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.



Saturday, 29 October 2011

GrassHopper part 1

It's been a while since my last post and although I haven't progressed the creeper 6x6 project, I have been pretty busy. I usually have a two or more projects on the go at any one time. It means you can move on to another project while waiting for parts, or waiting for a solution to a mechanical or design problem to slowly bubble to the surface of your consciousness. Sometimes you just need a change from one project and then come back to it refreshed later when previously insurmountable problems somehow don't seem insurmountable anymore, and you wonder what the hell you were thinking back then.
One of the projects I have been working on had its genesis in an impulsive purchase of a Tamiya fighter Buggy RX hard body on eBay in about 2000. At the time I thought it might make a good space ship cockpit.
A few years later I drew a rough thumbnail of a vehicle based on it.
And then maybe some years after that I drew another rough thumbnail.
This one was influenced by the famous ELF Tyrell p34 or "6 wheeler" formula 1 car that was success full in the Swedish Grand Prix in 1976. It was a pretty radical design and there were numerous models of all descriptions made of it at the time. Though of course Lady Penelope's Fab 1 from Thunderbirds four wheel steering preceded it by at least 10 years. Oddly enough the first RC car I ever had was the Tamiya Cheetah in 1981 and the first mod I ever did was to put 4 Tamiya Tyrrell rear Tyres and wheels all the way round. I modified a set of axles using my dads power drill and files and used it like a lathe to get them to fit. They were the fattest tyres I could find at the time. It appears that the mad urge to put massively fatter tyres on RC cars is still with me.

What I like about it is the concept of a ridiculously huge set of rear powered wheels, with the 4 proportionally smaller unpowered steering wheels up front.
More recently I did another rough sketch to try and figure it out. you can see why I called it the grasshopper, the shape of the rear wing on the body and the angle of the suspension remind me of the back legs of an insect. Of course there was a Tamiya GrassHopper which was the first RC car for many. It used to mimic its namesake quite accurately as the horrible rear suspension set up caused it to bounce out of shape over every bump and rut, making it extremely difficult to drive in a straight line on any offroad track.

My initial idea was to try and build a dual steerable wheel set on a single suspension arm.
Hence the trial layout below. I was thinking I would need a pretty large arm to hold 2 steerable wheels so I tried out a set of Traxxas Summit rear arms. After checking out a lot of crawler builds, I thought I would build a solid rear axle with a 4 link set up for the rear. The biggest off the shelf solid axle is from a Kyosho Twin/Mad Force which would do the job but is way too expensive at around $250.00 a set. I then had a look around at some cheap DIY solutions on the crawler pages and found some guys using Maxx diffs and suspension with some metal work added. I took the Maxx diff idea and came up with my 25mm aluminium tube sandwiching a Maxx diff with Revo Axles and Revo/Summit/Maxx Knuckles. Maxx diffs are amazingly cheap on EBay from the people that buy up new kits, disassemble and part them out. Most of my parts come from these eBayers and I like Traxxas parts for a couple of reasons. One is they are metric, they've always been metric and I like metric. Here in Australia we have been metric since the mid seventies but you are only just now seeing metric fasteners in hardware stores and its all because of the buying power of the USA. Most stuff  we get is made for the US market and they are stubbornly Imperial. As soon as the US sees the sense in the beauty of metrification the world will be a better place... for people that like to make stuff that is.
The other reason I like Traxxas for custom jobs and mods is they have a product line that uses many interchangeable parts, for instance the Revo E Revo, Summit, and Slayer are all based on the same platform. The Rustler, Bandit and Stampede share a good many common parts, so you can mix and match away to get what you want. The other important reason is that the parts are plentiful and cheap. You can afford to buy a part to try out and if it doesn't quite work out as you had hoped, you can consign it to the parts box for a later day and another project without feeling ill from wasting so much cash on something you didn't use.
One added tip to this, is to resist buying the expensive aluminium upgrades at first. Get the cheap plastic parts while you are prototyping, not everything will work as you intend. You may have to drill or enlarge holes or carve away plastic depending on the mod, then find actually it would have been better to go about it a different way and you have trashed the part. Once a part it is in and working satisfactorily then you can think about the bling because you know it will work. I speak from experience here, I am a sucker for bling, after all, in a manner of speaking, the SciFi thing is all about the bling.




