Power to Weight
Power to weight is exactly what it sounds like: how much power is used to move a given amount of weight a certain distance. Professionals use complicated machines to measure how many watts your body can produce. Then they use a formula using your weight in pounds to come up with mathematical calculations that may or may not be useful. That's fine for professionals, but for the everyday cyclist, power to weight is how you look and feel when you ride your bike. You probably have an ideal weight that you have established at some point in your life. If you weigh considerably more than your ideal weight, you will feel sluggish, lethargic and slow when you ride your bike. If you're close to your ideal weight, your bike will move faster. It's not rocket science, it's personal. Don't set unrealistic goals for your weight. Your power-to-weight ratio will stabilize and your bike will move at the most efficient speed for your body weight.
Water weight can vary drastically from ride to ride. If you're carrying an excessive amount of water in your body, you can feel bloated. This can affect cycling speed because you may not feel as if your body is operating at peak performance, and your power to weight ratio will be skewed. You might notice that you're water-heavy if you can't get a ring off your finger the way it normally slides off, or if your shoes fit too tightly. This condition will slow you down slightly, but it's nothing to worry about in the long run. It will dissipate. Don't attempt to ride without water to make your body lighter; that's counterproductive. Water weight can be caused from a fiber-heavy diet. If you have a problem with excessive water weight, choose to avoid high-fiber foods a few days before a big ride.
Mountain bikes are less susceptible to body weight than road bikes. For one thing, speed is not so much of an issue. Mountain bikes focus more on power, and even though mountain biking involves a lot of climbing, they have the gearing for it. To illustrate the difference, take a look at a mountain biker's body. It's typically more muscular, husky and compact. Mountain bikers are concerned with weight, but it's not so much about speed as it is overall fitness and power. In mountain bike races, you will sometimes see racers who appear to be slightly overweight. That's fine. These racers may even have the edge when it comes to overall mountain biker fitness and endurance.
The equations of body weight to speed are taken more seriously by road riders. Many of the races are won by fractions of a second. The difference between road and mountain biking is time in the saddle. Road riders will spend hours on their bikes. Even a small difference in weight can make a big difference after six or eight hours in the saddle. It takes energy to move weight. The more weight, the more energy is expended. It's a simple formula that gets more important as the miles add up. Weight taps energy faster. Road racers weigh things in grams. One gram weighs about the same as one paper clip. That's how seriously road riders look at weight, and they will make every attempt to stay thin. Just look at most road racers: They are willowy, lean and trim, testifying to the fact that less weight means more speed. It may be subtle, but the difference is there.