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 (ok, 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 a fan or 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: https://www.genesgreenmachine.com/direct-charge-grid-tie-battery-bank/

Parts list:   

 

Optional parts:

 

Tools:

 

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.

 

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 ensure you don’t 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.

Build a spin bike pedal generator

Here you will find everything you need to for an easy to build, powerful, quiet stationary spin bike pedal generator. I’ll cover tools and parts needed, along with diagrams for wiring and things I’ve learned along the way that can make your build a success. Let me start by saying that I am not an electrical engineer, nor a mechanical engineer, this was just something I was interested in and wanted to go about building. If you have suggestions or a better way to build it, I’d love to get your feedback!

Let’s get right to it! First you’ll need a spin bike, also known as a stationary bike or exercise bike. If you don’t have one already, check eBay, Craigslist or garage sales. Otherwise, there are several  models from Sunny Fitness that should work. Be sure to get one with a chain drive (not belt).

The next key component is a e-bike hub motor. Hub motors come in many different models, front, rear, 24 volt, 36 volt, 48 volt, 350 watt, 500 watt, 1000 watt and so on. For this to work, you’ll need a front hub e-bike motor (check eBay if Amazon is out of them), as front hub motors will fit between the typical spin bike front braces. You’ll also need to have a disc brake mount on the hub motor. More on this in a minute. For my build, I went with a 24 volt setup, using a 24 volt motor 500 watt motor. You can use a 36 volt brushless gearless front hub motor with disc brake mount if you can’t find the 24 volt variety.

Now you may be wondering how we plan to drive the e-bike hub motor with the chain drive of the spin bike. This took me a little while to figure out, but it just so happens that some people use the disc brake mount on a bicycle to mount a ‘fixed’ gear. There are a few companies that sell sprocket kits just for this purpose. The one I recommend is the 16 tooth sprocket from Origin8. Just take off the free hub adapter parts (the red things) and bolt the sprocket on the disc brake mount on the motor. Some Loctite might be advised.

Once the sprocket is on the motor, pull the original flywheel off of the spin bike and replace it with the hub motor, ensuring you route the chain over the newly installed sprocket.

When you turn the e-bike hub motor the permanent magnets pass over the the copper coils of the stator in the motor, generating alternating current electricity. To convert to more usable direct current, we need simple component called a 3 phase bridge rectifier – it sounds really complicated, but it is really just a basic circuit which takes the 3 wires of alternating current from the hub motor and converts it to direct current. The fat wire coming out of the hub motor consist of the 3 wires just mentioned, and some thin hall effect sensor wires used by a typical e-bike controller to determine RPM of the motor. These sensor wires can be clipped and/or tucked out of the way. For the remaining thicker 3 wires, we’ll put female spade connectors on the ends and connect them onto the 3 posts of the bridge rectifier marked with the “s” squigglies. The other two posts on the bridge rectifier deliver positive (+) and negative (-) direct current. The side of the bridge rectifier has a map of the posts, click on the image at right for a closer look at the map.

Next challenge – the voltage coming out of the bridge rectifier is anywhere from 20 to 40+ volts. This is assuming a 24 volt hub motor spinning anywhere from a leisurely pace to frantic sprint . We’d like this power to be a steady 12 volts so we can charge cell phones, iPads, and run things like fans and coffee makers. With 12 volts output, we could also use a DC to AC converter to charge our laptops, cordless drills or power a TV. Enter the step down converter! If you go with a 36 volt motor, you may want to get a 36 volt step down converter. This critical component is often used in electric golf carts to “step down” the 48/36/24 volt battery power in the golf cart to 12 volts to run fans, radios, lights and other things that plug into a 12 volt car socket. Here’s a diagram of the wiring, including an optional multi-meter – it really is very simple.

Wiring diagram for pedal generator

That’s it! Those are your basic components to make it all work.

This really is a fun project. With most of the challenges of how to make it all work spelled out above, what’s stopping you from building your own pedal generator?

