The Outer Foot Drive Parts

Mechanical & Electrical

There are several parts that you will need to complete the Outer Foot Drive Assembly. Note that you can assemble the foot with just the wheels and no drive components if you are building a static droid and want to be able to roll it around (instead of picking it up and carrying it).

Mechanical Parts
The Motor
I'm using the Razor 100/125 watt - 24v - scooter drive motor with a 9 tooth sprocket.
there are a lot of suppliers you can get them from at around $25 each. - w1311612030 - my6612b 100w chain drive - my6812 100w chain sprocket - #119-48 100w chain drive
QTY 2 - one for each foot.

The Chain
you will need two pieces of #25 (1/4 inch) chain for each foot.
One Piece 16 links (8 inch)
One Piece 40 links (20 inch)
QTY 1 -  a minimum of 60 inches of chain for both feet - I found it cheaper to get a package of 10 foot
see the assembly pictures here to be sure we agree on how to measure chain length.
you will also need coupling links for each chain. (I got mine from SDI-SI, see below)
A "chain breaker" is not necessary - see the instructions.

The Sprockets
you will need 
QTY 4 (2 per foot) -  22 tooth sprockets without hubs (flat - type A) , one for each wheel
you will need 
QTY 2 (1 per foot) - 22 tooth and QTY 2 (1 per foot) - 9 tooth sprocket with hub (type B)
your motor should have come with a 9 tooth sprocket mounted on it's shaft. If it did not
then you will need an additional 9 tooth sprocket for the motor.
I got my sprockets, chain links and ball bearings from SDP-SI -
QTY 2 (1 per foot) - A 6C 7-25B09  - 9 tooth sprocket with hub - (used on jackshaft)
QTY 2 (1 per foot) - A 6C 7-25B22  - 22 tooth sprocket with hub - (used on jackshaft)
QTY 4 (2 per foot) - A 6C 7-25022  - 22 tooth plate sprocket (no hub)
QTY 4 (2 per foot) -   A 7Y55-FSS5025G - .25 ID, .5OD flanged ball bearing - (used on jackshaft)
QTY 8 (4 per foot) -   A 6Q 7-25SCCL  - spring clip chain link 
QTY 4 (2 per foot) - S5000Y-25016 - shaft collar .25ID, .5 OD - (used on jackshaft)

An alternate parts source for the sprockets, Your milage may vary as I have not tried these:

Linn Gear Co. appears to offer 25B09, 25B22, (9 and 22 tooth with hub) and 25A22 (22 tooth no hub) sprockets.
I don't know if  they have order minimums or not.
Their parts are distributed by MRO Supply, Inc. MRO has small order minimums and free shipping.

The Wheel
Colson Caster - part number
it's on page 82 of that catalog, or search by part number

There are lots of vendors for colson casters.
I bought mine on eBay from
they sell it as:
QTY 4 (2 per foot) - Colson Gray TPE Performa Soft Rubber Wheel 5"x1-1/4" with Delrin Bearing 2-5-441
they take "best offers" at between 10% and 15% less than their asking price
and they do not charge much for shipping, no minimum order. (buy center foot caster at same time!)

Springs (main chain tensioner)
I use a pair of compression springs in each foot to provide the tensioning for the main
drive chain. While a google search will yeild many spring vendors (WB Jones Spring Co,
Century Spring Corp, Diamond Wire Spring Co, Lee Spring Co.) most have very high
minimum order quantites. McMaster-Carr carries springs, but only sells in packages of
12.  I found two vendors, Grainger and Zoro Tools, who supply springs in packages of 5,
and curiously enough, use the same spring manufacturer and reference the parts using
the same part numbers. Zoro has cheaper prices and a fixed $5 shipping & handling charge.
I used the #1NBY9 compression spring -  
Zoro's part number G0696963 -
QTY 1 (package of 5 springs, you need 4, 2 for each foot drive)

you will also need 2 pieces of 3/8 in Delrin Rod - cut to 2.75 in long, #4-40 tap and drill.
I enlarged the hole in the rod from #29 (4-40 clearance) up to #27 so it didn't bind (see pix!)
I got mine from US Plastic, - Delrin is also called Acetal
Black Acetron® GP Acetal Rod - #42228

The Shoulder Bolts
Shoulder bolts, Allen, Alloy steel black oil finish, 3/8" shoulder (5/16"-18 thread) x 1-1/2"
QTY 2 (one each foot) - Bolt Depot part #14078
QTY 2 - unless you can find it at the local home depot, or equivalent, get the mating 5/16"-18 square nut

The Magnets
QTY 10 - Dimensions: 3/8" dia. x 1/16" thick - Grade N52 - K&J Magenetics part number D61-N52
buy lots, you need 8 (4 for each foot) at a minimum, but they are fragile and easy to loose. (also used in the center foot)
I started with
Dimensions: 1/4" dia. x 1/16" thick -
Grade N52 - K&J Magenetics part number D41-N52
then went larger for
a stronger grip

