Space Truck 8X8 part 1

Fairly typically I have started a new project ( actually quite a few) without completing the old ones. I cant help myself, I do so much thinking about a model over a long period of time that once it is underway my mind starts to wander and think about the next one. A new idea can be sparked by some left over parts from some previous experimentation, or by something seen on various forums. It sometimes takes a while to amass the parts required and come up with concept thumbnails that lead to a design I like, and at other times I get obsessed and I just have to start now.
This one is the latter, the chassis at least came together relatively quickly.
As  the title states it is an 8X8, based mostly on traxass E-maxx parts. I got this idea from looking at this modellers site http://www.krohpit.ru/2012/10/8x8.html

My version is nowhere near as sophisticated and although much simpler mechanically, has not been as straightforward as it looks on first glance.

This is the thumbnail sketch I settled on for the body.

I then did some preliminary modeling to scale in Autodesk Maya to refine the design.

Then using the side view picture I knocked up a full size cad drawing in DraftSight.

I purchased a pile of E-maxx parts off Ebay, some new and some secondhand from a guy who also had some wheels and tyres he wanted to offload cheap. 4 of them were traxxas Summit wheels and tyres. I figured I may as well use these on this model and got 4 more new ones, again from E-bay.

While the wheels themselves are pretty solid ( fake beadlocks) and relatively cheap for their size from the E-bay parts dis-mantlers, the Canyon tyres are extremely thin walled, like a thick rubber glove. In consequence they do hug the terrain well but by all accounts are easily ripped on sharp stones. In this case they will probably do fine, time will tell.
Here is the chassis so far, it is big, bloody big... overall 900mm long. The body will add to this length eventually.

It is powered by a DeWalt brushed 14.4 volt motor via a EVX2 speed controller. The transmission is my usual choice, namely the Summit two speed for that low speed, high torque capability. We will see if this will be enough to move this beast, I have not as yet tested it. The two front sets of wheels are steered.

The transmission speed selection servo is an old mini futaba that I have had for years sitting in a box. I purchased this servo from an aborted movie production in 1987. The movie Total recall was originally being made in Australia, in Sydney by Dino Delaurentis and going to be directed by Bruce Beresford. Then the '87 stock market crash happened and the movie got canned after about 6 months worth of visual effects miniatures had been built. I had some friends who were working on the models for the film, they had a big firesale and I purchased a couple of the incomplete vehicles. One was  a 6 wheeled rig made from Thunder Tiger Silver Fox parts and one was the Tamiya 1/16th RC Leopard tank ( which I stupidly sold many years ago), the servo came as part of these.

The aluminium top plate has had a small recess milled into it to clear the spring lever for the two speed shifter. I don't have a milling machine and only a tiny Emco Unimat3 lathe. For this task I use a Dremel bit that looks like a tiny 3 flute end mill in my small 5 speed drill press and very slowly push the plate around by hand nibbling away at the metal. It is not the most ideal set up but it's all I have. I would dearly love a small milling machine and a bigger lathe but at the moment they are just a bit too far out of reach for my meagre purchasing power. The Unimat 3 which I have had since 1990 is very useful for making links and spacers but cant really handle much larger tasks. I don't even have a 4 jaw chuck to hold square items but its better than trying to file parts mounted in a spinning drill and way better than nothing.

The maxx steering post has had to be slightly modified so the lever comes off at 90 degrees to the original position. It is made from Pvc and is super glued to the servo saver part. I hope it stands up to use. If it doesn't I will have to engineer something stronger. The front most steering arm is slightly longer than the inner one. It has a hole further out from the centre so that the front most steering turns slightly less than the inner. The inner set of front wheels turns on a smaller radius then the outer set so the slightly smaller movement that results should compensate. Note that I did not actually calculate any angular mathematics for all this, thats just not my strong point, I just basically winged it. I am pretty sure however, that this vehicle is going to have a relatively large turning circle.

. The diffs  at each end are Maxx diffs, but the two inner ones are modified Revo diffs. Unlike the Maxx diffs which have a left and a right side, the Revo diffs have a back and front half so it is easy to get a second set of diff cases and bolt two front halves together along with a second pinion gear and bearing. This makes a diff that has an input and an output pinion for attaching to the driveshafts that lead to the transmission at one end and the end diff at the other. There is a little lip on the diff cases that has to be sanded off so they mate flush. Then there is a big job in dremeling out the Maxx diff bulkheads to fit these Revo diffs. This is the hardest part of the conversion. I had to also make some new pvc parts to hold the Revo diffs in the Maxx bulkheads so they don't rotate, much like the  transverse collar that comes with the Maxx diffs. It also helps that the Maxx and Revo diffs have the same ring and pinion gears so the axles all turn at the same speed.
The other catch is that the front diff bulkheads are designed to be mounted angled up for front castor purposes. In this case they are mounted flat and the transverse diff collar holes expect this. I had to make a new PVC block to act as a transverse collar holding the diffs at a corrected angle. Originally I made a large block that bolted to the bottom chassis plate but this ended up fouling the steering bellcranks. The blocks had to be removed and remade to bolt to the top plates. That is why there is a couple of unused countersunk holes in the front bottom chassis plates. It is really a very tight fit around the steering beelcranks with a thick drive shaft running through the middle of it and the locking the diffs in place. It was quite a fiddle to sort out. This is one of those occasions where a milling machine would have been handy. I could have made entirely new bulkheads that alleviate all the problems that adapting existing parts throws up.
You have to remember to alternate the ring gear side of the diffs as you go down the chassis so all the axles turn in the same direction, otherwise if they line up all on the same side some of the axles will be going forward while others are going backwards.

Maxx diff at both ends.

Modded Revo diffs in the middle.

Aluminium brace made up to clear the drive shaft for the inner rear diff.

