Plans and Instructions are Now Available!
I bought a toy King Tiger for my 5-year old son (the only reason we men have kids is so we can play with their toys) and, well, I just "got to thinking." Could this thing be scaled up and fitted with R/C controls? How big could it be made? Why not - dare I say it? - big enough for ME TO DRIVE AROUND? Well, that was a bit excessive, but one big enough for my son to drive around is well within reason. (Or at least more reasonable than most of the things I do.)
Just imagine the looks on your neighbors faces when you rumble into the midst of their barbecue in a Panzer, and paste the beer cooler with your 37mm potato cannon. That'll teach 'em to drink Canadian beer on the 4th of July.
Recognize it? It's a WW II era M4 Sherman Tank. It's about 1/5 scale, 28" wide over the tracks (so it will fit through a standard interior door - barely), and about 4 feet long. Except for the drive components, it's made entirely out of wood, even the tracks.
Image from OnWar.com, used with permission
The Sherman was a cool little tank. It was 19'-2" long, 8'-7" wide, 9'-0" tall, weighed 33 tons, carried a crew of five, and had a top speed on a smooth road of 24 mph. The 75 mm gun was replaced late in the war by a more powerful 76 mm gun with a much longer barrel, and some Shermans were equipped with a 105 mm howitzer. The British equipped theirs with 17-pounder (76mm) guns, and called them "Fireflies". The first Shermans had a Continental R974 C4, a 9 cylinder radial aircraft engine! This, in part, is why it was so tall - usually not a desirable characteristic in something people are going to be shooting at. Later versions had other engines, including the unusual "Chrysler Multi-bank", five 6-cylinder engines arranged around a common crank.
The hull of the model is cut from four quarter-sheets (2' x 4') of ¼" Lauan plywood. The pieces are attached at the edges with ¾" chines cut from scrap 2x4s. It took about two pounds of 1" and 1-¼" sheet rock screws to hold it all together. The resulting hull is very rigid and surprisingly light. The tracks and suspension easily weigh twice what the hull weighs.
The power source in this model is two 12v 75Ah deep-cycle Autocraft batteries driving two 24v 1/5 hp electric gear motors (see the bottom of this page for more on these motors). The batteries sit in the back, side by side, while the motors sit in front in the "sponsons" above the tracks, driving the front axles via a #25 chain and sprockets (I've since switched to #35 chain and sprockets - see below). There will eventually be a built in 24v battery charger and fan to ventilate the battery compartment.
Scale speed was designed to be 2 mph, or 3 fps, so the sprocket should have turned at around 150 rpm. As it turns out, the motors (described in the catalog alternately as 30 rpm and 45 rpm) actually turn at 67 rpm under no load. The gears I have available are all 40 teeth, so the final drive ration is 1:1. Assuming the motors turn ¾ of their free rpm under a normal load, speed will be 50 rpm, or 1.28 fps. Under 1 mph. That's fine - remember, this is going to be controlled by a child.
It is be controlled by an X-Arcade joystick, modified to make it shorter, mounted in the left (my son is a lefty) sponson. It is turned 45° so that pushing the stick forward triggers what is usually the upper and right (or left) switches, which control the left and right tracks. Pushing it 45° to the left will only turn on the right track, and 45° to the right turns on only the left track. Pushing the stick sideways turns on both tracks, but running in opposite directions, so the tank will "pirouette" on the spot. The joystick controls a set of 30A 24v relays to control the motors - I hope to eventually refit it with PWM control with a "momentum simulator" circuit like model trains use.
The bogies are modeled after the Horizontal Volute Suspension System (HVSS) used on the M4A3 version of the Sherman - mostly because it was easier to model than the original Vertical Volute Suspension System design. The bogie arms were cut from 2x4, and drilled 1/2" for the axles. There are no bushings, though I may go back and install some. The wheels are made out of 4-3/8" diameter circles cut from 3/4" plywood, drilled out 7/16" and pressed onto 1/2" shafts. The springs are valve springs off a Chevy 350.
