Lego Unimog U400 Kit – Modified by Harman Motor Works

This post has been a while coming, but I thought it was high-time I did post it, as the Unimog might not be around for much longer! :o

You may be aware from one of my previous blog posts, that I received the Lego Unimog kit as a gift a while back. At the time, I mentioned that I would kit-out the model with my own custom modifications in order to get more out of it and ‘make it mine’. Well, fast forward several months, and that has been achieved. Keep reading to see the details on just what it is I did to the truck…

Mod #1: Power to the people

One of the first things that I wanted to do with this model was make it powered. I figure that it came with a very capable all-wheel-drive system out of the box, so why not make proper use of it? So, I promptly ditched the fake piston engine provided by Lego in lieu of an M-motor, coupled to a hi/low range gearbox, to drive all four wheels. The result was certainly worthy of the effort.

Mod #2: High/low range gearbox

As I briefly mentioned above, I also added in a compact two-speed gearbox just behind the motor, to enable speed and torque to be adjusted to the conditions. In low-range, this thing really develops some decent torque given that it’s one heavy truck powered by only an M-motor. In high-range, the vehicle picks up some good speed as well. The gearbox ranges can be controlled via a lever which protrudes up into the cab beside the driver’s seat.

Mod #3: Air suspension (rear) & air tank

The next thing I did was add in an air tank to the truck’s chassis. I managed to neatly tuck it in within the chassis frame…it just turns out that there was a perfectly-designed ‘nook’ just waiting for the tank to make its new home. Cool! The air tank not only extends the capacity of the pneumatic circuit, it also means the electric motor which drives the air compressor (on-board) doesn’t have to run constantly when operating the air-driven devices.

Another thing I did while I was at it, was to replace the main drive gear from the compressor motor, with a gear which has a safety clutch it its hub; this basically means the clutch will kick-in when the air pressure in the pneumatic system reaches its upper limits, and prevent the motor and the related pneumatic components from strain and damage.

With the air tank fitted, I decided to ditch the rear springs and replace them with two pneumatic cylinders, which act as adjustable height air springs. The great thing about these things is that the ride height can be adjusted to suit no matter what type of load is carried on the rear of the truck, so no more spring-sag when carrying heavy items. The rear air springs are controlled via a pneumatic switch in the cab.

Mod #4: Auxiliary manual air pump

My efforts to cram as much as I could onto this model resulted in this next mod: a manual hand pump/crank system to drive the onboard air compressor when battery power was unavailable. I figured that now I have the rear air springs, it would be handy to be able to pump them up whenever I chose even if there was no electric power available. The system works well, and the manual pump handle is stowed away in a specially-made storage compartment by yours truly.

Mod #5: Lower-ratio raised hubs

This was actually one of the earlier mods I did the vehicle, and not major by any means, but necessary all the same, if I was to effectively motorise the truck. This mod involved removing the 1:1 gear ratio at each raised-hub, and replacing it with a set of gears offering 3:1 gear reduction. The result was perfect to give that M-motor the extra torque it needs to power such a large and heavy model. Combined with the low-range gearing, this truck seems to easily tackle most obstacles.


So that’s basically it for now. Unfortunately I may have to dismantle this model soon; I have a hankering to use its many parts on some new models of entirely my own design, and it pains me to see that I have several hundred Lego parts sitting on the shelf locked-up in this kit, that I could otherwise be using for my own creations. I do seem to enjoy building my own models from scratch moreso than building Lego kits to the letter. With that said however, I did thoroughly enjoy custom-modifying this kit, and even stock out-of-the-box, it certainly was an impressive kit to build. It’s just a shame that I did not get to add remote-control capability to this kit. Maybe next time.

I’ve included some pics below and also a link to a video of the truck on my YouTube channel, Harmanmotor.