The final work and testing on my Mëtal Scooter project has been very productive; especially in highlighting those areas of the design that need improvement. As only my second electric vehicle (and really the first meant to actually be regularly ridden on), Mëtal Scooter has been an highly educational project from start to finish for me. Maybe at some point in the future I'll be able to use these lessons as inspiration for a version 2...
Continuing from where last update left off:
I finally have a front wheel now, complete with an mega-sized aluminum fork! After getting more cutting fluid for the mill, the ShapeOko cut out the other half from last update. This is obviously a much larger wheel (8") than the Razor was ever designed to use, both in width and diameter. Conveniently, the addition of the aluminum plates to the outside of the existing fork mount was just wide enough to give the wheel clearance. The axle is a simple threaded rod with end cap nuts like the back wheel. This front end actually seemed to come out pretty well. The wheel+handle has room to turn 360° without getting caught (not that I intend to turn that far while riding).
Initial non-powered push rides with this arrangement quickly revealed the first design problem. The force holding the platform and rider to the main chassis is focused on 4 machine screws near the front. My brief and non-scientific test ride immediately lead to the stripping and pulling out of all these joints. While the riser to deck connection is more than sufficient for preventing carbon fiber flex and keeping pieces from falling apart, it isn't enough to support a rider. My first attempt to remedy this problem was a redesigned handle mount plate:
This new plate allowed it to spread the load over twice the number of screws, as well as reduce the deck flexing. Unfortunately, the 8 screw arrangement failed just as much as the first. For the moment, my solution has been to stick a C clamp on the front end of the scooter during test rides to keep this from coming apart. I've recognized the foolishness of trying to support my weight (and potentially many times more, especially given the force from going over bumps or small drops) on small screw threads. The two solutions I'm considering right now are bolts going from the top plate to the U channel aluminum beneath, or some kind of aluminum retainer piece stretching around the front or sides.
With the front end no longer falling off, I was able to take a short powered ride. It became immediately clear that the friction fit between the sprocket and rear wheel bore was not enough to keep it from slipping under medium acceleration. Instead of messing around with trying to create a tighter fit that would eventually slip at some future point, I just put a few threaded rods between the sprocket and wheel to ensure they would rotate together:
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| Two of the three rods can be seen here |
With the scooter in a ride-able state, I gave it an inaugural test by flooring it up and down a hill a couple times. Due to the brushless motor and sensorless speed controller, it was necessary to give it a good push-start before using the throttle. Once it was moving though, it rode well and was easy to control. I was concerned that having the rider so high above the ground would make balance a challenge, but this didn't seem to be the case. After another half hour of hill riding, I found that the scooter seemed abruptly less powerful. I brought it back in and checked out the battery by plugging it into the Smart Charger.
In short, it turns out that I managed to turn my 8s LiPo into a 6s LiPo according to the charger. After kicking myself for being so stupid, I checked out the damage by measuring the voltage on each cell individually. One cell read 0 volts, all the other cells read in the low 3 volts. The 0 volt cell was a lost cause. By slowly charging the rest of the pack at 6s, then re-charging it as 7s however, I was able to bring it back to a 7s pack. I think the cause for the problem is probably twofold. First, driving it irresponsibly up and down a hill until the pack was nearly drained was a terrible idea with LiPo batteries. Unlike the lead-acid batteries I'm used to, this probably caused some permanent damage to the cells. Secondly, back when I was initially soldering connectors to the battery leads, there was a quick short circuit when the two wires simultaneously brushed up against the motor exterior. I didn't detect any problems back then, but that might explain the totally dead cell.
I think I can live with the 7s pack for a little while, so until it proves problematic I won't order a new 8s. In the meantime, there are only a few small pieces to finish until the scooter is done and it can get a proper testing run.
