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I've been thinking about the tracks.
After playing with numbers and sketches, I decided that the dimensions I proposed before felt off. The 18 inch length with the 3 inch wheels I was considering using felt too thin (6:1 length to height ratio), and the 12 inch width was too narrow (2:3 ratio of width to length). Three inch wheels would also not leave much room for motors or other internals, because the bottom of the center section would need ground clearance and it also has those recessed greeblies on top.
Treadwell uses 4 inch wheels, so I decided to go up to that size, then boost the body length to 20 inches and the width to 15 (5:1 length to height, 3:4 width to length). I'm using a belt thickness of 1/4 inch for now. Half of that is for the belt itself, the rest for the segmented outer portion. Might reduce the thickness later if that isn't flexible enough.
As each belt occupies roughly 1/3 of the 15" width of the droid, I set them to 5 inches wide (the side plates appear to be entirely surrounded by the belts, so little or no additional width from those. I'll have to get clever to mount them).
![[Image: nuegmlk.png]](https://i.imgur.com/nuegmlk.png)
Though only the front and rear wheels are shown here, it will have at least one additional set hidden in the middle.
Assuming that the front and rear wheels are the same size, and that the belt runs straight between them -- i.e. the middle wheels are not larger like Treadwell's are -- then the inner circumference of the belt (blue line on the sketch above) is:
With the new numbers, the calculations for the belt look like this:
Axle_Spacing = Body_Len - Wheel_OD = 20 - 4 = 16
Spans = 2 * Axle_Spacing = 32 (the top and bottom spans of belt between the wheels)
Wheel_Circum = Wheel_OD * PI = 4 * PI = 12.566 (the length of belt that's in contact with the front and rear wheels).
Belt_Inner_Circum = Spans + Wheel_Circum = 32 + 12.566 = 44.566 inches. That is how much space you have to allocate to the outer tread segments and the gaps between them.
Belt_ID = Belt_Inner_Circum / PI = 14.186 inches.
Belt_OD = Belt_ID + (2 * Belt_Thickness) = 14.186 + (2 * 0.25) = 14.686 inches. This belt would be printable on my Anycubic Chiron (bed size of 400 x 400 mm or 15.74 x 15.74 inch).
Belt_Outer_Circum = Belt_OD * PI = 46.137
I decided to set the arcs that define each segment (the Segment_Length) at 2.2 inches, which looks like a decently rectangular aspect ratio given the 5 inch width of the belt. Could reduce the segment length to improve that further, but I also want to preserve the relatively small number of segments. It appears that the illustration is showing the droid with 18 segments per belt (I count 9 on the half of the nearer belt that's visible). I'd prefer to stay close to this number.
Once you know your Segment_Length and the Belt_OD, you can draw that arc length in Fusion using this formula to calculate the angle between the edges of the segment, in degrees:
Segment_Angle = (Segment_Length * 360) / Belt_Outer_Circum = ~17.2 degrees
![[Image: g85IP2P.png]](https://i.imgur.com/g85IP2P.png)
The maximum number of segments that you can fit on the belt is the integer portion of Belt_Outer_Circum / Segment_Length. In this case, that's 46.137 / 2.2 = 20.971 segments, or a max of 20. The gaps all come from the 0.971 that's left over (times Segment_Length) -- the lower this remainder, the tighter the segments will be spaced.
If you want to reserve a minimum gap between segments, then just add that to the Segment_Length before dividing. So for 1.4 mm, convert to ~0.055 inches, then add it, to get 2.255 inches. 46.137 / 2.255 = 20.46 segments ... still a max of 20 segments either way in this case. The nonzero remainder here means our gaps will end up wider than 1.4 mm (in fact they end up more like 2.7 mm).
From the sketch above, I first extruded the inner belt (the wider blue curve), and then the calculated segment. I reproduced the latter feature with a circular pattern around the belt in order to get the model.
