The NorthEastern portion of the United States is not particularly kind to avid cyclists, especially those who work during prime daylight hours. The electro-bike, herein referred to as E.B., was designed to keep the user aerobically fit while creating some extra power that may be sent to a bank of batteries that are mainly powered by photovoltaics. Any bicycle will do. However, bicycles with wheels of larger diameters, such as 27 inches as opposed to 16 inches, create more mechanical advantage as will be shown. Both street bikes, with very narrow, smooth tires, and mountain bikes, with wide, knobby tires, have been used with equal success. The bicycle is placed upon the stand, which is an Advent Mag-Trainer. It comes assembled and folds up easily for transport - even after the alternator is added.
First, we removed the roller and flywheel mechanism from the frame. Two nuts and bolts hold the roller in place. Then, a metal plate, with two holes drilled in it, was placed upon the bike standís swivel mount, right under the rear wheel of the bike. This plate was 11 inches by 7 inches and stiff enough to allow slight flexing. Two nuts and bolts were used to secure the plate to the swivel mount. The alternator was mounted upon this plate using four, two inch L brackets. There are two long bolts that run through the alternator, horizontally when the alternator is on its side. The L brackets can simply be fastened to these. It is not feasible to have the axle of the alternator pressed up against the bikeís rear wheel because massive slippage occurs. A small wheel needs to be fastened to the alternatorís axle. Anything with a circumference between 2 and 10 inches should do. The smaller the wheel, the greater the mechanical advantage, but the more likely slippage is. I simply used the flywheel that came with the stand. Since the alternatorís axle was too large to be fastened to the flywheel, I had to grind the axle down. Hooking the alternator to a 12 volt battery and running it as a motor allowed the use of a file to whittle down the axle to the proper size. Once this was accomplished, we put the flywheel on the alternator and drilled a hole through the flywheel mount and alternatorís axle to get a secure fit. A bolt was passed through this hole and fastened with a lock washer and nut.
The bicycle is secured upon the stand by placing the E.B.ís back wheel between the Advent standís two cup holders. A cycleís rear wheel has an axle which terminates in a lug nut at each end. These lug nuts are to be placed in each one of the cup holders. Then the cup holders are to be tightened down on the lug nuts until the bicycle is held firmly. This also allows perfect alignment (left to right) of the rear wheel directly above the alternatorís wheel. Now the tension of the alternator mount needs to be set. The knob under the metal plate changes the inclination of this plate upon which the alternator is mounted. The adjustment knob should be tightened so that you can hold the alternatorís wheel with one hand while trying to spin the bikeís rear wheel with the other and get no slippage. Do not overtighten. This will put undue stress on the components. It does not take much tension to eliminate slippage. Since the rear wheel of the bike is about one inch off the ground while in the stand, it may be necessary to place a piece of wood under the front wheel. This will make the bike level and prevent the rider from sliding forward on the seat while pedaling. Keep in mind that the folks at Advent constructed this stand so that you may easily remove a fully functional road bike and take it out for a spin on a sunny day. Simply unscrew the holder cups from the lug nuts of the E.B. and the bike easily comes away from the stand.
Math and Mechanics
The Univega Mountain Bike we used for most of the testing has 26 inch wheels. This is the diameter of each wheel. The circumference is approximately 82 inches ( Circ. = PI * Dia. or 3.14 * 26 = 81.64). This fact is important when deciding on the wheel you are going to use on the alternator. A wheel with a circumference of 10 inches will spin 8.2 times faster than the bikeís rear wheel ( 82/10 = 8.2). A wheel with a circumference of 4 inches yields much more mechanical advantage ( 82/4 = 20.5 times). The faster the alternatorís axle spins, the more amperage is available at the alternatorís output terminals. I had no way of accurately measuring work exerted on the bike, but I tend to spin a bikeís cranks at about 80 RPM using the large sprocket when I am on the road. This large, front sprocket has 52 teeth and the smaller sprocket on the rear wheel has 13 teeth, meaning the rear wheel spins 4 times faster than the cranks do. If the cranks are spinning at 80 RPM, then the rear wheel is spinning at about 320 RPM. As shown before, the rear wheel has a circumference of 82 inches to the flywheelís 10 inches. The alternatorís axle spins 8.2 times faster than the rear wheel. So, the rear wheel moving at 320 RPM means that the alternatorís axle is spinning at about 2,624 RPM. This alternator speed consistently creates about 4 to 5 amperes of power. Crank speeds closer to 100 RPM create about 6 amps. On sprints, I have watched the ammeter jump to almost 7 amps, but these speeds are not sustainable, even for the disciplined athlete. The amperage measurements were obtained by hooking an ammeter directly to the alternator. Actual throughput will most certainly be less, especially when a charge controller is used (see below).
In the setup we constructed, the alternator is wired to a second charge controller, which is wired in parallel with the main charge controller and then run to the battery. If one were to use blocking diodes (I suggest at least 10 ampere diodes) between the alternator and the main charge controller, the alternator could be wired in parallel with the photovoltaics using only a single charge controller. Two caveats: First, blocking diodes must be used along the photovolatic power line to the charge controller BEFORE this feed meets the alternator feed and then to the charge controller. This is to prevent some of the alternatorís power running up to the panels and being wasted as heat energy. Secondly, make sure this one charge controller can handle full panel amperage PLUS the 7 amperes the electro-bike could create at any one moment. My panels are able to create 6 amperes in strong sun and the E.B. can crank out 7 amps on a real spin. Hence, a single charge controller with a rating of less then 13 amperes could be troublesome if it is very sunny at the same time the rider exhibits real zeal. Voltage tends to be between 16 and 20 volts. Not very kind for direct connection to a battery.
The Next Step
Recently, we have added a second alternator to the stand which doubles the power output. I am searching for a larger alternator that would do the work of two American Bosch alternators. Please note, that although I have listed the supply house for the American Bosch alternator, they no longer list this item in their catalog. I believe they still have some in stock, though. All of this experimentation is a fine balance between power creation and the strength required to turn the bikeís rear wheel. The current configuration with one American Bosch alternator can be easily spun by people of all ages. Larger alternators would be more difficult to spin and might be feasible only for those looking to endlessly climb imaginary hills. Finally, a cyclocomputer will be added that will measure ground speed, time in training, average speed and top speed. This instrument will be used primarily to compare the E.B. feel to that of a bike on a road surface. If the average speed of the E.B. is much higher than that of the bike on the road for a trip of the same length, then it can be deduced that the E.B. is "too easy" and more load should be mated to the E.B.ís rear wheel. Since there is no wind present when using the E.B. indoors, additional resistance must be presented to the E.B.ís rear wheel to experience a life-like ride.