Tracked Monster Project

[Mad Ax]

What do you do when you're crazy busy with new builds and complete teardowns and chassis fabs for upcoming race meets?

Yes, that's right, you start something all new!  Well, that's what I do, anyway.

In fairness, this isn't really starting something new - I just wanted to progress this idea far enough to see if it will work before I plan to invest more time it in later in the year, when all the other stuff is done.

So, what's this all about?  Well, it's about my desire to create a tracked vehicle in the style of those specials that hit the 1:1 monster truck scene back in the 80s (and that's why I've put this in the Monster Trucks forum - because I'm effectively treating it like a monster truck build).

I've had this idea for a few years now, and had been keeping half an eye out for a good donor chassis.  There are those tracked chassis for robotics builds, but they all seemed too small and too fragile for something that was going to be ragged around the garden.  There is the old Kyosho Blizzard, but they're expensive, rare, and probably not built for thrashing either.  Tamiya tanks are way too expensive and so are most of the other genuine tanks, too.  Then I stumbled on a video of the FTX Buzzsaw, and realised it was just what I wanted.

I put one on my Christmas list, and lo and behold, one arrived!  

It comes assembled and RTR, with a USB charger and a little Lithium battery pack and a wheel controller.  I gave it its first run a few months ago, and was somewhat disappointed by the speed - it's not exactly slow, but not as fast as I expected based on the videos and reviews online.  It wants to lift the nose everywhere - adding weight up front is a common mod - it's got a very noisy transmission, and after some time in storage, the belts get kinked and it doesn't run entirely smooth.  That will probably get better in the summer when the rubber is warmer.  It also doesn't have a lot of traction - hit an obstacle and the smooth belts just slide, instead of gripping and pushing.  There are mods for that too, but more on that later.

The weirdest thing about it is the steering.  All the control is done by a board of wizardry inside the chassis, so it's just like driving a conventional car, except if you turn the wheel while it's stationary, it'll spin around.  The very most oddest thing is that steering is reversed when reversing.  Well, er, yeah...  Allow me to explain:  if you squeeze the throttle to go forwards and turn the wheel to the right, it drives and makes an arc to the right.  If you then push the throttle for reverse, you would expect it to make the same arc backwards - or, to go back exactly the way it came.  Except it doesn't.  Somehow the wizardry gets reversed and instead it turns the opposite way, meaning you can do a 3-point turn while holding the wheel in one position and just going forwards and backwards.  Except you don't need to do a 3-point turn with a tank, because it can turn around while stationary...

So, there's scope for improvement, then...

[Mad Ax]

I wasn't really planning on doing much with this project for some time yet, but I knew I wanted to change the body.  I wanted something monster-trucky.  My first thought was a Landfreeder / Bush Devil body, as these are fairly small and not too long, and I have a few.  Sadly it's way too long - the FTX really isn't as big as it appears.  The Bush Devil overhangs badly at both ends and basically looks daft.

I knew I had some spare M-chassis bodies, and I spent a few days looking for them.  I really wanted a Mini - if you can't make a 1:10 scale monster with a pickup, making a 1:10 scale monster with a Mini is the next best thing, IMO.  I thought I had a couple of old Mini bodies, but alas, they must have been thrown out or given away, as I couldn't find them anywhere.  I tried a lexan Pumpkin body by Parma, but it was too long; I tried a vintage Fiat Abarth body, but that was a bit long also, and didn't really have the right lines for it.  The best body of all was my old Tamiya Suzuki Wagon RR body, which I had thought was battered beyond repair, and would have been virtually perfect - but when I looked at it, it's really too good to cut up.  These are seriously rare now, and I couldn't bring myself to take the scalpel to it.

So, I put a shout out on the Iconic RC page for anyone with a spare, battered M-chassis body to bring it along to the Mendip round of the Iconic Cup, and I'd take it off their hands for free!

What I got offered was, basically, perfection personified.

Here's the tracked badboy, sitting on the bench:

And here's the body I got given, for free:

It's an L&L Mini, with a 225mm wheelbase.  That's longer than the regular Mini body, and also has bigger arches.  It's barely been used - the previous owner has sold all his M-chassis cars, so it was surplus to requirements.  Way better than the smashed up four-season-old race body I was expecting!  Almost too good to cut up, but it's not exactly rare and it didn't cost me anything, so, out came the scalpel.

Wheelbase check - perfect!

[Mad Ax]

The width is spot on, too

At first I'd planned to strip off the spoiler and bumpers from the FTX body to get the Mini body to sit lower, but there were electrical connectors that couldn't be removed from outside, so I unbolted the body entirely.

While I was in there, I had a closer look at the gearbox.  I was hoping it would have two independent motors, each driving their own transmission, but sadly that's not the case.  That would have made it much easier to upgrade - I could have added some hotter motors, and swapped out the complex electronics for two ESCs handled by the two vertical sticks of my FlySky FS-i6, for that proper oldskool skid steer experience.  That's going to be fairly difficult to achieve with this setup - some extensive mods will be required to go that route.

Here's how it looks inside.  I think this is a kind of differential gearbox - where that empty block is at the bottom left, should be a smaller motor.  There's a bigger drive motor underneath the gearbox (that slatted thing top left is the top of the can).

