It turns out that a big problem with player-made wheels was that the game was recognizing them as high-speed spinning weapons and adding Weapon_Blur_Cylinders to them. The Weapon_Blur_Cylinders are designed to correctly calculate the result of impacts when a steel bar is spinning faster than approximately 400 RPM. At this speed, the physics engine is essentially "teleporting" the spinning thing to its new location every tick.
Weapon_Blur_Cylinders do apply tangential forces, but they don't simulate friction, so driving with them is a terrible experience.
In the new build if you create a shape and set the material to "Rubber", the game will not create a Weapon_Blur_Cylinder at all. This means that the game will use its normal friction system, and the robot will drive exactly like it would if you used premade wheels.