So here's what's been consuming a bunch of my time....
An ex-neighbor asked if I could build an electric go kart for her physically challenged (is that the political correct term for today???) grandson. I said yes and the thing became unbelievably problematic. I won't get into all that went wrong, suffice it to say I've probably designed and built about three variations. Here's the latest and last.
Oh, when you look at the pics, some of the welds aren't as pretty as they should be. I had some welder problems and just let a few ugly welds go to save the time of grinding them down.
Ross, shown here in a fitting of an earlier model,
is 18 years old, and that's as big as he's expected to get.
The go kart is electric, powered by three small 12 volt deep cycle batteries which power a 36 volt, 600W (just a bit shy of 1 HP, 746W) permanent magnet DC motor with built-in 6.67:1 gear reduction transmission. The power will be controlled either by a microcontroller and MOSFET H-bridge, which I already have designed and made, but I'm tempted to go all analog, just to show the world it can still be done. The motor speed will controlled by pulse width modulation. Ross can only operate one foot pedal and I've designed that to make the cart go, brake, and panic brake.
When he presses the switch, the control circuitry, be it digital or analog, will soft start and accelerate up to cruise speed. If he releases the throttle pedal, it will soft stop via pulse width modulating dynamic braking. If he panics and presses too hard/far on the throttle, it will sharply engage dynamic braking.
The steering is a true Ackerman configuration. I designed the geometry in AutoCad to get the response to produce the Ackerman steering. Ackerman steering is basically designing linkages to turn the inside wheel slightly sharper than the outside wheel. You can see that in the pictures here:
The final design, morphed through several four wheel variations, into it's final three wheel version.
The reason I decided on a three wheel design is it puts all the weight on the rear end of the kart on the single drive and braking wheel. Plus, it is very compliant with uneven terrain. I started with a rigid frame with the drive wheel on one side as shown in the version with Ross sitting in it. I just couldn't get comfortable with that because the drive/braking wheel would only have half of the rear end weight and it was not compliant with uneven terrain. Plus a one sided drive wheel, which is often found on go karts exhibit a resist/aid in turns. That means when you turn toward the drive wheel side, it resists the turn. When you turn the other way the drive wheel helps with the turn.
Next was a pivoting rear axle which was compliant with uneven terrain but still the drive/brake wheel would only have one half of the weight of the rear end. And it had the asymmetrical steering problem described above.
I didn't even attempt to use a live axle because it resists turns in either direction and that just didn't seem like something Ross could manage. Sooo....the three wheeler!
The two small wheels at the rear don't contact the ground - they're just catch wheels should it tip. If I could have located more weight toward the front, I probably wouldn't have put the catch wheels on at all.
In an early fitting session, I discovered if the steering wheel was positioned so he could reach it from the seat, then he couldn't get into the kart. So I designed this. Here's the steering wheel in it's drive position - notice the little ring just under the steering wheel:
That little ring is a handle to unlock the steering wheel - here it is in it's unlocked position:
This allows the steering wheel to fold forward to this position:
And as you can see, he has plenty of room to get into the kart and seat.
Here's a rear view. You can see the catch wheels, the drive wheel and the 0.8 HP motor and gearbox. I don't have the chain installed.
The wiring and control stuff begins this week. I may get a buddy to help with programming the microcontroller board or make PC boards if I decide to go analog. That will give me time to get the wiring and E-stops, etc. in place.
Just about done - I hope this week finishes it up.