*--- UPDATE ----*
It's been a few months since settling into the new space and I finally was able to devote more time to my little project. It has grown quite a bit and I do mean quite. What started as a simple precursor to a half track is now something far bigger. Read on
*--------------------*
Some years ago, I had some scrap aircraft aluminum that I was using to practice riveting techniques with. As I got to drilling and countersinking, I realized that if I am going to be drilling holes and putting in fasteners, it might as well be for something more than just some scrap bits and pieces. My scrap aluminum became a Go Kart and it went very well. With some Home Depot materials and stuff lying around my garage, I cobbled together a 2 seater that at half throttle was clocked at 15mph. Since then, I moved twice and the Go Kart is gone....well, the frame is. But the engine, wheels and transmission (a Torque-A-Verter torque converter) are still here. This project started out as a prelude to building a 4x6 Go Kart/ATV/Replica mini APV (I have these huge ATV wheels to play with), I went puttering around the Internet and found the plans from SpiderCarts (www.spidercarts.com) and what intrigued me most was their 3 wheeler. Looked around my shop and lo! I have wheels, sprockets, bearings, chains, tons of nuts and bolts, cables.....Now if I can make the same kind of thing but NOT weld a darn thing....

As I got to work and I assembled my frame, I found that the use of a single back wheel posed some problems with the parts I have. Specifically, my rules were that I was not to use (or try real hard to not use) anything I did not already have, and if I needed it and couldn't make it, then and only then could I go buy it. In this case, the problem was the brake. I had these lovely disc brakes, but they were meant for a 1" shaft and they were meant to not mount to a wheel. After much trying, I found that the brake calipers just would not accomodate this concept, so I shifted gears. The first few steps take you up to the point when I realized my problem, and then I continue with the resultant design. Here's the thing though: the drive system as I have completed it allows my MMV to be a 2x6, a 4x6 a half track and by locking the steering, a tank, or a 4x6 skid steer. So follow along won't you?

As you might imagine then, this is a work in progress.

Step 1: Inventory and the Rear Wheel

Here are my basic rules again:

1) Buy very little extra stuff
2) If I must buy something, try to get it at Home Depot or Lowes or Ace or whatever hardware store I can reach
3) If it is something specialized (like a sprocke)t, get it on the cheap, even if it means cannibalizing something else
4) Use the maximum amount of parts I already have (I actually made this a design constraint -- if I had it, I had to make it fit/work)

If you have to, adapt my design, steps or parts to accomodate what you can get your hands on.

With that out of the way, let's start with the rear wheel. Here are the parts I had available to me:

1) 3 differentials, the sort you buy at Northern Tool. 38" axle with a 1" diameter with lots of slots for keyways.
2) 1x #41 chain sprocket from Northern Tool (hole pattern fits the differentials)
3) 2x 5" Brake disc with 1" bore hub (Northern Tool)
4) 2x 36"x1"x1" square steel tube (Home Depot) -- rear frame stringers
5) 4x 1" Pillow Block bearings (anywhere on eBay, I got mine for $15 delivered) -- frame step but needed it here to verify design
6) 8x 3/8" nuts, 8x3/8" bolts and 16x3/8" washers
7) 2x Airheart mechanical disc brake caliper (Northern Tool) -- we will need these when we do the frame
8) 4 right angle brackets (2" are good), again, Home Depot
9) 4 C-Channels, preferable out of steel, 3/4" steel is good. If you can't find any, improvise with square tubing. These are for the engine mount. I used left over pieces of channel from the wall mount for my LCD TV -- remember, recycle, re-use, re-purpose.
10) 6 wheels and tires, 4x1" hubs (hubs from Northern, wheels from Surplus Center) and 2x1" hubs with roller bearings in there (front wheels for steering don't need to be bolted down to the drive shafts). I used the aforementioned ATV wheels and tires, but what will drive you are the size of your brake discs and drive sprocket.
11) 4 smaller 1" #35 chain sprockets
12) A length of #41 chain
13) 2 lengths of #35 chain

Step 2: Perpare the Parts for the Rear Wheel

Critical to this is how you mount the drive shaft. The drive shaft is the back shaft (remember, 6 wheels here) and positioning it on the frame and relative to the second shaft depends on how big your wheels are, so measure carefully. So, first I built the drive shaft, then the frame and then mated them together and installed the wheels.

