Originally Posted by lastri
I heard that road bike is much lighter, so you can go faster.
Yes, road bikes are much faster with the same amount of energy output. But it's for many reasons, not just they are lighter. The tires are upwards of 90 psi. inflation, whereas mountain bikes may be around 60 psi. So more inflation, less rolling resistance. Also the tires are thinner on a road bike. Again, less contact with pavement, less rolling resistance.
Also with a road bike, your position is more forward towards the front tire than on a mountain bike. The further back you sit on a bike, the easier it is to climb hills, so mountain bikes are made where your weight is more shifted towards the rear (so you can use your leg muscles more efficiently for climbing). But this hurts forward momentum.
Road bikes also have different gearing. Smaller gears mean greater speeds, but the tradeoff is that you have less torque (power) for climbing steep hills.
With road bikes, the stem is lower so when you lean forward on the handle bars, you are in a more prone position, whereas on a mountain bike the stem is longer and you sit more upright (the handlebars play into this as well). This affects not only forward momentum but also increases wind resistance when you're sitting more upright on a mountain bike.
I love talking bikes. Hopefully I didn't bore you.