Small Car EV

Power wheels overhaul to compete in Power Racing Series

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An old mobility scooter hacked together with some RC components, go-kart parts, and some other random electrical components to create a grown-up power wheels.

The Power Racing Series limits competing vehicles to look like a "children's ride-on toy", operate on less than 48V with a maximum power input of 1440W, and cost around or less than $500. There are some other rules regarding safety and other build conditions, but those are the main ones. The features I've planned for the final result have changed over time due to budget constraints, scope-creep realization, and my desire to just get this finished because it's been in limbo for a long time. The unique features have boiled down to capability to remove the steering wheel and control the car wirelessly, "drive-by-wire" brakes (to allow for steering wheel removal), and enough infrastructure built in to allow for bolting on components for autonomous driving in the future.

  • Power Transmission is Iffy

    Eric04/21/2020 at 14:42 0 comments

    I finally switched out the chain and sprockets between the motor and transaxle for a timing belt and some pulleys. Although it's much smoother, I'm starting to think I still won't get as much speed as I could. The transaxle came from a mobility scooter and obviously wasn't designed to go 20mph, and it's starting to show. I haven't bought the 36V battery pack to test the whole drive system on full-power so I still have some hope, but I think there's a good chance I'll have to find a replacement that can do what I want.

    A new rear axle would also most likely give me some more space for a brake system that doesn't have to sit so close to the wheels. Currently, there's almost no room for the tires to flex.

  • Steering Wheel is Somewhat Functional

    Eric01/21/2020 at 03:50 0 comments

    The steering wheel has made some progress after a long time CADing and re-CADing, printing out pieces for sizing, and testing how it feels in my hands. It is now physical! I do have a regular sheet metal steering wheel that I bought online for cheap, but it's a little big and I couldn't fit as many bells and whistles as I wanted on it.

    I had a few iterations of shoulder levers that didn't feel right. The biggest problem was finding the right shape to get a good amount of contact with my pointer fingers while also allowing for a significant range of motion. In the end, I went with a simpler straight-bar shape but that extended a little bit deeper int the z-direction to contact more of the width of my finger. The metal tubing for the grips and torsion springs for the shoulder levels all came from a rusted out bike at GoodWill that I bought for a couple bucks.

    The electronics aren't anything special, there's an Arduino micro (cuz it's easy), a CAN bus breakout for sending info around the car, the pair of potentiometers for braking and accelerating, and a key switch. I don't exactly know what the key is going to do yet, I'm waffling between enabling acceleration and toggling maximum speed. If I have some extra time at the end, I want to put a speaker and amplifier somewhere for a cool horn and also some engine revving for when the key switch turns.

    I do have a lid CADed out but I'm waiting to print until I'm sure the contents of the wheel are final for this first iteration. I've toyed around with the idea of a dedicated button for the horn as well as some status LEDs to help debug.

    Here's another picture from the back. It's cool I guess.

    Yes, everything is still PLA. I know that PLA isn't great for things that take any sort of beating but PLA is cheap and super easy to print. Once these initial PLA parts start breaking, I'm going to replace them with PETG and see how it fares. I haven't ventured outside of PLA before but I'm sure I'll do so by the end of the first test drive.

    Also on the way: a new 1st stage speed reducer! The chain drive, as pointed out by others, isn't great. It's loud, spews grease everywhere, and gets hot real fast. I ordered some timing pulleys off of Amazon and am hoping they can more gracefully handle the high output of the motor.

  • From Inception to Now

    Eric11/28/2019 at 05:17 0 comments

    This blog is being transferred from my own website to Hackaday so this post is going to serve as a summary of progress from inception to present-day.

    Earliest post

    Since I'm starting from scratch and I don't know anything about go-karts, I decided to start out by putting together a BOM of obvious components and start tweaking in the direction of what seems like success. I found a really nice sale on some slick wheels at BMI Karts (along with a bunch of few other things).  I was also having a lot of trouble finding a Power Wheel and a cheap mobility scooter to salvage parts from and, as soon as I put out a notice on Facebook, I found a good pair of deals on Craigslist for exactly a Mercedes Benz 300SL Power Wheels and mobility scooter. High quality German engineering.

    After what seemed like forever building up a bill of materials, I finally ordered the first batch of parts. I would have spent longer designing and measuring stuff out in CAD, but BMI Karts was having a pretty irresistible sale on some stuff I would eventually need and sometimes you just need to have the parts in your hands to get a better understanding of how everything should go together. I got a set of wheels, a steering column, steering spindles and brackets, a pair of tie rods, a steering wheel, and a pair of pedals. After receiving the parts, the steering wheel is probably too big and the pedals too heavy but I paid $4 total for both of them so I’m not crushed.

