Cnuteneering, Part Seventeen

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.
Part Eight.
Part Nine.
Part Ten.
Part Eleven.
Part Twelvety.
Part Thirteen.
Part Fourteen.
Part Fifteen.
Part Sixteen.

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 the last episode of Cnuteneering with the sensors and Pi mostly physically integrated and working. I was happy to draw a line under it and call it done but leopard questioned the wisdom of using a breadboard to hold the cabling together on something that is going to be in motion. I uhmed and aahed about it for a bit, as the ribbon cable pushes down quite hard on the cables as is, then headed off to buy some Veroboard and a 40 pin solderable socket. Thanks leopard!

Actually that bit is not true; I bought a pack of six 40 pin 2×20 sockets because I could not buy a single one. Ah well, I am sure there will be Cnuteneering projects in the future so at least I won’t have to wait a month to get the parts like this time around.

In any case I can put the electronics to one side now and actually do some proper Cnuteneering. First up is to get the bloody gear changer sorted. It pushes the hex hub slider at an angle, which binds and the changer goes nowhere. I think this is because it is exerting pressure at the edge of the hub rather than close to the axle. So I tried a bit of a redesign so there were 3 points of contact:

Gearchanger Mk III

Only 15 minutes in to the days Cnuteneering and already we have one chalked up in the FAIL column. I did have a prior design rethink, of having a pivoting lever that pushes the hex hub about but the siting was inconvenient to say the least. It was in the centre of the gearbox and there was too much trouble getting the supports in the right place.

In theory and practice, the moving lever arm was good so I just re-sited it to be sitting above the hex hub. I need something to mount the lever on though, so I got ready to braze:

Brazing, I’m ready to go–oh!

Clamped together with the clips you see in the pic (minus the rubber gloves, obvs), acetone washed and neatly filed, I waited for dusk so I can watch the colour of the aluminium better.

Disaster! Just at the point of the metal being hot enough – white patina tending to a red glow, there was an almighty boom and red hot pieces of aluminium and bulldog clips showered the back patio workshop. Luckily they all exploded forwards or that could have been quite bad.

This is the detritus I could salvage-

Fabulous Disaster!

What I think happened was that the box section collapsed under the pressure from the bulldog clip when it got hot. I have no idea how the big chunk of 6mm aluminium snapped in half though.

And then back to the hacksaw, drill and files.

Here is the problem that I am finding with Cnuteneering time and time again – accuracy of drilling holes. In the pic below you can see clearly where I have etched the surface with two intersecting lines; where they meet I punched a centre point. And yet the drill still skids to wherever it bloody well fancies.

Skidding drillbit, pissed off Cnut

And so, back to the pile of bricks and angle iron balanced on top of a Workmate brazing station to redo the braze. This time I just hold the pieces together with the bits of roofing tile left over from the first gearbox proof of concept. They are some weird sort of composite ground up stone so I am guessing they won’t mind the heat.

Success! They hold the pieces nicely in place and suffered a minute or so of direct heat from the brazing torch.

And here we are again…

All went well until MBWKaren popped her head out the back door and El Woofador came bounding out. Attention distracted, I turned away from the piece and yet another disaster:

Melted box section.

2-1 to the fail column at the moment it would appear. No matter, the piece still seems strong enough for now so I finish up, and file then sand with P240 paper.

Doubtless someone, somewhere will be offended by the blackhand.

During this period we had yet another tick for the fail column.

Here is the Sword that was Broken and is forged again!

At this point I should explain how the gear changer now works – I need 2 fixing points to hold a pivot point over the hex hub. I need to be able to move that along the length of the shaft so it is centred above the hub, and I already have two 5mm studs in place where the old gear changer design used to be. So if I can get a threaded upright to sit on the 5mm stud, I am good to go.

Engineering youtubers simply select a nut from their vast stores, clamp it to a piece of mild steel and then Mig weld it in place before getting on and actually building something. Cnuts like myself have to go the long way around!

Tap my bitch up!

A little more drilling, tapping and general fettling and we are there – the Mark 4 Gear Changer.


