Off Road Electric Kick Scooter Build

[Technochicken]

I mentioned in x88x's E-motorcycle build thread that I was working on my own small personal EV, so here's my progress so far.

As with many of my projects, I got most of the parts first and then found a project to do with them, rather than designing a project and then getting the parts for it. Here's the background of how I got the parts.

Five or six years ago, my school's robotics team used to do a competition called FIRST. Now, we no longer have the budget to do that competition, which means that we have tonnes of left over parts from the old robots, such as motors, motor controllers, gearboxes, and miscellaneous other parts like bearings and scrap metal. The teacher that is in charge of robotics wanted to clean out his classroom this year, so he let the current members of the team scavenge any parts we wanted from these old robots. So that's how I got most the core components of this EV for free.

The platform of the scooter is based around an old aluminum Royce Union kick scooter I had lying around. It is very similar to Razor style scooters, but with built in front and back suspension.

Here's the scooter I started with:

Here are the new 12.5" wheels. I was originally going to get 10.5" wheels, but I found these epic snowblower tires that fit the bigger wheels. These tires are what I will use for serious off road/snow.

And here's the basic layout. Yes, over a foot of clearance may be excessive.

To fit these massive new wheels, I had to completely rebuild the fork and rear suspension assembly. I built the rear wheel assembly out of a 4" x 1/4" aluminum bar I scavenged, some 2" x 1/4" aluminum bar, and some 3/4" U channel. I also bought some 6", 1500 lbs/in shock absorbers meant for mountain bikes.

Here are the two new shocks compared to the old one:

Here are the swing arm assemblies:

Attached to the scooter platform:

Here's where the wheel will go:

The fork had to be completely rebuilt as well. I started out by taking apart an old mountain bike fork. I removed the springs and dampers from the legs of the fork, and cut off the legs. In their place, I bolted pieces of 2" x 1/4" aluminum, to create a leading link type suspension. I built new shock absorbers out of steel tubing, steel rod, and the old springs from the mountain bike shocks.

MTB fork on the scooter:

DIY shock absorbers:

Fork:

With a wheel:

I cut dropouts for the wheel, so it can be removed easily (I did this on the back as well, but don't have pictures right now):

To make sure the handling was okay, I pushed it around the driveway a bit:

As you may be able to tell from the pictures above, I had to bend one side of the rear suspension assembly to get the rear wheel's sprocket to fit. I did this by heating the aluminum with a torch along the bend line while it was clamped in a vice.


Now for the motor assembly. To power this vehicle, I am using 3 2.5" CIM motors, which are the standard for FIRST robotics. At 12V, which is the limit at FIRST, they only put out about 340 watts of mechanical power at their peak, which isn't very much. I'll be running them at a little over 18V, so I'm estimating that I'll get approximately 750 watts out of each motor (1.5x the voltage, 1.5x the current) That means I'll get a total of 3 horsepower. Now, I could buy a cheap brushless motor and get twice that for 1/5 the weight, but then I would have to actually buy the motor along with a motor controller, and from what I've read, hobby brushless motors/esc's are extremely finicky in personal EV applications. For motor controllers, I will be using 3 IFI Victor 884 controllers. Although these are listed as being max 15V 40A controllers, they are in fact underrated. They are identical except for the microcontroller to the older Victor 883 controllers, which are rated at 24V and 60A. So they should be fine at 18V

Since CIM motors are high speed (about 450 rpm/volt), I built a gear reduction out of two gear boxes I scavenged from school. The gearboxes were originally 2 stage, but I removed the second stage, as there will be additional reduction from the chain drive. The original gearboxes were designed to hold 2 motors each, but since I am using 3 motors, I chopped up the gearboxes and spliced them together into a 3 motor 14:50 reduction. I fastened everything together with this stuff called Alumaweld, which is basically a brazing material that works on Aluminum.

Here's the gearbox:

Here is the inside. I still need to buy one more 14 tooth motor gear:

The whole unit:

That's all for now.

[x88x]

Heheh, nice.

A few questions:

What thickness is the aluminum that you are using for the front and rear forks? The motor plate?

