Is my bike just faster?
10-09-18, 12:14 AM
#26
Senior Member
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...
https://youtu.be/QyeF-_QPSbk
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.
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...
https://youtu.be/QyeF-_QPSbk
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.
What may mean to be saying, or at least should be saying, is that all mass experiences the same pull of gravity, ie an acceleration of 9.8 m/sec squared
10-09-18, 03:16 AM
#27
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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.
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.
Sure some are a little bit faster but certainly not enough to make it a problem to keep up with them.
10-09-18, 03:22 AM
#28
I pedal in my sleep...
The debate here is all wrong. No one has brought up the color of the bike.
10-09-18, 07:01 AM
#30
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10-09-18, 07:05 AM
#31
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10-09-18, 07:17 AM
#32
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One of the guys on my group rides like to do the aero tuck sitting on the top tube. I only need to get a hair lower than my usual riding position to coast down faster than him. He's probably about 35lbs lighter.
10-09-18, 08:08 AM
#33
Non omnino gravis
If they get in front of you at any point, they're the reason you're going down the hill as fast as you are.
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10-09-18, 08:43 AM
#34
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I'm not sure why you're faster, but I am going out and buying a Giant TCR
10-09-18, 08:50 AM
#35
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Simple solution to descending faster;
Search this page for "mercury": https://en.wikipedia.org/wiki/Jean_Robic
Or buy a set of these: Tech: HED launches descending special | Cyclingnews.com
Search this page for "mercury": https://en.wikipedia.org/wiki/Jean_Robic
Or buy a set of these: Tech: HED launches descending special | Cyclingnews.com
Last edited by Shimagnolo; 10-09-18 at 01:53 PM.
10-09-18, 03:43 PM
#36
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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.
Did you ever ride a motorcycle? Maybe you're just naturally good at it.
10-09-18, 03:47 PM
#37
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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.
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.
So yes grandma - heavier riders do descent faster all things being equal.
Bodies are ONLY accelerated at the same speed in a total vacuum.
10-09-18, 04:00 PM
#38
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10-09-18, 06:14 PM
#39
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Do playing cards weaved into the spokes create an aero advantage? And, are they UCI legal?
10-09-18, 07:09 PM
#40
Senior Member
Heavier athletic muscular people are shaped different than lighter athletic musclular people. Not all their extra weight or bulk is frontal area subjected to wind resistance. That means their mass vs. frontal area profile is different. Usually it is higher. As the mass goes up, the frontal area goes up but at a lower rate. Therefore their terminal velocity going down a hill will be higher. Terminal velocity is the speed where frictional forces exactly equal gravity force. Yes in a vacuum all things fall at the same rate and terminal velocity is undefined. In a non vaccuum, friction (air mainly) plays a role hense terminal velocity is a factor. Not all heavier people are more dense, most are. If you are noticably faster down hill than your peers, you have a higher mass vs. frontal area ratio than they do. Given a long enough hill, you would just keep pulling away unless they add enough force by pedaling to overcome their terminal velocity to equal your terminal velocity as you are coasting at zero watts. Fill your frame tubes with sand and you'll go even faster down hill. You added mass and zero frontal area. If you dropped a 12inx12inx12in piece of steel and a the exact sized empty cardboard box from a bridge, the steel will hit the ground first. Same frontal area to wind, different mass, the cardboard will hit its terminal velocity first and at a much lower speed. Do that in a vaccuum, they will hit at the same time. There are hundreds of analogies that can show the same thing. If you oare still reading this long post... let's take this full circle. As stated. Mass vs. frontal area determines terminal velocity. You obviously can't change your mass at will at the top of a hill unless you fill your pockets with rocks but.. you can change your frontal area. So this mass vs. frontal area concept is exactly why getting more aero increases your speed on a hill. You reduce your frontal area, your mass vs. frontal area ratio goes up, your terminal velocity goes up, you go faster down the hill. Can your skinny low mass self get aero enough to get the same terminal velocity as your tubby buddy sitting upright? Maybe.
Last edited by u235; 10-09-18 at 07:47 PM.
10-12-18, 09:43 AM
#41
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10-12-18, 10:50 AM
#42
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OK, here's a thought experiment: you've got two spherical horses on top of a hill, in a vacuum. One has 2x the mass of the other but the lighter one is bright red in color. Which one rolls down the hill first?
10-12-18, 11:28 AM
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10-12-18, 11:29 AM
#44
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10-12-18, 11:49 AM
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Heavier athletic muscular people are shaped different than lighter athletic musclular people.... So this mass vs. frontal area concept is exactly why getting more aero increases your speed on a hill. You reduce your frontal area, your mass vs. frontal area ratio goes up, your terminal velocity goes up, you go faster down the hill. Can your skinny low mass self get aero enough to get the same terminal velocity as your tubby buddy sitting upright? Maybe.
10-12-18, 11:51 AM
#46
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yeh there's also this, even sometimes when you slip out of their draft and pass them. Because you're going faster than they are as you approach them in the draft.
10-12-18, 01:20 PM
#47
Senior Member
Be careful of these types of generalizations. My skinny ass is usually descending FASTER than the "tubby" guys, or even the "heavier athletic muscular" types. Whether or not they're getting low and aero. It is because I am more dense than they are, since most "heavy" guys are heavy due to higher body fat percentages, and my muscles are leaner than theirs' are.
10-12-18, 01:39 PM
#48
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But to refer to density as the cause is misleading. Density is mass per volume. Descending speed (without pedaling) is determined by mass per drag area (CdA). It's true that the denser the body, the the faster m/CdA grows, but it isn't the density per se that is responsible for the higher speed.
10-15-18, 04:31 PM
#50
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His body mass and road bike weight contribute to descents faster with the help of gravity! Flat and ascending will slow anyone but more with greater mass Newton's F=ma
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