[isidor3]

Heavier things do gain kinetic energy faster when falling, it's easy enough to confuse "being hard to stop" with "moving fast." Intuitively, the heavier object has "more" of something but in casual conversation "kinetic energy" isn't a common or familiar term. If you haven't studied enough physics to reach for that concept, I'm not surprised a lot of people just settle on "faster" when attempting to describe it.

[taeric]

Yeah, and we have words for that. Momentum, is a good one.

I should clarify the last time I had this conversation, I'm talking about 40+ year olds building bikes. They were thinking a heavier bike would accelerate faster down a hill. That just is not how that works.

[isidor3]

It actually is though! The force due to air resistance is the primary thing slowing down the bike and rider, and the force, which doesn't change with mass, slows down a heavier bike less, and so a heavy bike will be rolling faster than a lighter one at the bottom of a hill, all other things being equal.

[taeric]

Right, you can hit a higher velocity. You will not get extra acceleration. That is my entire point.

That is to say, the force from the air does not actively slow you down. It prevents you from speeding up, after a point. (Aerodynamics are, of course, more complicated. But this general model is pretty solid.)

Such that if I want to accelerate faster down the hill, I have to pedal to cause that to happen. Or add a motor.

[jstanley]

The heavier bike will reach the terminal velocity of the lighter bike sooner than the lighter bike will. If that's not extra acceleration then I don't think you're looking at the problem from a practical perspective.

Yes, acceleration due to gravity is the same, but air resistance has less effect, at all speeds, on the heavier bike. The rate at which the heavier bike gains speed is higher. It accelerates faster.

[taeric]

This is just disagreement over magnitude. And what it means to hit higher acceleration.

Take my statements to be, it won't hit a meaningful higher acceleration. Not that it won't necessarily accelerate for longer.

In general, you will accelerate down the hill at 9.8 m/s^2 modified by the incline. No matter how heavy the bike is. (Within the realm of realistic weights.).

Yes, the points you are raising are true. But within the realm of the biker and realistic bikes, not really relevant.

[SirLuxuryYacht]

The velocity of the bike+rider is proportional to their mass. Realistically this can range over two orders of magnitude, which can be particularly relevant in a racing context.