Perpetual Motion

Getting something for nothing or a free ride is something that man has always been on the lookout for. The economist will tell us that there is not such thing as a free lunch and our mothers will chide us that we must all pull our own weight. As long as the sun burns and we can utilize that energy in some form, we can create near perpetual motion devices such as this solar-electric powered bicycle.

The System

A mountain bike is the first choice for this project. It is the sturdiest of the common bicycle designs available today. This is necessary since in the process of constructing this electric vehicle (EV) we will be adding anywhere from 30 to 40 lbs. to the stock frame. Even for people who are light and intend to use this EV, 40 lbs. adds quite a bit of weight to the total weight of bike plus rider plus electric upgrade parts. Adding more than 40 lbs. would probably require reinforced framing at some points, a front suspension system and better brakes.

The electric motor is the core of the system. A 12 to 24 vdc permanent magnet, series wound electric motor is a good choice. A series wound motor has excellent torque current characteristics and is a perfect mate for the mountain bike. We will be buying something in the 1/4 to 1/2 horsepower range where one horsepower is defined as 764 watts. For those who have little experience with DC motors, they are true workhorses. The first thing one will notice is the amazing range of torque. DC motors have a much greater torque range (across a greater RPM width) than do their gasoline counterparts. EV cars that are powered by DC motors usually use second gear on the manual transmission for all speeds from 0 MPH up to about 40 MPH. That is quite impressive.

The choice of batteries is an important one. Basically, coming in under weight is crucial. Mouser Electronics has a tremendous selection of quality, sealed gel cells for such applications. Finally, mounting materials and some other minor electrical parts will be needed to complete the system. Initially, I will not be using a throttle, only a simple on/off switch that activates a contactor which will handle the heavy current flow from the battery(ies) to the motor. Mechanics and Torque Buying a motor that is just the proper size is a challenge. If the motor is too small (i.e., is not able to provide enough torque), it will not work well and will probably heat up and maybe even burn out. However, a motor that is too big will have plenty of power to push the bike, but may take a lot of amps just at no-load running speed and will weigh a lot. I saw one motor that I really liked that was rated at 1 HP. It weighed 17 lbs. and this is simply too much. I expect the batteries to weigh the most followed by the motor.

The mountain bike that we selected for this project weighs 25 lbs. and has 21 speeds. I put the chain on the largest rear sprocket and the smallest front sprocket. I am looking at purchasing a 60 tooth gear that works with #35 roller chain. I will also need to buy this size roller chain and another 8 tooth gear for a #35 chain that will be hooked to the axle of the motor. The 8 tooth gear comes in two sizes for different axle diameters, 5/8 inch and 3/4 inch. This will be a factor when buying the motor. I need to get one of these two axle sizes.

The 60 tooth sprocket will replace the large gear on the front of the bike. The motor will be mounted on the frame tubing just in front of the front sprocket. The crank arms will be removed. After I am done, there is no way you’ll be able to pedal this bike any longer. The roller chain will connect the 8 tooth gear to the 60 tooth gear and power is transferred from the front sprocket to the rear sprocket using the standard bicycle chain geared as mentioned above.

Using an 8 tooth gear connected to a 60 tooth gear is a differential of 7.5. This is important. Motor RPM and torque are inverse quantities. If speed doubles, torque is cut in half. Let’s say our motor has a no load speed of 3,000 RPM. Well, 3,000 divided by 7.5 equals 400 RPM. If the motor’s rated torque at this speed were 10 foot lbs., then the 7.5 differential would translate into 75 foot lbs.

Now, at the front sprocket we have a speed (no load) of 400 RPM and torque of 75 foot lbs. The same figures are present at the rear sprocket since there is no real differential between the small, front sprocket and the large, rear sprocket. They are both about 32 teeth. Since the wheel is a 26 inch wheel (diameter), the radius is 13 inches. Take that 13 inches and multiply it by the 75 foot lbs. present at the rear sprocket and we get 975 foot lbs. This is the force present at the road surface, right under the bike’s rear wheel. Keep in mind that this is a no load speed. Once you start loading the motor, the RPM measurement drops everywhere and torque increases. Think of it this way: As a kid riding a bike, when you started going uphill, you began to slow down and then you pedaled harder. Your speed had dropped, but the torque, the strength with which you pushed down on the pedals, had increased.

The Power Source

The on-board battery(ies) will need to be charged. If you read the Reclaiming Power article on this site, you’ll see an opportunity for powering our solar electric bike. I plan to create a charging port for this EV at home where I can just hook up the bike to the PV array we have on roof.

In addition to charging the bicycle from the PV array at home, I will bring along a small, 14 vdc 1 - 2 amp AC/DC wall charger when I bring the bike out. This will allow recharging at any 120 vac outlet.

I have seen the new flexible PV panels being offered. When riding, a panel such as this could be placed in a backpack and unfurled for charging when taking a break on a trip with the EV. The final option entails charging from the lighter socket of a car. Depending on the car battery’s voltage, it may be necessary to have the car running while charging the bike. This may not be the best way to get charging current.

Due to the many ways I envision charging the solar electric bike, I have decided to place a small, 8 amp, 12 vdc charge controller on board. This will allow any DC source from 13 to 35 volts to be connected to the bike’s charging ports. An ammeter will be part of the system so that I may get an idea of the current flow into the batteries. An onboard voltage meter will also be needed to get a picture of battery state of charge.

The Next Step

As of 3/1/97, I am testing various motors to see how well they work. This entails putting the 8 tooth sprocket on the motor and putting the motor in a vise. The 60 tooth sprocket is already mounted and the bike is sitting in a bike stand in the basement. The test involves powering the motor and running the bike in the stationary stand and then using my hand to provide resistance to the back wheel and measuring the ampere draw from the battery. Motors that are two weak cannot hold up to the hand test and motors that are too big, in addition to weighing too much, will pull too much power from a battery at a no load speed.