Lego Tumbler mk2 progress

I have been making slow but promising progress on my second attempt Lego Tumbler.  Whereas last time I was able to spent all day every day for a week working on the model, at the moment I only have evenings available to me, so it’s a little slower in coming together.  That said, the progress this evening has been really encouraging and it’s already looking better than the previous version.

I have also been taking plenty of photos, partly for my own reference and geekiness, and partly because I got a lot of comments on YouTube of people asking me how it all worked, and since I’ve not seen anyone else do anything like this on anywhere near the same level of technical accuracy I felt it might be useful to someone!  I’ll try to include some photos in this post, but I’ll be putting them up on a central gallery too, so take a look there if you’re interested.

Day One

My initial plan to put motors in has fallen through, unfortunately.  The first day of the rebuild comprised mostly of experimenting to try to get a reliable working rear axle, in such a way as to get an electric motor driving the wheels (via a differential), and have some sort of suspension at the same time.  As it turned out (despite all the mind-numbing research I had done previously) this wasn’t possible.  Lego is good, but it’s not that good.  At least not with the parts I’ve got and at the scale I’m trying to build.  So in the end I had to give up on the idea of being able to reproduce it all like that, and focus on just rebuilding the existing model in a better, more professional way.

I started off by looking at the steering mechanism.  In my original model I used a transverse rack and pinion system on each arm to convert the rotational action to a backwards-forwards motion, which in turn was linked to the custom axle to turn the wheel on its pivot.  All sounds rather technical, and in the original design it looked excessively complicated and messy, so my first job was to try to simplify it or at least make the mechanism more elegant.

The solution turned out to be a combination of my rack and pinion setup and a pivot bar as used by other models.  I used the same method of getting the action into the bar, by using a universal joint centred on the pivot point of the arm; this ensures that the steering isn’t affected by the angle of the arm in any way, which is a more reliable way of steering and avoids bump-steering.  You’ll see from the photos how it all works in practice, and how tidy it all is too.

Day Two

With one working proof-of-concept front arm sorted, creating a mirrored version for the other side was no problem, and soon I was on to making a subframe to mount the arms on.  That led on to linking the steering mechanisms each side together.  I’m using my usual approach of sending the steering to the back of the vehicle so I can steer and push at the same time, rather than what official Lego models tend to do and put the steering wheel on the top of the vehicle.  Just personal preference really.  And means one less set of cogs between controller and wheels.

Having concluded that a live axle wasn’t going to work, I fell back on the independent suspension setup I had used on the original model, modifying it slightly to take into account that the new model was looking like it was going to be slightly smaller.  I also took the opportunity to put in two suspension coils on each wheel rather than just one, which should make the suspension stiffer and more able to cope with the weight of the vehicle.  It may turn out to be overkill, but it’s easier to take one set out than to try to squeeze one in later.  If nothing else, it looks fun.

A quick play around back at the front of the vehicle gave me some basic suspension on the front arms.  It’s nowhere near strong enough to take the weight of the car yet, but it did allow me to take it for a quick test drive around the lounge to make sure everything was working nicely.  Unfortunately, not everything was.  It soon became apparent that there was just far too much ‘give’ in the steering mechanism, which meant that once the vehicle was pushed forward the front wheels tried to turn outwards.  I tried adjusting the central link to make them point in, but the excess movement just made them turn in on themselves instead.  Not good.

It looks like the problem is in the excessive use of cog wheels, the fault being in the older style wheels.  The newer style ones are much better and give a tighter connection.  Thankfully I happen to have just enough newer style cogs to replace the old ones, but it will mean changing stuff on the front arms to take the different size cogs.  Hopefully that will solve the problem; there may still be a little excess movement in the bar connecting to the axle via a ball joint, but my observations so far have shown the vast majority of the movement to be confined to the cogs, so I’m hoping my next evening’s attempts will bring the steering to an acceptable standard.

That just leaves refining the front suspension, doing some heavy-duty testing on the whole setup, and building some bodywork on the frame to make it look a bit more realistic.

One thought on “Lego Tumbler mk2 progress

  1. Hi Matthew.
    I was lokking after pictures of the Tumbler, i thought i´ll make one in LEGO, and yours came up when i googled it.Very impressive! The sterring is brilliant. I have some ideas myself, and i´m just starting on the design. I might lean on your ideas, i´m a bit rusty(i haven´t invented any models latly, i´m 41 years old and i think it´s 20 years sinse i made something up myself) I haven´t much sparetime so it´s going to take a while. (I also got my PS3 and remotecontroled car…:o) But i thought your work should have a coment. Very nice indeed:o) Cheers from Denmark.

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