nickc3

OK, sort of a newbie question - sorry....

I ride a Giant TCR.....I ride with some of the same rider and sometimes group rides. I have noticed - fairly consistently, upon decents and even minor decents, I can just coast, without drafting, and I just pass most, if not all riders....even ones that are pedaling
I run stock wheels - nothing fancy - while some of these folks have aero wheels, bikes like Venge, etc. How is that possible? Why would I be able to consistently just cruise by these other riders? Is the TCR just a fast bike? Could it be my body composition? My position? Etc.? Again, I'm NOT a very experienced rider- about 3 years - I'm probably technically not really a great rider. I am a strong rider- but that is immaterial here. Just wanted some thoughts.

Skipjacks

No.

You're just awesome.

Soak it in, brother. Soak it in.

rms13

I used to ride with a guy that was about 260 lbs and he would fly past everyone on descents. He was riding a Supersix at the time but I'm pretty sure the bike had nothing to do with it

merlinextraligh

Coasting speed downhill is predominantly governed by surface to weight ratio. Your surface to weight ratio is better than the people you're passing. This is because 1) you're heavier than they are , 2) you're more compactly built than they are, 3) you have a more aero position on the bike, or a combination of three.

ridelikeaturtle

Some cyclists are shockingly bad at descending. They brake at the wrong times, losing all momentum through a turn, or taking the worst possible line through a turn (forcing them to run wide or having to correct and brake mid-turn etc etc)... and sometimes that little bit of speed at the start will make a big difference later on.

Did you ever ride a motorcycle? Maybe you're just naturally good at it.

HTupolev

Descent coasting speed is heavily driven by how big a rider's aero profile is versus their weight. If you're lower on the bike than they are, that's probably a big part of it. Bigger riders also tend to have an advantage here, since body mass usually increases faster than aerodynamic profile as people get taller. And of course it helps to be heavier.

Also, people pedaling on a descent doesn't necessarily mean they're going fast. A good aero tuck can save a huge amount of power when you're going fast downhill, and some people don't get very aero when they pedal downhill... if they're relatively upright on the descent and only putting out 200W while moving at 40mph, they'd almost certainly go faster if they were to stop pedaling and tuck low.

Skipjacks

Gravity does not pull on you faster because you have more mass. That is not a thing.

prathmann

Sure it does. The force with which gravity pulls on your mass is called weight. So the greater your mass, the greater the force.

Of course it also takes a larger force to accelerate a larger mass. In the absence of air resistance things would cancel out and all objects would fall equally fast. But usually air resistance doesn't increase as fast as weight and therefore bigger/heavier people descend faster.

HTupolev

Yes it does. Gravitational force is proportional to an object's mass. This causes objects to all fall the same as each other when they're in a frictionless vacuum, but in the real world and atmosphere, it gets more complicated. If two objects are aerodynamically equivalent, but one weighs more, the heavier object will tend to fall faster.

As people get larger, their mass tends to increase faster than their aerodynamic profile.

datlas 

How much do you weigh??

Skipjacks

No.

I did amend the original post to make the overall point more clear.

Gravity does not pull you FASTER because you have more mass.

Gravity acts on all objects the same.

In a vacuum a feather will fall at the same rate as a brick. Drop a brick and a feather from the top of a building on the moon where there is no atmosphere and both will hit the ground at the same time. The brick will NOT fall faster.

Here...watch this play out on video...

On Earth it's a little difference because of the atmosphere. The feather has a low mass to surface area ratio so it's impeded by the air more than the brick which has a much higher mass to surface area ratio. Gravity is still pulling both the feather and the brick with the same force, but the air is holding the feather back more.

Back to the biker on the hill....the 260 lb biker is being pulled downhill with the same force that a 100 lb biker is being pulled down.

The 260 lb biker may reach the bottom first, but it's not because gravity is pulling him more. Other variables are in play.

You all mentioned aerodynamics. The 100lb biker has less surface area to trap air and slow him down. But he's also lighter and offers less momentum to push through the wind. Whereas the 260 lb biker has a larger surface area but 2.6 times the mass to push through the air. Depending on the exact weights of the bikers the heavier one might be slowed down by the wind more or push through it more.

