Following the theme of my bicycle trainer pedal generator, I wanted to build something with a more substantial flywheel in order to take advantage of momentum the added weight will provide. An exercise spin bike is a perfect candidate for making a pedal generator with just a few parts and fairly simple electronics. Like my other design, we will use an RC motor and a 3 phase bridge rectifier for the core of the design. This design should be adaptable to most other spin bikes with little or no modification.
Here’s what you’ll need for this project:
– A spin bike. I’m using one from Sports Authority, but most are of a similar design. If you don’t have one, Amazon has some for a reasonable price, also look on Craiglist, garage sales, etc if you don’t want to buy a new one
– 140Kv RC motor, this thing is capable of 1200 Watts and 34 Amps so this won’t limit our max potential!
– 35 Amp 3 phase bridge rectifier
– DC Meter or this bigger one, both are powered by the input voltage
– Wire, I use 12 GA, you can use smaller (14 or 16 GA) if you don’t plan to generate 100’s of watts
– Female disconnects, also known as spade connectors to connect to the bridge rectifier and the car sockets
– 3 to 4mm bullet connectors and heat shrink tubing OR
– MT60 3 wire bullet connector (my motor came with a Female MT60 connector so that’s the path I took)
– Zinc Steel flat punched bar
– All thread or bolts, go with .25 inch or whatever size the holes are in the punched bar for a snug fit. I went with smaller bolts so the drilled holes in my spin bike is smaller and not very noticeable if were to ever want to sell the spin bike in the future. If you are going all in, go full size, because the mount will be more solid!
– Lock washers
– Thread locker
– a bunch of fender washers that match your all thread rod or bolt diameter
– BaneBots T81 8mm Hub (get the right size for your RC motor shaft if you choose a different motor)
– BaneBots Wheel 2 7/8”
– bungee cord(s)
– Radio Shack Project box
– U bolt – to attach project box to handle bar – measure your spin bike handle bar to get the right size
– 12v car sockets, 2, 3 or more depending on the size of your project box and how many things you want to charge at once. You can also consider getting a splitter/extender as well.
– 12v battery jump starter
– 12v DC to 110v AC inverter
– USB car charging adapters (this 6 USB charger is a also a good choice)
– USB fan (must have for me – to keep you cool!)
– soldering iron
– some good solder
– helping hands
– hack saw to cut the all thread
– drill bits
– measuring calipers
– wire stripper/crimper tool
– metal file
– snap ring pliers
– Allen wrenches
– adjustable wrenches
Choosing the Motor – Math Time!!
Let’s figure out how to choose the right RC motor for this project. Our goal is to have a motor spin at a rate that will generate between about 12 and 15 volts when we are spinning on the exercise bike. RC motors are rated by how many volts they produce per RPM. A 1000Kv motor would need to spin at 1000 RPM to produce 1 volt. To get to our 12-15 volt range in this case, we’d need to spin the motor at 12,000 to 15,000 RPM. Let’s get some basic measurements from the spin bike.
– The circumference of the flywheel on my spin bike is 57 inches
– Pedaling the spin bike at a comfortable pace rotates the flywheel between 250-350 RPM, which I confirmed using a digital tachometer
The BaneBot wheel we chose is 2 7/8 inches diameter = 2.875″. You’ll recall that circumference is Pi times diameter. The circumference of this wheel is about 9″.
If we take the low end and use 250 RPM and multiply that by the 57 inch circumference of the flywheel, we get 14,250 inches per minute. Dividing the 9″ BaneBot wheel circumference, we get 1583 RPM. We want our motor to produce 11-12 volts at this RPM, so divide by 11 and 12 and get 131.9 and 143.9. So our target motor should be around 140Kv. We found this Balancing Scooter RC motor that fits the bill.
If you have a significantly different circumference flywheel, or choose to use a different drive wheel than what I have, you can use the above steps to determine an appropriate motor to use. Otherwise, just use the 140Kv motor as I have.
