Bicycle Frame Polar Tracker




What better to make out of an old boy's BMX bike frame than an equatorial style tracking mount that can be used for solar energy to get nearly 50% more power out of a couple PV panels! Most solar trackers are either big expensive things, or so small and flimsy as to be completely impractical. I wanted to come up with something in between. And just for fun I wanted to make it a true equatorial tracking mount instead of an Altitude-Azimuth style mount. The concept is simple enough: use the handle bar as the control arm for the main polar axis with the front forks, the bike frame is turned upside down, and then tilted up to get the correct polar angle for the latitude, then a frame that mounts on top of the forks and pivots on the original axle bolt locations provides declination adjustment and support for the PV panel. There's a little bit of work to be done on the forks and the frame to accommodate the linear actuators that will move the tracker, but it can all be done with basic MIG skills on most steel frames. Nobody could guess what I was building when I made it at TechShop ( using just MIG1, Ironworker, and basic metal shop class skills and tools. Final control is provided by a Home CSP Mega Tracker dual-axis digital controller.

Step 1: The Frame

Picking a good frame is important. This boy's BMX frame has a few features that make it ideal:
 - the seat tube and fork tubes are nearly parallel
- the seat to crank distance is short which helps actuator clearance
- the fork tubes are round and easy to cut and splice

The frame is prepared by stripping everything else off except for the seat because we'll use some of the hardware from the bottom of the seat later on. This would also be a really good time to sandblast if desired and really clean everything. Lubricate the fork bearings now as well if needed.

Step 2: The Base

Building a base to support the frame was my first objective. I used some 1.5" angle iron to make a double legged T. A couple welded brackets provided mount points for the rear axle bolt holes. 1/4" rod fit the clamps used to hold the seat on the post, so a couple pieces of rod were welded to the legs to provide support for the seat post mount.  By sliding the seat post in and out slightly and sliding the seat clamps along the rod, the angle of the front fork axis can be adjusted to point at celestial North. A piece of 1.25 sq tubing matches the inside rod spacing and provides triangulated support. This was welded on after this picture.

Step 3: Fork Modification

With the mount taken care of, attention moves to the front forks. In order to get enough clearance for the linear actuator (which is nearly 30 inches long), the forks need to be extended, and a mounting bracket for the actuator created.
The first step is taking the forks off and cutting them. I made sure I had a couple inches of the straight/parallel tube after the bent area and cut each side off as evenly as possible with a cold saw.

I got some 1" OD tubing that fit perfectly inside the fork tubes.
While I started out thinking that the extensions would be straight, I wasn't thinking about the tilt of the bike frame, and allowing the PV array to tilt below the celestial equator. Without a bend in the tubes, the PV array will hit the handle bar mount during winter tracking.
When finally welding the axle end of the forks on the extensions you may want to swap sides, or rotate them 180 degrees. the reason is that you want to make the width as wide as possible to help the array stability (6-9" is fine).

Step 4: Finishing the Forks and Frame

A couple pieces of bar were cut with the Ironworker, punced with mounting holes, bent with a sledge in a vise, and then welded in place on the fork inside edges to provide a mount for the declination actuator motor.  A heavy duty SuperStrut angle bracket (from Lowes electrical dept) provided a mount for the polar axis motor. In order to have clearance for the motor the actuator was mounted in a cross-armed fashion to a 14" piece of 7/8" rod that replaced the original handle bar. A bolt was welded perpendicular to the rod near the end for the actuator arm end mount 

1.25" sq tubing was used to make the final elevation frame to support the future panels.
Lawn mower wheel style axle bolts were used, and custom bushings made from 1" Delrin rod fit inside the short round tube pieces welded to the central part of the frame provide a fine pivoting movement. 
The important part with the elevation frame length is that it corresponds to your desired control radius for your actuator.

Something about all the crazy angles made me decide to paint it up in a black and yellow paint scheme.

Step 5: Adding Controller and Wiring

The final step is adding the electronic components needed to make everything work!
I used the Home CSP Mega Tracker, which provides dual-axis digital control using astronomical algorithms for solar position calculations. It's based on the Arduino Mega motherboard and is fully configurable for a wide variety of custom tracking mounts. The controller comes with a waterproof enclosure and membrane keypad with works for setup as well as manual motor control.
3/4" flexible PVC conduit, as well as a rigid 90" sweep, and a T type conduit body were used to enclose as much of the wires as possible, as well as the motor drivers and 24VDC step-up power supply.
#3 rubber stoppers were used for grommets to seal the wire entrance/exit points.
A standard SLA 12V battery 4-10Ah fits nicely in between the central base legs and can provide power for several days of tracking.



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    10 Discussions


    4 years ago

    Tracker could be simpler: A housing with a slight aligned over a photocell switched, turning off a small motor that turns the whole unit once the sun passes through the slit to hit the switch for the motor. you would want the direction of rotation to match the sun moving in your sky.

    1 reply

    Reply 4 years ago

    *slit aligned*


    5 years ago on Introduction

    Bravo! I wish I was smart like you. Here in Arizona everyone should have solar!


    5 years ago on Introduction

    Brilliant!  I love the McGiver-esque application using everyday parts.  Can you post some pictures with the PV panels mounted and your device in various positions?  Also, how stable is the thing with the panels mounted? Can it sustain much wind without having the base tied down?  How much greater is the watt/hrs obtained than the same panels on a typical stationary mount?



    Reply 5 years ago on Introduction

    Yes, find tracking trajectory of the heavenly body you wish to observe and plug it into the servos.

    I didn't check how many axis this (the project says plainer so I am thinking 2 axis) is so if the comet is 3 and this is 2 axis understand it will travel across the photo.


    Reply 5 years ago on Introduction

    Thanks for the comment. I understand the temptation. I've got a little 4" scope I may try to mount on it just to play with. I think It would be OK for wide field visual observing, but not planetary/photographic.
    The first change would be to replace the R.A. actuator with a clock drive mechanism because the linear actuator movements are too crude for photography. The second problem is because it uses a control arm (instead of a gear/slew ring) the rate of rotation varies relative to the actuator motion. This is most pronounced at the extremes of movement. A speed controlled DC brushless motor would probably be the best, you'd have to come up with your own controller most likely.
    The third problem is that telescopes are usually counterbalanced to help provide consistent tracking rates, and this design doesn't really have that, so your motor feedback would have to help you control the speed..


    5 years ago