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.

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.

No comments:

Post a Comment

Most Popular Posts