Yesterday, I took my new trailer design out for its first road test. You won’t believe what happened next!
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Learning to fail is a critical step on the road to success. I learned something yesterday. To explain what happened, allow me to recap some highlights from my solar bike trailer failures.
This was one of my first attempts at using a trailer, back in August 2015. I was so impatient to experience the difference between riding with just the 50 watt front fairing and the 200 watts of this rig that I failed to take the time to properly attach the flexible solar panels to the trailer. The rear-most 100 watt solar panel bounced like a trampoline and flexed so much that it failed during my first day of riding. I never found the problem but it was likely a broken connection between two cells somewhere within the laminate.
As long as you put some cross-bars or a rigid panel under the solar panels, this is the easiest way to add solar charging to any ebike. Just about any bike trailer will do. This could be a weekend project and only requires a few basic tools.
Two years later, I had distilled my ideas and completed this one wheeled trailer. The idea was to pare the trailer down to the minimum required to tow a solar panel behind a bike. The 1" (25mm) aluminum tube attaches to the bike rack using a universal joint so the trailer can tilt and turn with the bike without falling over. I rode about 100 meters, stopped and had to walk the bike back because it was so unstable.
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I don't have a video of that first tide, but here's a demo of the problem.
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Theoretically, it's possible to attach a one wheel bike trailer with a u-joint. There are a couple of commercially available trailers that do just that. I've never seen one in action but I assume they handle well. Tout Terrain even makes a kid trailer like this.
I attributed the problem with this design to the flexibility of the thin walled aluminum tube I was using. I assumed it was twisting under the load. My recent experience would seem to indicate that there's more going on here. I solved the problem by changing the boom tube to a larger diameter, moving the wheel further back and replacing the u-joint connection to the bike with a
Bob trailer fork which connects to the rear wheel axle. Effectively, it's just a bigger u-joint with a horizontal axis and a vertical axis. This is best photo I have showing how it attaches the trailer to the bike.
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This completely solved the handling problem but I didn't fully grasp how each of those changes contributed to the solution. This trailer weighed 27 lbs (12 kg) and had 188 watts of solar panels so about 16 watts per kg. For my current build, my goal is to increase the solar to 248 watts and bring the weight down to around 20 lbs (9 kg) so about 27 watts per kg.
I started over from scratch and re-examined every component for weight saving opportunities.
- The new solar panels are much lighter.
- The red foam/fiberglass support panel is being replaced with a lighter honeycomb/carbon fiber sandwich panel.
- The simple elastomer suspension was inadequate on rough roads so I upgraded it to an adjustable air shock. It's slightly heavier but should ensure that the trailer will survive thousands of miles of rough roads.
- I moved the trailer wheel forward to shorten the boom tube segments. They need to pack down to fit airline luggage constraints.
- The Bob trailer fork is made of heavy steel tubing designed for a trailer plus cargo combined weight that is nearly 4 times the weight of my trailer so it seemed needlessly heavy duty. I went back to the u-joint with the hope that the carbon fiber boom tube would be stiff enough to make this design work.
And here's where we are today. The plywood panel is a temporary stunt double for the solar panel assembly. It matches the weight and center of mass but is about 1/3 of the size of the solar panels.
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Possible culprits for the stability problem:
- The u-joint is too far back from the bike or maybe the bike rack to which the boom is attached has too much flex and sets up the oscillations? I'm leaning toward dropping the u-joint and going back to the Bob trailer fork.
- The trailer's center of mass is too high? I need some room for suspension travel but there's room to lower the the boom. Also, the photos above are a bit deceptive because I hadn't adjusted the shock preload yet.
- The trailer wheel is too close to the center of mass? I can test this with the current rig by mounting the plywood further forward.