 Various reinforcement blocks of 10mm Pvc have been made to tie all the bits together.
I am planning to build the body out of PVC foam sheet to try to keep the weight down a bit.
Next to go is the battery and ESC mounting positions, then I can test it out...

Mobile Factory 8X8

In the GrassHopper part 2 post I mentioned another project, well here is the very start of it. I have been mulling over this idea for a couple of years now and slowly drawing thumbnails and collecting parts to try out the concept. Below are some of my design attempt rough concepts over the past 2 years. A recurring theme in the concepts has been the suspension system comprising a sprung arm on which is mounted a pivoting bogie carrying two wheels. There are two of these arms reaching out from the centre of the vehicle on each side. The other design goal is that it should look massive, I mean really really big.

None of these designs has really floated my boat as it were. More recently I have been toying with the idea of something that looks a bit like the NASA crawler but with giant tyres instead of tracks. I thought that perhaps it could be a mobile industrial complex like a land based oil rig or a industrial processing facility.

NASA Crawler.

The suspension design first steps.

Mechanically, the idea is to put a motor in every wheel, which is the most efficient way to build an electric vehicle as there is no shafts or transmission. The largest mining dump trucks (used to be Haulpaks now Komatsu) are built this way with a diesel engine driving a generator to produce the electrical power to drive the electric motors built into the hub of the giant rear wheels.

I was thinking I could use Imex Jumbo Maxx Chevron tyres and Jumbo Maxx wheels, eight in total, all powered. For each wheel there will be a 540 sized motor with a gearbox. Each side of four motors and wheels will be driven by a separate speed controller making this a spin steer vehicle. It means I don't need to build any complicated steering mechanism into the suspension bogies and the vehicle can turn on the spot.
To work out what rpm would be required from the motors I temporarily attached one of these tyres and wheels to my Makita cordless drill and spun it up running along the ground to see how much distance it travelled at whatever speed. The drill has a two speed gearbox. At full depression of the speed control trigger, it reaches 400 rpm on low gear and 1200 rpm on high. It seemed that 400 rpm was about right but perhaps a tad slow at top speed. I wanted a bit in reserve so figured 500 rpm would be about right.

One of the issues preventing the realisation of this project is whether I could get another two pairs of the Imex Jumbo Maxx Chevron tyres. I already had two pairs, which I have had for a good number of years but they seem to not be making them any more so sourcing them is getting increasingly difficult if not impossible. When I scored the pair of Jumbo Maxx Claw Dawgs for the GrassHopper project I also scored a pair of the Chevrons so that made six. Could I ever find the last pair I needed? Having a brain wave recently I contacted the distributor of Imex products in Australia to see if they just happened to have any Jumbo Maxx tyres left and did they have any chevrons in particular? As luck would have it they did have 3 pairs of Jumbo Maxx tyres left, a pair of Claw Dawgs, a pair of Swamp Dawgs and... a pair of Chevrons. I ordered all of them through my local hobby store. I now had the required eight tyres. The four most recent tyres had black foam inners which are made from a nicer more damped and flexible foam than the original tyres which have a white rather stiffer bouncier foam. I will have to make sure to mount one of each on each bogie to even things out.

The next obstacle was to get the eight 540 sized 12 volt 500 rpm geared motors needed which I eventually located and purchased from China on eBay. Once the motors arrived I tested them out to see what sort of amps they pull. The plan was to employ two Traxxas EVX speed controllers (one for each side) so I needed to see if they would cope with driving four motors each instead of the two they usually have to cope with in say an Emaxx or E-Revo. No load they pulled about 2 amps each so about 8 amps all up. Under load they pulled about 4 amps each making a total of around 16 amps which I think easily falls within the design specs of an EVX ESC. Motors geared down like this can really produce quite a lot of torque without stressing too much, after all it is not the sort of thing that will be hammered around a race track at wild speeds. This thing is designed to crawl along and look freakin huge.

I laid out the wheels and started to gather some measurements to begin laying out some CAD for the bogies. I have begun to use Draft Sight which is a free 2D CAD program from the makers of SolidWorks. Its taken me a while to get the hang of it. I used to use DeltaCad which I found was very easy to use and simple to learn. Draft sight is different in many ways but once you get over the first learning difficulties, it becomes simple enough. It works more than adequately for what I need. As I will be cutting everything out by hand ( no CNC or even a milling machine) I do the usual trick of marking the centres of all the radii to be drilled on the CAD drawing. A full size print out of this will be stuck to whatever sheet material I eventually choose and the centre popping, drilling, cutting and filing will follow the printed marks.

I decided to build a mock up of the arm and bogie to check out the function of the suspension before I got too carried away. It was quickly built out of wood to check clearances and position of the  shocks for full travel. I managed to score two full sets of Venom aluminium aftermarket shocks for the HPI Savage at run out prices which I thought might do the job. That means there will be four shocks per arm, sixteen in total. They are mounted on a pivoting bracket at each end which I felt would equalise out the force. To stop the bogies flapping about I have mounted a couple of  Venom Creeper shocks from the bogie to the arm. They have to be mounted half way compressed so that they can both move in and out alternatively as the bogie pivots. They don't really centre the bogie as each shock cancels out the other, but they do dampen the movement. You can see in the picture below a strip of PVC clamped to the arm that the dampers are attached to. I moved this up and down and changed the holes on the damper attachment points until it the range of pivot was satisfactory.

Bogie and damping mock up.

Imex Jumbo Maxx tyres and wheels.

Venom Savage shocks.

Once I was confident the mock up had the range of motion required, I was able to transfer the measurements to a more refined bogie CAD drawing and drawing for the arm commenced.

Bogie CAD Draft Sight

Suspension Arm CAD

  More to follow...