Then there is friction of the tires on the road. A biker that's too heavy is pushing the tire into the road more and creating more friction. More friction = slower. A biker that is too light isn't creating enough friction to keep the tire firmly planted on the pavement at high speed. Too much bouncing on small bumps cases a rough ride which is slower. In practice the differences here are probably negligible. But they matter. I remember doing Pinewood Derby racing as a kid. A 5 ounce car was faster than a 3 ounce car, not because of more gravity, but because the 3 ounce car bounces on the imperfections in the track too much and caused it to slow down. Just like the heavier brick pushes through the air better, the heavier biker will push through the road friction more, unless he's so heavy that he creates more friction than he can offset.

The heavier biker also puts more friction on the wheel bearings than the lighter biker. That impedes the heavier biker. Though that might not be measurable depending on the quality of the bearing.

Fun fact....if you shoot a bullet from a gun in a straight line parallel to the earth, and drop a bullet from your hand at the same time, both the fired bullet and the dropped bullet will hit the ground at the same time. (Assuming the bullet doesn't go so far that the curvature of the Earth plays a role) Gravity pulls on both bullets at the same rate and doesn't care that one has a forward velocity while the other does not. The fired bullet will just be much further away when it hits the ground.

Skipjacks

Ultimately if you take the 260 lb biker and the 100 lb biker and neutralize all the other variables (basically by putting them in a vacuum chamber and dropping them straight down so air/road/wheel bearing friction are eliminated) both bikers will; hit the ground at the exact same time.

Gravity will NOT pull the heavier biker down faster or exert more force on the heavier biker.

If you want to get technical the heavier biker when in free fall is pulling the Earth up towards him as a faster rate than the lighter biker is pulling the Earth toward him. But the difference is so infinitesimally small it's not even a real factor. Also if you drop both bikers at the same time, they are both pulling the Earth up together so neither one has an advantage.

69chevy

rubiksoval

The mythbusters show did this. It was a cool experiment. Hardest part was getting the dropped bullet and the fired gun to go off at the same time.

HTupolev

No, the 260lb biker is being pulled downhill with 2.6 times the force that the 100lb biker is being pulled down. If this were not the case, then 100lb objects would fall faster in a vacuum than 260lb objects!

Remember, F=ma. Gravitational acceleration is 9.8m/s/s on Earth's surface, so the gravitational force for a 1kg object is 9.8N and the gravitational force for a 2kg object is 19.6N.
Another way to put it is, it takes 19.6 Newtons of force to accelerate a 2kg object as quickly as it takes 9.8 Newtons of force to accelerate a 1kg object.

Skipjacks

I mean that's a good way to sum up everything I used 5000 words and a video to say....

But um....yeah okay your way is better.

Brofessor

You are right, but this has no relevance to OP's example because 1) Air resistance matters, 2) Rolling resistance matters. Heavier object will create a larger force opposite to the frictional force between tires and the road surface, therefore heavier rider (holding everything else constant) will need less pedaling to overcome that force.

Skipjacks

Gravity is not making the fatter biker faster.

Period.

Other things are. Gravity isn't.

TimothyH

We went through this already.

Why do I coast faster than everyone else?


-Tim-

HTupolev

Your objections are all perspective. You've just divided mass out of the force diagram. These two statements mean the same thing:

1. "Both objects are accelerated by gravity at the same rate, but the denser object's mass causes air drag to decelerate it at a lower rate, resulting in it falling faster than the less-dense object."

2. "The denser object is being pulled by a greater gravitational force, but at any given velocity its air drag force is the same as for the the less-dense object, resulting in it falling faster."

RChung

If you dropped a light aero bike and a heavy non-aero bike from the top of a tall building which would hit the ground first?

jon c. 

I'm not sure, but if one was carbon and one was steel, the carbon bike would assplode when it hit the ground.

songa

it sounds like everyone agrees with each other, just nobody understands each other

DrIsotope

I don't know about no science, but I do know that it doesn't make a damn bit of difference what the 140lb guy is riding-- my 210lbs blows by him on the descents like he's standing still. Other guy can be pedaling in the drops, and I can be sitting straight up.

The amount of drag I generate relative to the smaller guy isn't enough to offset my greater mass. I weigh 50% more, but might only have around 10-15% more frontal area. So I go down the hill faster.

Litespud

Not strictly true - place both riders at the top of a hill and let them coast down. They'll both accelerate at essentially the same rate up to the point where drag kicks in. The difference is that the heavier rider will likely have a higher terminal velocity (his weight:drag ratio will be greater) and will keep accelerating after the lighter rider has reached his terminal velocity. If the descent is long enough (long enough for the lighter rider to reach his terminal velocity), the heavier rider will outstrip the lighter rider