Cnuteneering Part Eight

Welcome back to Cnuteneering, where the possible is made more difficult by bone headed ignorance, overenthusiasm and pointy metal things being brought together.

You may want to refresh your memory on the project in:

Part One.
Part Two.
Part Three.
Part Four.
Part Five.
Part Six.
Part Seven.

Design goals

  • Fast as possible on offroad; too big to have on roads. I will set a target speed of 50mph.
  • 4 WD.
  • Must be able to reverse, and brake.
  • Unbreakable, or as close to.
  • Must be able to mount GoPro or similar camera on it.
  • Cheap as possible.

We left last episode of Cnuteneering at the exciting point of getting the Sliding Hub With Giant Washer arrangement binned in favour of the hex hubs.

Now that my Brazilian proper engineer friend is back and rocking, we went for a walk together so I could hand over a pile of gears, axles, aluminium plates and polycarbonate plates. He asked me for a drawing of how the gears should fit together so I painstakingly measured everything up and sent him this:

Gear block engine side

Being a pro engineer and all that he knocked up this which I have to say does make my real life 3d modelling techniques look a little quaint.

The gearbox in actual 3D modelling

Even better than that he put this together:

Then he went on to cut the gears to specification:

Gear Milling (o how I covet this milling machine)
Hub – milled and attached to its companion hub

And here we have it In Real Life, doing the shifting bit.

At about this point he noted that the bottom gear, the central differential was a bit weedy. Before we cut the aluminium plate to size it might be a good idea to get a forever gear in there instead of some cheap 1/8th scale plastic crap I had bought off t’internet.

The plan is to have the bottom gear, the central diff, attached to the car chassis, and the gearbox itself to mount on top. That way I can remove the engine and gearbox if needed without having to worry about the rest of the drivetrain – I am guessing the gearbox is most likely to need removing for refinement but I want everything else to not need detaching too.

Engine mount to frame of chassis – these sizes matter so the driveshaft to the rear has clearance below the engine

At about this time it began occurring to me that I had no failsafe if the (currently non existent) comms failed. I have no idea how heavy this car is going to be, but I will guess 15-20Kg when finished and if that is doing 50mph on a full tank of petrol it will be over the horizon and gone forever unless it maims some passing unfortunate on the way. There are kill switches you can buy that connect on to the kill switch on the motor; once it sees there is no signal coming from the Rx unit (or if you trigger the channel it is on) it will kill the engine. This could be useful so off to t’internet and coughing up some cash, and some more… Waiting. There just hasn’t been enough waiting so far yet.

This is the killswitch on the motor. I took a look at it prior to buying the switch I did, which has 2 connectors for the switch. This appears to have just a single cable running to it:

Killswitch – Engage! (or how the hell does it actually work?)

I can only assume that it is using the casing somehow as the other connector, but I shall see how that bridge needs crossing when the killswitch finally arrives. Or possibly some kind puffin may have had experience with these types of killswitches and post a comment for me not to read.
I managed to source a cheapo 6 channel Rx/Tx unit from BangGood which seem to have Europe based warehouses and deliver in a week or two.


This was duly connected to the servos I bought off eBay and a new gear change servo. I had to get another adapter for my charger in order to charge the battery that came in the bundle only to find it is NiMh not LiPo. NiMh batteries take longer to charge than LiPo ones and I am unsure of how they deal with heavy usage. If I am going to power anything more than just the servos and Rx off this it may prove to be insufficient. We shall see I suppose.

On the plus side – I did not buy a dud. All servos work just fine and the range seems massive – I walked down to the end of my road withy my son keeping watch and the signal was still good.

We now have another riveting period of… Waiting. This covid bollocks is getting right in the way of an honest Cnut’s engineerings. While I am waiting for the postman again, I started doing more research.

Reading a lot of the RC car forums, no one had anything good to say about rack and pinion schemes. Too hard to keep the mud and grit out of so it wears quickly and needs a lot of fussing about with.

Looking at how my other actual proper engineered cars work, they have something called an Ackermann mechanism. Ackermann steering geometry is a geometric arrangement of whereby the inner and outer wheels of the car need to trace circles of different radius – the inner wheel must be pointing further in to the corner than the outer wheel, or one of the wheels will skid or needlessly engage with the differential. I’m going to need something like that for the Follymobile.

Having a quick shufty online, there was a double result – on sale, and proper two horn action (obligatory filth).

Steering rig, ackermann and revised servo holder. Above is the previous servo holder with all the electrics inside. Mostly

Whilst I was reading up on engine performance I saw that a decent exhaust can make all the difference.

Filthy pink silicon seals.

And it fitted nicely. I did want it to be rear facing but this was the only design I could find that would fit and not get in the way of anything.

Can the race can do the can can? No idea

Suspension arms are going to have to be of a certain width, which is still as yet unknown. Until I get an idea of how long the car is going to be and how long the driveshafts are…

In the meantime I idly sketch what the chassis will look like.

Chassis draft one

And of course, back to waiting.

As ever all comments and suggestions in the comments I shall not read.

Still to be designed

Brakes / reverse gear
Actual gear changer
Suspension arms

Still needed

Drive shafts from central diff to front / rear diffs and from front / rear diffs to wheels.

Pics, vids, words and music © El Cnutador 2021

The Goodnight Vienna Audio file