I think if you looked at the physics of a collision between a pump shoe and a rock/log/etc, you'd find that the shoe material would change the forces applied to the rest of the motor during a collision very little. Typically, there just isn't a lot of compression and deformation of material during a collision to absorb and dissipate the forces applied to the motor. If the metal shoe didn't break, but compressed and deformed when a UHMW shoe wouldn't, then it should make a difference.

*Disclaimer: Dan S. is no physicist. He has no degree in physics or any closely related field. His opinion should only be taken as gospel if you're really dumb and/or gullible.
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