Heavy bike riders descend faster?

Factual Questions
1

There is a very ingrained attitude in the bike world that heavier riders descend faster.

Of course I know that all objects fall at the same rate in a vaccuum.

Is there something else going on when you put heavy people on a bike that will make a heavy rider descend faster? Do 2 people with the same cross-section go down a hill at the same rate?

The notion seems wack to me, but there’s a lot of people out there firmly convinced of it, like this guy.

You can find mention of it on any bicycle bulletin board.

Related question. . .

Does a cannonball and a glass sphere of the same size fall at the same rate in air?

Do they roll down a plank at the same rate?

Do two cyclinders of the same size/different weight roll down a plank at the same rate if their coefficient of friction is the same?

2

I looked over the page, and with my cursory glances I did not see where he said heavier cyclists go downhill faster. The math he did looked OK, but I didn’t try to critique it.

All things being equal (bearing resistance, tire pressure, etc.) two cyclists of unequal mass should descend a slope at the same rate. However, it will take more braking force for the heavier cyclist to stop, and if the the heavy rider is significantly heavier, the effect will be noticable in the stopping distance. My guess is that is where the perception of heavier riders going faster really comes from.

  1. Yes

  2. Yes

  3. Given a surface that doesn’t compress or crush under a rolling object, friction plays very little into the dynamics of an object rolling down a slope. It’s there if you analyze the forces, but doesn’t have much of an effect except to keep the rolling object rolling instead of sliding.

3

Gravity powered racers. such as soapbox derby cars, have a maximum weight rule, and the cars are ballasted to be just under it. A heavier guy on a bike, unless his drag is increased proportionately to his weight (unlikely, since mass is roughly proportional to the cube of linear dimensions and cross-sectional area proportionate to the square), will have a higher maximum speed downhill. We’re talking here about a situation where he’s maxed out his pedaling and is coasting.

4

We’re assuming here that the light and the heavy guy are both tucked in good aero positions. Actually, if you are insanely brave, you could go faster by putting your legs straight back, laying on your stomach on the seat. Cornering would be a problem, though.

5

So, Hoodo are you stating that a heavier guy and a lighter guy, tucked into the same aero position will descend at different rates?

Forget about pedalling. . .assume they just crested the hill at the same rate and now are just tucked.

If so, I’ll let you and Vunderbob hash it out. I’m just the instigator.

6

Let’s start from the bottom:

(Assuming you mean a cannonball that is much denser, therefore heavier when the same size)
Initially, when they’re both moving slowly, so air resistance doesn’t matter, yes.
But once air resistance becomes important, the cannonball will go faster, so no.

There are three things going on: force of gravity (which is proportional to the mass), inertia (how much it resists acceleration, which is also proportional to mass) and the force of air resistance (which is more or less proportional to surface area, not mass). In a vacuum the cannonball has a larger gravity force, but also a larger inertia by exactly the same amount, so they fall at the same speed. But the air resistance force is the same for both balls; and because the glass sphere has less inertia it is slowed down more.

For rolling down a plank, it’s the same thing, except usually speeds are low enough that air resistance doesn’t matter much, so the cannonball will only be a tiny bit faster, probably undetectably so.
Now on to bicycles:
You’ve still got gravity, inertia and air resistance, plus some rolling resistance (friction on the wheels and bearings). Now rolling resistance is partly proportional to weight, and partly constant, so the lighter rider is affected a little more. And air resistance is important at hill-descending speeds, so the lighter rider is significantly more affected by air resistance.
Therefore the heavier rider has a speed advantage.
Plus, there’s the idea that a heavier rider might not only have a slight advantage descending but also a disadvantage climbing, so relatively speaking, descending is even better. Consider a rider that’s, well, let’s not mince mince words, he’s fat. He’s got the same amount of muscle for power as the skinny guy, but has to lift a lot more mass, so he’s much slower going uphill. Going downhill, though, gravity is more important that muscle power, so the fat guy has no disadvantage. And if fact a slight advantage, so he’s a lot better off descending. This works even if he’s not blubbery fat and out of shape, but just broad-shouldered with big arm muscles that don’t push the bike forward.

So putting these together, I think it’s not ridiculous to think that a smaller, lighter rider should plan to try and gain ground on uphill sections, while assuming he’ll lose some on downhill.

