Chopper's Foot Drive

Mechanical & Electrical Parts List

03/27/18, 09/20/18 - I've been spending time learning about BLDC motor drives to use for Chopper's foot drive. This narrative is going to ramble a little as I've got a lot of ground to cover.

I started out planning to use a modified version of my Scooter Motor Outer Foot Drive that was designed for R2. To allow that drive to fit into Chopper's foot shells I proposed two modifications to the shells. 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. For Chopper I wanted something simpler, and more compact. I started looking at Brushless DC motors (BLDC) for use in a drive. Fortunately, others in the R2BC have already done lots of work in this area.

I stumbled around the R2 Forums until I came across the posting Brushless motor alternative for Jaycars by Olivers. Basically the drive consists of a 268Kv BLDC motor and a Banebots P60 16:1 gearbox. The motor has an unusually low 268Kv rating, most motors are much higher. This means the motor turns more slowly so a lower gear ratio gear box is adequate. The P60 gearbox is 1.9in deep, with a 1.5in long .5in dia. drive shaft. The two together make up a 2in dia. package 4.2in long. By comparison the Scooter Motors are 2.68in dia and 5 to 5.25 in long. This makes component placement within the foot shell a lot easier.

When I designed my R2's foot drive I used 5in dia Colson Wheels with .5in dia dead shafts. The wheels turned on their own internal bushings, driven by plate sprockets that were bolted to the wheel. Drilling the mounting holes in the wheels & sprockets and enlarging the sprocket center hole were all tasks left to the builders. 
For Chopper's foot drive I have configured a design that works with off the shelf components. What follows is the list of parts I used.

Wheels: We already knew that we needed a 4in wheel.
Banebots offers Colson 4in wheels with a press fit 3/8 bore clamp hub  - The 3/8 in shaft is more than adequate for Chopper's load. [I only used a 1/2in shaft on R2 because that was the size of the bushing in the 5in Colson Wheel!].
They also offer to grind down the wheel's crowned surface so it's flat, That provides more traction and, coincidently, saves space!

You choose the grinding option and hub after you select the wheel, I chose to have my wheels cut down to 4in and the 3/8 shaft clamp type hub press fit.
On the Banebots order page you should see:
Colson Wheel 4in x 1-1/4in

Gearbox - P60 Gearbox Standard, 16:1, For RS-500 Motor -
Remember that 16:1 gear ratio is about as high as you can go and still be able to push your droid by hand (if you have to :-(  )
(When you order your Banebots parts remember you need 4 wheels and 2 gearboxes -  yea, I got that wrong!)

Sprockets - I used a 22 tooth (type B) sprocket on each wheel. Now the OD of the sprocket is only 1.89in so space isn't going to be a problem.  Fortunately, the 22 tooth sprocket is also available in both a 3/8in bore and 1/2in bore which pairs with the standard shaft on the P60 gearbox. However, if you want to use sprockets that are all the same bore, you can get a 3/8in drive shaft for the P60 from Banebots (at extra cost!). Sprockets are available from a number of suppliers.  I used
B&B Manufacturing, 712 N. Fail Road, La Porte, IN -

I ordered qty 4: 25BF22x3/8 and qty 2: 25BF22x1/2 sprockets

The rest of the parts list is derived from the forum posting note above. I have added additional parts suppliers and comments:

268Kv Brushless motor -
    Find more detailed info is here:
Brushless motor controller -
    If you use a different motor controller make sure it can drive the motor in both directions.
Programming card for above -
LIPO batteries - Multistar High Capacity 4S 5200mAh Multi-Rotor Lipo Pack (my choice based on what I used with R2 - your mileage may vary!)

You might also want to buy mating connectors for the ESC to Motor and Battery connection (The ESC does not come with connectors!).
Tank Conversion: I used a Pololu ESC when I built R2 partly because it did the conversion from Left/Right and Front/Back R/C controller Joystick signals to the Tank Drive signal needed for R2 & Chopper. The ESC noted above does not do that. Fortunately, PaulG pointed me at DasMikro RC Signal Mixer For Tank Dual Bidirectional Motor Control from

Grease: The standard P60 comes without grease. Banebots recommends Mobil XHP 222 Special grease,
available from a number of suppliers - Grainger has Mobilgrease XHP 222 Special, 13.7 oz  -

[I purchased from Grainger only because I was also ordering other parts from them at the same time - there are lots of suppliers]

Replacement Motor Shaft - The gearbox pinion is sized for a 3.2mm shaft, the motor has a 4mm hardened shaft. It's easier to replace the motor shaft with:
SCP-HK-22-SHAFT317 Scorpion HK-2221 Motor Shaft Kit (3.17mm) -

Miscellaneous Parts
Since the wheel shafts are now live in this design you are going to need bearings and shaft collars for them.
Bearings: I used FR6ZZ 3/8" x 7/8" x 9/32" Metal Shielded Flanged Ball Bearings - I got them them on Ebay, a package of 10 for less than $10.
Shaft Collars: I got a package of 10 of 3/8 in bore Shaft Collars on Ebay for $10. I suspect I could have done better if I kept looking!
Shafting: 4pcs 3/8 steel shaft cut to 4.2in long
You'll need need 2pcs of #25 chain each 22.5 in long measured inside link to inside link (see pix). Lots of Suppliers.
Chain Master Link: Master links are used to splice chain, Get extras as the small clip parts are prone to flying off into corners of your workbench never to be seen again! I got a package of 10 from an eBay supplier there are many!
Chain Tensioner
: I used a W75-8372 Chain Tensioner for the Razor E100, E150, E175, & E90 Scooters from Monster Scooter Parts

Parts Used to Hang Battery Box from Foot Shell
The Battery Boxes are hung at the top from a pair of studs that extend out from the top of the Foot Shell and are held in place at the bottom by a pair of magnets.
Studs consist of a 4-40 x 1/2 in bolt held in place by a 4-40 nylon insert lock nut. Over the Bolt is an Aluminum Spacer and a Fender Washer.
Fender Washer: 3mm x 9mm Metric Fender Washer (Bolt Depot #17828)
Aluminum Spacer 6.35mm OD x 3.1mm ID x 6mm Long (
    The length of the space is chosen to be slightly longer than the thickness of the plastic (.1875) the Fender Washer provides an edge.

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

Chopper Home Page