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Designing the Sun Lover Part 2: Build Story

The Sun Lover Solar-Powered Tandem Bicycle

In part one of this essay I describe how stupid cars are. They are incredibly unsafe, especially if we take a holistic view of safety that includes the safety of people outside the car as well as inside the car. And I describe how cars are disgustingly wide and heavy, in order to be stable at highway speeds. 

What if we decided we didn’t want the vehicles in our cities to go highway speeds? What if we bifurcated our idea of what a car is into small cars limited to 30 mph for travel within cities, and larger cars capable of 75 mph for travel between cities? These larger cars already exist, but for some reason no one is building these smaller, lighter, narrower cars. Is it possible to build such a vehicle? Yes. The technology exists. But I don’t know of very many people working on it. Our engineers and our entrepreneurs have a false set of assumptions about what transportation should be like. They are mired in group-think created by the automobile and fossil fuel industries. And so I am called upon to create such a vehicle myself. For about 14 years now, I have been on a quest to build a lightweight narrow solar-powered vehicle. My design criteria for a “bicycle that can replace a car” are:

  • Top speed of 30 mph

  • Range of at least 60 miles

  • Able to carry enough solar panels to recharge from four hours of sunlight or less 

  • Weighs less than 200 pounds

  • Able to fit through a standard door, so less than 30 inches wide and 78 inches tall

  • Less than eight feet long

  • Costs less than $5,000

  • Able to carry two people and 100 pounds of their groceries or gear

  • Performs well on trails as well as asphalt

My first attempt to build such a vehicle was in 2010. I started with a Surly Big Dummy cargo bike. I fitted it with an electric motor, batteries, and three 65 watt flexible solar panels. Results were so-so, so I waited a dozen years for solar and ebike technology to mature until I tried again. In 2022 my wife Judy and I toured on our ebikes with a 170 watt flexible solar panel mounted on a bike trailer. That worked pretty well, so in 2023 I took the time to mount two of those solar panels on a Worksman tandem trike that we named the Sun Pony. That also worked well. You can read about those experiments, along with a short history of solar powered vehicles in general, and furthermore including instructions for making your own Sun Pony, in my blog post entitled “Here Comes the Sun Pony”. 

The Sun Pony Solar-Powered Tricycle

There were however some drawbacks to the Sun Pony. One unexpected issue is that many New York State country roads now have "rumble strips" on their shoulders which makes it very difficult to ride the Sun Pony on the shoulder. And much bicycling infrastructure is too narrow for the Sun Pony to pass through, including some gates on rail trails. Lastly, because lightweight tricycles are in danger of tipping over when turning at speeds above 20 mph (as I'll describe in great detail in part three of this essay), the Sun Pony had limited usefulness in city traffic where vehicles are expected to travel at 30 mph.

Opportunity for Improvement

This year, however, an opportunity presented itself that has enabled me to dramatically improve my design. That opportunity was: a friend was throwing out an old steel-frame inline tandem bicycle. The bike was a Northwoods 18-speed. It was not a great bike. The components were mediocre. The brakes were the older cantilever type rather than modern disc brakes. It was rusty in places. One of the bottom brackets had bad bearings. But then I realized, I would enjoy replacing the mediocre components with good ones. I would hesitate to make such dramatic modifications to a brand new expensive tandem bike. But I wouldn’t hesitate to mod the crap out of this bike. Plus it was free. And it was steel, which is much easier to weld than aluminum. It was perfect!

Crappy tandem out by the trash.

Making It Electric

One of the first critical decisions when designing an ebike is: hub motor or mid-drive? The best answer in this case turned out to be: both. 

I began making the Sun Lover electric by adding a Grin All-Axle direct drive hub motor to the crappy tandem. I had the motor lying around for another project that I had aborted. This is the same type of motor that I had added to the Sun Pony’s front wheel. Grin the ebike company is the master of regen, and I was excited at the prospect of being able to give the tandem bike regenerative braking. Regen is especially important for a tandem. Bicycle brakes are designed so that a front and rear brake together have just enough stopping power for one person. With two people on a bike two brakes are not quite enough; the braking on a tandem can feel sketchy. (The Worksman company who built the tandem trike we used as the basis for the Sun Pony solves this problem by putting a sticker on the bike that reads “Do not ride this vehicle in hilly areas”). Quality tandems of yesteryear solved the flaky braking issue by adding a third brake called a “drag brake”, a shifter-operated drum brake in the hub of the rear wheel that has a large heat sink. Hub motors with regenerative braking are a great replacement for the drag brakes that tandems used to have. They can provide about the same braking force as a drag brake, but instead of producing waste heat they produce useful electricity. On an average round trip a bike with regenerative braking can gain back about 10% of the energy it used to get up the hills on that trip by braking on the way back down those hills.  

