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"The Descender"

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Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

"The Descender"

Old 04-22-08, 10:32 PM
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Arrowtalon
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"The Descender"

The buds I ride with and myself recently conducted an experiment with my old steel frame, 27 inch wheeled road bike.

After out-"diving" them on a few hills and carrying momentum better, they decided to relinquish their modern bikes to try mine. We started two of them at the top of a nearby hill, at rest. We timed how long it took for my bike, without peddling, to get to a certain point down the hill (maybe a 1/4 mile or so). We then tried the same with their bikes.

No matter who was riding, my bike was significantly faster, outdoing the others by several seconds. We think we each took similar paths on all bikes, never rode the brakes, and stayed tucked as low as possible.

In the end they jokingly called my bike "the Descender" and tried to figure out what gives. Their bikes are nicer than mine. A new Cannondale Synapse and a new Mercier Aquila. The Aquila was a bit faster than the Cannondale, but not notably so.

Could the difference be in weight? My bike weighs maybe 30 pounds loaded out as compared to 17 or so for theirs. I also thought maybe my wheels (27 inchers) carry more angular momentum. Or perhaps my hubs are better quality albeit older?

Any theories?

*EDIT: Two of the three riders were nearly identical weight. One was significantly lighter. No matter which rider was riding which bike, the differences in speed did not compare to difference in speed of the bikes themselves.

*EDIT: If anything, the wind resistance is greater on my older bike. It does not have "aero" rims and has a higher geometry, even when the rider is in a full tuck.

Last edited by Arrowtalon; 04-23-08 at 09:30 AM.
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Old 04-22-08, 11:04 PM
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I'd say weight has a lot to do with it. There is one hill in town that I get up to 61 km/h on almost every time. That said, once when I was loaded with +/- 16kilos (35lbs) in a messenger bag I got to 72km/h without even pedaling full out.
The weight of the bike+rider will make all the difference...on both sides of the hill.
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Old 04-22-08, 11:46 PM
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nothing surprising there
heavier objects accelerate faster down an incline. due to more mass being acted on by gravity
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Old 04-23-08, 12:47 AM
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I am the descender.

That should do it.

No really, how's the weight dif. between you and your riding buddies? cause if the weight of your bike is enough to overcome it, then that's your answer.
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Old 04-23-08, 01:44 AM
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Originally Posted by mista_chewey View Post
nothing surprising there
heavier objects accelerate faster down an incline. due to more mass being acted on by gravity
Umm, you may want to update your physics understand a tad here. Have you heard of Galileo dropping different mass balls from the Leaning Tower of Pisa?! Heavier objects accelerate at 9.8m/s^2. Light objects ALSO accelerate at 9.8m/s^2! In the absense of forces countering gravity all bikes would descend at the same speed.

However there are such forces - rolling resistance and air resistance. Assuming the same aerodynamic profile, air resistance will decelerate a heavier object less. Why? F = ma. Same force, more mass therefore less acceleration. That is why i, at 69kg have to work much harder on the descents than my mate who's 90kg. On the plus side, climbing is a whole different question...
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Old 04-23-08, 03:07 AM
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Originally Posted by Stu07 View Post
Have you heard of Galileo dropping different mass balls from the Leaning Tower of Pisa?!
Have you heard of the tooth fairy?
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Old 04-23-08, 03:21 AM
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Originally Posted by Stu07 View Post
Umm, you may want to update your physics understand a tad here. Have you heard of Galileo dropping different mass balls from the Leaning Tower of Pisa?! Heavier objects accelerate at 9.8m/s^2. Light objects ALSO accelerate at 9.8m/s^2! In the absense of forces countering gravity all bikes would descend at the same speed.
Whoops. You missed day 2 of physics class.

Just don't go dropping CF bikes off towers.
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Old 04-23-08, 06:42 AM
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Originally Posted by Stu07 View Post
Umm, you may want to update your physics understand a tad here. Have you heard of Galileo dropping different mass balls from the Leaning Tower of Pisa?! Heavier objects accelerate at 9.8m/s^2. Light objects ALSO accelerate at 9.8m/s^2! In the absense of forces countering gravity all bikes would descend at the same speed.
ummm...im not talking about a vacuum here buddy.
so my statement which refers to the planet where i am located on right now "Earth" has an atmosphere
the atmosphere which causes wind resistance will hinder lighter objects rolling down an incline at velocity, which drag will increase exponentially with velocity

you should also know light doesn't accelerate, because it's already at it's terminal velocity leaving the source.

Last edited by mista_chewey; 04-23-08 at 07:03 AM.
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Old 04-23-08, 07:14 AM
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it's definitely the weight.

I can out descend 95% plus with my fat ass.
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Old 04-23-08, 07:35 AM
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It's not about the bike.

If it's a simple case of riding down a long straight road, then I'm always the fastest.

