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:
- iPhone 7 Plus: 10 watts
- iPad Pro: 12 watts
- USB battery pack: 5 watts
- Quick Charge 3.0 battery pack: 18 watts
- USB C battery pack: 15 watts
- USB C Lenovo Laptop: 15 watts
- 32 inch LCD TV: 50 watts
- 12 volt fan: 6 watts – although this one uses USB and has a built in battery which is great for emergencies, camping and other uses beyond keeping you cool while pedaling
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/
- 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!
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:
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.
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.
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.
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:
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.
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.
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.
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!
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.