 The end brackets are 3mm thick Aluminium deep channel, with a wedge cut from the open end. The Ends are tapped so the pillow balls can be screwed in. I am thinking about putting a lock nut on as well. All the Ally is brazed together using low melting point aluminium brazing rods using only a propane gas burner. The rods go under a number of brand names such as Durafix and HTS 2000. A good joint is stronger than the parent metal and the bond works extremely well as long as you clean all the surfaces to be joined by sanding and using a stainless steel wire brush, don't use a normal steel wire brush, it will the impede the bond. It all has to be held securely in position before joining, lightly clamped. The heating anneals the aluminium and if you clamp too tight it will distort the piece wrecking your work. Its  a bit tricky to get right and the joint is quite a bit lumpier than you get with normal processes. Unlike silver brazing brass or steel it does not get pulled into the joint with capillary action, its more like a weld where you can bevel the mating surfaces leaving a valley which you fill with the molten rod. The other way is to tin both parts and then reheat to melting point and push together. This only works where you are able to align the two pieces accurately while reheating to melting point and then squeeze together without them slipping. Part of the process is to scratch the rod onto the joint surface as it is melting this helps to float the aluminium oxide off the joint getting the metal to bond. You can also use a piece of stainless steel wire to scratch away in the the molten pool to further manipulate it. I use a piece of stainless wire from an old kitchen whisk.
If there is no other way to join pieces then I use this method but if it can be mechanically joined I prefer to use fasteners.
 The end of the Aluminium tube is tapered to fit into the taper inside the Revo Knuckles. Once the pillow balls are screwed tight the knuckles are prevented from turning. You can still set a toe in amount or caster. I may still add the toe in turnbuckles if it looks like it could be knocked out of alignment in bash testing.


After reconsidering the layout I felt it was all looking way too wide so I thought I might try Slayer arms which are much shorter then Revo arms. While waiting for these parts to arrive and after a failed attempt, I found I couldn't come up with a satisfactory way to mount two steerable wheels and a servo to operate them out on a single arm. I then decided to just go with two whole Traxxas Jato front suspensions and I narrowed the rear axle to what you see below.

I did a CAD drawing laying out the rear suspension which uses some shocks and rocker arms from a Revo. As well as a couple of Maxx aluminium braces, upside down as the main frame, which I found going cheap in my LHS. In fact these braces came before the drawing, the whole idea of the rear chassis set up stemmed from these chance finds. Then a cage was made from 6mm aluminium solid rod uprights and an 8mm transverse rod to hold the rocker pivots. I didn't end up putting in the central post as on the CAD drawing. It may still need it, not sure yet.

The two main links are Super Crawler machined aluminium links. I happened across them on one of my many eBay trawls.They worked out to be in the range of the length I needed. I have put a couple of rubber O-rings at the axle ball joint to prevent them from flopping sideways on suspension compression. The other red anodised links are leftovers from the Creeper 6x6 project.
 The rear Tyres are Imex Jumbo Chevrons on dyed Imex Jumbo diamond rims. I have had these tyres and rims sitting around in a box for several years, saw 'em in a Hobby shop at half price, couldn't resist, its that big tyre thing again. They got carted around to 4 different houses, across Australia and back again. It's good to actually be using them for something after all this time.
I may make some sort of detailed hubcap insert to go into these rims eventually.
The front Tyres are 2.2 Imex Claw Dawgs forced onto my favourite wheel the Venom Creeper Bead locks.
 I think I may substitute a pair of Jumbo Claw Dawgs for the rear chevrons so they match up. Those chevrons look pretty grunty, but the Claw Dawgs are perhaps a bit more SciFi.and from the same tread family as the fronts. Mmm... what was that about "good to see them actually get used for something"?


 The Jato Front ends on a Jato Chassis plate. I had to braze on an ally plate at the bottom because the chassis gets a big hole  to accept the second Jato bulkhead and slots to allow the second arms to droop. Here I tinned both parts then placed them together and remelted squeezing them together. Of course all the anodizing has to be removed from the joint first, as the rods will not stick to anodizing which is a coating of colour dyed oxide. The chassis was still one piece when the bottom plate was attached  across the hole for the bulkhead and the slot cut after joining. Thus the chassis is still perfectly aligned. It also means there is a weak point here as chassis is effectively cut right across with only that bottom strap of aluminium holding it together. There will have to be a top plate brace made up to strengthen across the gap which will also hold the two steering servos.

 The transmission and motor is from a Traxxas Summit which is a two speed. There will be a mini servo mounted for shifting duties. the transmission will have to go in reverse in this configuration so I am hoping the motor has no timing built into it. It appears to go the same forward as reverse in testing.




 I still have not actually worked out where the battery and esc (which is a pretty big, also from a Summit) will go yet, but am thinking about a big jet /turbine engine thingy above the rear axle which the  two batteries could slide into. This thing runs on 16.8 volts, it needs it,as it already weighs a tonne. The Imex tyres are heavy beasts with a lot of hard rubber, fronts and rears.