Chopper's Foot Shell & Apron, Drive & Battery Box

Background Discussion

During the design of  what are 4 separate but interdependent designs: foot shell & apron, battery box, and foot drive there were a number of compromises made in order to be able to deliver an integrated assembly at a reasonable cost.

Chopper's Battery Box was derived from Cleofett's .dwg drawing files. However, as noted by TinyP the original 6.625 in tall battery box hits the body in 3-leg mode.

See TinyP's build log, page 8, scroll down to post #74

"One compromise that I already anticipated was the battery box
hitting the body when in 3-leg mode so I had to adjust the
height of the box closer to 6" instead of 6.5".

Picture Left & text above from TinyP's Build log
In addition to changing the height of the Battery Boxes, In my version of Chopper's Battery Box the OD of the ends was changed from 2.625in OD to 2.75in OD. The change was driven by the cost of obtaining any kind of tubing that could be used to fabricate a 2.625in OD design. See the discussion about Acrylic Tubing costs for price comparisons. 
When I announced my intent to cut Chopper's parts from Extruded Acrylic tubing, a number of folks warned me that it was difficult to machine. While I don't disagree, it's both hard and brittle, but I've found it manageable. See here for how I've done it.

At least one builder has noted that there's not much space in the foot for wheels! Here's an outline of the problem and the solution I've used. The drawing on the Left is a revised version of Cleofett's design. It's hard to find 1.1299 ID tubing. I've changed the curvature of the center mounting channel (shown in magenta) to 1.125 from it's original 1.1299 dimension and planned to use readily available Acrylic tubing. Then I checked the dimensions of the rest of the foot. Allowing for a sheet of  3/16 in styrene under the tubing, and presuming a 5/8 in reveal of the wheels leaves only 3.9424 in of space! Not enough to use a 4 in wheel. My next design iteration, in the center, utilizes a 1.5 in ID tube (center shifted up to keep the channel depth the same). That change was enough to get 4.073 of clearance. While it might work, It's a little tight for a 4 in wheel. My final design iteration was to use a 1.75 in ID tube, yielding 4.126 in of clearance.




I started out planning to use a modified version of my Scooter Motor Outer Foot Drive that was designed for R2, changing it from 5in wheels to 4in wheels. To allow that drive to fit into Chopper's foot shells I proposed the modification above for the 4in wheels.  However, even with those changes, from my perspective, Chopper's feet are still too small to comfortably fit a 2 wheel scooter motor drive where both wheels are driven. When I designed my R2's foot drive I used Colson Wheels with .5in dia dead shafts. The wheels turned on their own internal bushings, driven by sprockets that were bolted to the wheel. Enlarging the sprocket center hole, and drilling the bolt holes in the wheels & sprockets were all tasks left to the builder. For Chopper's foot drive I'm going to try to configure a design that works with off the shelf components. To make the drive system more compact I've adopted the brushless DC motor design published by Olivers on the R2 Forums. See my notes on the design here (not yet posted) and the components used.

Now we're set to "peel the layers on the onion" that is the Foot Shell & Apron, Drive and Battery Box assembly.
Except for the rainbow of colors, this assembly looks like most other Chopper Foot designs.

The Image to the left links to a 3D .pdf file that allows you to rotate and zoom in on features of the design.
The Battery Box hangs from two studs on the Foot Shell and is held in place by a pair of magnets. Note that the cutout in the side of the Battery Box is not symmetrical since the box is not centered on the Foot Shell. ie. there's a Left Hand version and a Right Hand version.

The Image to the left links to a 3D .pdf file that allows you to rotate and zoom in on features of the design.
With the Battery Box removed you can see how compact the drive components actually are! All of the space in the Battery Box is available for the LIPO battery and the ESC to drive the motor.

The Image to the left links to a 3D .pdf file that allows you to rotate and zoom in on features of the design.
The Foot Shell and Foot Apron are assembled separately and then glued together. The Shell assembly slips onto the the Foot Drive Structural assembly and is bolted in place from the underside of the foot.

The Image to the left links to a 3D .pdf file that allows you to rotate and zoom in on features of the design.
The Foot Drive Structural components serve as the mounting point for the Drive parts and provide the mounting point for the legs. Not shown on this drawing is the chain that links the three sprockets. You can see the chain path in the detailed assembly drawings.

The Image to the left links to a 3D .pdf file that allows you to rotate and zoom in on features of the design.

For a description of the Mechanical Components, a list of  part numbers and designations, and the suppliers I used go to the Chopper Foot Drive - Mechanical Parts page
The last piece in the onion, the Foot Drive Structural assembly shown here without the Drive parts.

The Image to the left links to a 3D .pdf file that allows you to rotate and zoom in on features of the design.
You can download a .zip file that includes both .pdf drawings of the parts for the Foot Shell, Apron, Drive Structural & Battery Box components of the design.

Follow the links here to the detailed descriptions of each of the components:
Foot Shell & Apron
Foot Drive
Battery Box

This design is copyright Media-Conversions LLC and is made available for non-commercial use only.

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