Grin All-Axle hub motor with Box Prime 9 cassette.

Then I added a Box “Prime 9” nine speed drive train to the crappy tandem. This has become my go-to drive train upgrade for ebikes. The Box components work very well and the Box cassettes have an insane range, with a 50-tooth cog the size of a dinner plate that enables you to climb up any hill. At this point Judy and I took the tandem for a test ride on the rail trail near Penn Yan. In spite of a rusty brake cable breaking (instead of braking) as we set out, the trip went well. But we found that with two people on the bike the motor threatened to overheat on hills. 

The solution was to add a second motor, specifically a BBSHD mid-drive motor, to the rear bottom bracket. I use this dual-drive setup on my cargo bike and I find it to work well. I use the smaller lower-geared mid-drive motor for powering the bike from zero to ten miles per hour, and I use the large hub motor for powering the bike from ten to forty miles per hour. I can use both motors at the same time for getting up hills, even with a passenger or hundreds of pounds of cargo. 

I had a spare BBSHD on my parts shelf. But I fretted a little bit about how to mount it onto the crappy tandem. The width of this mid-drive motor made it impossible to attach the “synchronizing chain” crank arm to the left side of the motor. The synchronizing chain on a tandem transfers power from the front crank to the rear crank. On most tandems the synchronizing chain is on the left side of the bike and the drive chain is on the right side. Here is where the ebike hobbyist community came to the rescue. I read a post on the Electric Bike Forum by “Apeekachu” who explains how he had encountered this very same problem. He describes how he solved it: he moved the synchronizing chain to the right side of the tandem. In his setup the BBSHD’s right side has two separate chainrings: an inner chainring driven by the motor with its chain extending back to the rear derailleur, and an outer chainring with its chain extending forward to the front chainring. Brilliant! 

BBSHD mid-drive motor with two chainrings attached.

There was another minor obstacle to implementing this setup. Once I added the BBSHD to the rear bottom bracket, the new front and rear chainrings of the synchronizing chain did not line up, and the chain would likely fall off during normal use. This is where Lekkie, the renowned New Zealand BBSHD aftermarket parts company, came to the rescue. Lekkie makes axle extensions to correct this very issue. I ordered these extensions and eagerly installed them when they arrived. 

Lekkie axle extensions to improve the synchronizing chain line.

One last hurdle presented itself. The synchronizing chain needed some sort of mechanism to take up slack in the chain since the original tensioner was now on the wrong side. I found a slick spring-loaded tensioner intended for fixies, welded a derailleur mount for it onto the bottom tube, and it worked perfectly. 

Chain tensioner for the synchronizing chain.

This build has two “shark” style 52v batteries attached to the tandem using Grin’s clever “Triple Bob” battery mounting system.  

The Triple Bob uses pipe clamps to attach battery cradles anywhere on the bike.

This build has another innovation from Grin: a "trip-wire" ebrake cutoff switch. This tiny switch attaches to a brake lever and when the brake lever is pulled it cuts off the motor and activates regenerative braking.

Trip-wire ebrake cutoff switch.

The bike now had a lot of range and a lot of power. The only thing it was missing, in order to achieve my design criteria, was solar panels.

Adding Solar 

A friend of mine who had heard of my interest in building solar powered vehicles asked what it would take to make her small EV solar powered. She liked the idea that she could get her fuel for free from the sky, and she imagined that that ability could relieve her of the range anxiety that plagues EV owners today. 

