In Search Of Cheap Fast

Since last time’s disappointment with the performance of the “Alt-Kart” set up with Mars BLDC motor and malfunctioning (now dead) Sevcon PMAC controller it’s been on my summer list to make the kart fast- not perfect, just cheap dirty and fast. To that effect, and to celebrate the ending of summer vacation (classes start tomorrow, Monday) Micah and I got together on Friday to make it happen.

I picked up a new sprocket hub and keystock from Comet to get the chain drive back in working order.

I had to use a puller to remove the old sprocket hub, you can see the gouges where the deformed edges of the axle keyway scraped through the bore.

I filed the smooshed edges of the keyway to bring the OD back in under 40MM, you can see where the damage was. I cut the new key to fill the keyway end-to-end, so I was not concerned about the torque-transferring ability of the connection.

All fixed. The new sprocket hub has a much improved clamping design than the old one, the whole assembly feels very tightly put together. I can’t imagine it coming apart in the way the old one did.

Micah had this D&D 6.7″ series wound motor from his electric RX-7 project earlier this year. We judged that it would be a highly suitable motor to take the kart to stupid levels of quickness, with the added bonus of sliding in and bolting right up with the same pattern and shaft as the Mars BLDC. Of course it weighs three times as much and lacks the convenient axial flux design, so all that weight is on a significantly longer lever arm off the thick but admittedly not-designed-for-stiffness mounting plate.

In our proud tradition of shady fixes involving lumber, Micah devised a 2×4 strut resting on various chassis pieces and held together with conduit clamps and screws, plus a ratchet strap around the motor. Shady indeed- but it kept the slop in the chain drive from causing the rear of the motor to bounce up and down- score!

After fighting with the hardware configuration of a Sevcon 4qd 48v/300A controller for a while (even with the *expensive* calibrator dongle there are a bunch of settings you can’t change without a secret password) and indeed seeing it sort-of work we settled on switching it out for the simple, brutish but eminently functional Alltrax. You may note the toasted main fuse- it’s actually still intact, and after blowing our nice looking fresh fuse in a regrettable flying-lead incident it got the job done.

With the Alltrax’s simple requirements we ended up driving the Kilovac contactor with an entirely independent 12v supply composed of four (counterfeit) A123 cells. Just like the Killacycle! Except theirs are doubtless genuine.

At this point we took it out for a drive on the same 36 volt set of batteries that were installed with the ill-fated brushless system. Aside from issues with our hose-clamp jury rigged throttle pedal slipping around the results were quite positive- good pickup and speed, remarkably quiet and smooth.

But as I assured Micah, it had to happen- we strapped another three battery packs onto the kart for a full 72V.

With about 180 LBS of lead batteries in tow and somewhere around 40hp peak the kart was entirely transformed. I doubt I actually ever hit full throttle on account of the acceleration being so incredibly strong. Any easy jab of the throttle resulted in instant neck-snapping torque-thrusted acceleration. It felt quicker than the Tesla Roadster I rode in in July. I was even impressed with the handling and steerability. The steering and chassis felt incredibly direct and communicative. Maybe it’s just because I haven’t driven karts before, but I loved it. It didn’t have much trouble with scrubbing on tight turns even with the immense extra rear-biased weight. The good design of the chassis really shows in that regard.

We ran the motor up until the speed leveled out with the wheels off the ground- the axle was spinning about 2700 RPM max, corresponding to somewhere around 90MPH ground speed. I think I only hit 30 or 35 in the tiny space we tested in, but it accelerated extremely responsively at any speed. In trying to hit the highest speed I could I found myself unable to keep on the throttle for any more than a few seconds at a time without the kart starting to lose traction and get squirrely. If only we had access to a larger flatter area to test drive…

I was curious about the slidability of the kart, so I turned hard and laid on the power- it started sliding beautifully for a second before ping-whizzzzzzzzzzzzrrrrrrr the chain popped off. I figure the combination of the new chain stretching, the overall slop of our setup and the chassis flex in the hard cornering conspired to allow the chain to hop off- fortunately unlike last time there was enough slop that the sprockets, shafts and hubs seem no worse for wear. At this point it was getting late, we were satisfied with our results and didn’t want to push our good luck with not getting in trouble further so we took the kart apart and went to go get some cheeseburgers.

Of course, now that I’ve had a true taste of just how incredibly fun this kart can be to drive I’m seriously looking forward to doing it better and more permanently- but for now I’ll be storing the stripped kart to work on other things. It’s time will come.