I would like to use the PF servo motor for steering, however it is very large - I was playing with the idea of having it mounted vertically, and then using bevel gears to change the direction of the motion, however that introduced too much play into the steering system (even with only two 12t double bevel gears)

I would like to know if there is any good gear/spacing for driving a rack via a bevel mechanism, preferably as small as possible and with a vertical PF servo motor

(P.S. - yes, I know the micro-motor would probably be better for this..)


Too much backlash (play) in LEGO gears is an universal problem that unfortunately can't really be mitigated since all compatible LEGO gears are designed with very loose tolerances compared to professional gears.

According to Sariel the bevel gears you tried using already have less backlash than regular gears, so you can't really expect any improvement by switching to another gear type. You could however try using worm and larger bevel gears, this would supposedly provide the most precision achievable with gears.

If you plan on mounting the servo motor vertically then you could skip rack and pinion steering and connect the steering rods directly to the axle of the servo, eliminating all gears and their inherent problems.

  • Any comment on using the bevel to drive the rack? – user2813274 Jul 15 '15 at 17:14
  • You can certainly do that, I can easily imagine a setup with a vertical servo, a gear attached to its shaft, and a rack turned 90 degrees to the side. The only problem I could imagine is when in the extreme positions the plane of the rack could move away from the shaft, possibly disengaging the gear. – zovits Jul 15 '15 at 19:12
  • Any way to drive the rack without having to rotate it? (as that's kind of the point of a bevel gear.. to change the plane of the torque) – user2813274 Jul 15 '15 at 19:13
  • Honestly I haven't seen this kind of setup, but it could work. I wouldn't bet on it though. If you have more space in your vehicle you can also try using two universal joints at 45 degrees each and see if their backlash is less than the two bevel gears'. – zovits Jul 15 '15 at 19:19
  • 1
    I haven't seen the setup either and haven't been able to assemble a decently sized and right-distance one yet, which is why I am asking - two universal joints seem to have more backlash than the 12t double bevel joint though – user2813274 Jul 15 '15 at 19:40

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