The Motor Mount
There are many ways you could mount the motor, but in this project we choose to use 2 flat punched steel braces along with some nuts and bolts/all thread and washers. First you’ll need 2 equal lengths of flat punched steel.
7 or 8 inches each is fine – use your hack saw to cut, then file the rough ends so no one gets cut on the burrs.
Now the trickiest part – measuring and drilling the holes to screw onto the motor.
– measure the distance across the face of the motor from one threaded hole to the other hole on the opposite side of the motor shaft. Do this with calipers preferably. Check and double check your measurement. Mine was 38mm center to center. Now, measure and mark where you should drill two holes on each side of a punch hole in the flat steel in about the middle of the bar.
The motor shaft will go through the punch hole. The bolts for my motor were 2mm, so I drilled holes slightly bigger so the bolts would fit in. Drill the holes and mount the motor with the Allen head bolts that came with the motor.
Mount the T81 hub on the motor shaft using the right size Allen wrench.
Mount the wheel and install the locking snap ring that comes with the hub. This task is much easier if you have snap ring pliers, but can be done with other pliers, a screwdriver and determination.
Motor Mount Bracing Install
Next step for the motor mount is to use all thread or a 5 to 6 inch bolt to connect to the second bar/brace. Tighten the bolt, nut and lock washers on the non-motor side flat punched steel bar. Put on 2 nuts and lock washers on the other side with the motor mounted bar sandwiched in the middle, but leave them loose. Place the non-motor bar against the right inside brace of the spin bike just above the flywheel. Adjust the bolts left or right to get the BaneBot wheel in the center of the flywheel, then tighten the bolts.
On the spin bike, carefully measure and drill 2 holes, just above the flywheel, you want to be just high enough to not touch the flywheel with the all thread or bolt. Use a stack of fender washers, along with lock washers and nuts to mount the motor assembly to the spin bike. Use enough washers to center the BaneBot wheel with the spin bike wheel. Once all lined up, tighten all the nuts. Some thread locker can also be helpful as vibrations from spinning may loosen the nuts.
Final step in the mounting is to add a bungee cord (I used 3 little ones) to add some tension from the motor wheel onto the spin bike fly wheel. The ends of the bungee’s hooked onto the transport wheel brackets of my spin bike, hopefully yours is similar. Only hook up the bungee when using the generator, otherwise you may get a flat spot on the rubber wheel.
We have the motor mounted, now we need to make some electrical connections. We need to take the 3 phase alternating current of the RC motor and convert it to DC using the 3 phase bridge rectifier. The motor I have comes with a female MT60 3 wire bullet connector so I just made a 3 wire whip with one end having a male MT60 bullet connector and 3 female spade connectors on the other ends of the wires.
These 3 female spade connectors plug into the alternating current spade contacts (marked with a squiggly that looks like an “S”) on the 3 phase bridge rectifier.
Order of connecting these wires doesn’t matter. Next we need to come off of the DC plus and minus posts and go into the DC meter (optional, but highly recommended). The Drok meter I used has some wires with alligator clips that seemed to connect securely enough to the rectifier. Maybe I’ll add some spade connectors later. On the output side of the meter, we’ll connect to a 12v socket or two. One socket is easy (use the alligator clips), two or more will need a splitter of some sort. I used some 5 wire connectors I had, but you can splice wires together, use a distribution block, some wire nuts or whatever.
Once you have it all hooked up, give the bike a spin and see how many volts the meter reads. The goal is to have 11-15 volts, which is ideal for using car socket chargers and inverters. If the voltage is too low, pedal a little faster. If that isn’t working, you may need to use a smaller wheel on the motor to get the RPMs up. If the voltage is too high, pedal slower or use a larger wheel on the motor. With the combination I have, the 140Kv motorwith a 2 7/8” BaneBots wheel puts me right in the sweet spot for voltage.
Box It Up!
