I am studying implementation of 9398 crawler reduction. Here is the stripped part, in order to see the gears and their relation - the image is made with the excellent tool which is LDCad.

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My idea is to use this Lego design for other model and I need to know how would I calculate the effect of changing it a bit. My intention is to replace the two 16 teeth gears 94925:

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with a pair of 12 and 20 teeth (32270 and 32269):

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and to remove the last reduction pairs, the ones in portal axle:

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so it would be as follows:

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Would this become faster/less powerful or slower/more powerful? My aim would be to keep it pretty close to how it performs now.

Also, is there a way to combine gears in order to add a distance of half of a stud between the motor axle and the next or the second one? In the image below, I have moved the parts and created a gap between the gears, in order to point what I intend to move, but the contact must be assured by some gears, obviously by replacing the existing ones. Would be there possible?

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2 Answers 2


This modification won't change the rate of reduction.

You can calculate the ratio of the angular velocities of two adjacent gearwheels as

i12 = w2/w1 = n1/n2

where w is the angular velocity and n is the number of teeth of the gears. Hence the original reduction reads as:

i12*i23*i34*i45*i56 = 12/20 * 16/16 * 20/28 * 12/20 * 12/20 = 27/175 = 0.1543

while the modified is:

i12*i23*i34*i45 = 12/20 * 12/20 * 20/28 * 12/20 = 27/175 = 0.1543

What you did is you simply removed the 16/16 and replaced the 12/20 gears, which results in the same reduction because the order of the multiplication is arbitrary.

To the second question:

You can change the 12-20 gears to 16-24 ones to fit the increased distance. In this case, the new reduction is

i12*i23*i34*i45 = 16/24 * 12/20 * 20/28 * 12/20 = 6/35 = 0.1714

Note that the new gears may intersect parts not shown in the figures.


If the only change is replacing the two 16-tooth gears with a 12-tooth gear and a 20-tooth gear, then you will reduce speed by a factor 12/20 and increase torque by the same factor.

If you want to increase the distance between gears by 1/2 stud, then you can experiment with using bigger gears and offsetting the motor sideways so the gears don't touch vertically. This may induce some play in the gears.

  • I would drop the portal axles, so the performance will be kept. The reason is for implementing this on other model with less axle height (42069). Well, I did it partially, but using a horizontal transmission and keeping a similar design, with only 2 reductions instead of 3 (as this model has). It performs acceptable - a little faster than 9398, but powerful enough to run over decent obstacles for its tracks. I couldn't find a combination of gears for that 0.5 stud displacement, though. The gears are usually too tight to work safely together.
    – mike
    Commented Jun 22, 2018 at 5:44

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