How Much Electric Kart Can You Build in a Day?

Last week I rebuilt the kart with fresh fasteners and axle bearings, plus picked up a steering wheel and some #35 sprockets. Then, once the Mars motor looked repaired and good to go onto the kart I picked up a handful of 7/8 keyed #35 sprockets from Surplus Center. The parts were forecast to arrive on Thursday- so I made plans with my friends to bring everything to Laney and try to get the kart running. I came in at two and got started- about eleven hours later the kart was driving, a half an hour later it was broken, and finally at about four in the morning we all went home.

Miles drew up the motor mount plate in CAD and cut it out of 3/16 mild on the CNC plasma table. It’s bolted right to the outer bearing frame support with slotted holes to adjust the chain tension- the stick he’s using is to help support the motor while he tightens the plate in.

Micah made up all the cables out of 6GA welding cable. We didn’t find enough lugs locally at any sort of reasonable price so we made “lugs from the ghett-o” by smashing copper tube around the wire, filling with solder then drilling. A little trick I learned from Home Power Magazine about ten years ago. I mounted the Sevcon Millipak controller, contactor and fuse, and made up the control panel, throttle, and hall sensor wiring harnesses.

See the rest–

Micah set up battery mounting using only the best pallet-wood and ratchet straps

A dead cushman carcass was ransacked for the 0-5k throttle pedal mechanism, which Micah cleverly secured using hose clamps.

Igniton and forward/reverse switches. Classy purple switch cover courtesy of Surplus Center. Of course.

(warning: there’s a little F word in the video)

So, what happened? Let’s just make a list:

• The axle was binding in funny ways at first, we assumed it was the chain but realized the chain was not tight in any particular way while the axle was binding. Turned out it was the axle bearing set-screws interfering with the slot-shaped cut-out in the motor plate. Since it was slot shaped there was non-constant interferance, and because the rubbing pushed the screws counter clockwise it wasn’t possible to simply “wear in” as the screws kept backing out and rubbing worse. I simply pulled the setscrews out (badly mangled, but really loose) and that problem went away.
• Violent motor cogging- somewhere in the electronic commutation system (hall sensors, wiring, controller) it’s getting bad sequence information, causing awful cogging and jerking torque output. Not sure what’s at fault exactly. You can really hear the effects in the wheels-up video above. It seems to be worst at around 1/3 speed and throttle. It usually clears up with higher speed and doesn’t really seem to manifest at very slow speed while starting out. It did this on the bench before we did the motor repair hacking as well.
• Directional confusion- the Mars motor is set up to spin CCW, the fan is only effective while spinning CCW- which I think is correct for most motorcycle applications. In order to put the motor inside the frame not outside we need to spin it CW. I wired up the motor exactly to the recommended diagram which meant we were operating the controller in reverse. Either due to a reduced speed reverse algorithm or a significant timing bias in the motor the kart ran far faster backwards than forwards. Which is somewhat entertaining but mostly stupid.
• Fusing Stupidness- Before even putting the kart on the ground we blew the $20 150A fuse I had just bought. Not sure exactly why, given that it’s a 250A controller and slow blowing fuse- I suspect it had to do somehow with the cogging problem. It’d be real nice to get that solved- good for the batteries, controller, motor, drivetrain… well everything.
• Parking ticket- I got a big effing parking ticket for being parked on the street at about 12:50. Street sweeping. To their credit they did then bring a sweeper through. But it was one of those great ones where you’re walking to the car as they’re writing the ticket. Ok, moving on…

All of these things were non-optimal but did not prevent driving the kart around.

Then the mishap that ended things for the night- Due to a combination of the sprocket carrier axle-clamping screws not being tight and the awful jerking cogging-torque from the screwy commutation the sprocket holder key jiggled out, then the chain derailed. Casualties- the key was smushed up pretty good, the keyway suffered a bit, and the sprocket carrier became a bit pretzel shaped. Turns out that if a chain in a system without any give in the chain-path length derails it’s going to deform the weakest part of the system. In this case the aluminum sprocket holder. At least it wasn’t the motor shaft (though I should probably check that carefully)- that would be significantly more expensive. So that’s probably one of the reasons kart suppliers sell “sprocket guards” – a bit of insurance against chain derailment. Maybe worth it.

So I will pick up a new sprocket carrier and a new key, this time I’ll get one that is the entire length of the keyway- so that there’s nowhere for it to escape to, and to fortify around the mooshed part of the keyway. Then we should be back in business, for some faster gear ratios and higher voltage…