![[Image: L1uqxbv.png]](https://i.imgur.com/L1uqxbv.png)
After playing with numbers and sketches, I decided that the dimensions I proposed before felt off. The 18 inch length with the 3 inch wheels I was considering using felt too thin (6:1 length to height ratio), and the 12 inch width was too narrow (2:3 ratio of width to length). Three inch wheels would also not leave much room for motors or other internals, because the bottom of the center section would need ground clearance and it also has those recessed greeblies on top.
Treadwell uses 4 inch wheels, so I decided to go up to that size, then boost the body length to 20 inches and the width to 15 (5:1 length to height, 3:4 width to length). I'm using a belt thickness of 1/4 inch for now. Half of that is for the belt itself, the rest for the segmented outer portion. Might reduce the thickness later if that isn't flexible enough.
As each belt occupies roughly 1/3 of the 15" width of the droid, I set them to 5 inches wide (the side plates appear to be entirely surrounded by the belts, so little or no additional width from those. I'll have to get clever to mount them).
Though only the front and rear wheels are shown here, it will have at least one additional set hidden in the middle.
Assuming that the front and rear wheels are the same size, and that the belt runs straight between them -- i.e. the middle wheels are not larger like Treadwell's are -- then the inner circumference of the belt (blue line on the sketch above) is:
- Twice the distance (once for the top span and once for the bottom) from the center of the front axle to the center of the rear axle
- plus the circumference (PI * diameter) of one of the wheels. (It's actually half the circumference of the front wheel and half of the rear, but those are the same in our case, so they add up to one whole circumference)
With the new numbers, the calculations for the belt look like this:
Axle_Spacing = Body_Len - Wheel_OD = 20 - 4 = 16
Spans = 2 * Axle_Spacing = 32 (the top and bottom spans of belt between the wheels)
Wheel_Circum = Wheel_OD * PI = 4 * PI = 12.566 (the length of belt that's in contact with the front and rear wheels).
Belt_Inner_Circum = Spans + Wheel_Circum = 32 + 12.566 = 44.566 inches. That is how much space you have to allocate to the outer tread segments and the gaps between them.
Belt_ID = Belt_Inner_Circum / PI = 14.186 inches.
Belt_OD = Belt_ID + (2 * Belt_Thickness) = 14.186 + (2 * 0.25) = 14.686 inches. This belt would be printable on my Anycubic Chiron (bed size of 400 x 400 mm or 15.74 x 15.74 inch).
Belt_Outer_Circum = Belt_OD * PI = 46.137
I decided to set the arcs that define each segment (the Segment_Length) at 2.2 inches, which looks like a decently rectangular aspect ratio given the 5 inch width of the belt. Could reduce the segment length to improve that further, but I also want to preserve the relatively small number of segments. It appears that the illustration is showing the droid with 18 segments per belt (I count 9 on the half of the nearer belt that's visible). I'd prefer to stay close to this number.
Once you know your Segment_Length and the Belt_OD, you can draw that arc length in Fusion using this formula to calculate the angle between the edges of the segment, in degrees:
Segment_Angle = (Segment_Length * 360) / Belt_Outer_Circum = ~17.2 degrees
The maximum number of segments that you can fit on the belt is the integer portion of Belt_Outer_Circum / Segment_Length. In this case, that's 46.137 / 2.2 = 20.971 segments, or a max of 20. The gaps all come from the 0.971 that's left over (times Segment_Length) -- the lower this remainder, the tighter the segments will be spaced.
If you want to reserve a minimum gap between segments, then just add that to the Segment_Length before dividing. So for 1.4 mm, convert to ~0.055 inches, then add it, to get 2.255 inches. 46.137 / 2.255 = 20.46 segments ... still a max of 20 segments either way in this case. The nonzero remainder here means our gaps will end up wider than 1.4 mm (in fact they end up more like 2.7 mm).
From the sketch above, I first extruded the inner belt (the wider blue curve), and then the calculated segment. I reproduced the latter feature with a circular pattern around the belt in order to get the model.