Hi everyone - quick question on something that's been bugging me. So, train wheels, which are solid steel, have incredibly low rolling resistance, an order of magnitude less than average car tires. This is because they don't bend significantly, so you're not wasting energy to heat as in the bending rubber of car pneumatic tires. Indeed, to minimize losses in pneumatic tires, one technique that's used is reinforcement to stiffen the sidewalls to minimize their flexure. Okay, got it. So then why is it that everything I've read about solid tires versus pneumatic tires in wheelchairs says just the opposite? Everything I've read, including a paper doing rolldown tests on a dynamometer, say that the pneumatic tires have lower rolling losses. Why is this? What is the factor here that's causing the difference? Is it possible to have a solid tire that gets low rolling resistance like a train wheel but has a weight like a bicycle/wheelchair tire, or is it somehow inherent that in order to get super-low rolling resistance you have to significantly increase the weight, and thus negate your benefits?