The drive sprocket was designed in AutoCAD, printed to scale, and glued to a piece of 1/2" plywood. I cut it out on my band saw and used the first one as a template to cut three more. Two sprockets were then glued and screwed to a hub of five 2" circles cut from 3/4" plywood, and the whole thing was bored out 5/8". A 1/4" steel rod passes through the sprocket assembly and the axle. The axle is mounted on ball bearings.
The track is modeled after the T-41 track used on the Shermans late in the war. (Okay, it's actually modeled after my watch band, but don't tell anyone.) Each track link consists of a pad, two links, four screws, and a bell (the thing on the inside of the track that keeps the bogies "on track".) Building the track is tedious as hell, as each link requires:
It takes about ten minutes per link, even though I'm doing them in batches - all the drill press work at once, for example - to make it go faster! I need 70 links, plus a few extras. And worst of all, I'm not entirely sure a wooden track will be strong enough - I may end up having to start over again with something stronger.
The turret ring posed some real challenges - after a great deal of fiddling, I settled on a simple "bushing" style turret ring, although this required me to bend ¼" plywood to a diameter of 13.5" or so.
I was lucky in that I found a piece of 1/8" plywood in the corner of the basement, left by the previous owner of the house. This was perfect, as it bent easily and - even better - cost me nothing. I cut this piece into four 1-½" by 36" pieces and used the base of the turret as a form to bend, glue, and clamp two of them into a ring. They weren't quite long enough to go all the way around, so I spaced the gaps 180° apart and cut one of the remaining pieces to fill them in.
After letting it dry overnight, I sanded it carefully and glued it into place in the turret base, filling the inevitable gaps here and there with home-made wood filler (wood glue and sawdust). The turret sits nicely in the hole in the hull and turns easily, but I still added some wheels swiped from some Burger King toys (BK toys are full of all kinds of neat little bits) so it rolls, rather than slides, and sits about ¾" above the hull. A couple of 3/16" bolts though the ring below the hull keep it in place; two of the bolts are mounted on springs so they can be pulled easily to take the turret off.
The gun is simply a piece of 1" schedule 40 PVC pipe stuck through a piece of 4" DWV PVC pipe. This gun is a mock up of a pneumatic cannon I've designed (but not yet finished testing) specifically for this tank. It will be supplied by pressurized air stored in a 60 cubic inch air tank stowed in the rear of the left sponson. Tank pressure will be about 100 psi, regulated down to a much lower pressure for the gun.
The sides of the turret were cut from corrugated cardboard and glued in place - I had to take a box cutter and slice up the inside to make it bend smoothly, but it worked quite well and is surprisingly strong. I almost used papier-mâché (French for "chewed paper" - how romantic) to replicate the rounded shape of the original cast Sherman turret.
It took three cans of grey Krylon primer and two of Krylon Hunter Green to paint the tank. The stars were painted with Krylon White using a stencil cut from the back of a pad of paper.
On November 16th, I installed the batteries and gave it a try! Frankly, it worked great. It was surprisingly powerful and easy to control, even when I was crawling rapidly along side it with one arm over the side holding the joystick. (I've since hooked up a remote control.) The tracks made a lot of noise, but didn't give me any problems. The motors didn't drop too far below their free-rpm of 66 rpm, and the lights didn't dim so I know the motors weren't drawing excessive power. I drove it across the basement to the furnace, pirouetted, and drove it back, a grand total of about 20 feet...
...and it suddenly stopped moving and starting making an awful grinding noise.
Investigation revealed that the geniuses who designed the motors used a plastic worm gear in the gearbox. Naturally, the gears stripped. The motors weren't heavily loaded, as they didn't slow down, draw excessive current, or even get warm... so why did the gears strip? Shouldn't a transmission be at least as strong as the motor it is attached to? Is there ever an excuse to put a wussy little transmission on a big hefty motor? Am I missing something here?
So now I trying to figure out what to do: cut new steel gears or buy new motors. I'll keep you posted...