Some optional components, basic tools and supplies you may need to complete this project:

Project box to make it look nice
Fuse and Fuse holder to protect components
Multi-meter to quantify watts and watt-hours being generated
12v car sockets – I like to use 2 of these
Anderson Powerpole Connectors
12 Gauge Speaker Wire
Wire Crimper Stripper
Adjustable wrench
Metric Allen Wrenches
Multi-meter, can be handy

Please support this site by purchasing any needed components through the links above. It will help pay for hosting this site, and will fund future adventures in alternative energy projects I’d like to pursue.

Direct charge, grid tie or battery bank?

What is the best use of your pedaling time on a pedal generator? There are 3 typical avenues you could direct your watts:

  1. Direct charge: Directly use the watts coming out of your pedal generator, once converted to useful 12 volt current, and charge your devices using car chargers.
  2. Grid Tie: Use a grid tie inverter to send all your pedaling effort into the nearest wall outlet.
  3. Battery bank: Charge a bank of lead acid batteries.

There are advantages and disadvantages to each of these.

Method Pros Cons
Direct charging Efficient – 90%+ charging efficiency. You will need to round up enough gadgets to create the appropriate resistance for a good workout.
Grid Tie Directly offsets your utility bill with watts generated. Grid tie inverters are about 80% efficient in practice.  Put in 100Wh from pedaling, only about 80Wh will be put back into the grid.
Battery bank For those living off grid, augmenting their battery bank may be a necessity. Lead acid battery charge efficiency is only about 85% – so if you put in 100Wh, you only get out 85Wh.  Couple this with the efficiency losses of a charge controller and an inverter to get it back out as AC, and you might get 50% of what you put in.

I advocate for direct charging, and I’ll explain why.  Going out on a limb here, but I’ll assume everyone uses a mobile device or two, and maybe a tablet. Assuming you charge these things regularly, let’s look at the scenario of charging these using the methods above.

Direct charging:

Pedal generator -> Charge controller (90%+ efficient) -> 12v car charger  -> device(s)

Grid Tie:

Pedal generator -> Grid tie inverter (80% efficient) -> 110VAC wall charger -> device(s)

Battery Bank:

Pedal generator -> Charge controller (90% efficient) -> Battery bank (85% efficient) -> DC to AC inverter (90% efficient) -> 110VAC wall charger -> Devices(s)

As I discovered in the previous blog post, wall chargers have losses up to 30%+, whereas 2 out of the 3 car chargers tested had losses of just over 1%. Using either grid tie or a battery bank may result in 50% or more of your efforts wasted on conversion losses.  I do want to point out that the charge controller used in my pedal charger design is 90%+ efficient, so you will lose up to 10% in that conversion, resulting in a max of about 12% loss if you include the car charger losses – but far better than the other two alternatives!

Wall Charger Verses 12v Car Charger Efficiency

Have you ever wondered which kind of charger is more efficient?  Are wall chargers we use to charge our devices just as efficient as 12v car chargers?  In this blog post, you’ll find out!

I grabbed 3 quality wall chargers and 3 quality car chargers with the intent to see how many watts each consumed, and how many they actually put into an Apple iPad Mini.  To find these measurements, I used a wall AC meter, a DC power meter, a USB dongle meter and an Anker 1 ft Powerline cable.  Here are the results.

Charger Watts In Watts Out Percent Loss
Apple 5 watt 6.6 watts 4.6 watts 30.3%
Apple 12 watt 11.6 watts 9.2 watts 20.7%
RAVPower iSmart 11.0 watts 9.1 watts 17.3%
PowerGen 20w car 8.9 watts 8.8 watts 1.1%
Anker 42w car 10.7 watts 9.9 watts 7.5%
Anker 24w car 8.2 watts 8.1 watts 1.2%

Wow!  I didn’t expect to see such a difference in efficiency between the wall chargers and the car chargers.  After a little digging, it makes sense – the wall chargers need to convert AC (alternating current) into DC (direct current), whereas the car chargers are just changing 12 volt direct current into 5 volt direct current that is the standard for USB.  Even the worst car charger was 10% more efficient than the best wall charger – crazy, right?

