Erg Rower Generator

This pandemic has given all of us an opportunity to try something new or explore an idea that we would have otherwise not had time to do.  Thanks to being stuck at home, here’s what I decided to do with my new found time.  For many years I’ve been had an interest in human powered electricity, and my first adventures pursuing those interests resulted in my pedal generator designs (exercise bike, bike trainer and spin bike versions).  I thought it might be a fun challenge to build a rowing machine generator.   

Building a rowing machine generator has some unique challenges, mostly due to the 2 cycle nature of the rowing machine.  Cycle one where the chain is pulled and power is created, and cycle two is the return stroke where no power is generated.  In this design I do a couple of things to solve the challenge of smoothing out this cyclic power: 

  1. Use a heavy flywheel 
  2. Incorporate some ultracapacitors into the charging circuit 

To make the story short: these two things did the trick and I’m able to generate 100-120 watts steadily and get in a great full body workout. 

Let’s get into how I built it.  A great big shout out goes to Jim Flood for coming up with and sharing his rowing machine plans that can be built with common lumber and wood tools and a few other parts.  Check out his website at https://openergo.webs.com/ 

I took Jim’s plans for the Mark 1 design, followed them almost entirely as he describes, but instead of using a bicycle wheel and some fins for resistance, I put in an e-bike motor along with some a circuit to charge or power electronics.  The circuit is basically this: 

E-bike motor->3 phase bridge rectifier->meter->ultracapacitors->charge controller 

The charge controller is a 24v->12v step down controller that connects to a couple automotive sockets which can be used to charge phones, laptops or power a TV or other devices with a DC to AC inverter.   

Let’s talk about the build. 

The first minor hurdle: Jim’s plans are all in metric.  I did some quick conversions to come up with similar dimensions in inches.  If I’d had a metric tape measure, I probably would have just used his dimensions and been done with it.     

I picked up some fixed casters to use as part of the sliding seat, and decided I wanted to put a couple grooves for the wheels for a track.  I used my router table to add a groove with a round nose bit the top of a couple 2×4’s, which would be the basis of the rower.  A couple small boards in the back for one support, some 30 degree cuts here, 60 degree cuts there for other boards along with some leftover deck screws and the frame was together in a couple hours.  One tip, I first put on the back base and cross support, then shimmed up the front until it was level, then clamped and screwed the front braces on.  The key distance you need to keep in mind is the width of the top posts needs to be around 135mm, the width of a standard bicycle rear end.  I used a 2×4 and a 3/4” pine board to get the right thickness.   I ended up adding a cross brace later on (see top photo or photos further down) in the front just under the e-bike motor as I noticed some twisting due to torque while rowing.  The cross brace addressed this.

The sliding seat was pretty straight forward.  I used some more of the 3/4″ pine board I had to make the base, adding a couple 2x4s for bracing.  I then measured the distance from one groove to the other (bottom center to bottom center) and used that distance to center the caster wheels on the seat.  Once built and placed on the track, a fancy chair cushion makes for a comfortable rowing session!

To create a mount for the e-bike motor (generator) I notched out a couple pieces of steel so the e-bike shaft fits in the notch, then mounted those two brackets onto the frame. 

The e-bike motor shaft has flat sides on it, 10mm wide.  So I made 2 mounts out of slotted flat bar using my reciprocating Dremel MultiMax.

For the flywheel, I had a local water jet cutting company create a couple 1/2″ thick steel donuts.  I gave them the inside diameter just inside of where the machine screws mount the side plate of the e-bike motor, and a little under 18″ for outside diameter.  These each weigh about 28 pounds, total of 56 pounds.  With the flywheels in hand, I then took off the side plate of the e-bike motor and overlaid it on each flywheel marking where the machine screws needed to go.  Using my drill press, I drilled out the mounting holes (slowly, applying oil as I went for cooling and ease of drilling).  Due to the 18″ diameter of the flywheels, I needed to take the head unit off my drill press, overlay the flywheel, then re-mount the head unit in order to get the drill bit in the right location. 