Under normal forwardsy-backwardsy-motion, the small motor stays still and the big motor drives the belts.  I think it sort of drives both sides, but there's a bevel gear on each side that transfers drive to the opposite side through a 3-gear differential to the opposite side.  With the smaller motor removed, the diff spins freely and the truck doesn't go anywhere, as if it's diffing out.  With the motor in place and locked stationary, drive is forced to both tracks and the vehicle drives forwards.  If the smaller motor starts spinning, it adds some diff action which speeds up one side and slows down the other.  If the main motor is stationary and the smaller motor turns, the tracks counter-rotate and the vehicle spins around.  It's really all very clever and I find it quite hard to understand, but the gears are all nylon without any bearings and the box of wizardry is all hardwired onto the board, so adding a higher-voltage battery or hotter motor is likely to end in tears, one way or another.

Delving deeper into how the transmission is attached to the drive wheels might yield more joy, perhaps for pulling out the transmission entirely and replacing it with two separate units, but that's a job for another day.

[Ray_ve]

The one motor propulsion, 1 motor steering setup is somewhat unusual indeed, the 'standard' 2 motor transmision is simpler is design.

Sounds like something similar to this gearbox:

 

 

[Mad Ax]

@Ray_ve yes, I think it's very much like that.  The Lego design shows how a conventional diff gear is used to perform steering, I guess something similar is taking place inside the FTX gearbox.  It's a clever design and fits in a compact space, but I can't be sure what the benefits are over a conventional 2-motor setup.  Unless there's a potential problem of mismatched motors causing the vehicle to not run straight?  It would be interesting to test that theory.  There can be a big difference in performance between two identical-spec motors, especially at the budget end of the spectrum, plus I guess once all those gears have been designed, manufacturing costs are low for this configuration.

Anyhoo - on with the project!

While I had the chassis open, I added some weight up front to stop the nose from constantly going skywards.  Quite a lot of weight, in fact.  120g, in fact.  The stock body is actually really heavy, I think part of it is die cast.

The next trick was to get the body mounted over the tracks.  First I tried removing just part of the body sides - I figured I could do something cool with the arches, like make up some V8 headers to poke out of them.

Unfortunately it didn't quite fit like that - the body was slightly too short to make it work, so I had to raise the cut line to the top of the arch and cut into the front wings.

The rear of the body is just right to fit between the tracks

[Mad Ax]

With the rear fitting neatly, I had to keep chopping away at the front to get it to sit down properly

almost there, but the front drums are rubbing on the inside of the body

That's got it!

[Mad Ax]

So - there you have it!  Main goals achieved - the body fits neatly over the chassis and covers all the plastic, and it still very definitely retains the key features of the Mini, from the front, sides and back.

The body isn't properly mounted yet - just plonked in place - but it works well enough, I even drove it around the workshop like this and it didn't come off.

There are more changes required, though.  The lack-of-traction problem can be solved by adding teeth to the tracks, but that will make the tracks taller and necessitate raising the body.  That in turn might require more clearance at the front, where the bumper is.  I'm not 100% sure the bumper needs to stay - it definitely helps retain the classic mini look from the front, but it will create ground clearance problems and ultimately might look better if it comes off.  Alternatively I can chop it away from the body and mount it separately onto the chassis to retain the Mini look without having clearance problems with the track teeth.

That's all I really wanted to do at this stage - prove that the body was going to fit and look right.  I think it does that.  Obviously there's loads more to do here, but right now it's looking real good just sitting on the shelf.  I think this will be a fun project for later in the year once the crazy summer race season is over  

[wolfdogstinkus]

Track tensioner on a spring?. Just throwing an idea out there.

Cool project, these things are never as easy as you imagine.

Slot the bottom gear and fit a spring in-between it and the top gear to aid traction on the tracks?

Slot the bottom gear and fit a super small CVA in there?

[Mad Ax]

5 hours ago, wolfdogstinkus said:

Track tensioner on a spring?. Just throwing an idea out there.

 

Thanks - the track tension doesn't seem to be the issue, the tracks run properly on the drive wheels, and they are sprung in a couple of places for both tension and I guess to reduce shocks on the running gear.

The issue is that the tracks themselves are smooth where they meet the road surface, so when it runs up against a steep obstacle it just spins the tracks on the ground instead of climbing over it.

That's not to say it isn't effective - it'll run around the patio and the grass and go over a 4" wooden beam on the bottom of the climbing frame without too much trouble, but when pushed over less even ground, the tracks just slide.  There's no bite on the surface like you'd expect from a good off road tyre.  I think I've seen some examples of people gluing L-shaped blocks of something onto the track plates to give it some grab.

[Ziddan]

21 hours ago, Mad Ax said:

can't be sure what the benefits are over a conventional 2-motor setup

I think the benefit is less advanced electronics/cost savings.

Sure if the controller has a sticks for each track the 2 motor setup works but if one wants one stick for throttle and another for steering on a two motor setup there needs to be mixing done.

Its still about the same amount of gears but diff steering cuts out the need for an electronic mix.

Here is another look at the style of system that is a bit slicker than the lego one as there are only gears and axles visible, no obscuring bits:

 

[Mad Ax]

Good find @Ziddan, that's a neat vid