Best DIY Bike Trainer Generator




Introduction: Best DIY Bike Trainer Generator

About: Interested in green energy with a keen interest in human power.

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 generate electricity, allowing you to easily take your bike for a ride when you are not.

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:

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 watts in gives you only 85 watts out. Read more about this in my blog post:

Step 1: Parts List

Optional parts:


Step 2: Remove Magnetic Resistance Parts From Trainer

First we need to remove the guts of the trainer's resistance unit. Unscrew the three Philips head screws holding the outer shroud and remove the shroud. Pry out the metal ring with magnets from the outer shroud.

Use Allen wrench to remove the Allen bolt holding the other magnetic resistance parts. Pull out inner metal housing. Take care not to get your fingers pinched when placing near the other magnet ring.

Turn the cable tension adjuster so the cable is loose, and with fingers or a pair of pliers work the cable end out and remove.

Step 3: 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 created a 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 4: Attach RC Motor to Outer Shroud

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 Ref:
  • 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 Ref:
  • 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.

Drill the center hole of the shroud a little bigger so the RC motor shaft won't rub. From here, there are options on how to attach the RC motor to the shroud, here are two ways:

1st way: 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 may also want to do some Dremel work to remove some of the plastic in the shroud in order to secure with a nut and washer, but I've found that it wasn't necessary, the only purpose of the bolts is to keep the motor frame from turning.

2nd way: Attach the motor without the + bracket, drilling and bolting directly through the housing. I found I needed to add a couple washers for spacing so the shaft lock washer and bearing didn't rub. This way is more challenging to measure where the holes go, using the 1st way you can just use the + bracket as a guide to center, mark and drill.

Once the motor is mounted to the shroud, we need to slide the motor shaft onto the shaft coupler, while lining up the 3 screw points. Once pressed on all the way - tighten the Allen/hex bolts onto the motor shaft. I found drilling a 1/2 inch hole in the bottom of the shroud made it easier to access the hex bolts. Put the 3 screws back into the shroud that were originally used to hold it in place.

Step 5: Turn AC Electricity Into DC

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 (if you use a different motor, check motor bullet connector size with caliper before ordering) 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 AC posts are marked with a ‘~’. The order of the connections to between the bridge rectifier and the motor make no difference.

Alternate: If soldering is not something you are comfortable with, this project can be completed with a pair of wire crimps and suitable connectors. You would need to cut off the bullet connectors on the motor, strip wire ends and crimp on 3/8" connectors. Where I have used XT60 connectors, just use spade connectors or Anderson Powerpoles.

Step 6: Make a DC Connector Assembly

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 Socket Adapter

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: Done!

Get charging! If you just plug in car charger adapters, most will start charging at around 9v input, and the good quality chargers (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 if you followed the design as I've outlined, and not worry about limiting voltage. 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 375 watts. It can go higher, but that’s nearing the limit of the 50 year old pushing the pedals (me!). The motor itself is capable of up to 1820 watts. Want a challenge? Brew some coffee!

If you want to power something that plugs into a wall outlet using an inverter, 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.

Let me know if you build this, I'd love to hear your feedback!

Step 10: Detailed Build Video



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6 Discussions

This is very cool. one question, how difficult would it be to put in some sort of warning system (either a blinking LED or a beeping device of some sort ... or both) for when you are approaching the upper end of your safe output, so you don't pedal too fast and melt your devices? this way you don't have to monitor the read out while watching the TV run on pedal power :)

1 reply

Very good point, Lorddrake! One of the very reasons I like this Drok meter as you can set it up to blink the screen when you reach an upper limit on voltage. I used this meter for the box you see in the video mounted on the bike. Another nice feature of this meter is it 'remembers' the total watt hours generated even if you stop pedaling.

I'm still in the mist of what motor I need to use considering that I'm able to flow 300W at once into a lipo battery using the above stated battery balancer. My problem is the two intervals we'll be facing: pedalling speed and voltage input (cr. Attachment).

Following your definition of Kv, it makes sense to spend my own human energy as economical as possible. A motor with the lowest Kv value (63Kv), serves this goal: I can pedal at a slow pace while generating the highest voltage output (28v). However, I know I can pedal much faster and release more human energy. As I don't want anything to blow up, I should limit the generated power with a motor with a higher Kv. In this case, I would choose the 220Kv motor. This is because I am at hitting 28v as the preferable output instead of a lower 10v.

While writing this process, one question comes to my mind: what voltage output SHOULD I be aiming at? You did not mention anything about current and amps. I reckon if we know this, we could determine what motor is the most appropriate choice for use in combination with the 300W battery balancer?

1 reply

I see your quandary - let me try to clarify. To your last statement - the current (amps) will be defined by the load, so when I’m charging my 5500mAh 11.1v Lipo battery, the proper settings to charge it are 5.5 Amps and the volts are managed by the balance charger, starting out at whatever the battery is at, usually around 11.2v. Watts = volts * amps, so we get 61.6 watts of resistance while pedaling. Not exactly - the charger isn’t 100% efficient, so that load is more like 70 watts. The wattage will actually increase as the battery charges until the battery gets to 12.6v, at which time it will taper off as each cell reaches a fully charged state (which makes for a good cool down). To your question about which motor to choose, what you really need to determine is what RPM range you will be in, from low to high. If you’re using a bike with multiple gears, you can always adjust the RPM by shifting up or down. The load will remain relatively constant, and as long as you are in the voltage range of the charger (10v-28v), the charger will be happy. I think your calculations to go with a 63kv motor are off a bit as this would put your voltage at about 68 volts @ 15MPH (using a mountain bike tire as I have in my calculations). Don’t overthink the motor, I’d go with a 320kv for a mountain bike tire and a 350kv for a road bike tire.

Great first instructable! I'd love to set something up like this for my computer desk.

1 reply

Thank you! You could easily make this work as a pedal desk, using a DC to AC inverter to power the computer. For better efficiency, get a 12v DC charger for your laptop. Another great trend is USB-C charging on newer laptops - you can directly charge with a USB-C car adapter, like this one: