Bike Forums - Does weight affect downhill speed?

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Quote:

Originally Posted by imi (Post 9217270)

From your answers (sorry I can't follow the math) would I be correct in concluding that:
1. The heavier rider's extra mass causes him to accelerate faster down the hill solely due to the effect of gravity in an atmosphere? No. Gravity would cause him to accelerate at the same rate as another rider. However the gravitational force would be stronger.
2. That his assumed larger cross section (which I called girth) would slow him down due to greater air resistance? YES.
3. That his tires have a greater contact area with the road thus increasing friction (heat) thus slowing him down? YES, but only a miniscule, trivial amount
4. That the air resistance and friction, however, are less than his greater gravitational acceleration meaning that he would get to the bottom of the hill first? The air resistance is less than the gravitational force, not the gravitational acceleration.

To the other part of my question... Assume there is another hill after the first, the guys are still coasting, would the lighter guy catch up on the way up the hill or come to a stop further up the hill than the heavier guy... ? NOT if they are coasting the whole way. In a vaccuum they would coast down and roll up the same amount, but in atmosphere, since the light guy loses more of his momentum to air resistance on the way down, he won't coast uphill as far.
Hope this clarifies my question
Thank you for your time and knowledge
:)

The difference between force and acceleration can be illustrated thus. If you hold a pound of butter in one hand and a penny in the other, you can feel that the butter has a greater gravitational force ie. it pushes down harder on your hand. However if you drop them, their acceleration is about the same - they both pick up speed at the same rate (until air starts to impede their acceleration). Similarily the light and heavy rider will initially accelerate at the same rate until they get fast enough that air starts to interfere with further acceleration.
Hope that helps.

Quote:

Originally Posted by umd (Post 9191912)

I think part of the problem is that people are using the terms weight and mass interchangeably...

I don't see that being a problem in the least assuming we are talking about a situation here on earth where, in addition to an atmosphere to complicate things, weight is directly proportional to mass.

Ah sorry about writing "acceleration due to gravity" when I should have written "gravitational force"... I meant "go faster" ;)

Is the bigger guy's greater cross section such a minor factor? When you pull your knees in or hunch down it makes quite a difference coasting down a hill, but maybe big guys aren't as big as they seem (I'm one of the slender variety)

"The bigger they are the harder they fall"... uh true or false?? :D hihi

QUOTE: since the light guy loses more of his momentum to air resistance on the way down, he won't coast uphill as far. END QUOTE

uh, wouldn't the big guy have a greater gravitational force slowing him down up the hill? (sorry if this was stupid)

Quote:

Originally Posted by trekker pete (Post 9217301)

As for the coasting uphill part, I honestly don't know. Assuming a higher speed at the bottom of the hill by fatso, I will guess that he would coast a little farther up the hill, but, I'm not as comfortable with this assumption as with the downhill part.

If they are pedaling the uphill side, I suspect fatso will get dropped like a hot potato, assuming comparable fitness levels.

Correct on both counts. If they're coasting the heavier guy goes further before he stops (because he has greater momentum at the bottom). If they're pedaling he gets dropped, and not just because he's out of shape -- even if he's a lean muscular dude, he's got more weight to carry up the hill.

Thanks guys! This makes everything much clearer :)

Quote:

Originally Posted by imi (Post 9217383)

uh, wouldn't the big guy have a greater gravitational force slowing him down up the hill?

No. He will experience greater gravitational force, but the same gravitational acceleration. He has greater momentum. That is, because he's heavier, it takes greater force to slow him down. Since he was going faster to begin with, he ends up going further.

Quote:

Originally Posted by imi (Post 9217383)

Yes, but the resulting acceleration (deceleration, if you prefer) would be the same as for the light rider. Gravity pulls harder on the big guy, but his mass resists it, and they balance out.

Think of trying to pull a standing freight train up to speed. If there are 50 cars, and it takes one locomotive 5 minutes to accelerate it to 40 miles an hour, then if there were 100 cars, it would take two locomotives 5 minutes to accelerate it to 40 miles an hour. Twice the mass, twice the force, same acceleration. Gravity is like that. It pulls harder on a heavier mass than on a light one, but the acceleration is the same.

Another interesting result (which I bring up just because this thread needs more discussion ;)) is that if they were to reach the bottom at the same speed (say the heavy guy had been breaking) and then coast on a level surface, the lighter guy will slow down faster.

In fact, I believe that even if they were to reach the bottom at the same speed and then coast up a hill, the heavy guy would go further because they would both suffer the same deceleration from gravity, but since the force of air resistance would be the same on both, the heavier guy would receive less deceleration from air resistance (a = F/m).

So if it's a short hill, the heavy guy might not get dropped after all.

Quote:

Originally Posted by Andy_K (Post 9217455)

So if it's a short hill, the heavy guy might not get dropped after all.

I can attest to this. Assuming I have a nice down hill to get over 40 mph, I am a lardass hill climbing machine, so long as that hill isn't more than 50 yards or so long. A good stretch of my commute is along a road with a lot of small rolling hills. If I pound the down hill sections I generally manage to stay above 20 mph. Some mornings I'm just not feeling it though and end up spinning up them in a stump puller gear at 12 mph.