Update 11/26/2002 - I think I've identified the manufacturer of these motors: AM Equipment. These motors appear to be their model 224-1103. If it weren't for the plastic gears, I'd say these were very nice, inexpensive, powerful motors.
Update 12/21/2002 - After wasting two weeks trying to get replacement metal gears cut for less than $300 each, I gave up and ordered some MONSTEROUS motors from botparts.com. Brock Schippers, who runs the show over there, was very helpful in recommending AM Equipment 265-1002 motors: 48 ft-lbs of torque at stall, and 82 free rpm! That's a scary amount of torque.
They arrived on the 19th, and the first thing I did was pop the cover and see what kind of gears were inside. I suspected they'd be metal, as the covers were held on with screws, and not rivets as they had been with the original motors (as if the engineers were too ashamed to let anyone look inside...). I was right, the gears are metal! Woohooo!! I have begun installing them - which will take some rebuilding, as they're frickin' HUGE - and will post pictures soon!
Update 12/24/2002 - IT IS ALIVE! (Maniacal laughter.)
The new motors are installed, though it took a major rebuilding effort. They are MUCH more powerful than the old motors, and can pirouette the tank with ease. They're quiet, too.
New motors in place.
I spent the time waiting for the motors to arrive building headlights - two 50 watt MR-16 halogen bulbs. They're not very realistic looking, but they're incredibly bright.
Finally being able to test the tank, I've found a small problem - the road wheels keep falling off the axles. I had thought that a friction fit would be sufficient to keep them in place, but alas, I was wrong. I am working on a fix using e-style retaining rings.
Merry Christmas/Hanukah/Solstice, etc!
Update 2/11/2003 - Further testing
I fixed the road wheel problem using e-clips. I chucked each axle in the drill press and used a hack saw to cut a small grove in each end for the clips - works great!
Further testing revealed yet another problem, however - the chains refused to stay on the sprockets for very long. Turns out I was over-optimistic when I hoped that I wouldn't need a way to tension the chains. The fix was pretty easy, though - I just modified the motor mounts to allow the motors to shift slightly backwards to tighten the chain.
I've also started work on the gun. The trick will be to fit all the necessary components into the available space between my son's nose and the front of the turret! The current incarnation uses a 3/4" copper pipe as a barrel, and a specially modified ball valve for a breech.
Update 3/10/2003 - Plans and Instructions!
Several people have urged me to write up a set of plans and instructions and offer them for sale. So - I am writing "Building a 1/5 Scale M-4 Sherman Tank Riding Toy", 56 pages and 81 illustrations and photos. It will be in PDF format, and I think I will charge $10 for it delivered over the web, or $15 on CD by snail mail. I am hoping to get it all up and running in a week.
I still working on the pneumatic gun - free time has been in short supply lately. I've got it mounted in the turret, but no air supply hooked up yet.
Boy, I can't wait for warm weather so I can take the tank outside and get some decent pictures. I've been very lucky in that my son hasn't grown an inch since I started this project!
Update 3/21/2003 - Beware the Ides of March
On March 15, this site received one million hits. (One million hits isn't quite as impressive as it sounds - a mere 84,000 visitors.) Turns out that this site was featured on Fark.com, and from there news of it spread to some other sites.
It even spread to Radio - I am to be interviewed for "The Other News", a one-minute blurb put out by the Associated Press Radio Network! I'm really quite amazed at all this attention.
Update 4/5/2003 - Plans are now available
Well, I finally reached a point where I think the plans are fit for human consumption. I haven't started offering CDs yet, but the plans are available for download here. Too bad I didn't have it ready before I got slashdotted.
Also, I've made a lot of interesting progress on the pneumantic canon. I've managed to fire a 2.6 gram foam-and-penny projectile sixty feet straight up. I'm working on a web page for just the gun, and hope to have it in place later this weekend.
