So Why do heavy guys do better on Flats?

Maybe I'm off on this one, but I've come to understand that heavier guys are better suited to cruising on the flats. Is this because they have more momentum, or what?

Yup. More momentum means more inertia. More inertia means it will take a greater force to slow the bike/rider down. Your case is a perfect example of Newton's First Law of Motion.

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I think it's mostly just because they are heavier in terms of muscle, which means more and stronger muscle mass overall. Bigger guys have this advantage in most sports.

Power-to-weight ratio favours them on flats when power matters but weight itself doesn't matter as much once the bike is rolling at speed, whereas it favours smaller guys on climbs where weight starts making more difference, assuming all else is equal.

Weight does matter in terms of inertia. A 200 lb rider moving at 20 mph has 33% more inertia than a 150 lb rider. That a very significant difference. That means a given amount of aerodynamic drag causes the 200 lb guy to slow down significantly less than the small guy. The same goes for the effect of drivetrain friction and rolling resistance.

I don't think the OP was asking why stronger riders are fast on flats. That's a non-starter. Of course, I could be wrong about his/her intent.

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I agree that inertia may be part of it too... but I think bigger frames mean bigger, longer muscles, bigger heart, bigger lungs, etc.

By fast, I really meant the ability to keep going at a fast pace almost indefinitely. Fitness being equal, I've always found bigger guys can do that better than I can, but as long as I can hang in there, I'm better on the hills.

Hmm. Generally speaking, if both people were in a draft, what would be comparable efforts (in %'s) that the heavy and the light guy would have to support? Assuming both are physically equal in fitness, and they're riding flats.

They don't do better on the flats. You're assuming they do because the light guys do better on the climbs.

Mark Cavendish has won more flat stages this year than anyone. Is he a 'heavy' guy?

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Not really, but Hushovd and Cancellara are "massive" by cycling standards (180 lbs).

As always, it depends on what we're talking about in terms of weight. Most people will be somewhere in between the two extremes, but in cycling, 180 lbs is a lot more than what the 125 lb climber weighs. It's much more likely that the 180 lb guy will be a better "rouleur" than the 125 lb one.

You need to restate your question differently. By the responses, it's pretty clear there's no consensus on what you meant.

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I don't know if I can make it much clearer, but...

If someone weighs more than someone else, but has the same BF%, why will they be faster on flats.

According to Contador, they don't!

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My guess is that they are bigger, so they have more output. (bigger as in muscles, lungs elc). Someone already mentioned this.

Now put them on a curvy hilly area and I think the lighter guy will win, for his less rolling resistance (heavy on tires in corners elc) and gravity pull when climbing.. well more gravity pull in corners too, thus the more rolling resistance on the tires?

Im new to road biking especially, but thats what I think =P

This perception exists because some of the heavier riders also have the advantage of being able to generate bigger wattage by virtue of their physique. Unfortunately for most of the bigger riders, this advantage is negated when the road turns skyward.

Weight matters to it. Believe it or not heavier guys don't use as much energy on flats as lighter guys. You can do a simple experiment, buy some groceries and put them in your backpack, and that goes on your shoulders. I guarantee you will ride faster and be less tired. Essentially, weight in your backpack is doing the pedal work. A light guy has to use his muscles far more in this scenario.

I'm a city rider, that means a road bike along side of cars, difference between going to grocery store and back in terms of speeds is a lot. I could sustain 18 mph going to the store with empty pack, while maintaining my regular breathing somewhat mind you, back with pack fully loaded I could be going 25 with the same close to normal breathing. And this is not race, this is city pleasure riding, keep that in mind. And it only works on flats and down hill, going up hill with more weight is hard, I have triple and I down shift onto smallest cog when I have extra weight, without any bags I could take my hill in middle cog. Which is the cog I use for flats as well.

You guys ever tried riding in a flat with loaded backpack and 20 mile wind in your back? I once reached 38 mph like that.