Bafang BBS02: Speed Sensor Nut Reshaping

A Bafang BBS02 (for a friend’s upright bike) arrived with a deformed speed sensor nut:

Bafang BBS02 - Deformed speed sensor nut - end view
Bafang BBS02 – Deformed speed sensor nut – end view

It traveled halfway around the planet while trapped underneath the motor and, if it rode in the top layer or two of containers, the combination of pressure and heat would be irresistible.

The plastic was stiff and I couldn’t force the nut over the connector using as much force as seemed reasonable:

Bafang BBS02 - Deformed speed sensor nut - test assembly
Bafang BBS02 – Deformed speed sensor nut – test assembly

On the upside, the nut just compresses the silicone washer between the connector and the sensor to make a waterproof joint, so it need not have perfect threads or a uniform shape. Once the nut is in place, it will likely never be removed and should never bother anyone else.

Being unwilling to apply a hot-air gun near the cable, I decided to try slowly cold-forming the nut inside a mold:

Sensor Nut mold - solid model
Sensor Nut mold – solid model

The gap isn’t a kerf: the two halves meet to form a cylindrical pocket. The smaller holes fit a pair of brass tubes keeping the halves lined up while I arrange things:

Bafang BBS02 - Deformed speed sensor nut - clamp detail
Bafang BBS02 – Deformed speed sensor nut – clamp detail

A pair of swivel-pad clamps apply the pressure:

Bafang BBS02 - Deformed speed sensor nut - compression clamp
Bafang BBS02 – Deformed speed sensor nut – compression clamp

A few days of squashing made it round-er, whereupon I applied the clamp directly against the remaining high point with the other side cradled in the mold. It still doesn’t slide over the connector body, but I’m not in a rush.

Bafang tech support generously sent a speed sensor extension cable from which I can extract a good nut, which will require cutting and splicing the cable from the motor.

I’m still hoping gentle suasion will prevail.

The OpenSCAD source code tucks into the overall file producing various useful bits:

// Mold to reshape speed sensor nut

SensorNut = [0,14.4,13.0];
SensorMold = [SensorNut[OD] + 2*WallThick,SensorNut[OD] + 2*WallThick,SensorNut[LENGTH] + WallThick];
MoldSides = 20;
RodOD = 1.6;

module NutMoldBlock() {

    difference() {

        translate([0,0,SensorMold.z/2])
            cube(SensorMold,center=true);

        translate([0,0,WallThick])
            rotate(180/MoldSides)
                PolyCyl(SensorNut[OD],2*SensorNut[LENGTH],MoldSides);
        translate([0,0,-Protrusion])
            rotate(180/8)
                PolyCyl(SpeedOD,2*SensorMold.z,8);

        for (i=[-1,1])
            translate([i*(SensorMold.x/2 - WallThick/2),SensorMold.y,SensorMold.z/2])
                rotate([90,0,0])
                    PolyCyl(RodOD,2*SensorMold.y,6);
    }
}

module NutMold() {
    gap = 1.0;

    for (j=[-1,1])
        translate([0,j*gap,0])
            intersection() {
                translate([0,j*SensorMold.y,0])
                    cube(2*SensorMold,center=true);
                NutMoldBlock();
            }
}

… snippage …

if (Layout == "NutMold")
    NutMold();

I haven’t worked on a safety bike in years!

4 thoughts on “Bafang BBS02: Speed Sensor Nut Reshaping

  1. How about approaching it from the inside? Print out two split halves of a tapered pipe threadish insert that you can put over the cable and then screw the nut onto. Every once in a while tighten the nut a bit which may expand it enough to allow it to screw on to the fitting (after the insert is removed of course).

    1. It finally roundified enough that I could force it over the connector, engage a few threads, and slowly proceed as you describe, minus the taper.

      The thread seems to be M11×1.25: making a split tapered plug tap is exactly why 3D printing was invented! The next time I’m confronted with this problem, that’s how I’ll solve it. The job might even justify a resin printer, too.

      Thanks for the suggestion!

  2. Well that’s just nuts.

    I probably would have just put a piece of glue-lined shrink tubing over the whole mess and ignored the nut.

    1. Perhaps borderline obsessive? [grin]

      Were it my bike, I’d probably make the damn nut, but this is for a friend’s bike.

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