Spin Bike RC Motor Powered Pedal Generator


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!
– Nuts
– 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.

– 5 Conductor Wago Connectors to wire connect the meter to multiple car sockets
– heat sink or cooling plate for the bridge rectifier if you don’t go with the Radio Shack project box

Additional/optional componnets:

– 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!)

Tools:
– soldering iron
– some good solder
– flux
– helping hands
– hack saw to cut the all thread
– drill
– drill bits
– ruler
– 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.

Electrical Connections

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.

Charge!

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, tabletse-readersportable 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.

Best Design DIY Bike Trainer Pedal Generator

 

Why would anyone want to build a pedal generator?  There are many reasons.

    • To be prepared for the next hurricane that takes power out for days, weeks or longer
    • To supplement your off-grid system
    • To have one of the coolest interactive science fair projects
    • To be more environmentally friendly and create a smaller carbon footprint
    • To have a backup plan should terrorists or nation states take out our power grid
    • To be prepared in the event of a zombie apocalypse (okay, a bike generator probably won’t help in this case – but add a set of the key electronics in your Faraday box will help should we see a Starfish Prime type attack)
    • Or like me, a fair weather mountain biker, you want turn your efforts on an exercise trainer in the off season into tangible outcome (in addition to better health).  For me, that outcome is a charged cell phone, tablets and other mobile devices, and a satisfaction that I contributed, if only in a small way, to preserving the earth we live on.

Whatever the reason, you’ve come to the right place for an easy to build, efficient bike trainer generator.  In this post I will provide step by step instructions and all the information you’ll need to source the parts for this project.  I’ve always had an interest and fascination with alternate energy and human powered energy in particular.  As a fair weather mountain biker, I find pedaling on a trainer or spin bike in the off season uninspiring, and often think “what if I could harness some of this energy”, or “I wonder if I could power the TV I’m watching” while I pedal.  I wonder no more.

I’ve checked out many pedal generator products on the market as well as in the DIY world and found the commercial products for sale were too expensive, and the DIY projects were often really complicated and/or required you to take your bike apart to hook it to the generator.  My first attempt at a pedal generator was expensive to build, although not extremely complicated.  So I set out to design a low cost and easy to build bike generator that you just drop in your bike when you want to use it, allowing you to easily take your bike for a ride when not generating electricity.  

I built this bike generator so I could charge my iPhone and other mobile devices while I get a workout.  If I want an easy workout, I’ll just charge my phone and a battery pack or two.  If I want a more challenging workout, I add more stuff to charge, or power TV!

Some things I’ve charged or powered with my bike generator, and the typical watts they require:

The more watts you plug in to power, the harder it is to pedal. Most people should be comfortable with anywhere from 40 to 150 watts.  Trained athletes can generate upwards of 600+ watts, but not for extended periods of time.

Some people have a need or desire to charge a 12 volt batteries, and this bike generator will do that if desired, but I would suggest that direct charging/powering is more efficient due to losses in charging lead acid batteries (15%), so putting 100 watt-hours in gives you only 85 watt-hours out.  Read more about this in my blog post: http://genesgreenmachine.com/direct-charge-grid-tie-battery-bank/

Parts list:   

Optional parts:

Tools:

    • Soldering iron kit  –  I started out using an old 15w Radio Shack soldering iron, which was an exercise in frustration. Once I got this 60w iron kit, my soldering skills were much improved!
    • Helping hands – these make holding wires/connectors and soldering much easier – highly recommended.
    • Drill  – if you don’t have one, I really like my Makita, they make solid tools.
    • Drill bits – you’ll need a few different sizes – I like the titanium ones, don’t dull as quickly as standard bits.
    • Metric Allen Wrenches – the ball end ones work well for tightening the shaft coupler
    • Wire stripper 
    • Rubber mallet – helps with tapping the shaft coupler onto the motor shaft
    • Calipers – perfect for confirming measurements so you get the right parts!
    • Pencil

For less than a couple hundred bucks you could have a working bike generator (assuming you have most of the tools) – far cheaper than most products on the market, and much easier to build than other DIY designs.  Let’s get started!

Detailed build video:

Step one:

Unscrew the 3 screws holding the outer shroud on, remove shroud.  Take magnet resistance parts and resistance cable out of bike trainer, along with the metal ring of magnets inside the outer shroud.  The metal ring was glued in a few spots on mine so took a little work to get out.  

Step two:

Add the shaft coupler.

The motor I am using has an 8mm shaft, and the trainer has a 10mm shaft. This shaft coupler connects the two shafts together quite nicely. I tried a grub screw style shaft coupler, but it made a really bad vibration, this one worked great! If you use a different trainer or RC motor, be sure to measure what you have before ordering the shaft coupler – they make many different sizes and you should find one that will work.

Put on the coupler, tap with a mallet if needed to get it seated all the way in – being careful to not tap the shaft out (brace the flywheel side when tapping). Tighten the Allen screw on the trainer shaft side.

Step three:

A bit about the RC motor selection process – math alert!

