The tire pressure, it seems like, is something that one could set at a standard value fairly easily. However, I don't see how weight makes any differences though, or tires. As long as the tire doesn't "slip" when it's on the trainer, it seems like that shouldn't matter. The internal resistance of the trainer seems like it's the only variable, as long as the contact between the trainer and the tire is tight enough such that the wheel doesn't skid. What am I missing?