ElectricBikeSquatch
100 µW
What I *want* to build is just an electric tractor out of old truck parts, but I don't have 50kw brushless money, nor do I have '1500 pounds of steel tubing and angle' money, nor even 'an absurd number of kilowatt hours in lithium titanate cells' money, but I do have some junk laying around.
I have a spare set of Toyota Land Cruiser axles sitting and waiting with the offset differentials on both ends, and a 3.70:1 ratio in those pumpkins. I found some half-bald 32" tires on rims on the side of the road with the right bolt pattern a couple weeks ago, and I have a massive overabundance of trees the right size and variety to do some goofy semi-structural stuff with.
My thinking is: 200 Nm at the back wheel on my ebike moves 150kg (330 pounds) of me and bike up a pretty steep incline from a stop.
Just because the motor specs are easy to look up, and they're readily available and pretty cheap, I thought I'd see what I could do with a QS205 (that's approximately what the green disk is sized to match). Since my current and soon-to-be next ebike build are 52V, and I'm thinking of building a 100ah LiFePO4 aux pack (and a trailer to put it and camping gear on), I'm gonna assume this 'stuff I have laying around, plus motor and controller' tractor thing is gonna have 100 amps of 52v available, so I'm gonna call it the '3kw' version of the QS205 that'll do 100A.
Ok, so, the yellow bracketry between the logs, absolutely subject to change, is a little like a cross between a transmission crossmember, a bike's rear dropout, and a bearing retainer for the, uh, single shared driveshaft (light blue).
To turn the shaft with a hub motor bolted there, I'm thinking: CNC-lasered-by-internet-company steel sprocket for the appropriate type of chain (thinking: whatever I can get for cheap which is stronger than 8spd bike chain?) with a 6-bolt brake rotor pattern on it. Or the 3-bolt scooter(?) rotor pattern on some of their housings. I can find the spec for the motor I get and order a sprocket really easily.
That gets a sprocket onto the motor. I think the shaft, modeled as just a single tube, is actually just an ebay/etc. kart 'rear axle' with some tube ending in a flange to match the Toyota input on each axle, so I get a sprocket and some splined or flanged ends to adapt to my tubes. Then, use matching 1" or whatever bearings on the axle, in the downward part of the 'dropouts'?
52 volts into a KV 11.9 motor gives us 618rpms. So, okay, let's just go ahead and call it a 2:1 reduction we need between the driveshaft/kart axle sprocket and the sprocket we bolt where the hub motor's brake rotor usually goes. Works out to 8mph/12.8kph spinning on jackstands without tires, maybe, and maximum torque from a stop to something like 2/3 of that speed. That's also a 7.4:1 total reduction at the wheels. At 100A, that QS205 supposedly makes 182Nm of torque, which is 1350ish at the wheels with the 7.4 total reduction.
That's like seven times what my bike puts to the back wheel, admittedly to about 2-3 times that speed, but also with a ~29" tall tire instead of 32. I figure you could still conservatively move like a full metric ton (including ~360kg/800lbs of axles and wheels/tires) on flat ground, and half that up steep slopes at jogging speed.
I'm thinking that the logs would just be u-bolted rigidly to the axles and further boxed in, and the tires run at low pressure to keep it from shaking itself apart...at all of 4 miles an hour? Typically?
So, okay, if all of that works, you put a cargo platform on top with a spot for the battery pack under it, and you adapt the...brakes and steering. That part is in a space I kinda grok, at this point, for doing manual or regular hydro-assisted controls like on cars/bikes/motorcycles/planes/etc. I know how to use those ideas at least as well as people in the 1920s did. What I want to do is add some beefy linear actuators and an RC rockcrawler control interface so I can drive it from somewhere beside the electric mule, instead of on it.
I don't really know how I'd do the brakes, though. I guess if you use a manual brake master cylinder for an old pickup truck or something, and push REALLY hard on the linkage equivalent to the pedal with a linear actuator, it's probably gonna give similar pressures to the wimpy vacuum boosted power brakes Toyota put on the truck that these axles are a spare for. It also has some mediocre but workable parking brakes on the rear drums, so I theoretically have those as, well, an emergency brake which can keep a 2000kg vehicle from rolling when parked. Maybe I use a beefy brake on the rotor on the kart axle for a central drivetrain brake, too? Oh, hey, and it would actually regen brake like a monster for coasting downhill, right?
That's a lot, I don't blame anyone who doesn't read all of that, but I hope I at least amuse somebody with my baby's-first-CAD work. Right now, I'm thinking I may have to get into RC crawlers just so I can do practical scale models and not be totally out of my element when it's time to make something the size of a horse without direct controls...
Last edited:
