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.
|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|