So this one started a very long time ago. I was given a little Stihl MS180 chainsaw without a bar or chain and instead of doing the logical thing and buying these two parts to have a handy little chainsaw around, I decided it would be a great idea to give it wheels and make it radio controlled. Well my naive young self started pouring ebay for possible wheels, shocks and a decent diff, figuring I needed these to base a design around. Instead, I bought some cheap HPI Savage wheels (not nearly big enough), some HPI savage shocks (not nearly big enough) and a HPI savage gearbox with "centre differential" which turned out to not have a diff at all. I also bought 10m of aluminium tubing because obviously with an ambitious project, you have to go all out and pick a notoriously difficult metal to work with right?
I decided I wanted a nice big, flat base to make mounting everything easy. I hadn't figured out anything about the design yet but I wanted to start now I had parts. I mocked up a rough shape on Pro/Engineer and decided "that'll do" and set to work. You'll notice with my projects that design tends to be very loose as I work with what parts I have lying around and can make easily with limited tools when I get to it. Well who knows what I was thinking when I made the front end up, certainly not me when I look back. It turned out the CAD mock-up was far too fat in reality so I shortened the suspenion arms...forgetting that the wheels still had to steer. This lead to some very hacky steering knuckles which really are terrible.
The steering wasn't planned properly, so that's also terrible, and I guessed at where to mount the shocks so it bottomed out as soon as I rested the engine on....again, terrible. The shocks were a fairly easy, if ugly, fix but the steering is still bad and really if I want to finish this properly, the whole front end needs a bit of a redesign.
It stayed in this state for quite a while. When I got the motivation to pick it back up I thought I'm going to do this properly and use maths and planning. I figured out the spring-rate of my shocks and used some trigonometry to work out where I wanted to mount them. I then set to work on Solidworks, designing how the drive system (I'd decided I didn't need a diff and the Savage gearbox would do) and suspension would all fit and work together. Again 'designing' was based on what was easiest to achieve with the materials and tools available so it's far from ideal. When it all came together it actually worked pretty well, although the wheels are out of line by a couple of mm which puts the whole engine and chain drive on an angle....oops.
Next came the RC throttle. This was fairly simple to set up using a cable guide system but it's still one of the best moments being able to rev it up from afar. I had facilitated a brake but I was too eager now that I had a throttle and steering, to bother fitting it before I took it for a test drive. This was a bad idea, espesially in an area as small as my garden. Not satisfied driving in small circles, I took it to the bottom to open it up and see what it could do, forgetting I had nothing to slow it down (the gearbox has a one way bearing so no engine braking either!) Needless to say it crashed, and snapped one of the steering knuckles. This is where it's currently up to pending further motivation.
Ideally I'd like to redesign the front end using rose bearings but due to the geometry I have, it's difficult to get any with enough range at the right price. The steering clearly needs a complete rethink as well, making the push-rod travel parallel to the suspension arms might be a good start!
There's also supposed to be a proper, removable, body shape which gives a little structure but is mainly for aesthetics. You can see the sides already mounted in some of the picures. A very rough visualisation of what it should look like is in the slideshow along with various random pics I took as I built it up. All of the pictures are commented if you go fullscreen and show info.