LEGO! “Technic” To Be Exact

Mk1

This is the first post in my blog where I talk about the custom LEGO Technic  Custom Flatbed Truck that I’ve had since about May 2009 when I started to get back into building LEGO Technic models. This model truck (purely my own design if I may say so) has gone through quite a few iterations since May ’09 and I’m still working on putting the finishing touches to the latest version as I type this.

Let’s take a step back though; this first post will introduce you to the development (from scratch) of the prototype truck and the subsequent (initial) MkI “release” (I’ll leave the latest-and-greatest version for a future post).

Prototype me!

Chassis

When starting any new LEGO Technic vehicle project, I tend to start off by getting the basics down-pat first: the chassis frame “rails”. The chassis will form almost the back-bone of the vehicle and will function as the main component which everything else bolts onto (much like real-world trucks). I normally go through a few design “phases” before I settle upon the length and width of the chassis unit. It can be difficult to envision just how long and high I want the vehicle to be, so I try to get the chassis as close to “perfect” as I can before I move on (because changes to the chassis rails can become very hard once everything else is attached to it!).

Rolling gear

Once the chassis is sorted, I start thinking about the next most important thing: the wheels! Obviously the wheels and their mounting system will form an integral part of the chassis unit and are obviously an essential part of the truck. Considerations such as wheel-base, wheel-track, steering and ground clearance start coming to the forefront of my thoughts.

Once I’ve got all these considerations sorted and perhaps after experimenting for a little while, I’ll normally end up with what you see below – a rolling chassis!

Not much to look at...yet

Wait a sec!!??

Ok so I cheated a bit here. What you see above is actually (believe it or not) the truck at a slightly more “advanced” stage of completion. You may (or may not) notice a longitudinally-mounted electric motor connected to a 2-speed gearbox, so let me fill you in those components now.

Electric motor

The electric motor I settled-on for this project was one of LEGO’s own: the “M-motor”. One thing I’ve noticed with most of LEGO’s motors, is that they mostly come with their own integrated gear-reduction. This is helpful as it means I don’t need to worry about tweaking the overall gear ratios too much before the power hits the ground. For this truck however, I still opted to build a gearbox of my own design…

Box of gears anyone?

For this project, I decided to custom-make my own LEGO gearbox from scratch. The gearbox offers 2-speeds: LOW and HIGH, plus a “neutral” range (which obviously transmits no power to the wheels).

The actual design of this ‘box is one which I’ve used for ages in my LEGO models now…it utilises an “input” shaft which connects to the electric motor, and from there, transmits power down to a “layshaft”. Finally, the power reaches the rear-end of the gearbox where I generally mount a drive shaft to the rear wheels, or in this case, a “final drive” unit which will run a chain/sprocket drive on the rear axle. If I’ve lost you several lines back, here’s a pic:

Gearbox explained...maybe

Selection of gear ratios is obtained by sliding the lower half of the gearbox (the layshaft)  back and forth so that it engages LOW, neutral, or HIGH. It’s a relatively simple design but one that works admirably. As any gear head will tell you though, only one gear ratio must be able to be selected at a time, otherwise (as expected) the gearbox cannot rotate and will lock-up, so I ensured that the gears were aligned sufficiently to avoid any issues.

Here’s a YouTube video of the gearbox in action…you’ll also get to see the rolling chassis in this video (incidentally, the proposed “RC” [radio-control] feature never materialised – at least not yet anyway):

Gear ratio-wise, LOW gear offers 3:1 reduction, while HIGH gear doesn’t change the ratio at all, offering a true 1:1 ratio (direct) drive straight through the gearbox. That’s not the end of the gear reduction though; at the rear end of the gearbox, the final drive reduces the ratio (no matter what gear is selected) by 3:1 again. In other words, the trucks enjoys some fairly serious gear reduction with LOW gear engaged, to the point where the motor can easily break the rear rubber tyres loose on slippery surfaces.

Chain me up

Eventually, I hooked up a chain drive from the rear-end of the gearbox down to the rear axle of the truck (which incidentally added another 1.5:1 worth of gear reduction!). The rear axle features a fully-functioning differential unit with three spider gears. I won’t bore you too much with the details, but some of the pics you’ll find at the end of this post should hopefully tide you tech-heads over.

Frame/Body/Cabin

OK, so I’m speeding things along a little bit here (mostly for the sake of readability)…to talk about the other major component and consideration for the vehicle. The frame! Otherwise known as the body or cabin or “cab”.

Here’s one of the earliest iterations of the framework which I came up with for the truck (note the battery box mounted immediately behind the cab, which takes 6x AA batteries):

First version of frame/cab

But alas the angles in the front pillars drove me crazy from a geometrical perspective (they could never seem to line-up 100%), so I scrapped that design and went for something a little more rigid (and at more sane right-angles!):

Final frame - awaiting gearbox

Once the frame was mostly settled-on, I could focus on getting the interior sorted and other somewhat minor details such as hooking up a gear lever and linkage so that the gears could be selected from the cab (as you’ll see in videos further down this post). But there was still one of vital component remaining…a steering system!!

Steering

Steering systems are always a challenge to build. On the one hand you have two giant front tyres which need room to be turned about their steering axis (this is where you need to make sure your chassis rails are narrow enough at the front end to allow this!), and on the other hand, you need to devise a way with which you’ll get the motion of the steering wheel inside the cab all the way down to the front axle to do the actual turning of the wheels (read: a pile of linkages!). On top of this, all these linkages must clear the rest of the vehicle, the motor, etc and everything else as they move back and forth, to and fro, to steer the wheels.

The system I ended-up using for the truck could perhaps be considered a type of “drag link” and you can see a bit of how it works in the YouTube video at the end of this post. The steering system you see in action here, while satisfactory, didn’t make me as happy as I could be – the steering gear ratio was too high – so after MkI, I lowered the ratio significantly. I’ll fill you in on this in future posts.

Mk1 ready!

So I eventually got the truck built to a level of my satisfaction and thought I was done. Hah! Unfortunately (or fortunately), in LEGO land, things are never finished…there’s always something more to be added, adjusted, broken-down and re-made or just plain changed. I will save all that subsequent development for future posts though, and instead leave you with pictures of how MkI turned-out. Enjoy!

Mk1

Side-on

Rear 3/4 view

Heli view

Front 3/4 view

Belly of the beast

Chain drive to rear differential

Looking down into cab...

Rear end

Mk1 videos