[MOC] "Subtractor Racer"

[davejsp]

Welcome and thank you for checking out my MOC, i really appreciate it.

As the name implies, this MOC features a "subtractor mechanism". If you don't know what it is, a quick search online will answser your questions, show various versions and what it is used for, but in a nutshell, it allows tracked vehicles to take on curves very smoothly and be remote controlled the same way as any regular RC car: one lever to go forward/backward, and another to steer (check the picture below for reference).

I decided to do this MOC because there are plenty of subtractor mechanism examples and variations on the internet, but not many MOCs actually taking advantage of it, so here's my contribution.
The set 42095 was the main source for parts, so you're up to a good start if you have it, only needing additional liftarms, gears and some miscelanious pieces.

Since this MOC is intended to be a racer, i wanted it to be as fast as possible, but the subtractor mechanism requires so many gears to function, that it generates a lot of friction; the MOC itself is somewhat heavy for its compact size (760g) despite several design optimizations. Keep in mind it's only one l-motor pushing the vehicle as the second l-motor is dedicated to steering (unlike conventional tracked vehicles in which both motors push the vehicle). This means i had to reduce the motor to wheels gear ratio (which is 1,667:1) to give it some extra torque.
I don't consider it to be slow (check the video below to get a better idea), but would love to see someone powering it with a buwizz on ludicrous mode as i don't have one.

This MOC was interesting to develop. As always, for anyone interested, parts list and building instructions are available at MocsMarket

 

 

As you might guess, it took several tries, optimizations and redesigns before achieving the final design and functionality you see here. I don't usually share any of my preliminar version MOCs as their quality is subpar, but i decided to share the first complete beta version i built while developing this MOC.
I forgot to take real pictures, but here are some studio renders of it. Enjoy!

Edited January 24 by davejsp
link update

[Hrafn]

Thanks for sharing!  I have noticed the subtractor designs recently and it is good to see them used.  It looks much easier to control smoothly than skid steering.

As for weight, do you have a rechargeable battery box?  Per Philo's site you should be able to shave off 125g by switching to one of those.

Can you share a picture of the gearing you used?  

[davejsp]

45 minutes ago, Hrafn said:

Thanks for sharing!  I have noticed the subtractor designs recently and it is good to see them used.  It looks much easier to control smoothly than skid steering.

As for weight, do you have a rechargeable battery box?  Per Philo's site you should be able to shave off 125g by switching to one of those.

Can you share a picture of the gearing you used?  

Hi, thanks for your feedback. I don't have any other type of battery unfortunately.

In relation to the gearing, it's basically impossible to take a photo showing it all at the same time as it is very compact, intricate and spread both horizontally and vertically inside the MOC, but here are two pictures, one from the underside and another from topside.

Edited August 4, 2021 by davejsp
typo

[ScT]

This looks like a nice build, keeps in with the original "idea" of the set, but improves on it a lot. Use of the subtractor is a great idea too!
I want to purchase the instructions - could you post a small preview of the quality? (I understand it's an export from studio)

[Hrafn]

Thanks!  Right angle gearing is always tricky to brace well - no matter what you do it seems to introduce friction.  That said, I wonder if the friction might be reduced by using a 7x11 frame or two like SaperPL did here:

 

Or there is the flat subtractor design at sariel.pl but that uses the old 4L differentials and a stuffed frame, and is even width.

[amorti]

16 hours ago, Hrafn said:

do you have a rechargeable battery box?

Even if the extra gears add a bunch of friction, the subtractor mechanism is a cool way to do it mechanically, and I really like the MOC.

But, if he had a Buwizz, all this could be done via software using the tracked vehicle module. Not as interesting in terms of gears and mechanical principles, but lighter and much less friction...

 

[davejsp]

5 hours ago, ScT said:

This looks like a nice build, keeps in with the original "idea" of the set, but improves on it a lot. Use of the subtractor is a great idea too!
I want to purchase the instructions - could you post a small preview of the quality? (I understand it's an export from studio)

Thanks for your feedback and interest. Here are some screenshots:

[ScT]

Beautifully clear, thanks. Will purchase shortly! :)

[davejsp]

2 hours ago, Hrafn said:

Thanks!  Right angle gearing is always tricky to brace well - no matter what you do it seems to introduce friction.  That said, I wonder if the friction might be reduced by using a 7x11 frame or two like SaperPL did here:

 

Or there is the flat subtractor design at sariel.pl but that uses the old 4L differentials and a stuffed frame, and is even width.

Hi, i might try using that frame and test it out.

I actually considered using a flat version but didn't go forward with it because it uses the old 4L differential, which a lot of people don't have.

1 hour ago, amorti said:

Even if the extra gears add a bunch of friction, the subtractor mechanism is a cool way to do it mechanically, and I really like the MOC.

But, if he had a Buwizz, all this could be done via software using the tracked vehicle module. Not as interesting in terms of gears and mechanical principles, but lighter and much less friction...

 

I understand and you got a point there, i just mentioned buwizz because it can deliver higher voltage to the motors, making it go faster.

The objective with this MOC was really to achieve this kind of control using standard lego parts and source most of the necessary parts from a set that a lot of people have.

3 minutes ago, ScT said:

Beautifully clear, thanks. Will purchase shortly! :)

Thanks.

[Teo LEGO Technic]

Very nice! Compact subtractor solution 

[davejsp]

17 minutes ago, Teo LEGO Technic said:

Very nice! Compact subtractor solution 

Thanks, yeah, it's my own version.