Pink Jeep

Vehicle: Random Pink Jeep-like Vehicle for Children

Drivetrain: 72v 3kW Eletric 3 Phase Motor

Current State: Will put a smile on your face

Fun Features:

• Solid suspension to give you a leg workout
• All your weight over the solid rear axle so sometime you have to fight understeer
• 12V Bluetooth Speakers that BUMP

This project was a fun gift for my wife’s birthday (and let’s be real here it was also for me). She’s always wanted something fun and wacky to ride around in since we saw the Power Wheels Racing Series in highschool. Finally acheived 10 years later

One goal with this thing was to be fun and for us that means fast with just a hint of scary. First up was estimating how much torque it would take to keep this thing at a speed by roughing in rolling resistance plus some factor on torque.

Once the torque was figured out, I wanted to find a motor solution that was capable of making sure we were not going to have any issues going fast. After searching around for some kits of motor and controller, I came across a 72V rated 3kW system that also had a motor speed/torque chart. Cross-referencing the chart and the calculated torque gave us a motor speed of ~4800rpm. That was calculated with an approximate ratio and tire size to say we would be able to maintain about 35mph which was plenty fast in a plastic kids jeep.

Now that I had a motor picked out, next was figuring out how I was going to power this whole thing. After some discussions with friends and colleagues I got a lead on a source for recycled batteries from rental scooter like Lime or Bird. They were rated at 36V so that meant I needed 2 in series to get the voltage i wanted. As for current, I knew what type of 18650 cell was in the pack but I was not sure what the battery management system (BMS) was limited to. This pushed me to put 2 packs in parallel to guarantee current. So in total I needed 4 packs wired 2s(eries)2p(arallel) (or 20s8p if you are looking at the individual cells).

The packs came in there original extruded aluminum housing which was a plus for protection and mounting. These packs also contained the original BMS which was a bit of con not only because the limits were unknown, but they required an activation signal to keep them alive. Thankfully I am not the first person to reverse engineer these packs and the serial protocol used to keep the packs awake was known. That also meant there were good solutions for the electronics to communicate and I did not have to spin up my own board.

The rest of the project from there was just a matter of fitting the adult sized powertrain into a childs sized car. The car was tore down to the bare shell. The rolling bits are all go kart components (bearings, wheels, axles, etc.) so first up was to make a steel frame that fit the plastic body and gave me a solid foundation for the kart parts. Once the basics were worked out and the electronics started to come in it was a matter of making them fit. I was hoping to fit the batteries under the hood but they ended up being too large, so that is where all the power electronics ended up. The motor ended up under the seat which is also right above the rear axle. That only left 1 spot for the batteries, fashioned into a belly pan. So those were mounted on the underside.

To fit the batteries, I needed to figure out where the steering shaft was going to come down. Unfortunately for me, visualizing linkages isn’t my strong suite. So after I mocked up the steering for the first time I ended up with a system that was backwards (left turned right). So it was back to the drawing board and about 3 more mockups that corrected the steering direction and gave me the amount of steering change to get it to turn (more on that in the payoff).

So the frame was assembled, all the wiring was done, and it’s finally time to enjoy this thing. And let me tell you it is SCARY in the best way possible.

The understeer was something I knew I would be fighting going into this. All the weight is over a solid 1 shaft axle and the lawn tires are grippy, so if you have traction (like on pavement) steering underpower can be somewhat unpredictable. The steering geometry though allows you to weight transfer to the outside corner when you let off and turn on a dime (short wheelbase).

The power. I knew it would be fast, but I was shocked when my math was close. Having not weighed the vehicle or verified any of the tire size, I am able to hit 33mph on flat concrete. It accelerates hard and my wife (who ways less than me) is able to lift the front wheels up on a launch.

Where this really shines though is in dirt/mud. Growing up, we always had toys at my parents we drove around (4 wheelers, dirt bikes, etc.) so that is our preferred location to have some fun. In dirt, the understeer is rarely there and while being solid suspension it really isn’t that uncomfortable to ride even if you catch a little air. You get a nice leg workout though.

Overall, this is by far one of my favorite projects I have ever worked on and I hope to get a few good years of fun hard use.