Steel Shafts
1/2 in steel shaft - 2pc 3.25 long for the wheel axles
assm: drill two holes at each end for 8-32 retaining screws 
1/4 in steel shaft - 1pc 4in long for the jackshaft

12 - 8-32 x 3/4 screws to attach sprockets to wheels
3 - 10-32 x 1/2 screws to mount the motor
4 - 4/40 x 1 in screws as guide pins for the chain tensioner (longer is better..)
2 - 1/4 in shaft collars (2 jackshaft) -  see above under sprockets
2 - 1/4 in ID, 1/2 in OD flanged bearings (jackshaft)
-  see above under sprockets
2 - .3 in pc of 1/2 in copper pipe - used as spacer

You should already have almost everything  I use during the assembly process.
The assembly process calls for a number of holes to be drilled and tapped in a variety of sizes. If you don't already have a set, consider the Vermont American 21698 13-piece set. It includes six high-carbon steel taps and six matching high-speed steel drills plus one T-handle tap wrench. Taps, drills and wrench fit into handy, tough molded plastic case. You can get it, or an equivalent set, from a number of internet suppliers, including Zoro tools, the supplier I used for the springs (above).
One thing I had to buy was a the Step Drill shown in the assembly instructions. I used a 1/4" - 3/4" Titanium Nitride Coated High Speed Steel Step Drill Bit  # 44460 from Harbor Freight Tools -

Electrical Parts

The ESC's (Electronic Speed Controls)
I'm using a pair of Pololu Robotics & Electronics #1381 Pololu Simple High-Power Motor Controller 18v25
This version operates from 5.5 to 30 V and is efficient enough to deliver a continuous 25 A without a heat sink.
Consult their Pololu Simple Motor Controller User’s Guide -
This ESC is extremely flexible, it's programmable using their provided program, over a usb port, and does the 2 servo channel to tank-drive conversion for you.
Since there are two separate ESC's it's possible to mount them, and the batteries in the battery boxes.

R/C control, Batteries, Battery Charger
Since I am just starting out I thought I'd start with the simplest form of R/C control and upgrade later on. There are any number of R/C suppliers on the internet. One that was recommended to me was HobbyKing - The 6ch 2.4Ghz servo system was only $30!
From them I purchased -
2x #N5000.4S.35/20740 Turnigy nano-tech 5000mah 4S 35~70C Lipo Pack
1x #ACC6/18066 Turnigy Accucel-6 50W 6A Balancer/Charger w/ accessories
1x #HK-T6A-M2/15140 Hobby King 2.4Ghz 6Ch Tx & Rx V2 - Mode 2
I also bought a set of rechargable AA batteries, you'll need 8 for the transmitter. The ESC's have a BEC (battery eliminator circuit) so for testing you don't need a power source for the reciever. If you locate your reciever in your Droid's body you will need a separate power source since it won't make sense to bring reciever power up from the legs.

Power Connectors
There are lots of different types of power connectors available from a variety of R/C sources. I chose the XT60 style as they are available from a number of sources.
In addition to the connectors, I needed an adapter for the LIPO battery packs, which come with 4mm Bullet connectors.
I found a supplier via Amazon: eCRATER - They could supply me with both:
XT-60 Female To HXT 4MM Male Bullet Connector / Adapter
30 Pairs XT60 Plug Connectors plugs Male / Female RC lipo battery connector
at low cost & free shipping. They are a broker for Chineese suppliers so delivery can be long (plan ahead!)

I used 16 Gauge Red/Black Zip Cord (pair of wires bonded together). My supplier for the wire was Powerwerx -
I bought a 25ft spool.

Reciever Wiring
Consult the
Pololu Simple Motor Controller User’s Guide for a wiring diagram. Section 4.3. Connecting an RC Receiver - scroll down to the heading "Driving a Motor" (pg 38 of the .pdf download I'm using) for a wiring diagram. You are going to have to connect the 2 servo channels you use to both ESC's so you'll either need Y-cables or some custom wiring arrangement. While I purchased it for a different project, Coleman Cable 51112, 22AWG, 4 Conductor Cable UnShield PVC  from Intertex Electronics, Inc.- is convenient. (4 wires are just enough, gnd, servo-1, servo-2, and the err signal). Since you don't need much, you could also use
Fry's Electronics
Micro Connectors 6' 24AWG 4-conductor Stranded Shielded Cable - #3550934 - [That's a smaller gauge wire and I wanted the 22 AWG for the project I was doing so I didn't use Fry's]

I used both Weldon #3 and Weldon #16 cements for the assembly. Each at different points in the assembly process. You should use both, or equivalent cements.
I used Squadron Green Putty - it also comes in white. It's easier to see the green against the white styrene. There are obviously lots of other suitable products.

A Note on suppliers cited
I have no relationship with any of them, except as a satisfied customer. Most were chosen because of their low prices, no minimum quantity order requirements, and cheap shipping.