In selecting an RC motor, we need to determine which motor will give us between 9-15 volts at normal pedaling speeds:

    • A typical 26 inch mountain bike tire is 2068mm in circumference https://www.cateye.com/data/resources/Tire_size_chart_ENG.pdf.  
    • The drive wheel diameter on the bike trainer is 30mm, circumference is 94.25mm.
    • For every rotation of the tire, the drive wheel will rotate 2068/94.25 = about 22 times
    • A comfortable riding pace is about 15 MPH.
    • At 15 MPH, a mountain bike tire is spinning at around 194 RPM https://endless-sphere.com/forums/viewtopic.php?f=28&t=16114
    • At 15 MPH, the bike trainer drive wheel is rotating at 194 x 22 = 4268 RPM
    • RC motors are sold in xKV, meaning to get x RPM(K) it will need (or generate) (V) volts, so a 1000KV motor will generate 1 volt at 1000 RPM, 2 volts at 2000 RPM, etc
    • To get to around 12 charging volts at 15 MPH (4268 RPM / 12v), we need a motor with around 355KV rating.  I wasn’t able to find any RC motors at that exact rating, I went with a motor with slightly lower (320KV) RPM because I’m lazy and don’t want to pedal as hard to get to 12v.
    • Vary your RC motor selection based on your expected riding MPH and wheel size using the reference links above.  If you’re a faster road rider, you may want to use a higher KV motor than I am, if you’re looking for a more casual pace or will be using this trainer with a 24 inch wheel bike for instance, then a lower KV motor might be a better choice.

Attach RC motor to bike trainer housing. Drill the center hole of the shroud a little bigger so the RC motor shaft won’t rub. Screw on the + bracket to the RC motor.  Place over the shroud hole as close to center as possible.  Align holes in bracket with solid part of shroud, mark holes, drill and bolt the + bracket to the shroud.

You can also connect through the inside of the shroud as shown below.

Both ways work – the real purpose is to keep the motor base from spinning, so there isn’t much pressure on these connection points.

Step 4:

Replace shaft housing, connecting to the shaft coupler.

Attach 8mm RC motor shaft to shaft coupler and attach trainer housing back on trainer.  I found drilling a hole in the bottom of the housing made it easier to tighten the Allen screws to the RC motor shaft.  Replace the 3 screws that hold the housing on, check for clearance by spinning the shaft, if all good – tighten the Allen screws onto the RC motor shaft.  If something is rubbing, you may need to move the shaft coupler further onto the 10mm trainer drive shaft and try again.

 

Next well cover the electrical – here a diagram of the wiring:

Step 5: 

Connect motor to bridge rectifier.

The 3 phase bridge rectifier sounds fancy but serves a simple purpose, it will convert Alternating Current (AC) coming from the 3 wires of the RC motor into Direct Current(DC) which is useful for charging.  A small amount of voltage is lost in this conversion process (about 0.7 volts), and some heat is generated, but this unit has substantial cooling fins so heat should not be a problem at the amperages we will be working with.  Okay, let’s make three (3) wire connectors between RC motor and 3 phase bridge rectifier – we’ll need bullet connectors on one end and female spade connectors on the other.  Solder 3 x 4mm bullet connectors to 3 equal lengths of wire, then cover with heat shrink tubing to insulate from shorting with the other bullet connectors. I’m using 12 AWG wire, you could go as low as 18 AWG wire. Crimp and/or solder 3 female ⅜” spade connectors to the other ends of the wires.  Finish with heat shrink tubing if desired.  Connect the bullet connectors to the RC motor wires, connect the 3 other ends to the 3 Alternating Current (AC) male spade connections on the bridge rectifier.  The order of the connections to between the bridge rectifier and the motor make no difference.

 

Step 6:

Connect to the DC side of the bridge rectifier.

 

Add ⅜” spade connectors to a black(-) and red(+) XT60 wire assembly and connect to the 2 Direct Current (DC) male spade connections on the bridge rectifier, ensuring to put the red(+) on the + connector and black (-) on the – connector.

 

Step 7:

Add a meter.

Adding a meter is optional, but strongly recommended to help you not go over on voltage, and to help measure how many watts you are actually producing!  For our build we used an RC power analyzer connected using XT60 connectors.  This meter will show Watts, Volts, Amps and scroll through Watt Hours (Wh), Amp Hours (Ah) and other measures.  The XT60 connectors make solid circuit contact and prevent plugging things in the wrong way.  Wire so the “source” is the bike generator, soldering each connection and sealing with heat shrink tubing.

 

Step 8:

 

Add a car socket adapter – in the parts list we link to a 3 port socket connector that should be suitable for 80% of users.  The 18 AWG wires limit the total wattage to about 150 watts, which is fine for most people and has been plenty for me, but strong riders may want to build something with 12 AWG wire using separate sockets and a project box.  To hook up the 3 port socket adapter, just solder on the XT60 connector to the matching wire colors, add some heat shrink tubing (put the tubing on before soldering, far up the wire so it doesn’t get hot) and plug in!

Step 9:

 

Get charging!  If you just plug in car charger adapters, most will start charging at around 9v input, and the good quality ones (like the Anker models referenced in optional parts) will handle up to 24v input without a problem.  If you only charge with these type chargers, you can pretty much pedal to your heart’s content and not worry about limiting voltage if you followed the parts design outlined above.  Need an easy ride, just plug in a cell phone or two.  To add more resistance, add more car adapters and devices.  I’ve tested this generating up to about 225 watts.  It can go higher, but that’s nearing the limit of the 50 year old pushing the pedals (me!).

If you want to power something that plugs into a wall outlet, or have a desire to charge a 12v battery, then you’ll need to be mindful of the voltage you are generating, and keep it to under 14.7 volts or so.