Update 4/23/2003 - Sprocket troubles
It was such a nice, warm, sunny day that my son and I tried to take the tank on an outing to photograph it. Alas, it promptly threw a drive chain, which got jammed between the sprocket and the side of the hull so hard that I had to take it all apart to get it out again.
I also discovered cracks in the 3/4" plate at the back which tension the track, so I replaced it with a thicker piece.
We'll try again soon.
Update 5/3/2003 - KAPOW!
I finished the gun and have it installed in the tank! See the gun page for more info on the gun.
I also jacked up the tank 3/4" by adding shims between the axles and the hull. Why? Because I was fiddling around with some photos I took of the tank and figured I'd overlay a line drawing of The Real Thing - the line drawing was taller! Can't have that, can we? It's amazing the difference 3/4" (less than 4 scale inches) makes in the way it looks.
I also made a non-very-accurate 1/6thish scale model of an M3 37mm anti-tank gun to tow behind the tank. Don't ask me why, I don't know.
Update 5/4/2003 - More sprocket troubles.
I give up on this damn #25 chain. It just WILL NOT stay on the sprockets! I've aquired some #35 chain and sprockets to match, and have started installing them. I had some 35B15 sprockets with a 5/8" bore, which I used on the drive axles, but I had to order sprockets for the motors. They have 12mm shafts - ever try to get a #35 sprocket with a metric bore? (I need something called "ISO 06C-1" sprockets - ISO 06C being the metric version of ANSI #35 chain. Just try and find any!) I ordered sprockets with a 1/2" bore, and I'll install sleeves to make them fit.
Update 5/15/2003 - SUCCESS!!
Last weekend I installed the #35 chain and sprockets, and while I was at it I added a better, stronger chain tensioning mechanism.
To test the new setup, my son and I drove it around the block! It took a good fifteen minutes of squeaking and rumbling, and by the time we got back we had an entourage of about fifteen kids following us. Everything worked!
The track held up very well - it even withstood the full tension of the motors at stall (when they produce their peak torque) when I climbed it over some rough ground. The surface is all scuffed up now, but there are no cracks or splits. One of the screws wanted to unscrew itself, but that was the only problem.
The rest of the suspension held up well, too. It was quite a blast watching the road wheels follow the terrain over rough ground.
One effect I noticed - when my son isn't in the tank and it's being operated remotely (by a long wire plugged into the back), the center of gravity is WAY aft, causing it to do some pretty spectacular tailstands over relatively small obstructions. I may add a second battery tray in the middle to move the CG forward when the driver is out.
Now I just need to take some video and get it converted to mpeg or avi format for the website!
The chains did skip a few times during the trip, mostly when climbing over obstructions (once I began to think it was going to make it all the way around the block, I started hitting every obstruction I could find). As a result I RE-re-designed the chain tensioner. This one is REALLY robust.
New chain tensioner, chains and sprockets
By the way, anyone who bought the Plans and Instructions will be receiving an updated version incorporating all these changes.
Update 5/24/2003 - An Idea
I had an idea to improve the support of the half-shafts. (Refer to the 12/24 update above for a pic of the original design.) The function of the center support was just to hold the two half-shafts in a straight line, but the plywood would bend slighly under load and misalign everything. This, in fact, was probably a large part of the problem with the #25 chains. My solution was to eliminate the center bearing entirely!
Improved drive axle arrangement.
I replaced it with a piece of pipe with bearings in it. This arrangement requires two more bearings than the old arrangement, and a piece of pipe with the proper inner diameter, but it places all the forces on the outer axle bearings so everything remains in line.
Experiments with a 12v wireless B/W video camera are under way - I borrowed a security camera and monitor from my folks, who run the Coxsackie Antique Center. In the long run, I want the tank to be radio controlled, and a camera will allow the driver to better see where he's going. In the mean time, it will just be neat.
Those LEDs are invisible, but my camera
can see them. Cool!
As it turns out, it's an infra-red camera, so the tank will have "thermal imaging" just like the big boys!
One seriously happy kid!
© 2003 W. E. Johns