Home > Ideas > Subtractor


January 3rd, 2009 Leave a comment Go to comments

A mechanism for tracked vehicles, makes them easier to steer and more effective to drive.

Subtractor is an effective and easy to steer drivetrain system for tracked vehicles. Basically, it allows to steer a tracked vehicle like a regular car: one motor drives it forward/backward, and the other makes it turn. It can be used as an advantage, by using a more powerful motor to drive, and a less powerful one to steer. An unique feature of a subtractor is the possibility to use both motors at once, and thus to make the vehicle turn in an arch. The arch can be calibrated by choosing a specific gear ratio for the steering motor. I have so far developed three types of subtractors:

Transverse subtractor

In this case we have two differentials, each driven with a separate motor. The basic principle is to have the differentials connected by two sets of gears (left and right), that offer different direction of rotation. I usually use two t16 gears with a chain on one side and four t8 gears or two t24 gears on the other. Different combinations are obviously possible.

Longitudinal subtractor

Here both differentials transfer drive from a single drive motor. The steering motor makes them rotate in the same direction, which causes one tread to run faster, and the other one to go slower.

Dual longitudinal subtractor

This version is very similar to the one above, except is uses two drive motors. Each of them drives one tread, which is very useful for particularly heavy vehicles. The steering motor, again, makes one tread run faster, and the other one run slower. If both motors are powered from the same output, one of them needs reversed poles, which can be obtained via a PF switch. Such switch can be actually used to make the vehicle turn in place if the steering motor lacks torque for that.


miniversion1.jpg miniversion2.jpg miniversion3.jpg substractor1.jpg substractor2.jpg s_longitudinal1.jpg s_longitudinal2.jpg s_longitudinal3.jpg s_longitudinal4.jpg s_longitudinal_dual1.jpg s_longitudinal_dual2.jpg s_longitudinal_dual3.jpg s_longitudinal_dual4.jpg s_longitudinal_dual5.jpg

Video (longitudinal version):

Media reference:


Categories: Ideas Tags:
  1. Sariel
    January 4th, 2016 at 15:56 | #1

    Sure, you can build subtractor with new differentials, my first book has examples with instructions. As for help, thank you, simply keep watching my YT videos, that will generate some money for me.

  2. hateofallhatreds
    January 4th, 2016 at 15:41 | #2

    Hi Sariel,

    thanks for the great tutorial. To your knowledge, is there any way to build the subtractor with the new differentials (part no. 62821)?

    On another note, I read that you’ve had to downsize the instructions to cut domain expenses. What can we users, who benefit of your talent and kindness, do to support you?

  3. Sariel
    June 6th, 2015 at 15:04 | #3

    There’s also a smaller version with the new differentials in my first book.

  4. Mikko
    June 6th, 2015 at 13:10 | #4

    Ah you are right, the layout with the old diffs looked just so different I didn’t realize it first…

  5. Sariel
    June 6th, 2015 at 11:59 | #5

    Sounds exactly like transverse subtractor.

  6. Mikko
    June 6th, 2015 at 10:31 | #6

    I just thought of a new way of building a subtractor. First build a drivetrain as in a car; a motor driving a differential, that drives the tracks. Then add a second diff driven by the steering motor, parallel to the drive diff. Then connect the outputs of both diffs together so, that the direction of rotation is reversed on one side. In this way you’ll use the diffs in a more conventional way, so they should be able to handle more torque.

  7. Sariel
    January 21st, 2014 at 22:50 | #7

    No, I don’t think you can increase it. A lot depends on the weight of the vehicle, I happened to break some gears at 3 kilograms.

  8. Alexander
    January 21st, 2014 at 21:31 | #8

    how much torque can the longitudinal one handle? im using two pf xl on the drive and either a pf xl or pf m on the steering end. is there a way to increase the torque the differentials can handle?

  9. Sariel
    January 21st, 2014 at 09:27 | #9

    No, it’s not possible. And differentials are very cheap.

  10. Witherley
    January 21st, 2014 at 05:46 | #10

    Is it possible to make a subtractor using only one differential? Because I only have 1 differential so I can’t make a subtractor using the tutorials above.

  11. Sariel
    December 29th, 2011 at 15:43 | #11

    No, it’s not.

  12. Magnus
    December 29th, 2011 at 15:35 | #12

    so is this basically a differntial for tracked vehichles? Sorry for my spelling. -_-

  13. Phillip
    January 13th, 2011 at 22:20 | #13

    Okay, thank you!

  14. Sariel
    January 13th, 2011 at 18:32 | #14

    If you’re planning to use them with PF IR receivers, then yes, of course.

  15. Phillip
    January 13th, 2011 at 18:14 | #15

    Okay, I plan on purchasing two 71427s and a micromotor, does that mean I’ll also have to purchase PF Extension wires?

  16. Sariel
    January 13th, 2011 at 07:47 | #16

    Doesn’t matter. Subtractor works with any motors.

  17. Phillip
    January 13th, 2011 at 00:43 | #17

    Are you using the old or new motors? I ask because bricklink has both and I’m not sure.

  18. will
    December 15th, 2010 at 00:23 | #18

    ok thanks

  19. Sariel
    December 14th, 2010 at 08:03 | #19

    Speeds don’t sum. Torque does.

