In this post I’ll give some instructions and pictures that should allow you to build a compact rotacaster platform like the one you see on the picture below.

The base of the platform is made out of 3 15 M beams. They are connected in a triangular shape using 3M and 4M levers on each corner. The 3M cross axles will stick out on top. They allow you to attach a sensor to the corners when the platform is finished.

This triangle is enforced from below with a smaller triangle made of 11M beams.
As you can see from this picture there is a 9M beam placed on the smaller triangle, these are actually 2 beams placed on top of each other. When the platform is finished these beams will hold the NXT brick. You can ignore the 3M beams, they are used to hold my port splitter.

Each of the motors is held by a frame. Two are equal, but one is mirrored. Although the frames are simple it is hard for me to explain how the frames are made, but I think the pictures below will give you enough guidance.

each of the three motor frames is attached to the platform by the four connectors, the blue ones go into the corners of the smaller triangle, the black one go into the bigger triangle.

The platform is made rigid by a A-frame that interconnects the three motors on the top of the robot. This A-frame also serves as a base for sensors. The top of the A-frame is made of a 3×3 T-beam, its two sides are 13M beams and the bottom of 2 5M beams and one 9M beam. The bottom might seem overcomplicated but this way you’ll have more room to press the NXT buttons.

The last step is to add the brick to the robot. Make sure you get two connectors in the top back side of the brick. Shove the brick in from the front of the robot and connect it to the rim of two beams that rest on the smaller triangle. If all goes well the bottom of the brick will fall nicely over the 2 3M axles that make the front sensor connector.