Building Big, Bigger, Biggest!

While building George (and Bob) I learned quite a few things about building big moving Lego models. Don’t worry, I still have plenty to learn however I try to give quite a bit more thought to a project before I start prototyping and testing things out.

The main issue I always face is how do you make a huge model move! Now George and Bob both struggled on non slip carpets, however most surfaces were not a problem for them. As I plan to make my next project a lot bigger than the last 2, I instantly hit issues. Both George and Bob were quite thin models with a main long beam. George had a height of 5ft 7in. They both probably weigh between 20 and 25KG each.  So imagine building something 6-8ft tall, with a main body 2 times fatter than George and a horizontal arm beam of around a meter long with a model of a mini gun on each end. Once built, if this things weighs less than 40KG I would be impressed!

For this next project I have given a lot of thought to the weight / moving issue. I have researched a few different options and the first one I hope to prototype is using Rotacaster omni-direction wheels to take the pressure off the tracks.  These wheels can basically move in any direction and support up to 25kg per wheel!

Currently the omni-direction wheels only come in 48mm size which is a little small to use as the main platform, however if I keep with the normal track design but place these wheels behind the track to support the main body, which is the heavy part of the model, then the resistance on the tracks will be less and should then have an easier job moving the robot.

Lately I have also been investigating Lego compatible metal axles. Trying to transfer the power from several XL motors to the robots tracks is actually quite hard work because the motors are strong enough to twist plastic axles and snap cogs.  They can also bend quite a bit when used as axles for the wheels / sprockets as they are supporting quite bit of weight. Metal axles would cut down on issues but could damage the plastic. I do hope to get hold of a few to test out and review. If anyone else is interested in metal axles and cogs that are compatible with Lego, then check out the Bricklink store Brick Machine Shop.

Another big issue for building large models is the cost of parts! Most Lego technic sets have maybe 10-15 large Technic beams in it. They might have 1, 2 or even 4 motors. When you start planning to build a large technic model, you’re looking at 10+ motors and thousands of beams! Also as everything is further apart, you need to invest in quite a few extension cables for the motors and they aren’t cheap. Lately I have even run out of Technic pins!! The last couple of models I have built used the Lego Starwars Hailfire Droid with wheels from set 4481 as a main bearing. These wheels only exist in this set (pretty rare) and cost over £20 per wheel to buy from Bricklink.

A few ways I have gone about keeping costs down is by parting out my sets and using them. A few years ago, I didn’t build but only collected sets. It was when I had to build something for a show that I did then switch over to building instead of collecting. I have now decided after many years to start integrating my sets into the stuff I use to build. Any rare sets I keep separate. I also asked on the Lego forums (e.g Brickish) for parts that people might have spare. A few people have been very generous to the cause! Generally I don’t need brick and non Technic parts and so I can use these parts to swap for bits I need.

Lego weight test using the multi-directional Rotacaster wheels.

I was lucky enough to win a set of 3 48mm Rotacaster multi-directional wheels from a competition held on BotBench.  These wheels are built to be compatible with Lego axels.

I have great plans for these wheels but first I thought I would put them through a simple weight test. Now according to the website the wheels can take up to 35KG per wheel however I don’t think the plastic Lego axels can!

I spent around 15 minutes building a rather simple platform to test out the wheels on. 2 of the wheels were directly connected to 2 PF XL motors (1 per wheel). The main point of this test was  to see is how much weight a simple layout could move and more importantly turn.

Once I get a rough idea of the basics then I can scale it up for my bigger models using more gearing, more motors or even more wheels.  As stated in my previous post, moving heavy weight is my biggest issue when building my robots.

In my opinion, 2 non geared xl motors moving and turning 12.5 kg of weight is pretty impressive!  Maybe I should use them to move my son around the house:)

For more information on these wheels, please visit Rotacaster.

Lego Automatic Gearbox (3 speed)

While I take the train to London, I have plenty of crazy thinking time and so I think about what I would like to try out.  So when I get home I build little simple projects like a 3 speed automatic transmission.

So this works by centrifugal force, the faster the motor turn the yellow thing (will think of a more technical name) the more it expands and pushes the center axle up.  This forces the gear changes.  The motor is controlled via a RCX as it was part of a display.

It started with BOB (T1 v1)

One of the robots that really fascinated me that I thought I could build was a T1 from Terminator 3.  It was a tracked robot with twin mini-guns (another thing I wanted to build).  So I went about building a rather large Lego version for the 2010 Great Western Lego Show which is our club’s (Brickish) largest event.
My first giant robot based on T1
More pictures can be found here:
Bob Flickr Stream
Bob suffered from a few major issues.

Due to its weight (well over 20KG) it could only turn on low friction floors.  The base of it was nearly a meter square running double tracks each side powered by 4 XL motors (directly linked to sprockets). Going forward was very quick but turning just did not work well. I only actually found this out on the day of the show where I discovered the floor was anti-slip carpet.

The robot had very clever communications in my opinion. It used 4 NXT’s (2 slave, 2 master)  which communicated via bluetooth. One set of NXT’s had an accelerometer which when moved, would move one of the robots arms so that it would mimic the users moments. The other set of NXT’s also had an accelerometer to control the other arm however it also had a mode button so that you could control the robot’s other functions (moving, body rotation and lift, head movement) via the same accelerometer but in different modes. Each master NXT also had a fire button to fire the Zamor launchers on each arm.
The problem was that the bluetooth communication just was not stable enough and also suffered from lag and loss of data. You would put it in arm movement mode and it would instead move the tracks. There there would be a large amount of lag which meant the robot was always a few seconds behind. In practice, this caused the robot to drive itself in to a table 30 minutes after the show opened causing a partial rebuild in front of many people!  Quite embarrassing.

The robot also had a large main body beam (around 4ft long) that had to be counter balanced via many boat/train weights which caused even more weight issues. This main beam actually snapped a few technic beams while it was being built. The main beam could rise up via 5 pneumatic rams. However it required over 30psi to start it off (see below). This caused other issues.

I also completely covered Bob in black plate, which had a habit of falling off and also added more weight.

The Lego Pneumatic Beam Engine

A great love of mine, is to build Lego pneumatic engines. If you check YouTube you’ll see that tons of other people also enjoy building them. It’s like building a real engine but running it off of compressed air. When I was little I used to go to Steam Rallies and Steam has always interested me.

So after playing around I managed to build a working pneumatic beam engine.


This is one of my favourite mocs, I can watch it for ages. It can be a bit temperamental, though.