 

Rather than use the multiple gears the motor came with, I swapped it out for a BMX style single speed freewheel.  The first one I tried was super loud, the second one was a little more expensive (Shimano) that is much quieter.  

You could use the multi-gear cassette if you like, I just wanted more control over the chain line going to the jockey pulley to limit the chance of chain derailment.

Flywheels mounted on motor, motor mounted onto rower with the customized slotted flat steel plates using a couple nuts and bolts. 

For a handle, a 1.25″ oak dowel felt good in the hand.  It came in 36″ lengths.  A 20″ cut, with a hole in the center for an eye bolt seems just right.  A couple wood screws with duct tape on them serve as the stop or rest for the handle when not in use. 

For the chain, I used 2 full length bicycle chains, connecting them together, and on the ends with Nite-ize 25# clips.  I used a jockey pulley as a guide just below the e-bike motor.  At first I mounted the jockey further back, but found the stroke was too short and the Nite-ize clip would hit the pulley and derail the chain. 

I used an exercise band to provide tension to pull the chain on the return stroke.  I bought a set with different tensions or “weights”, thinking I might need to adjust to a stronger or weaker band.  I started with the 10# band, which worked perfect. 

I ran the band to a wall mount pulley mounted to the rear brace and used a metal hook through the loop on the band to the front of the rower frame, just under the e-bike motor

 I decided to get a little creative with the other 16″ of dowel.  Using a 1.25″ Forsner drill bit, I drilled through the rower frame and inserted the 16″ dowel.  I then drilled a couple 8″ sections of 2×4 with the same bit, right at the edge.  I mounted those on the 16″ dowel, then screwed on a couple 3/4″ by 12″ pine boards to the base 2x4s on the dowel.  Adding large washers and wood screws to the ends keeps the foot rests from going off the end.  Finishing touches included adding a heel rest (make this 1/2″ or more tall to keep your heal from lifting out on the return stroke) and some velcro straps (screws with washers to limit tear out).

Let’s dive into the circuit used on this rower generator.  Looks like a mess, I should have used a bigger project box for a little more room, but it all tucks in there nicely.

It’s actually not too complicated.  The 3 thicker wires out of the e-bike motor are the 3 phases of AC (alternating current) that we want to convert to DC (direct current) so we can charge/power stuff. To convert AC to DC, we use a 3 phase bridge rectifier.  I chose to make my own using Schottky diodes, but you can use a pre-built one if you’re not up to making one.  I like the Schottky diodes one better because it doesn’t generate as much heat, and is therefore more efficient.  I soldered some 4mm banana connectors onto the 3 AC wires and onto the bridge rectifier.  I find using helping hands makes soldering tasks much more manageable.  I covered the exposed wires and connectors with heat shrink tubing.  I used 12 guage wire and Wago connectors where I could to limit how much soldering I had to do, and to make it easier to change stuff around if I chose to.  The charge controller takes the 18-36 volts of DC coming from the bridge rectifier and turns it into a useable 12 volts suitable for automotive chargers, like USB chargers, laptop chargers and inverters.  I connected a couple automotive sockets to the charge controller, you could add more if desired.

I’ve been able to consistently generate 120 watts with this rower generator.  That’s a comfortable workout for me, but if I want to step it up, I’ll need to invest in some bigger capacitors, then I believe I should be able to go well over 200 watts.  The little ultracapacitors I used are the limiting factor.  I could add more of them in series, but the connection point on these for the wire only allows a really small gauge of wire to be used and I suspect will limit the watts that can be pushed through.   Once I get some extra spending money, I’ll pick up 3 of those monster ultra capacitors and report back how it goes!

I welcome any questions on building this, and please let me know if you do build it!  Your support motivates me to keep doing these projects and following the Amazon links helps me pay for hosting this site along with funding my crazy experiments!

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