“Couldn’t I carry some solar panels in a trailer on long trips, and get them out for a few hours to recharge my car when I stop for lunch?” It sounds good in theory. However, what EV owners don’t realize is that a typical EV is stupid heavy. It has to be that way in order for it to have the same performance and low center of gravity (CG) stability as gasoline-powered vehicles, in order to play in the same ballpark, so to speak. I did some quick calculations. 

“You’d need to carry an array of 40 solar panels in order to make that idea work,” I told her. “And then you’d need a few more panels in order to have enough energy to carry the panels themselves…” I started to do some more calculations. 

“That’s okay, forget about it,” she told me. “But how do you make your solar vehicle work?” she asked. 

“Well the key is in making the vehicle very light in weight,” I explained, “which requires a high CG strategy for achieving stability. There are some trade offs of course, like being limited to 30 mph, but it’s totally worth it because there is also the side benefit of a lightweight narrow vehicle being incredibly safer, if you think about safety in the holistic sense…” I could see her eyes glazing over. “I’ll write a blog post about it and send you the link.” 

“Okay,” she replied.

So how did I add solar power to the Sun Lover tandem ebike? It was a tricky engineering challenge: how does one add a lightweight but sturdy two foot by eight foot surface to the top of a bicycle? Just imagining a solution was hard. It was difficult for me to visualize a solution because my human imagination only works well in two dimensions at a time. And so as a first step I built a 1:6 scale 3D model of the Sun Lover that I could use to try different solutions in 3D. I attached two adjustable wooden mannequins to the model so that I could see how the human limbs would have to work around the struts that would be necessary to hold up the solar array. I glued dowels onto my model in various positions. I concluded that the solar array could be attached to the tandem's top tube at two points, but that it would need some sort stabilizing bar that curved around the waist of the front rider.

1:6 scale model of the Sun Lover.

My intuition told me that tetrahedrons, the strongest 3D structure possible, would be necessary. I also applied Buckminster Fuller’s idea of “tensegrity", which is basically the idea that you can best make a lightweight structure by combining structural elements with integrity (like a bicycle rim) with structural elements under tension (like bicycle spokes). In my case I combined a central stiff structure in the middle of the bike with cables at the front and back of the bike holding the ends down. This arrangement had the added benefit of allowing the cables to be loosened when stopped so that the whole solar array can be tilted forward or backward to face the sun. 

The Sun Lover's solar array can be tipped forward when stopped to gather more sunilght.

I wanted to keep the width of the array to less than 30”, and I wanted two solar panels, one for each of the ebike batteries. (I like having two redundant drive systems so that I can still get home if one of them fails.) So I purchased two 100 watt flexible panels that are about 2 by 4 feet each. I welded up a sturdy frame for them out of 1” square tubing and attached them so that their wires hang down by the central post. I’ve since learned about a newer better kind of solar panels that are lighter, more flexible, more robust, and work better in partial shade. I was especially impressed by a Youtube video in which three types of solar panels are struck by a baseball bat, and only this type of solar panel survives. And in another Youtube video this type of solar panel still manages to put out half power even with most of it shaded by a cardboard box. I may redo my array soon to accommodate these new “CIGS” type solar panels. 

Lastly I welded two thick mounting plates to the top tube of the tandem. The solar array can be attached to and detached from these mounting plates with only two bolts. 

Solar array mounting points on the top tube.

One last step is to add electronics that convert the voltage of the solar panel (typically around 20v for a 100 watt panel) to the 58v required to charge my ebike batteries. I mounted two mppt charge controllers (also a Grin product) to the solar array structure, plugged everything in, and it worked great!

MPPT charge controllers.

Judy and I have taken the Sun Lover for several test rides and we’ve only encountered minor problems such as loose wires that needed to be tidied up. Future refinements I plan for the Sun Lover include adding a giant kickstand, upgrading to CIGS solar panels, and figuring out how to add some sort of canopy for camping. 

The Sun Lover just about meets all of my design criteria, but I encountered a new problem: the perception of my friends and family that the Sun Lover is somehow unsafe. Car-centric people seeing the Sun Lover react with several safety concerns: that it’s top heavy, that its light weight makes it more susceptible to sliding on turns, and that the large surface area of the solar array might cause it to lift up at high speeds or be blown over by moderate side winds. These are all dumb concerns and I will address each of them in turn in Part 3: Why Bikes Are Safer Than Cars.


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