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January 26th, 2014 Leave a comment Go to comments


Simple lightweight model of a high-speed tracked vehicle. Features realistic suspension system


Completion date: 25/01/2014
Power: electric (8878 batteries)
Dimensions: length 42 studs / width 16 studs / height 21 studs
Weight: 0.86 kg
Suspension: pendular bogies on trailing arms with shock absorbers
Propulsion: 2 x RC motor geared 1:1
Motors: 2 x RC motor
Top speed: unknown

The original Ripsaw vehicle is what happens when you strip a tracked vehicle of everything except a very basic body, soft suspension and a powerful engine. It’s an attractive subject to anyone interested in tracked vehicles, and having been asked to build it a couple of times I have finally decided to give it a go.

My model was by definition very simple and lightweight. I have used two RC motors for propulsion, but with a Power Functions system instead of the RC unit, which would make the model much larger and heavier. The narrow chassis housed two PF 8878 batteries, one for each motor, and two PF V2 IR receivers in the same system. It was a solution much smaller and lighter than a complete RC unit, and it also allowed simple steering, which would be greatly complicated with the RC unit.

The suspension system was modeled after the real one, including a total of 6 independent arms connected to soft shock absorbers, with a 4-wheel pendular bogie on each arm. The entire system was adjusted to work very softly, which also made it very sensitive to model’s weight. It was quite a challenge to make the bogies strong enough to stay together without losing any wheels, and then I had to make the tracks very tight to stop them from falling off.

The model performed nicely indoors, it had a powerful acceleration and steered greatly, but it felt like it somewhat lacked power to handle any serious obstacles. I was curious how it would perform outdoors, with a nice clean weather and cold reaching -20°C, hoping that the cold would stop the RC motors from overheating. The model turned out to perform nicely for a short while, and then, as the batteries got colder, it would lose all power repeatedly after a few seconds run. This was a bit surprising, as the type of power cells used in the 8878 batteries is supposed to handle the cold well – the same type of a power cell is used in the GoPro camera, which worked flawlessly at this temperature. It remains to be seen whether AA batteries, especially the rechargeable Eneloop ones, which are supposed to work well in cold too, perform any better.

In the end I was rather disappointed in the model. It was tiny, ugly, extremely loud indoors and struggling with the cold outdoors. Still, the suspension system, which was the most complex part of it, worked exactly as intended.


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Categories: Misc. Tags: ,
  1. Sariel
    April 25th, 2014 at 07:57 | #1

    No. But you can build it from the photos, it’s dead simple.

  2. benjamin
    April 25th, 2014 at 00:29 | #2

    could you make instructions for the ripsaw please

  3. Sariel
    March 18th, 2014 at 16:14 | #3

    Very few.

  4. technic-master
    March 18th, 2014 at 16:09 | #4

    one question how many things did you break in all of lego videos ?

  5. Sariel
    March 11th, 2014 at 09:22 | #5

    Thanks. The Chinese edition is already in the works and will be published soon.

  6. benhuangua
    March 11th, 2014 at 01:52 | #6

    hi, Sariel.

    there are always awesome works in your site.

    i have a question. if i want to translate your book — unofficial lego technic builder’s guide into Simple Chinese version, what should i do?

    Thank you. there are lots of fans of you in China mainland and Hong Kong.

  7. Sariel
    March 8th, 2014 at 20:09 | #7

    Thanks. It’s a dead simple model, I don’t think you need a tutorial to build it.

  8. March 7th, 2014 at 23:02 | #8

    That model is amazing! My friend has the CRAWLER, that stupid truck always destroys me in battle he always runs me over quite literately. O and could you make a tutorial on this? It would be greatly appreciated. Thank you.

  9. Sariel
    February 15th, 2014 at 20:51 | #9

    No idea.

  10. February 15th, 2014 at 11:24 | #10

    Hi Sariel!

    When I saw the video of your workshop, I just needed to ask you this question: How much is your lego worth? It may seem as a strange question, but I was just wondering.

  11. Sariel
    February 6th, 2014 at 16:39 | #11

    No, it would not. Speed control remote takes ages to get any accuracy, and subtractor is simply not needed here, not to mention it would make the model two times as big.

  12. technicfan
    February 6th, 2014 at 16:36 | #12

    Why don’t use a subtractor and speed control remote? This would enable sensible steering.

  13. Sariel
    February 1st, 2014 at 13:57 | #13

    Of course the size of wheels affects the speed and torque. There is a ratio between the wheels and the ground just like between to gear wheels.

  14. Witherley
    January 31st, 2014 at 23:03 | #14

    This is an edit: Both tanks were also roughly the same size and had 1:1 gear transmission.

  15. Witherley
    January 31st, 2014 at 22:39 | #15

    I’m not sure where to put this because there isn’t really anywhere specific to put it, but it relates to vehicle construction, but I’m assuming you noticed that the actual size of the wheels also affects the speed and power of the model?

    I’ve noticed it when making tracked vehicles. I made an RC tank with very tiny wheels tracks and it was extremely slow. I then made a second one using the 42021 Snowmobile tracks with two 40-tooth gears as the drive wheels and it could go ridiculously fast and even do wheelies because of the receiver placed at the very back.

    I’m assuming the reason it went faster is because the wheels were bigger and presumably increased the torque, and also because the wheel can make less of a turn to go the same distance, which when combined with a motor going at the same speed as a tank with smaller wheels would make it a lot faster. What do you think?

    EDIT: Both tanks were approximately the same weight.

  16. Renato Prata
    January 29th, 2014 at 16:00 | #16

    Good idea, it’s simple and efficient. Maybe you can use it like base to other projetcs more elaborates.

  17. Carsten
    January 26th, 2014 at 23:24 | #17

    Hi Sariel,
    maybe the problems come from the fuse inside the battery pack. The RC motors may take more than 1A.


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