I suspected AC wall chargers were less efficient from my experience using my pedal generator.  I could see the wattage used by the wall chargers put more of a load than the DC car chargers, but it was great to quantify the real difference.  I had no idea how much more inefficient wall chargers were!  I will definitely be using car chargers whenever possible for the most efficient charging of my devices.  On the other hand, if I need more of a challenge on my pedal generator, I can always substitute in the AC chargers!

If you’re living off grid, you may find this information very helpful in deciding how you should charge your mobile devices.  Directly charging with solar or a pedal generator would be the most efficient way to go, rather than dumping all power to a battery bank, then converting to AC and charging with a wall wart (thoughts of all the efficiency losses in this cycle is making my head spin!).

Comparing USB chargers for charging the iPhone 7 Plus

I use a stationary bike pedal generator to charge my cell phone and other devices.  Getting the most watts into my devices helps me make the most of my pedaling time. A typical 1 amp charger puts about 5 watts into most phones, but will take several hours to charge even the smallest device battery. In this post, I’ll compare the output of several chargers and how many watts they can put into my iPhone 7 Plus.

Chargers included in this test:
1. Original Apple 5 watt wall charger
2. Original Apple 12 watt wall charger
3. RAVPower 24W 4.8A (2.4A x 2) Dual USB Wall Charger
4. PowerGen 4.2Amps / 20W Dual USB Car charger
5. Anker 24 wattdual USB PowerDrive 2 Car charger
6. Anker Quick Charge 3.0 42W Dual USB Car Charger

While testing each device, the phone was plugged into each charger with a low state of charge (~15-25%) and left on the charger for a couple minutes to ensure the circuitry of the charger had time to identify the device and provide the maximum charge it was capable of delivering. We use a low battery as the rate of charging slows once you approach 80-90% charge on the battery. You may have noticed getting the last 10% charge into your phone seems to take an eternity. This slower rate of charge is to protect the battery from damage and is by design.  Let’s see how each of the devices fared in this test.

The results

 

Device Watts delivered  
Apple 5 watt 4.74
Apple 12 watt 9.94
RAVPower 9.49
PowerGen Non-Apple port: 2.36
Apple port: 9.13
Anker 24 watt 9.82
Anker 42 watt IQ Port: 10.18
QC 3.0 Port: 4.81

As expected, the 5w Apple charger delivered just under 5 watts.  The 12w Apple iPad charger kicked out over 10 watts initially but settled into just under 10 watts after half a minute or so.  The PowerGen 12v car charger non-Apple port was the worst of the bunch, pushing only 2.36 watts, however it’s Apple specific port cranked out a respectable 9.13 watts.  The 24 watt Anker managed about 9.82 watts out of each USB port.  The final USB car charger in the test, the Anker Quick Charge 3.0 and IQ charger managed less than 5 watts out of the QC3.0 USB port, but cranked out 10.18 watts out of the IQ only port.

Overall winner: Anker 42 watt!

I suspected based on my experience using these chargers with my stationary bike generator that this would be the outcome I would get, but it’s was good to have metrics on the actual watts going into my phone.  Pushing only 10 watts on the pedal generator requires almost zero effort, so I typically change my iPhone along with battery banks (I’ll review what works best among those later) and any other devices around the house that need a charge.  Adding more devices to the mix creates more resistance when pedaling.  Devices I typically include are iPads, other family members cell phones (if I can pry them away long enough), an iPad mini, an iPod Touch, my Microsoft Surface Pro 3 , Chromebook, bluetooth headset, FitBit, and cordless drills to name a few.  I try to get 60 watts or more in order to get enough resistance to make it feel like I’m doing something.  I’ve been able to generate over 180 watts with the pedal generator, but I can’t sustain that for too long.  I’ve found the best range of wattage resistance for me for any length of time has been between 60 and 130 watts.  In future posts, I’ll test the limits of the pedal generator (and myself!) to see what the upper end of wattage is that can be produced.