7

In terms of rolling cylinders down a slope, I remember at university covering the angular moment of inertia of an object.

The moment of inertia of an object, say a cylinder, is a measure of the mass distribution of the object. It’s to do with angular momentum, and to simplify matters, the greater an object’s moment of inertia, the harder it is for it to speed up or slow down rotation.

If there were two cylinders of the same weight, one hollow so most of the mass was concentrated at it’s edge, and one uniform in density, the unform one would roll down a slope quicker. This is because it would have a lower moment of inertia, and hence would need more energy to get it spinning and therefore rolling down the slope. Similarly it would need more energy to stop.

Not related to the two bike riders (unless one of them has hydraulic tyres instead of pneumatic ones for some reason). :wink:

8

On an anecdotal level, it’s certainly true that bigger riders descend better than little guys, but it’s not due the force of gravity, it’s the force of competition.

Little guys generally have the highest power/weight ratio and therefore are generally better at uphill. No problem there.

In the real world of competitive cycling, there are no straight line descents. There are always turns, switchbacks, sand, water and motor oil on the way down. That’s the conditions that allow the bigger guys to catch up. The little guys (generally speaking) have little to gain by going really fast downhill, but the people following might have a lot to lose.

9

Quercus, heavy riders are at a profound disadvantage in hills. I only weighed about 210 in my road bike riding days and it was always a struggle to keep up with my friends in the hills around San Diego. No one goes uphill fast so the amount of weight one has to lift if the biggest factor. It might have been an advantage going downhill but drag is the biggest factor at higher speed which marginalized any advantage I might have. In our suicide runs down the hill on Torrey Pines near Del Mar I might have slight edge but only by spinning out a 108" top gear.

Oh, bulky guys never win the polka dot jersey but upper body strength is a factor when standing in the pedals to climb hills.

10

I beg to differ…

  1. They shouldn’t do. If we assume the same aerodynamic drag, the cannonball will be heavier and should fall faster. And since air was specified as the medium, neglecting drag would be naughty…

  2. Not sure. Even if we can neglect aerodynamic drag, the situation is a little complicated. As the balls roll down the ramp, they are converting their gravitational potential energy into linear kinetic energy and also rotational kinetic energy. The cannonball will convert more energy per metre of descent because it is heavier, but it also has a greater rotational moment of inertia. Hmm. Going to bug me, that one. (Actually, think I’ve nailed it, see below.)

  3. Similar considerations to (2). But wait! As far as rolling is concerned, there’s no difference between a cylinder of fixed dimensions but double the density, and one that is twice as long and the original density. The twice-as-long cylinder has to roll at the same rate as the original cylinder - it’s exactly the same as two of the original cylinders rolling side by side. So the double-density cylinder will also roll at the same rate. By extension, if we can leave aerodynamic drag out of it, all uniform cylinders of a particular radius will roll down a ramp at the same rate, regardless of their weight. The same argument applies to the balls as well - the glass ball and the cannonball will roll at the same rates.

Re the bicycle, bigger things of the same density fall faster than smaller things, just because of their smaller area-weight ratio. E.g. sawdust falls a lot slower than chunks of wood, even though they have the same density. So the bigger guy should descend faster. A guy exactly twice as big is going to be eight times as heavy (twice as tall, twice as wide, twice as thick) but only four times the cross section.

11

There was nothing to differ with. Those were questions.

Except you’re not standing on the bike with your arms spread out to the sides.

Imagine if both riders were in the same aerodynamic shell, if you will. . .heavier guy still go faster?

That is decidedly NOT the meaning of the people who claim heavier riders descend faster.

I do understand what you’re getting at though, cycling-wise.

Too many dissenting opinions in this thread so far. . .can someone assert their authori-TAY?

12

Two riders, on bikes as close to identical as possible, in a shell will descend a slope at close to identical speeds provided they are passively riding.

I failed to take into account active riding. There, the heavier rider might have a slight advantage.

13

I think you might be confusing the reason for heavier soap box racers. It’s the same reason that, all things being equal, a heavier biker may appear to go down hill faster.

Let’s first establish that acceleration due to gravity is the same for all of these objects. It is; and we aren’t going to cram mass into a meter per second per second unit of measure.

However, gravity is not the only force at work. Friction and air resistance both will impart some force against the forward movement. Here’s where the difference is observed.

