August 13, 2013

Scooter Mega-Update: Piling on the Absurdity

Lots of progress has been made since the revival of the all-terrain scooter project, although a reasonable person would probably describe it as progress in the wrong direction.  As I've stumbled across random components in the treasure trove that is N51/N52, the scooter has become progressively more ridiculous.

Over a month ago, I finished up the dual-mag-box: a gearbox, which much like the triple-cim-box on the original scooter, combines and reduces the speed of the outputs of multiple (in this case 2) motors.  This time, rather than chopping up and sketchily aluminum-zinc brazing together a gearbox, the gearbox housing was milled out of a 6.5" x 3" x 1.5" aluminum billet.

Here's the brick marked up and ready to mill:

SAD: Sharpie Aided Design
I bored out holes for the gears.  The big one in the center proved to be a real challenge to machine.  The boring head for the mill was too small, and the geometry of the cutting tooth on it prevents it from making flat-bottomed holes.  I ended up chucking the billet on the four jaw chuck and boring out the cavity on the lathe.

Rather than using my homemade mill-broach-thing I used on my tricycle differential gears, I borrowed a real broach set from Charles, and broached the keyways on a mini arbor press.  Real tools are nice.

The motor mounting plate, with more sharpied in dimensions:

I reused the output gear and shaft from the original gearbox.

To mount the gearbox, I had to make cutouts in the u-channel frame for the motors.  These turned out to be difficult to make, because we didn't have any proper tool large enough.  I ended up using a fly cutter and taking very shallow passes.  Even so, the process generated lots of unpleasant noises.

One major problem with the last scooter was the chain path.  Because of the location of the suspension pivot relative to the gearbox output shaft, I had a dreadful chain tensioner assembly to compensate for changes in chain path length.  While eventually it became reliable by replacing the spring with bike inner tubing, it still severely limited possible suspension travel (the springs were stiff enough that this wasn't ever a problem).

To eliminate this problem completely, there will be a pair of coupled idler sprockets that ride on the axle the suspension pivots about.  I bolted together two sprockets side to side, and pressed some bearings in to make the idler.  The idler also accomplishes a bit of gear-reduction, so this vehicle will have a no load top speed of a bit south of 30 mph.

The rear suspension is where things start getting really silly.  Because there happened to be a big aluminum billet lying around MITERS, the rear swingarm is milled from solid blocks rather than plates.  During one of those brief periods where the MITERS bandsaw had an un-borked blade, I chopped the billet in half diagonally.  

I face the edges to have a nice stripey pattern, because I liked how it looks.  Credit for this technique goes to Julian.

To hold the rear axle, I drilled a hole in the end of each arm and added a clamping mechanism.  To create an offset for the disk brake on the rear wheel, I made another diagonal bandsaw cut down the middle of one block.  I bolted the two bits back together like so:

The pivot for the rear swingarm was made out of...more billet.  Each side has a pair of bearings pressed into it.  The pivot block was bolted to the gearbox with four long bolts.

The entire assembly is held to the deck with 7 countersunk 1/4-20 bolts.  Normally, tapping all those blind holes would be miserable, but I recently purchased a forming tap for MITERS.  Forming taps are a wonderful thing.  Instead of cutting threads, they actually cold-form the metal, squishing it into a thread shape as you tap.  You drill a slightly larger hole (#1 drill, as opposed to #7 for a normal 1/4-20 tap), and you can just crank the tap down until it hits the bottom.  Since it does not produce chips, you never have to back out the tap.  If you have a drill with a strong chuck, you can just power tap until the tap hits the bottom, without worrying about chips clogging it.  Even without a drill, blind tapping is way easier than with a normal tap.

In the piles of parts left in the remains of a certain electric vehicle club I found this very fancy downhill mountain bike shock.  One of the washers that held the spring in place was missing, so I turned my own and attached it with a two piece shaft collar.  It features adjustable everything, including spring preload, compression damping, rebound damping, and internal pressure.

I milled a block to hold the shock absorber to the swingarm out of some more billet.

And here's the suspension fully assembled:

The shock absorber attachment on the frame was turned from some 3" round.  Then a large flat was milled on each side.  The shock pivots about a 7/16 shoulder bolt tapped all the way through the mounting block.

The milled-down part of the cone makes a nice hyperbola shape:

Because leaving the rear swingarms solid aluminum would be silly, I milled out a channel on each side.  The center of the milled-out region is now 3/8" thick, instead of 1.25" thick.  I plan on doing the same on the other swingarm as well.

And here's the frame assembled.  Once I attack a proper steering column and handlebars, it will be pushable.  I did some jump-testing of the suspension.  The back seems just right, but the front is definitely too soft.  I haven't decided whether it's acceptable, or if I need to stiffen up the shock absorber somehow.  Either way, the suspension travel is epic, with something like 5-6" in the front and at least that in the back.  Clearance is similarly ridiculous at over a foot with my weight on it.

Still left:  Making a wider deck, mounting the electronics, and riding off into the distance to the whine of straight-cut spur gears.

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