread angle unlike the other standard British stuff (Whitworth and BSF) which is 55 degrees.

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    it certainly looks like most dutch bikes i ever had the crank out of.
    threads were a pain in the back as they seemed to only be used on bikes.
    if you can get some exact measurements i could have a look if they match the standard older dutch bikes of which there still are may around.
    if so finding a shaft should not be a problem.

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    IMO the only practical way to repair such an item (rather than make a new one) would be to EDM matching hex shaped recesses in both pieces (so the fracture mates) and use a piece of a high grade Allen key secured with epoxy or high strength Loctite as an internal splint. If it is a heat treated part weld repair may only lead to a fracture next to the weld.

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    Bore into the broken ends on each part, and fit a steel pin with about 0.001 diametral clearance.
    Flux the parts and the pin, and sliver solder the assembly. Fixture so the gap is pretty much closed
    up between the parts.

    You want to make the pin diameter such that the area of the bored hole is about equal to the
    area of the remaining original material. Do it correctly and you will retain most of the original
    strength of the assembly.

    This is a cottered-type crank, pretty common. What's unusual is the threaded portion is on the
    crank spindle,

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    As stated above find someone with an optical comparator to get your thread pitch dead on. Isn't there a couple PM members in Asheville?

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    Quote Originally Posted by Bill D View Post
    I believe it may be a British thread. Check to see if it is 55 or 60 degrees.
    Bill D.

    Bottom Bracket threading and other specifications for bikes with traditional three-piece cranksets by BikeRaceInfo
    This website gives the clearest description I have seen - thanks, Bill.

    British cycle threads were all 60° angle, though the root and crest radii were different from Sellers and Metric threads. They were originally called CEI threads, for Cycle Engineers' Institute. They were later rationalised and adopted as BSC, or British Standard Cycle, threads.

    George

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    If you are going to make a new one and can't find anyone local to measure the threads, you can send it to me. I have an optical comparator, tool makers scope, pitch mic and thread wires. I also have some 4140 remnants I could send back.If the threads are 60 degree I could even thread an end for you. Just cover the postage. I am in Central Va about 400 miles away. Probably 1-2 days in a USPS small flat rate box.
    Last edited by Dualkit; 10-30-2019 at 11:56 AM.

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    Respectfully: I'm happy you asked here before kludging something up. If you can't make the threads you should tell him your not the guy for that job but that you can help direct him to some people who are. I hope we can connect you to a bike nut that would do it right and for cheap.

    Human muscle / weight loads are in fact quite significant cantilevered off of a shaft the size of your thumb. The original Swiss engineered and heat treated part broke after all.
    Last edited by dsergison; 10-30-2019 at 01:43 PM.

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    Bore hole in each piece for a steel locating pin like what was stated before.
    Now a professional welding place could mount that to a rotating fixture and make a perfect set of beads. They might even recess the fractured line to get deeper
    weld penetration. Then maybe grind it or leave it as is.

    My BMW motorcycle had a worn a gear at the rear wheel where the drive shaft connects. The gear section was cut of and a new gear welded on as described above.


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