In putting together my shaft, I bolted on the sprocket to the first differential and drilled holes in the hubs of the brake discs and tapped them for set screws so that the holes were directly above the keyway in the hub. I have seen hubs where the set screw is offset away from the keyway, but I like it on the keyway because if I drill a second set screw diagramatically opposite, one holds the hub to the shaft while the the other holds the keyway AND hub to the shaft. In the case of the differential shaft I had, the slot for the keyway allows for the keystock to slide out, so I needed the set screw to not allow that to happen. Of course, I could have my set screw also go all the way into the slot for the keyway, but I wanted a bigger surface for the hub to push against rather than just a set screw tip and haft.

The picture here is of the brake disc I have, ignore the sprocket, that's not the one I am using.

Step 3: Rear Frame -- Drill Holes for Bearings

The square steel tubing you buy at Home Depot is pretty strong stuff. For this step, you will need a drill press and a suitable drill bit that can go through steel. I bought my drill press from Harbor Freight. Not the best thing in the world, but it works. During this step, the bit got jammed in the hole it was cutting and the chuck fell off! I fixed it, but be careful.

First, line up your bearings and put the end of one about 1-1.5" from the edge (I am assuming both pieces of square tube are exactly the same) and line it up to be in the center of the tube. Mark the holes for bolting the bearing on. My bearings have 3/8" holes, so I made 1/2" holes with my drill press. Once you have made the first set of holes (make sure you go through the tube), find a bigger bit, or a round file and debur the holes. You should always debur holes to make sure your fasteners, washers, etc. sit flush with the surface you are working on, and also because the burs eventually break off and will likely rust, or get into stuff you don't want them to. To debur, just get the larger bit and put it over the hole and turn it by hand. It should smooth out the edges of each hole.

Once you have your first set of holes, clamp the second tube to the first, lining up the edges and mark the second set of holes and drill them out. If your bearings have oblong holes, this will give you some play to make sure you can square your axle later.

Step 4: Finish the Basic Frame

In this picture, you can see the frame largely assembled. The wheels in the middle can be ignored. Originally this was to be a 3 wheeler, but it evolved to have 6. Once you have the frame squared, get your shaft, installed your bearings keeping everything loose so you can slide things around. You will need to center the shaft(s) before you mark and drill the holes for your bearings.

Be sure to use a drill press to make your holes go straight through and use the square brackets to square your frame.

Note that the hinges you see are going to be removed. Again, these were there when this was going to be a 3 wheeler --- what was I thinking?

The black metal C-channels are what came with the TV wall mount. I am using them here to provide a base where I can mount the engine and allow me to slide the engine right and left to get it lined up with the drive sprocket.

Step 5: Shafts, Chains, Sprockets, Oh My!

Ok, so here is a picture of the frame, with the shafts in place. Look at the way it is laid out to understand what is going on. The measurements should not matter, as what you gather for material determines how it all goes together, but the concept is the same.

Aren't the wheels HUGE? Ok, so here's what we have:

The shaft to the right is the drive shaft, it has 2 brake discs and one drive sprocket. If you hit both brakes, it stops like a car. If you hit only one brake it uses differential braking. This is how a tank works.

The shaft to the left is the secondary. If you leave the chain on between the 2 shafts, you have a 4x6. Take the chain off, it's a 2x6. I imagine it would stop better as a 4x6 or skid steer/tank because braking on one side of the drive shaft also brakes the same sided part of the secondary shaft.

On each side, you have 2 small #35 chain sprockets on the OUTSIDE of the frame put in the #35 chain. This is what gives you the 4x6 function. If I remove the chain and put a track (this will have to be a much much later update), you won't need it. The track goes over these 2 wheels (half track) or over all 6 (full tank). I am working on a modular track design, BTW.

The sprocket in the middle of the drive shaft is the #41 chain sprocket. If you look closely, you will see I have temporaily clamped the brake calipers in place.

I have put in a second picture so you can see how the frame was transformed and how everything sits.

Step 6: Wheels On!

Here it is with the wheels in place. Look how big this thing is. I can't believe I started out thinking Go Kart! One thing you have to make sure of is that when you mount your wheels you will have to retension your side chains. The extra load might shift your pillow block bearings if they are not on real tight. Make sure your chains are tight here, you don't want any slippage (although if they do pop off, you are still going to be moving, but the noise and the wear on one shaft will be ugly.

Now, in theory, I could stop here. I could mount the engine, seat and a pair of brakes and make this just a 4x4 that operates as a tracked vehicle at some point. You wouldn't need to do much. The engine I have set aside for this is a 6.5hp one from Central Machinery with a #41 chain clutch. Simply build a frame for your seat so that you sit towards the back of the vehicle with the engine below you and to the front. Install 2 foot pedals to operate the brake calipers and that's your steering system. Add a throttle and you are all set.

But where's the fun in that? Stay tuned...