The rim is the outer section of a wheel to which the spokes and tyre attach. Probably the most important factor in a strong wheelbuild. Rims generally come in four sizes for trials; 20 inch (mod bike front wheel) 19inch (mod bike rear wheel) 26 inch (stock bike front + rear wheels) and 24inch.

Most trials rims are "drilled" or cnc'd to remove excess weight. Trials rims are around 35-45mm wide, whereas a normal mountain bike rim is only 20-30mm wide.