  20. will
    December 14th, 2010 at 00:26 | #20

    does an adder combine the speed of two motors

  21. kyle
    November 3rd, 2010 at 22:07 | #21

    thanks will try that

  22. Sariel
    November 3rd, 2010 at 07:57 | #22

    It is 1 stud and they will work that way if you make sure the bricks between them stay together. Or, you can replace t24s with t20s and decrease spacing to two plates instead 1 stud.

  23. kyle
    November 3rd, 2010 at 00:15 | #23

    sorry what is the correct spacing between the two 24tooth and the two 16thooth gears, i tried 1 stud, it worked but the gears didn’t look fully meshed?

  24. Sariel
    September 18th, 2010 at 19:22 | #24

    You could knock out this “awesome” thing if you don’t want to be annoying.

  25. hector
    September 18th, 2010 at 19:11 | #25

    uhm,not to be annoying but could you maybe make a vid with a better quality.
    because i dont see very much.

    and again you projects are AWESOME

  26. Sariel
    April 20th, 2010 at 21:19 | #26

    Absolutely not.

  27. Tom
    April 20th, 2010 at 17:11 | #27

    would it work if I replace the diferentials with normal gears

  28. MB
    October 2nd, 2009 at 20:18 | #28

    Let’s try again..
    “Worked so well that it broke one of the differential bevels”

  29. MB
    October 2nd, 2009 at 18:04 | #29

    Thanks for the tip!
    Worked so great that managed to broke on of the differentials 🙂
    Still, I’m going to invent some kind of cut-off for the steer motor, before that, cannot sleep well 😀

  30. Sariel
    October 1st, 2009 at 20:38 | #30

    Gear 24t between differentials means no gear reduction (I assume you drive the 24-teeth side of the differentials). You could try replacing that 24t gear with three 8t gears (two of them as idlers). That would generate some backlash, but you would have 3:1 gear reduction and your steering motor would require three times lower torque to work.

  31. MB
    October 1st, 2009 at 20:06 | #31

    By a fortunate mistake, I now have only gear 24 between differentials and that is connected directly to steering motor. My creation works fine with both remote controllers.

    But still I’m not happy about using steering motor when drive motors are off. My creation is going to be used by children, so it would be good to have some protection to the steering motor in those cases..
    Any ideas? Somekind of power cut-off?

  32. Sariel
    September 30th, 2009 at 23:40 | #32

    Yes, I understand these difficulties with the controller. It doesn’t work like the analogue controllers common among RC models, it’s much less intuitive. Perhaps the problem is that this controlled was designed with Lego Trains in mind, and it’s not really suited for different tasks.

  33. MB
    September 30th, 2009 at 22:22 | #33

    That solution has the problem I was taking about. I tested it and it jumped steps all the time.
    I made it little more robust, but it didn’t help. But I think it does work, if you use it with patience.

    But bigger problem is the controllers unlimited dial steps. At least with a remote design like yours, combined speed dial controls.
    For example if you push throttle (to full speed) two steps too far, say +9, stopping is now on +2, not zero. Next time you overstep, say +11, stopping is now +4 and so on. Very soon the “back wall” blocks and prevents getting full speed. And that step jumping did made this even more annoying :/

    But like I said, no problems like those when using subtractor! And with simpler controller, just two wheels. Left wheel (speed) tilted to the side.

  34. Sariel
    September 30th, 2009 at 21:36 | #34
  35. MB
    September 30th, 2009 at 19:27 | #35


    OK. Thanks.
    I think I got it to work now. Steering is fast, I could be even faster (maybe with gear 40 ;), because when my vehicle it’s ready, I’m gonna use it with the new 8879 remote control and turn arch can be controlled with those dial’s speed levels.
    Without subtractor steering tracked vehicle is hard with 8879 because the dials unlimited rotation. It’s easy to loose the syncronizing between those two dials. It does work better with slow “turns”/changes, but that’s boring.
    Well, probably someone creates working solution some day 🙂

  36. Sariel
    September 30th, 2009 at 16:30 | #36

    You need a greater gear reduction for the steering motor. Clutch is useless in subtractor, because you don’t usually need a clutch to steer a tracked vehicle.

  37. MB
    September 30th, 2009 at 16:27 | #37

    What happens on (working) subtractors when swithing steering motor on and drive motor(s) are off?

    I’m testing dual longitudinal subtractor with PF XL motors to drive and medium to steer.
    I seems when using steering motor and drive motors are off, the steering motor is on heavy strain. It does not have power to move anything (my prototype is very light weight).
    That strain is not a good thing..

    I have gear 24 between differentials and two 16 before steering motor, are that the reason for my problem?
    I had to replace gear 8 with 24 because XL motors doesn’t fit on the original drawing.
    Maybe I just have to add a clutch after the steering motor…?

    But when drive motors are on, steering works like a dream.

    Comments anybody?

  38. Sariel
    February 25th, 2009 at 11:50 | #38

    I prefer double bevel gears in there. For instance the 12-teeth one and 20-teeth one. You may consider knob gears for extremely heavy duty.

  39. Nic
    February 25th, 2009 at 11:12 | #39

    was playing around with this the other day to use in a model. but was wondering what size gears you used to transfer power between the motor drive shaft and the secondary drive shaft in the single longitudinal version. i believe i tried the two sizes of gears shown in the pic but found that i could only get them to mesh with considerable backlash? am i using the wrong size gears?

  40. February 6th, 2009 at 17:33 | #40

    I developed this gear some years ago (2006 or so). But in my experience the diff-gears are broken several times.

  1. No trackbacks yet.