Power Wheels Cars

[Jeremy_BP]

Hi all,
I've been gone a while, but I've been busy.

I'm involved in the power racing series (PPPRS). We take kid-sized ride-on toys, re-build them, and race them (the goal being to have a good time more than win). It's been an excellent test of my fabricating skills, and now that the season is wrapping up and I can think straight, I thought it seemed like something a lot of you might be interested in. There's a different race in October of this year, and I'm building a car for it. I'm planning on documenting the whole process (kludges, zip ties, fires and all) here if anyone's interested.

A little background from this year. My hackerspace (Pumping Station: One from Chicago) build a car with a "Blues Brothers" theme. It started off like this.

The flashing lights worked, but it only had a top speed of 3(ish) mph. Something had to be done.

We took off and saved the body, then threw away the original "frame," wheels, and motors. We sketched out and then welded together a new frame (0.5" square tube).

I machined a 2-speed transmission for it:

We put the whole thing back together, and took it out to the races.

A few specs:
Rear Wheel Drive
Rear Disc Brakes
2200W Brushless DC Motor
2-Speed Transmission
Top Speed: 22 mph (in first gear)

If anyone has any questions, ask me anything. I know this isn't really machining-related, but I thought you guys might get a kick out of it.

Cheers!

Edit: formatting

 

[Darren English]

That looks like great fun.

 

[rhitee93]

Oh, now that looks like a good time!

 

[Jeremy_BP]

Here's a video from our most recent race in Detroit. We finished 8th.

[ame]http://vimeo.com/46605204[/ame]

 

[Admin]

That looks like a lot of fun. How did you get started?

I have a friend who rebuilds lawn and garden tractors to race. Aside from the fact you fabricate it seems to have a lot in common.

 

[Jeremy_BP]

Awesome. I've always thought that those people were a little nuts. Then I started this.

Most of the teams that race are from hackerspaces (communal workshops). After I joined my local hackerspace (Pumping Station: One in Chicago) I just joined because it looked like fun. And because the biggest machines we had at the time were a sherline mill and a 7x12 lathe. And because I'm short and actually kind of fit into the cars.

Going back to the hackerspaces, I encourage everyone who has one nearby to stop by and visit. The people are always fantastic, and there's always something neat going on.

Also, if anyone has any ideas about why I have this annoying thing (at the end of my post) appended to my posts, I would appreciate it.

 

[dsquire]

Jeremy

I have removed the appendages from both post for you. I don't know where they came from but they were annoying. When I first logged in to the first post the screen was about 4 screens wide and it took several minutes until it transitioned back to a single width. I thought it might be from the video formatting but it seems to function OK without it. If you need more help just shout.

Cheers

Don

 

[Jeremy_BP]

And a quick update.
Here's the video from the Chicago race.
[ame]http://vimeo.com/47237915[/ame]
We took 2nd place in the road race, won the moxie for the weekend, and ended up in 5th place overall for the series. I'm pretty stoked, and now that it's over, I can start working on the car for the October Classic.

And so here's the bit where I ramble on about design ideas. It's my version of throwing everything at a wall and seeing what sticks. But first, about the race. Here's the course.

It's a drag race with things in the way. Each obstacle is a box 7" deep full of mulch, water, sand, and gravel. There's a fairly steep ramp leading into each box. So you need to be able to go like hell, but the car will self destruct going over the ramps if you hit them too fast. Therefore, acceleration is key. And so this leads me into the star of my design, the motors.

When I built the drivetrain for the bluesmobile, I used a 2200W brushless airplane motor. It had plenty of advantages and disadvantages. Brushless motors have an extraordinary power density. The motor I used was barely the size of my fist. But with that much power in that small a space, I ran into thermal issues very quickly. The motor is designed to be air cooled by the prop pulling the plane along. In other words, it needs 50+ mph of airflow to stay cool. I had next to none. As a consequence, until I got a cooling system worked out, I melted 5 motors. Eventually, though, I hacked together some water cooling and everything was just peachy. But in the end, especially for a race like this one, the advantage of the brushless motor is the torque they can provide. Even with such a small motor, the bluesmobile accelerated hard enough to really press you back into the seat. It went from barely moving to top speed (~22 mph) in less than 30 feet. So it seems like a good idea to use another brushless motor for this build. Except I'll be using a 700W one this time. Just for kicks. And wheelies, because wheelies are awesome.

The other big problem with using brushless airplane motors is that they use sensorless commutation. Brushless motors need to know where they are in rotation in order to commutate. Large vehicle motors usually have sensors to read out the position. Smaller (or cheaper, as the case may be) motors don't. They rely on reading the back-emf from the unused phases to determine position. This requires them to be moving, and means that they can only start when they're either already moving or in a no-load state. The motor can't really start in a no-load state if it's driving a car. In the bluesmobile we had to give the car a small bump to get it going. For this car, that just won't do. So, I'm planning to use a brushed DC motor geared way down as a starter. It will keep the car rolling at ~5 mph at all times so that the brushless motor can get it going. The brushed motor has to be on an overrunning clutch in order to keep it from burning up.

So to summarize that (probably incomprehensible) ramble, I have two motors. One to keep the car moving slowly no matter what, and one to make it go like a bat out of hell. When I have more designed, I'll post it here. Sorry if I'm not making much sense, I'm going straight from my brain to the page. I'd be happy to clarify anything.
Cheers!