The drive wheels supporting the main rods saw much higher inertial forces at speed than the other drivers. Therefore, they were the most prone to crack, and often the only wheels replaced by Boxpok or other disk-type wheels in locomotive upgrades.

Balancing of steam locomotives is inherently imperfect because the vertical inertial forces imposed on the crank pin are only from the side rods (and the rear part of the main rod on the wheels where they attach). But in the fore-aft direction, the mass of the pistons, cross-heads, and the forward part of the main rods come into play. The counterweights cast into the drive wheels are sized to compromise between the vertical and the fore-aft inertial forces on the crank pins. Thus the counter weights are undersized for the fore-aft forces (under-balanced) and oversized for the vertical forces (overbalanced). Also, there are smaller counterweights 90 degrees from the main weights on later locomotives to achieve what is called dynamic balance, or "cross balancing."

The vertical direction overbalance forces are reacted by the track, meaning they are transmitted through the wheel. There is also some bending on the wheel due to the crank pin being offset in the outward direction relative to the plane of the wheel and the center of the rail. These radial and bending forces would contribute to cracking of the spokes where they join the rim.