Depending upon the slope and where you are in the decent, it’s possible for a heavier rider to begin accelerating more slowly, since the weight can increase the friction at the axel. The steeper the slope, the less you will see this particular effect, and at some point during the decent, mass as it applies to momentum (mass is a factor in momentum) may overtake the friction at the axel and overcome a portion of the decelerating forces.

Traction is also higher for the heavier rider, but I’m not sure it is a major effect here.

14

The same thing is true with downhill skiing - heavier guys have the advantage.

As others have said, weight (and therefore the gravitational force on the person) goes up roughly as the cube of linear dimensions, whereas surface area (and therefore the drag force acting against movement) goes up as the square of linear dimensions.

So, the bigger a person is, the faster they are going to go, in general, as the ratio of drag to gravitational force is smaller. The famous “feather and hammer” experiment is an extreme demonstration of the same effect.

Of course, on skis you have the added factor that more weight means more pressure on the snow, which makes more of a melting effect and better lubricates the passage of the ski over the snow surface.

15

I take it you are referring here to HPVs with aero fairings. If so, you are even wronger than before, since th bigger guy now has** no** additional aerodynamic drag. Same force slowing it, greater force pushing it - the big guy wins.

Waverly, with bikes at high speed the aero forces are so much greater than the rolling friction that the latter can be ignored.

Tapi’s point that in the real world of competitive cycling, downhill speeds are limited by cornering speed is of course correct.

16

There is no dissent. People are just getting lost in the minutiae.

With varying degrees of approximation:

[ul]
[li]Two balls of different mass, dropped in vacuum (i.e. no air resistance) - this is the classic high school thought expeirment. The two balls fall at the same rate.[/li][li]Two balls of identical size and different mass, dropped in air (air resistance not ignored): the heavier ball falls faster. Air resistance is the same for both balls, but the heavier ball is pulled downward with a greater force (of gravity).[/li][li]Two objects of identical size and shape, and different mass, rolling down a hill on identical wheels: The heavier object rolls down faster, for the same reason.[/li][li]Two objects of identical density and different mass, rolling down a hill on identical wheels: the heavier object will roll faster. Since the density is the same, the heavier object has a larger mass-to-surface-area reatio. That means if you make the object larger, mass (and therefore grativtational pull) increases faster than air resistance does.[/li][/ul]

So no matter how you think about it, the heavier rider wins.

17

So, you’re telling me that in a VACCUUM, the objects fall at the same rate because there’s no air resistance. Fine.

But your list item 2. . .in AIR, two identical balls fall at different rates EVEN THOUGH THE AIR RESISTANCE IS THE SAME because the force of gravity was higher on the second ball?

Isn’t that the same gravity that was acting on the balls in the vaccuum?

You’re saying air resistance is NOT ignored, but then you’re saying the air resistance is the same for both objects. FINE. But the claim you’re making (that the force of gravity is stronger on the denser ball) applies just as much to the experiment in a vaccuum.

Yes, there is dissent. You’re telling me the heavier rider goes faster downhill. Vunderbob is saying the opposite. That’s what I’m calling “dissent”.

18

We’re talking about terminal velocity here. No acceleration. Inertia not a factor. Just drag vs force of gravity.

19

Imagine two motor vehicles, a souped-up truck and a small Japanese car. The truck obviously has a larger engine, but also more inertia to overcome - in other words, the power-to-weight ratio is the same. So they both do, say, 0-60mph in 6 seconds. That’s roughly analogous to two balls falling under vacuum: one is heavier than the other (i.e. more power), but it has more inertia so it cancels out, and they both accelerate at the same rate.

Now put identical drag chutes on both cars. These act as brakes and provide identical braking force to each car. Will the two cars still have the same 0-60 time? Of coures not, the same braking force will have a larger effect on the smaller car. The bigger truck wins hands down.

20

Who’s talking about terminal velocity?

When did we start talking about terminal velocity?

I thought we were talking about a bike rider going down a hill.

Besides, not that you don’t know this, but there is no terminal velocity in a vaccuum.

Saying that the air-resistance is the same on two spheres (as SCR4 said) is pretty much the same thing as negating the effect air resistance which is the main point of talking about what a vaccuum does.

If two spheres have the same air-resistance, then how does one get going faster than the other?