Lets tidy things up a bit by putting the electronics in a project box. I like the ones from Radio Shack that have an aluminum bottom plate on them because we can use it as a heat sink for the bridge rectifier, which is especially important if you are pushing 50 or more watts for any period of time. The size of the project box really depends on how many 12 volt sockets you want to use, along with the size of the DC meter. In the project box shown, I put in two 12 volt sockets, a Drok DC meter and the 3 phase bridge rectifier. I could have easily put 2 more sockets in this box. Cutting the hole for the meter was the most challenging, but here’s the approach I took. Measure the dimensions of the multi-meter. Place masking tape over the area you plan to cut out for the meter. Measure and mark on the tape where to cut, then use a sharp utility knife cut out the hole. Start gently to be sure to stay on the lines. Keep tracing the lines with the blade, pressing firmer and firmer until you cut through the plastic. I’m sure there is a better way to do this, but with patience this approach works. For socket holes, a 1 3/16″ hole saw does the job very nicely. Drill a hole in the metal backing for the bridge rectifier, and use a nut and bolt to secure. I also used a U bolt to mount the box to the handlebar (measure your handlebar and get the right size U-bolt). A couple more holes in the metal plate will be needed to mount the U bolt. You can either drill another hole in the metal plate for the wires to go through, or you can cut/drill in the side of the plastic box. You can get fancy with a wire clamp if desired. I didn’t this time, I just notched the plastic with some wire cutters for the wires to go through.
Okay, the moment we’ve been waiting for – let’s start charging and powering things with our human powered generator! Drop in some 12v car phone chargers, plug in some devices and see how easy it is to put power into them. You may want a splitter to allow connecting more devices. A basic 12v to USB socket charger will only put about 5 watts to your mobile devices, which is why I suggest using some better quality chargers that can put 10 or more watts into your phones, tablets, e-readers, portable battery banks and other devices. You’ll also find having a fan a welcome addition once you start pedaling.
The more devices you plug in, the more resistance you’ll feel as you pedal. For me, anything under 50 watts doesn’t feel like much resistance at all. 75-100 watts begins to feel like a good workout. Around 150 watts is about my limit for a 30-60 minute workout.
Here’s some typical devices and the watts to power them:
– iPhone 7/8/X – 10 watts with a decent charger
– QuickCharge 3.0 phone – 15 watts
– USB Fan – 3 watts
– Makita 18v automotive battery charger – 70 watts (I use this in combo with their USB adapter to charge “slow charging” devices like iPads, Kindle’s and other portable battery packs)
– 32″ HD LED TV through a DC to AC inverter – 30-60 watts
– 55″ HD LED TV through a DC to AC inverter – 50-120 watts (varies widely by model/features)
To Add a Battery, or Not to Add a Battery, That Is the Question
For generating and powering mobile devices like cell phones and USB battery banks you won’t really need to add in a 12v battery as buffer. If, however, you are going to power a TV or other devices through a DC to AC inverter, you’ll want to consider adding a battery. One of the nice things about this design is adding a battery to the circuit is quite simple and requires no additional electronics because the 3 phase bridge rectifier acts as a blocking diode (it’s actually a circuit made with a handful of diodes), preventing the battery from putting power into and spinning your RC motor! The easiest way to add a 12v battery is with a car power bank or ‘jump starter’ along with a socket to socket adapter. Plugging in the battery will power the DC meter and provide a voltage reading on the battery. Anytime you are producing more volts that that reading, you’re putting amps into the battery and powering your devices. Be careful, however, to not spin so fast that the voltage goes over 15 volts as this could damage the battery.
Adding the battery will act as a buffer for powering devices that are “spiky”, in other words, that need a bunch of power to start or during various phases of operation, then settle into a more manageable power draw. I found trying to power a 55″ LED TV was difficult without having the battery as a buffer. It wasn’t that I couldn’t generate enough power, it seemed that my pedaling wasn’t “smooth” enough current. Adding the as battery as a buffer solved this problem.