abdul10000

Today, is the first time I took a rout with many hills and I noticed something odd, my mate could role down hills without pedaling much faster than I could. I checked my bike, in case the brakes were slightly pressed, and they were fine. I can't see a reason for this, except that he is heavier than me. I am 67kg he is probably 85kg. Our bikes have similar weight. I asked him about this and he said that without pedaling the heaviest person always roles down faster. Is this true, and if so why is this the case? I mean isn't gravity pulling us both at the same rate?

99Klein 

Yep. That's the only place weight is a good thing.

gsa103

Aerodynamic drag.

Below about 15 mph you will accelerate similarly. Once you start picking up speed, the heavier person will typically have a higher top speed.

wphamilton

Yes, gravity does pull you at the same rate but think about the forces. Your actual speed depends on the balance between the force of gravity and the forces slowing you down. The forces slowing you are rolling resistance and air resistance. Air resistance dominates at the high speeds you get coasting down a big hill.

The force from gravity is the gravity acceleration times the mass so it increases at the same rate as the mass increases. The force of air resistance increases at the same rate as the area in the front. Area increases less than the mass increases, so the heavier guy (if he's the same shape in the same position) has an advantage at the higher speeds downhill.

NatUp

I'm not a judgmental person, so I say fat people can play whatever role they want.

abdul10000

true, he had a hard time uphill

I understand aerodynamic drag increases much more as the speed goes up, but how is weight related to that? I thought a bigger physical size increases aerodynamic drag. In that case it would make sense because I am 193cm he is 165cm, both around medium build.



So far so clear, rolling resistance should be almost the same for both of us because he is using all around 23mm tires and I am using race 25mm tires. Physically they look the same width. Air resistance is where he would do better because he is much shorter, his bike is smaller, and he is wearing biking vest (I am not).

Quote:

Originally Posted by wphamilton

The force from gravity is the gravity acceleration times the mass so it increases at the same rate as the mass increases. The force of air resistance increases at the same rate as the area in the front. Area increases less than the mass increases, so the heavier guy (if he's the same shape in the same position) has an advantage at the higher speeds downhill.

This is the part that I dont understand.

mfowler95

Gravity!!! When your a clyde like myself it has it's advantages (going downhill) or disadvantages (going uphill). Think of it this way if you were to go onto your roof and drop a feather and a rock at the same time, which one would hit the ground first?

SteelCan

Incorrect per Galileo with an orange and a grape and again by Astronaut David Scott (using a hammer & feather) on the Moon (the second demo is cool just for the fact that it's on the Moon and yes I am aware of the objections that will point-out a lower gravity).

wphamilton

It's a square/cube thing. Simplifying to a simple shape, if you expand your mass (volume) say to 1.5 times, every dimension would be the cube root of that, ie, you'd be 1.14 times as wide and 1.14 times as thick. Your frontal area would be wide times thick, or 1.14 squared as much. 1.3 times as much area.

So the forces pulling you down are 1.5 times greater than they were, but the forces slowing you are only 1.3 times greater.

NatUp

Actually, correct, in a way. A tall, skinny guy like me is analogous to the feather in that I have low weight and high frontal area, whereas a "fat" guy is like the rock in that he has a similar frontal area but a higher weight. So: the wind slows us down roughly the same amount, but his higher weight pulls him down with more force. More force + same resistance = free speed, baby.

rpenmanparker 

It is not so easy to see without actual case numbers, but here is a generic derivation:

The acceleration of each rider is the gravitational acceleration minus the effective headwind deceleration. We know the gravitational acceleration as a constant for any give slope which we will just call g'. The wind deceleration, w, we only know as it is produced by the wind force f proportional to the frontal area of the bike and rider combo (approximately) and the speed of the bike and net headwind. That force can be expressed as the mass of the bike-rider system multiplied times the resulting deceleration w. So we have a=g'-(f/m). For rider 1 that is a1=g'-f1/m1. For rider 2 that is a2=g'-f2/m2. Now mass is a function of volume and wind caused deceleration is a function of frontal surface area. So mass is a third power scalar while deceleration is a second power vector. And if rider 2 is heavier than rider 1, then we can assume rider 2 has more frontal area than rider 1. So the decelerating wind force is greater for rider 2, but the mass of rider 2 is MUCH greater than the mass of rider 1. So when you divide larger force by much larger mass, you get f1/m1 > than f2/m2. Subtracting these results from the gravitational constant you get g'-f1/m1 < g-f2/m2. In other words the heavier rider has a higher net acceleration at any speed. Viola. The other frictional forces in the bike and contact with the road and non-frontal drag forces have been discounted for this simplified treatment.

loxx0050

You do realize in a complete vacuum that both will hit the ground at the same time? The difference in falling rates is the feather has more air resistance.

But, I have out coasted featherweight guys with Tri/TT bikes and aero wheels on a road bike with clip aero bars (in a few Tri Races) with regular wheels (bladed spokes though) downhill plenty times before. I am not a clyde but not lightweight either as I was about 175lbs during that race and I'd guess I had 20 pounds on most of those guys. In shape too as I am carrying more muscle than most competitive age grouper triathletes last season as l still lifted heavy weights during that time twice a week on the days I swam laps in the pool.

Thulsadoom

https://wiki.answers.com/Q/Why_does_a...an_a_light_one

Heavier objects move downhill faster than lighter objects because the effects of friction and aerodynamic resistance are reduced by higher mass.


If two carts are subject to the same gravitational acceleration (9.8m/s^2 at sea level on Earth), wind resistance, and ground friction, the cart with more mass will gain kinetic energy faster and will be better able to overcome the drag forces that accompany acceleration.


This assumes that both carts are the same size. If one cart is twice as heavy because it is twice as large, then the larger cart would be subject to more air resistance and possibly more ground friction.

znomit

There is a drope the hammer joke here somewhere.

rebel1916

We all have roles to play...and my role is to drope the hamer and dial it up to 400w!

asgelle

Let's just save ourselves a lot of time shall we. https://forums.roadbikereview.com/gen...ml#post4525219

gc3

Wasn't enough to beat it to death in another forum, someone. just had to come here and do it ask over again....

znomit

Swing and a miss.

rebel1916

Coming from the kind of ignoramus who thinks its spelled I think I'll be OK!

cafzali

Actually, you're in no way defying gravity on a downhill. You're still as firmly planted on earth as if you were standing. The reason clydes can go downhill much faster is because the heavier weight of the total person/bike package causes them to gain speed on downhills faster. This is especially true if you can get in an aerodynamic riding position.

Cyclists in this situation are encountering the same phenomenon as tractor trailers going down steep inclines, which is why you see truckers shifting into lower gears before steep downhills to avoid overheating the brakes.

mfowler95

You might want to read this https://ilovebacteria.com/gravity.htm

rangerdavid

I beat lots of folks on downhills with my superior weight!!

EdIsMe

Drope the hamer is the correct spelling.

\o/
*******

p
_______

^droping the hamer -win-

Cyclelogikal

Newton discovered that principle many years ago called "Gravity!"

AScoda

Acceleration of gravity is 9.8 meters per second per second, regardless of mass.
Remember that an object in motion tends to stay in motion? This is absolutely affected by mass.
The air/wind resistance is what is slowing you down. The lighter object is going to be slowed down more easily than the heavier one.
You can hit a baseball(you, the lighter one) traveling at 60 miles an hour and change it's direction relatively easily. Not so much, a car(your heftier friend).