With the semester winding down (or, perhaps, finally ramping up!), many of the 2.00gokarts are in the process of being wired up and tested. The final product is due next week, and our competition (last year’s video) is on May 5th!
Some of the students have been industrious and scheduled their checkoffs and inspections early. Here’s a preview of the action that will unfold in a much larger space next week:
Because conventional controls and riding postures are for wussies, apparently. I’m both amused and somewhat terrified at the prospect of there being three (out of eight) karts in which you ride head first. As it was my stated mission to not interfere much with the design and construction of the karts to let students experience as much of the design process, I might have to start padding BurnoutChibi and run interception for wayward karts.
Speaking of which…
Here’s a picture of the aftermath of BurnoutChibi’s motor detonation. As I would later find out, the sparks seen in the video were not the magnets grinding on the can, but rather them cutting up the phase wires.
Here’s a better picture of the ownage. The red wire, in particular, was cut almost all the way through. The annoying thing about this is that the wires were so close to the stator. If they were further out, patching would be a simpler job. I’d have to loosen the epoxy holding the wire stubs in place and also trim the heat shrink selectively.
While I await better motors, i decided to try and repair these. First step was to pop them open. There is a front retaining ring that comes out, then 2 set screws loosen up to free the shaft from the rotor. Then it’s a matter of pushing the shaft out to the right in the picture – this step was done on an arbor press.
Ouch. In total, five magnets broke loose. I figure this must have been a chain reaction where one magnet ditched first, and the resultant imbalance caused can deformations which broke the rest loose.
This is why I recommend motors that have “rotor bearings” or “skirt bearings” to everyone who asks me about them for vehicle apps. Even though it adds a little drag, the distal end of the can is properly supported on its own bearing. The only exception is if the motor is very short, like a more “pancake” style design.
I mixed up a generous dose of long-cure epoxy with glass microsphere (microballoon) filler, to slightly under nutella-like consistency. The offending magnets were pried out, the mating surfaces cleaned, then this epoxy smeared into the new joint. I replaced the magnets and used as much of the remaining epoxy as possible to completely fill in the gaps between them.
Evidently, I didn’t add enough microballoons, as the mixture did sag a little. To keep the cure symmetric, I actually chucked this thing into Tinylathe and ran it on a very low speed for several hours.
After the mixture was firm (but not cured), I set it on a radiator to cure with heat. Luckily for me, the radiators in the building were still on; they were switched off successively as recently as 2 days ago!
I didn’t get a good picture of the wiring repair before, but it basically involved exactly what I described before – carefully scraping away the heat shrink tubing to expose as much wire as possible. The wire was actually all magnet wire, so it would have been difficult to solder. To combat this, I “frayed” each lead as much as possible to expose the maximum amount of magnet wire surface area. Then I cranked the 80W soldering iron up all the way to 850 fahrenheit and literally burned away the enamel by embedding the frayed ends in a big ball of solder for heat transfer.
I think I managed to get back 75% of the red lead. The rest were patched similarly, but did not need as drastic soldering measures.
After the real epoxy fully cured, I reassembled the motor and crammed it back into the left side transmission.
I have yet to ditch a single magnet. Though I figure it’s only a matter of time before the right side lets go…
And with that, BurnoutChibi is ready to lasso its rogue… brethren? Bastard children? Offspring conceived via assistive reproduction technologies? Something. The only thing it does not do very well, sadly, is burnouts! Because the rider weight is basically square in the frame, and is up so high, it really just like to drag the front wheels along even if I’m holding the brakes. The same reason contributes to its severe power understeer (and associated lift-off oversteer!) behavior. Oh well…