I would be a bit concerned about the controllers, just because I am skeptical how well that fan can push the heat away from the transistors/mosfets without a heatsink on them. This is the point of failure I have seen most commonly in people who have used RC motors on small EVs; they try and use an RC ISC, and it can't handle the heat of the continuous load and blows up. I don't know about Victor in particular, but a lot of these manufacturers rate their stuff to a peak current, or an occasional load. So, they might be able to deal with bursts of 100A just fine, but expect to be at 0 load an equal amount of the time. The problem with dropping that into an EV is that they will instead probably not hit the peak very often, but will spend most of the time at something not 0. So a controller rated to 100A in RC applications will fail utterly in a small EV application where it has to deal with 15-20A maybe 80% of the time.
I know you don't want to buy anything, but it might be worth looking into something like the below. Based on the prices I'm seeing, you could probably sell those Victors and more than cover the cost. Going that route, I would connect the motors in series, use one of the below controllers at ~72V, and run with it.
http://kellycontroller.com/kds721001...ler-p-396.html
http://kellycontroller.com/kds722002...ler-p-288.html

Also, I'm sure you've realized this already, but those tires are going to be incredibly inefficient. Should be fun though.

I look forward to seeing this get moving! Any thoughts yet on batteries?

[Technochicken]

Heheh, nice.

A few questions:

What thickness is the aluminum that you are using for the front and rear forks? The motor plate?

Both the forks are made from 1/4" thick aluminum. The plate used for the rear forks is 4" wide, and and the front is 2" wide. It should be way more than strong enough, especially since I weigh under 140 lbs.

I would be a bit concerned about the controllers, just because I am skeptical how well that fan can push the heat away from the transistors/mosfets without a heatsink on them....
http://kellycontroller.com/kds721001...ler-p-396.html
http://kellycontroller.com/kds722002...ler-p-288.html

I've played around with the idea of using a high voltage controller and wiring the motors in a series, but I think I'm going to at least give the victors a shot. I have six of them in total, so even if I toast one of them, I could probably sell them and get a Kelly. I've seen the Victors used on robots that weigh more than I do, so hopefully they'l handle the abuse.

I look forward to seeing this get moving! Any thoughts yet on batteries?

I'm definitely going to use LiPo's, but I'm not sure what configuration yet. It depends on how much space under the platform I end up having after getting the motors and controllers mounted.

[TLHarrell]

Not sure about the geometry of the front end. Since the wheel is forward of the center of turning, it's likely to want to swivel around 180 degrees (like a caster on a shopping cart). I would at least add some width to the handlebars to help counter that. Still, just looking at it scares me. I had a gas powered one that somebody gave me. Sold it once I got it working. Too much likelihood of eating pavement real hard.

[Technochicken]

I the low-speed tests I've done, the steering geometry has not been a problem. Because of the kick scooter frame I used, I had to choose between having a decent amount of suspension travel and having the wheel set back closer to the steering axis. For better or worse, I chose more suspension.

[nevermind1534]

Those CIM motors and gearboxes look familiar.

[Technochicken]

I've attached the motors and gearbox to the scooter platform using some 1" angle aluminum and my last left over bit of 2" x 1/4" bar.

I'm thinking about making a new platform to stand on that is wider then the current one, and just bolts on top. A slightly wider platform should give me enough room underneath to fit 3 5Ah 5s LiPo packs side by side. I could have two rows of three without the batteries extending below the bottom of the gearbox, giving me a 18.5V, 30 Ah battery pack. If I chose to get a single controller to run all three motors in series, I could easily reconfigure the pack to be 55.5V, 10 Ah.

[d_stilgar]

Cool stuff. I can't wait to see more.

[Technochicken]

I've pretty much finished the chain routing. Because the drive sprocket is not mounted on the axis of rotation of the rear suspension arm, as the suspension is compressed, the effective chain length is increased. To compensate for this, I built a chain tensioner that can take up the slack in the extra chain required to accommodate the suspension, similar to how a derailleur on a bicycle lengthens or shortens the amount of chain used to allow for different gear ratios. Unlike on a bicycle, where the the chain tensioner pushes the chain outwards, this chain tensioner pulls the chain in, so that the chain does not reduce the clearance under the scooter.

Here is the assembled drivetrain. I still need to borrow a chain tool so that I can remove the extra links and close the loop. The idler sprocket above the tensioner routs the chain under the suspension arm, which it would hit otherwise:

Here is the chain tensioner. I pulled the spring out of an old bicycle derailleur:

Here is a template I made for the new platform:

And here are a couple pictures of the whole thing so far:

The next step is to order batteries, charger, and a few other small electronic bits, and an 1/8" aluminum sheet to make the new deck.

[x88x]

Nice wrap on that chain layout.