About this Blog

This is about the combination of two interests, Radio Control vehicles and Science Fiction models. Its also about vehicle design. The models have to satisfy two main precepts.

1. The vehicles have to work, ie be driveable, but not nescessarily win any races or rock crawling competitions.

2. The main thing is that they have to look cool.

Sunday, 30 October 2011

SpinSteer 6X6

Originally I called this a skid steer vehicle but have recently come across a definition which explains that skid steer is when one side is braked and the other side powered to effect a turn to the braked side. In the case of this vehicle, it is more rightly termed "spin steer" because both sides can be powered to effect a turn and one side may even be put into reverse thus spinning the vehicle 360 degrees on the spot. A skid steer vehicle generally has one transmission that goes into a diff. Each side of the diff is a brake which can be alternately engaged by moving the two control sticks so that one side or the other has varying degrees of brake applied thus sending more power to the unbraked side. The Spin Steer vehicle, of which this project is an example, uses two power sources ( a separate motor and throttle per side) which can be independently controlled forward and reverse.

This project has been kicking around for a very long time. I started this when Pro-Line first released the Masher 2000 tyre, which I think was just before the year 2000, so lets say 1999. Again these were the tallest tyres you could get for a 2.2" wheel. They became the original rock crawling tyre which set the form factor for the 2.2 crawler tyre as we know them today, but originally were sold as a monster truck tyre for such rc vehicles as the Traxxas Stampede. of course I was attracted to both their size and the slightly SciFi look of the tread.

I had an idea to build a 6x6 vehicle loosely based on the cool Explosives truck vehicles in Gerry Anderson's Joe 90.

I was also a huge fan of the Banana Splits show as a kid and always wanted one of those amphibious 6 wheeled skid steer atv's they used to hoon around in the shows credits. Since then I have always tended to go for anything with more than 4 wheels.

Originally I was thinking of a shaft drive system with a left and right shaft driving the wheels on each side, with two speed controllers and motors. Some time later Losi introduced a radical belt drive layout for the xx s touring car and I thought something like this might work and be easier than bevel gears to implement.
 This is the Belt layout drawing showing how the belt wraps around the centre wheels pulley with the aid of two idlers. It then goes up over the drive pulley which is the highest circle in the drawing. This drawing also helped me to work out the belt size I would need which turned out to be pretty close to a size that was available. I have since found that the belt slips over the drive pulley in one direction of travel so I am going to have to add another idler to increase the wrap around the drive pulley it will also help as a general belt tensioner. The belts and pulleys are 2.5mm pitch. The idlers are simply two 11x5mm bearings on a stub axle with a collar retainer. It works out that only the smooth back of the belt runs over the idlers. There is no transmission as such, the motors have small 18 tooth pinions driving the biggest spur I could find at 101 teeth. That is mounted on an HPI RS4 slipper clutch on a shaft which then passes back under the motor to drive the 10 tooth pulley. The belt then transfers the drive to the larger pulleys which are 40 tooth.
 Each side is a mirror of the other so one of the motors has to run in reverse. The timing of that motor has been rotated 180 degrees. At the moment I have got two 50 turn crawler motors mounted but may well change to less turns for a bit of extra speed. I was going to run a couple of LRP runner plus reverse ESC's but found they have an annoying amount of delay before reverse kicks in after braking. In this application, so I can Spin Steer I really want  no delay reverse. So I am on the lookout for a couple of cheap no delay reverse ESCs.

 The chassis is a length of 3mm thick 100mm wide aluminium. The blocks which hold the drive and bearings are made from 10mm thick PVC sheet. Its easy to cut accurately on saw table and drill and tap. It also takes super glue very well as long as the mating surfaces are sanded. All the suspension arms, hub carriers cups and axles are Traxxas rear Bandit gear. The cups and axles are the hardened steel universal joint types. The wheels once again are the Venom Creeper wheels. I originally had Tamiya Stadium Blitzer 2.2" rear wheels but after an order of a couple of Creeper wheel pairs arrived, I found the packs contained 4 wheels not pairs as listed. I had these left over so used them instead. They hold the ProLine tyres quite happily and look way better than the Stadium Blitzer wheels. The other thing to note is the suspension arms are molded with about 3 degrees of toe-in built in so all the tyres point inward at the front.

The shocks are plastic bodied front truck types from an unknown Kyosho.
 All of the mechanical aspects of the chassis were worked out and built many years ago. So recently I turned my attention to the body design. After many rough sketches I found I liked the bottom right one the best.

I then drew up a rough CAD version of the outline and made up some bulkhead lines to start cutting material.
I had a box full of balsa wood that I've had for years so decided to put it to some use. The 6mm thick balsa bulkheads were glued up onto longitudinal frames with thin zap-a-gap super glue and then covered with thin sheets and thick planks of balsa to build up the varying levels of the hull skin. A solid chunk of balsa was carved for the nose. I actually went through a couple of bottles of super glue on this.

 The intention is to cover this with thin woven glass cloth and epoxy, making a rigid surface. The balsa bulkheads can then be stripped out of the interior or even left in as it is still fairly light in weight. the Balsa has had a few coats of shellac which gives it that honey colour . Cellulose dope would have been better but I didn't have any at the time. A very lightweight water based premixed interior filler was used in some areas to shape and fill in holes.

 The chassis still needs some top bracing added to prevent flexing of the bottom plate which affects the belt tension. Then the sides, front and rear will be boxed in with styrene sheet and detailing. Some method of battery access and hold down in the chassis is yet to be determined.

 I've got at least another two Sci Fi projects in the works, at least in the very early mostly conceptual stages.
As well as a racing conversion project on the go as well. More soon.

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