# Tank tracks distance problem

Hey,

I’ve modelled some tank tracks with the usual array following a curve. Problem is I tried to model the “wheel” with the teeth that push the tracks round but I cant get the teeth to be the right distance to go between the track parts.

I’ve been taking a cylinder and extruding the faces to make the teeth but it’s just off every time.

Is there a trick to get this right?

The circumference of the circle should be a multiple integer of a teeth and space between them … remember: circumference of a circle = 2 * pi * radius. Yes the track is longer but it has to fit because the wheel has to be at the same position after one turn (and that is measured by the track length) to fit the teeth again.

You know what. I managed to get it really close by mashing my face into the keyboard. And I’m kind of glad because I have no idea what you just said.

But many thanks for your reply and I’ll try to learn what you taught because I’m going to need to do this a lot for different assets and there’s only so much amount of face mashing I can take.

Cheers

@Okidoki has this math right — and you already know the distance between the cogs (the pitch, Pitch) because you have used an array modifier.

Here it is in a 5 images. First some simple (massive!) tracks. Note that I use a constant offset of 1.1,

Let’s assume you want 12 teeth (Z) on the cog. You have the circumference,

Circumference = Z * Pitch = 12 * 1.1 = 13.20

C = 2 * pi * r
so,
r = C / (2*pi) = 13.20 / 6.2832 = 2.10

Make a cog that fits the width of the tread slot. I’ve made a temporary array (using 1.1 constant offset). You should be able to overlay the track and the cogs will fit (sorry, didn’t take that image),

To setup for spinning, first remove the array modifier from the cog. Shift the cog to the origin and move it up 2.10. Note that the origin of the cog is centered at the base of the cog to make it easy to get it into position.

Now spin around the Y axis 12 steps. Remove doubles. Shift the origin to the center.

You have lots more work to do but this image shows that a gear wheel built this way will “roll” along the tracks.

To experiment with different size gears, the dimension of the cog doesn’t change, you just need to calculate the new radius based on the tooth count.

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Aaaahh I will give it a try, thanks!