Introduction
The LWB electric bike is great but… it weighs a ton, it’s ungodly long, and it steers like a pig. Plus, it’s quite complicated. This leads me to the requirements.
Requirements
This next bike must be exactly the opposite. Light and portable, so I can actually lift it and get it on the train. It must handle like a regular bike. It should also be far less complicated.
Design
Unfortunately all the electric pieces weigh an amount that is basically fixed. I won’t find a lighter motor, and the batteries won’t improve past a certain point. I am probably not going to be able to reduce the weight of these components below 15 lbs. A good road bike weighs about 20 lbs. So how can I possibly keep the weight down?
The boom on the last bike was far too heavy. It was just too much metal. Ok, less metal…
The derailleur is a big pain. Setting it up, making it happy, keeping it clean, all very annoying. Internal hub gears or single speed. That would be much easier. Drum brakes would make things easier too. No maintenance!
How can I make a portable bike? Small wheels would help. Big honking 27″ wheels mean the bike must be at least five feet long. Rather than finding some small wheels, I decided just to start with a kids bike. An adult can ride a childrens bike if accomodations are made for the extra leg length. Extending the seat tube and handle bar tube is one solution, but it’s possible to get aerodynamic gains by building a short wheel base recumbent out of the kids bike. A semi recumbent rider lowers the frontal area of the vehicle considerably.
Poor Aerobic Rider. Your days of sitting neglected in the backyard, a constant guilty reminder of broken New Year’s pledges past, are over. You will be reborn…
Build
The first step was to tear down an unused exercise machine. This would provide with plenty of square tubing.
I then tore down the bike down. It had a 1 piece crank. All the threads on the left hand side are left hand threads (reversed from normal). I removed the pedal on the left, then loosened the nut on the crank, then pulled out the crank entirely (it comes out as 1 piece). Next I removed the front fork and handlebars by loosening the bolt at the top of the head tube. I took a scrap piece of metal and placed it against the head tube and found it was just about the right length that if I placed a seat on the main tube and cranks at the end of the scrap, I could fully extend my legs. The only difficulty was sourcing a tube that I could place the cranks in. Mr. Sawzall fixed that problem. I cut out the entire crank tube from the frame and welded a piece of square tubing in its place. I then jigged the tube up with the scrap metal and welded that on. Next, I used the exercise machine as a jig to hold the bike frame and welded the scrap tube to the head tube. That was it for the welding.
Next, I took the long flat seat from the exercise machine and attached it to the main tube of the bike using bolts. I then dremeled the soft part of the seat off from the frame of the seat and screwed a piece of plywood to the frame of the seat. I tilted it so that it turned into a backrest. I then replaced the cranks in the tube where they came from.
I tried this contraption out by walking it up a slight grade and then coasting down. The handlebars proved to be problematic; there is no way to get them high enough. I would have to weld another tube to the end of the fork and attach another set of less stylized handlebars. When the handlebars are right between your legs, they don’t need to be very wide.
The above took fours hours and then I decided to hold off on more work for now. Until next year!
The bike at the end of day one. Note the cutesy stickers on the front sprocket, and the burnt rear wheel. Remove the tires before welding, or have water ready to extinguish the inevitable tire fire.
January Progress
Google maps blew up Firefox so I lost a rather lengthy addition to this page I was working on.
I attached a generator motor to the boom. The generator motor is a 36V, 250W 9:1 internally geared DC series motor. I brazed the gear set from a mountain bike to the no. 35 sprocket already attached to the motor. I made a motor plate out of some steel sheet. The 40t crank gear lined up with a 22tooth sprocket on the gear set, giving a gear ratio of 1.8:1. With this gearing and a DMM attached to the motor output wires, I pedaled as fast as possible and generated 13v. If the motor output is linear with speed, it will take a 5:1 ratio to generate 36v. But, the drive motor can only accept up to 28v. To achieve this I would need about 3.9:1 gearing. The smallest gear set sprocket is 13 teeth, and the largest crank I have is 48 teeth, or 3.7:1, which should generate slightly more than 26v. Due to clearance between the gear set and the cranks, I need to attach the 13 tooth sprocket in some other way (if I put spacers underneath the motor mounting plate, the pedals hit the gear set).
Two different views of the original generator setup – the gearset is brazed to a no. 35 sprocket
An additional problem is that at 13v with the throttle wide open, the drive motor, a Kollmorgen 300w, does not turn. It requires a threshold voltage before turning on.
The whole system is suboptimal, and one of the following may be better:
- Use a battery with a PWM controller to raise the voltage to the low threshold, and modulate the speed using the throttle until the threshold is reached by the generator. Above the threshold, the generator output limits the power output of the drive motor. This adds the expense of a controller and batteries and charging hardware.
- Use a super capacitor with a PWM controller to raise the voltage. The super capacitor could be charged initially by pedaling.
- Use a buck boost converter to raise the output voltage of the generator. Additionally, the output voltage could be lowered when above 28v to 28v. Gearing using standard bicycle chain would be impossible in the latter case with the parts I have.
I also needed to gear the drive motor. It operates at up to 3500 rpm. To figure out the gearing, we need to calculate how many turns a 20 inch wheel must make to go one mile. Then we set a maximum speed, and from there figure out how many turns the wheel must make per minute. The ratio from wheel turns to motor turns is the gear ratio.
1 mile / (20 inches * pi) = 1009 turns. At 3500rpm we would travel at nearly 200mph. 25mph is the legal limit. Lets aim for 30mph for the sake of the calculations. 30mph would require a wheel RPM of 500. 3500/500 = 7:1 gear ratio. The coaster sprockets on the rear of the bike are only available in 16 and 18 teeth. This would require a 2 or 3 tooth sprocket on the motor, which is not possible. The wheel must be replaced or a larger sprocket fabbed. I have a 40 tooth sprocket with a small bore hole that could be milled out to fit the coaster wheel. 40/7 = 5.7, which is also unreasonable. Lets work backwards. The smallest bike chain sprocket is 13 teeth. 13 * 7 = 91, which is far larger than any bike chain sprocket available.
I went by two bike stores just to see what was available; crank sprockets start at 22 teeth and go up to about 52 teeth (these are easy to mount because they are mounted using 4 bolt holes– making an adapter plate to the motor shaft is easier than using gear set sprockets). Coaster brakes, as I said before, are only 16 or 18 teeth.