    Instead of pulling apart the car right away, I decided to piece out the scooter so I could figure out what parts I’ll be able to use. All the scooter yielded was a transaxle and a pair of two-piece rims, but that's all I was really hoping to get out of it. The wheels I bought online are all free wheels so the plan is to pull the tires off of two of them and put them on the scooter rims. Luckily, the tires happened to be the exact same size that the scooter used which was pure serendipity and something I did not plan for. Very neat.

    The Power Wheels I bought was way too small for a full grown man-child as myself to fit into, so I had to do a bit of modification. I put down a tape outline of the planned wheelbase of the car on my floor and sat in it to get an idea of how big everything would need to be. The dimensions I had come up with seemed to be just big enough to fit me but the Power Wheels was much too small to wrap around the potential frame.

    I initially cut out the base of the body to see if I could fit inside without the seat and floor to no avail so it turned into a full on quartering. I stitched the thing back together with some good old cardboard and duck tape so its width fits nicely around the salvaged transaxle and the rest of the shell lined up with the lines taped on the floor. The next step with the “biggening” of the body was to get some bondo or modeling clay and fill out the holes so it's smooth and streamlined again.

    Making everything nice and pretty took much longer than I had hoped. I severely underestimated the amount of putty-ing and sanding and re-putty-ing and re-sanding that’s required so the process went on for quite a while. The detailing power sander had become pretty much my best friend along with second best friend belt sander. It’s super satisfying to just hold a power tool against a rough patch of Bondo and watch it turn super smooth.

    It took forever, but I finally got a smooth body over which to apply a fiberglass shell. Creating the buck for the fiberglass was time consuming, but fiberglassing is probably one of the worst things ever. The resin is sticky, dissolves everything, and just ruins everything it touches. Fiberglass frays everywhere and sticks to everything via splashed resin. It took several grueling hours of brushing and tapping resin into every dry crack of fiberglass but I finally finished and will never do that again unless it's absolutely the only answer.

    I ordered some of the major electronic components...

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Teslafly wrote 11/29/2019 at 19:40 point

Your gearing setup greatly concerns me.

you know the AquaStar has a max speed of ~30krpm right? With that chain you are certainly exceeding the maximum linear surface speed spec by a lot.

I would recommend somthing with directly meshing gears, (like an angle grinder gearbox, Ala mini mukivan) or a belt at the very least (though you will likely still violate the max surface speed, just by Less.)

The body looks like it's coming together real good. Hope to see you at a race this next season!

  Are you sure? yes | no

Eric wrote 12/01/2019 at 05:21 point

Yeah the plan is 36V in Y config, so it would be at 26,280 in a perfect world. I never considered the chain having a speed rating, but you're undoubtedly right about exceeding whatever that is.

I have this 36V/12A power supply I've been using right now in testing which nerfs the motor pretty hard so I guess once I get the electronics set up I'll go back and redo the drive train before hooking it up to batteries.

Thanks for the heads up!

  Are you sure? yes | no

Teslafly wrote 12/01/2019 at 07:55 point

If you are pulling the full 12a for in the air bench testing to the point you can't reach full speed you have really high losses in your power train setup. With a top speed of ~22 mph I draw 6ish amps at 48v to spin the wheels to full speed.

That may improve once you get rid of the chain, but those losses don't decrease when driving under power, so you can think of it as whatever power it takes to spin in the air at a certain speed, won't be going to the ground at that speed when driving.

Also I recommend a vesc for a speed controller (6.0 hw deravitive, stay away from 4.2), as it will do current (aka torque) control for you. Unless you plan on using an Arduino and current sensor with your esc to approximate that.

One last thing. For a perminant setup, stay away from 3d printed bits in your drivetrain. They are good for testing, but every time I've tried it, it has blown up sometime on race weekend. People forget that races aren't bumpercars. Other places in the cart are fine and great though. Except steering.

  Are you sure? yes | no

Eric wrote 12/01/2019 at 18:12 point

I haven't tried running it free-spinning, that would probably be good to do. The VESC looks cool, but the site says it's $245 which is almost half the budget, I got my current ESC heavily discounted from the PRS Facebook page. I knew going into it that 3D printed parts wouldn't last long, they were more for placeholders until I figure out a better way to replace them. I have no experience with this kind of stuff so the goal is to just get a full system set up that works at all and then watch it break and improve each component from there.

  Are you sure? yes | no

Teslafly wrote 12/02/2019 at 05:39 point

This vesc based esc (fsesc6. 6) is currently on sale for $99 and is easy to add a fan to. Was on sale for $80 a few days ago.¤cy=USD&gclid=CjwKCAiA5o3vBRBUEiwA9PVzanRonXiCdxkaGWN0T5tpJqMgSOp0CKveUcmBYGqJKjXYHwDN63m_fRoCp44QAvD_BwE

As for the prototyping, that's how I built my cart too. And why the frame is wood. I just never got around to making it not wood. And it works well enough. I'm the mystery machine if that rings a bell. Been doing this for 3 years now.

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