Online Design Tools

The menu at the left shows a few online tools that I use on a regular basis to design my bots. Over the years I have used each of them many times. I spend a lot of time laying out the design before building them to save some of the cost of unused parts or ones that don't fit. These tools help you gain some confidence without spending any money.

Spinning Weapons

The Spinning Weapon calculator provides an estimate of how much energy your robot's weapon will store. It seems to only work well in Internet Explorer. You have to provide the MOI of the weapon and how fast it is spinning. This particular calculator allows you to build up the MOI of the weapon by compositing several different shapes. If your weapon is not a simple bar, you will need to find a way to model it with the shapes provided.

I did this for Scurrie's Blade by making a cylinder for the outer ring, four bars for the eight spokes, and two teeth for the.. uh.. teeth. When I was done I put in the top speed of the blade to find out how much energy it stored. It turns out that Scurrie only stores about 520 Joules of energy. There are some beetles that claim to store that much energy but it has been plenty for me for a few years. Like most things, it's not the size of the hit that counts, it's how you use it :).

Conical Shells

The Conical Shells tool helps you make the 2D drawing that could be rolled into a cone with a flat top. The final output of the calculator is the dimensions of two arcs and the lines that connect them. I wrote this calculator to help me design the shell for Steel Shadow. I wanted to be able to send the drawing off to a waterjet company but the material was expensive so I didn't want to just guess on the shape. In the end it turned out just like I wanted.


I can hardly imagine designing a new drivetrain without the Tentacle Drivetrain Calculator. It allows you to calculate how much current your drivetrain will pull at wheelspin and so will help you pick speed controllers, batteries, motors, and wheel size. It is invaluable for exploring available options. Remember that the results are just theoretical. For small bots, I've been pretty happy shooting for 50% or so of the stall current at wheelspin. Steel Shadow has an acceptable drivetrain at 13.2 volts, even though I rarely use it to it's full potential. Scurrie's numbers show a seriously weak drivetrain but it is enough to move around in the box. 5 MPH in a 16 foot box is the absolute lowest that I consider acceptable. Notice that I'm running the motors below their rated voltage. This is an artifact of the battery system in Scurrie that didn't provide me a good voltage for my motors. It was a sacrifice I had to make.

Timing Belts

Stock Drive Products / Sterling Instruments is a great place to get small timing pulleys, belts, and bearings. All of my bots have SDP products in them. The shipping and handling costs are a bit high so make sure you get everything in one go, but they stock lots of tiny stuff that is hard to find elsewhere and will let you put in small quantity orders. This calculator is their web tool for determining how far apart the shaft centers should be for a given timing pulley and belt system. I use metric 5mm HTD pitch belts on Steel Shadow for both weapon and drive. These belts have really deep grooves and don't slip at all, which makes them really good for drivetrains. Because the grooves are really deep, you can be a tiny bit off on the center distance and still use them well. My calculations showed that I would need to add an idler to take up a little slack in the belts. The buttons at the left of the calculator are supposed to link to the parts you picked but my experience has been that they don't work very well. I recommend going to their e-store directly and searching for the parts. You need to switch to the "metric" index to find the belts, but the pulleys can be found in either the "inch" or "metric" index.