I'm building a (what I hope to be) high-speed car using two PF-XL motors together and some gearing up. However, there's a point where I have to rotate the axle with power (that comes from the 3 speed gearbox) 90 degrees to line it with the wheels (this is probably a long way around the barn to say it in english, but I got a picture to fill my lack of words). This is what I want to do:

enter image description here

(the position of the gears is incorrect: in my project the 8t gear would be one hole to the left)

My question is, assuming the structure holding the wheels is solid and won't bulge, what's the best gears to use so that they won't snap?

So far I've considered four combinations:

  • The same as the image, except the 8t is a 16t
  • The fat 20t with the fat 12t
  • The skinny 20t with the skinny 12t (this one doesn't actually work for me unless I do some changes to the car)
  • Two of those 4t gears that aren't very smooth

It stands to reason that the last option is the strongest because they don't snap unless they break apart. However, they're more suited to low speeds. With high speeds they aren't fluid at all and make a lot of noise. But my biggest problem with them is that they sometimes get stuck (the gear without power sometimes stops in a position where it blocks the gear with power and when the motors start again they can't rotate - this is really bugging me).

So, excluding the last option, what do you think?

  • This question would be easier to answer if the application was made clear. Are the axle locations in the picture (edit accounted for) in a fixed location, aka have to be there? Are you wanting a gear reduction or a 1:1 transfer? Is there a height restriction that would prevent larger gears from being used? Is a differential used in the solution an option? Do you have any restrictions on what gears you have access to? If a gear reduction is desired, which axle in the input?
    – JohnnyB
    Commented Sep 3, 2019 at 8:27

1 Answer 1


I would tend to recommend some options which you didn't consider yet: using bevel gears, but actually use a combination of two gear on each axle: a 12t one (the thin one, as you say), and behind it, a 20t one (there you can use a fat one, which will probably be better).

Another option is to use a worm screw, which will also allow you to change the direction of the axle, but which makes the axles non coplanar.

But it really depends on a variety of factors. Gear reduction is one (keeping in mind it has implications on torque, but frankly I'm already way over my head here so I won't tackle that aspect); but torque transmission is another, and so is strength/reliability.

For example, the knob wheel that you did mention has some uses, as it practically can't slip, but at the cost of lower torque transmitted (or something). Worm screws again have another particularity which is unique to them in that they can transfer torque only one way, which also has its uses (locking a mechanism in place).

And then don't forget that the gears aren't all there is to say: sticking your gears closer to the beams will help, as will replacing the two separate beams by an actual frame. You can also make sure one of the axles goes all the way to the other side of the frame.

And last thing, if you're going to transfer power to two wheels, don't forget to use a differential between them (not sure the new one is very solid though).

  • Thanks. You mean on one axle use those two gears, but what about the other axle? What gears would you recommend? Also, worm screws aren't really what I'm looking for, as they reduce speed dramatically. And I have already added a differential (the old one, since I only have of that type).
    – Alex
    Commented Apr 10, 2013 at 21:12
  • I meant, use both gears on both axles, for a total of 4 gears
    – Joubarc
    Commented Apr 11, 2013 at 17:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.