11 November 2012

MelonChopper: a High-Performance Reverse Trike

Ever since the end of 2.007 (2.00Scooter) and the Cruscooter build, I've been looking for an excuse to put the experience I gained in designing an electric vehicle to good use. In 2.007EV, I had the opportunity to test out the motley array of vehicles my peers came up with. 
"Splinter Cell", Credit to Tyler Hamer (tyhammer@mit.edu)
Some of my favorite designs were my friend Tyler's wooden reverse trike (fun to drive and incredibly stable, but was severely underpowered and "steered like a battleship"), the melon*-powered Brickscooter my friend Bayley built (a scooter with a custom-built high-power motor controller), and the impressive David Wise/Jacqueline Sly Melonkart. (as the name suggests, it is a gokart driven by a melon motor). Also in the mix were class instructors Shane Colton and Charles Guan's TinyKart and Chibikart, respectively. 

*Melon: a Turnigy brand 80mm diameter class brushless outrunner motor from hobbyking.com, capable of outputting about 7kW of power. 

I wanted to combine the designs: make a gokart-style vehicle powered by a melon, with a custom controller, that happened to be a three-wheeler. 
Because reverse tricycles are freaking badass
The Reverse Trike (Probably the incorrect name, usually actually called "Tadpole" configuration) gokart geometry is pretty much just like that of a regular 4-wheeled gokart. The front two wheels steer just like a normal 4-wheeled vehicle, and instead of powering two rear wheels through a differential (Which is a complicated device to make or find), there is a single powered wheel in the center. This offers both simplicity and stability, though you still need to deal with two-wheel steering, which is never trivial. 

This is WAY more stable compared to a regular tricycle geometry (Also referred to as Delta geometry), where the rear two wheels are kept facing the same direction (and can be driven through a differential if you'd like) and the front wheel steers. Assuming your configuration contains no leaning (which would mitigate these issues somewhat), if you build up enough forward momentum and want to make a turn, there is the possibility of tipping over the vehicle the opposite direction of the turn. 

An excellent analysis of Delta vs. Tadpole trike configurations can be found here: http://www.jetrike.com/tadpole-or-delta.html

Here's what I came up with. The six A123 ALMs are there for no reason other than to get an estimate for the storage capabilities. I first put together the basic structure, defining parts to pretty imperial lengths with little though, making this preliminary splatter of ideasauce more art than engineering. The wheelbase is reasonably wide for stability while keeping the 8020 happy. 

I used 10" pneumatic tires instead of the trusty 8" ones on my scooter to give me a bit more added traction when maneuvering at high speeds. The tractor seat is one I found randomly on MIT campus for free. The 8020 framing is all held together by waterjet aluminum plates a la Chibikart and Tinykart. 

Here you can see the basic steering mechanism and uprights close up. The uprights are pretty much copied from Chibikart's: stacked plates enclosing a steel hex cap bolt (which is the axle). It's a solid design, so I just scaled it up to work with the larger 5/8" shafts required for my 10" wheels. 

Now I needed to work on the steering geometry. For most cars/gokarts, Ackerman Steering is used to make sure both wheels experience minimal slippage (translating to damping losses and wear on the tires) when turning. To get the whole Ackerman Steering geometry going, I used the following simple diagrams off the Wikipedia page: 
See how the front wheels are angled slightly differently?
Thank you Wikipedia for pretty and useful diagrams! 
This tells me that when the kart is steering straight forward, the line created by the steering pivot point and the tire rod pivot point should pass through the center point of the rear axle. To get this working with Melonchopper, I temporarilyforced the front wheels to be parallel and face straight. I then computed the required angle needed for the above-described line to work.

Now it was time to really flesh out the steering. Here you can see my initial Designarrhea, before my hours of tuning which actually took into account the fact the the vehicle's operator has legs. I took my "no need to reinvent a perfectly good wheel" approach even further, basing the steering column/tirerod assembly almost directly off of Shane Colton's TinyKart. 

However, rather than decouple part of the tire rods to certain axes, which Shane did to allow part of the tire rod to remain completely planar and pass beneath Tinykart's frame, I opted for a single set of tire rods which passed under the driver's legs. The lower and more planar I could place the tire rods, the better, because less nonlinearities show up in the steering configurations. Remember, small angle approximation! (See the link to Chibikart above, where the steering linkages are almost exactly planar.)

Melonchopper, now with More Legit Steering.
With some tuning, I was able to get the steering wheel in a comfortable position while removing much of the nonlinear behavior usually seen in this not-planar nonideal steering configuration. 
Driver's view. Pretty sweet looking for my derpy design process, right?! 
To test the minimum turning radius (sharpest turn I can perform) and whether or not the steering behavior remained consistent throughout the entirety of the turn, I took the long plates in the uprights and mated them to the kart's structure, where they would hit in real life to limit the steering. 
Turns out I can steer pretty sharply while still maintaining the behavior I desire! One issue both Melonkart and Splinter Cell had (to different degrees) was the steering geometry "toggled" to a strange, nonlinear configuration before the uprights hit the physical limits of the geometry (usually the outer wheel was angled more than the inner wheel, which we lovingly dubbed "Anti-Ackerman" steering in 2.007). In my steering configuration (which, admittedly, took HOURS of tuning to get right), the toggled configuration only exists beyond the physical limits of steering imposed by the kart's frame. Therefore, no Anti-Ackerman mode for Melonchopper! 

The above picture also accurately demonstrates the usefulness of the Ackerman steering geometry: note the left(starboard) wheel is angled a little more than the right wheel. 
More POV steering action!
He's going to KILL me for posting this pic :p
Oh yeah, there's a whole other side to this project, the Motor Controller! This enthusiastic fellow is Bayley Wang, fellow MITERS denzien, taker of 2.007EV (He made Brickscooter) and self-proclaimed high voltage expert. Just look at this cute Tesla coil he and some friends designed, meant to be a coil design highschools everywhere can easily use: 

Bayley is designing the motor driver with one thing in mind: 7 KiloWatts, which is the full rated power of the Melon motor. Why is this a YAMEB-Certified Big Deal? MITERS has never ever seen the full power of a Melon motor, and this is the perfect excuse to make the 70-volt 100-Amp motor controller we have all been dreaming of. With my mechanical design and his electronic magicking, Bayley and I make an excellent team.    

Just to demonstrate how much of a Big Deal this controller is, let's plug it into WolframAlpha with a 15-tooth sprocket on the Melon and the stock 55-tooth sprocket available on the Razor E300 10-inch wheel we're planning on using.

Mother of God...
So, after some highway cruising, we'll stop in New York City for a Broadway Musical, then Of course, we don't actually plan on using this gear ratio. We ended up with something like a 9/110 gear ratio, giving us a ~35MPH top speed and 2MPH minimum speed. And SHITTONS of torque. I'm seriously dreading the day I do my Garage Run. 

Oh and we got full ($500.00) Techfair funding for this project. Which is totally enough to get all the parts and stuff. Booyah I can't wait to do some snow drifting this January!

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