Now I need to get the gear change servo mounted and in place, and connected to the gear change lever. I used a bit of rod cut from an old grill tray that I had somehow hung on to when we got a new oven last year, dunno how that happened, it was almost like planned or something.

Using one (1) spring from my box of 200 (there are 200 springs you know, 200!) which receive abuse from far right misogynist trolls for being only 200, and a bit of hollow tube with a side hole cut and tapped to make a grip for the spring on the grill rod, I finally get it all together once I saw off the extra bit of 6mm plate that is not needed.

Servo and push rod in place

At about this point, I bite the bullet and take the gearbox apart for what feels like the 100th time and cut down the hex spacer to the exact size. I save about 13mm from the length of the gearbox which is great. Everything is taken apart, put back together and the bearing retainers plus gears that are fixed are all locktite’d as they are now in their correct spots.

I notice that the centre diff that I had to correct is still a little off kilter from where I filled it, so it is out with the file again to reduce the overcorrection. This time around, I use the bearing itself as a mould to keep the epoxy in the right place.

I don’t want the bearing to be glued in place though so in a fit of inspiration I covered the bearing with a bit of clingfilm, a sort of glue condom for the part to keep the glue away from it.

Lovely job

While I am waiting for the epoxy to set, I do some testing with the bearing mount goop I got. This is to cover for the slightly larger than required hole in the aluminium plate that I have to hold the other end of the big centre diff. It is about 0.2 mm too big so this should sort that out admirably.

I do need to test how this stuff works – no idea if it will bond to polycarbonate or epoxy. So I put a test together – epoxy and steel, steel and aluminium, steel and polycarbonate.

The old grey Loctite test

As I write this 2 days later, the steel-aluminium joint is rock solid, but for polycarbonate to steel and epoxy and steel the compound just slithers impotently. I count this as a success though as at least I know that Loctite 741 is no good for gluing to polycarbonate or epoxy.

I put the gearbox back together once the epoxy has hardened and it all seems to fit together. The gears seem to mesh a little too snugly for my liking but we shall see how that pans out. Hear the clunk as another item hits the “Too Hard To Think About Right Now” pile.

On the plus side though, now that the gearbox is totally assembled, I can finally get the width measurement and start thinking about how the wheels are going to attach to the chassis. The sockets seem to work quite nicely, I might have to 3d print some supports at the end but we will see how that pans out in the future. For now at least, width is not going to be a problem.

Never mind the length, feel the width!

Things are starting to take shape now:

Getting there – one bodge at a time!

The servo to change gears is now mounted, so I move out to the back patio test area. The engine starts easily and I let it tick over in idle for a little while to warm up before stopping it (must fit the remote kill switch) and connecting up all the RC gubbins.

The engine really needs holding still and I struggle to hold it and the throttle cable, while trying to watch the gears and not burn my fingers on the spark plug top. The supposedly free wheeling gears are not free wheeling and this is a problem. I think they are too tightly bound to each other, so this means I will probably have to move the axles a mil or two further apart so there is no pressure on them.

This and coupled with the fact I do not have a 3rd hand to operate the remote control, cause me to abandon the experiment as if the 2 freewheel gears are in play at the same time, bad things will undoubtedly happen. It does vindicate leopard’s advice from earlier though – anything not lock bolted, threadlocked or soldered is going to get shaken loose the minute the engine starts up.

This, of course means taking the gearbox apart, again.

Testing the gear changer seems to go OK though. I need a stronger spring (only 200) in there I think, and the angles are a bit off so a small rethink is needed.

But in principle it seems to work:

Still waiting for:
Heat conductive glue to stick the temperature sensor onto the engine.
Veroboard and 40 pin socket

Still left to do / think about
Go through the rear end design again so that the shocks and rear suspension arms are properly sited. This means I can finally attach the engine to the chassis
Sort out the car underside – at the minute it is just a gaping void
End point adjustment for servos and control mechanisms Dumbo RC setup
Petrol tank and centre of gravity
Siting of brakes
Build RPM counters
Integrate code to read sensors, temperature and RPM into the video capture
Siting of RPM counters
Should I add a microphone to the Pi to record sound?

Pics, vids, words and music © El Cnutador 2021

The Goodnight Vienna Audio file