Having no shoulders and an aerodynamic cottage of wattage has it's perks.
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Old 04-23-08, 07:49 AM
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Is your bike red?
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Old 04-23-08, 08:09 AM
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Unless the descent is steep and technical, weight is the most important factor. Even 10 or 15 lbs makes a noticeable difference.
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Old 04-23-08, 08:45 AM
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Didn't you do pinewood derbies in cub scouts? You have to tape pennies to your car to add weight, otherwise you get get destroyed.
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Old 04-23-08, 08:57 AM
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Originally Posted by mista_chewey View Post
the atmosphere which causes wind resistance will hinder lighter objects rolling down an incline at velocity, which drag will increase exponentially with velocity
Exponentially. I wish that term were banned from the language until people understand what it actually means.

Air drag is a squared function. Bernoulli said so.
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Old 04-23-08, 09:00 AM
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Originally Posted by Fat Boy View Post
Exponentially. I wish that term were banned from the language until people understand what it actually means.

Air drag is a squared function. Bernoulli said so.
looks like an exponent to me...?
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Old 04-23-08, 09:05 AM
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Originally Posted by mista_chewey View Post
ummm...im not talking about a vacuum here buddy.
so my statement which refers to the planet where i am located on right now "Earth" has an atmosphere
the atmosphere which causes wind resistance will hinder lighter objects rolling down an incline at velocity, which drag will increase exponentially with velocity

you should also know light doesn't accelerate, because it's already at it's terminal velocity leaving the source.
You were still wrong. Admit it and you'll be left alone.
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Old 04-23-08, 09:06 AM
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Originally Posted by NeezyDeezy View Post
looks like an exponent to me...?

Depends on whether your using the phrase in its strict sense, or a more general sense.

https://en.wikipedia.org/wiki/Exponential_function


It's technically more precise to say that wind resistence is a squared function of speed.
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Old 04-23-08, 09:06 AM
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Originally Posted by NeezyDeezy View Post
looks like an exponent to me...?
Exponentially means the variable (velocity here) goes in the exponent. Technical term here is "quadratically"
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Old 04-23-08, 09:22 AM
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In response to rider weight question: Two of the riders were nearly identical in weight. One is significantly lighter. But we all rode each other's bikes at least once, with no measurable difference in how the bikes performed. Again, none of us pedaled, we just tucked and coasted.

My theory is that the weight of the wheels matters... What's the addage, "A pound in the wheels is 5 pounds on the bike." I don't know if that's true, but my wheels seem to carry angular momentum better. (I have Araya hubs and 27" AL alloy rims).
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Old 04-23-08, 09:35 AM
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Originally Posted by NeezyDeezy View Post
looks like an exponent to me...?
Exponential growth means that the exponent is a variable. i.e., "2 to the xth power" is exponential growth. "x squared" does have an exponent (the "2" in "squared"), but since the exponent never varies, you don't have exponential growth.

Think of it this way -- go to your boss and tell him that you'll work for $1 today, $2 tomorrow, $4 the next day and so on. If you're still employed in three or four weeks (you won't be, because the company will be bankrupt), you'd be making enough $$$ to buy whatever country you want and retire there. That's exponential growth.
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Old 04-23-08, 10:04 AM
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Originally Posted by Stu07 View Post
Umm, you may want to update your physics understand a tad here. Have you heard of Galileo dropping different mass balls from the Leaning Tower of Pisa?! Heavier objects accelerate at 9.8m/s^2. Light objects ALSO accelerate at 9.8m/s^2! In the absense of forces countering gravity all bikes would descend at the same speed.

However there are such forces - rolling resistance and air resistance. Assuming the same aerodynamic profile, air resistance will decelerate a heavier object less. Why? F = ma. Same force, more mass therefore less acceleration. That is why i, at 69kg have to work much harder on the descents than my mate who's 90kg. On the plus side, climbing is a whole different question...
You're missing the part where, when an object is on an incline, you can give the force of gravity two vectors - one straight down onto the slope, one parallel to the slope (the direction in which the force gravity causes the object to accelerate). For a heavier object, the vector for acceleration is longer. Drag and rolling resistance have nothing to do with it. More force of gravity on an object on an incline = higher speed.
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Old 04-23-08, 10:08 AM
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To the OP - this is simple to control for weight. Just have all the guys weigh themselves, holding their bikes, on a regular scale, and wearing an empty backpack.

Then add weight to the backpack so everybody's at the same weight (with bike), and coast.

I bet the winner will be the smallest guy / lowest spoke count.
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Old 04-23-08, 01:01 PM
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Originally Posted by grolby View Post
You're missing the part where, when an object is on an incline, you can give the force of gravity two vectors - one straight down onto the slope, one parallel to the slope (the direction in which the force gravity causes the object to accelerate). For a heavier object, the vector for acceleration is longer. Drag and rolling resistance have nothing to do with it. More force of gravity on an object on an incline = higher speed.
Now you've totally f*cked it up too. The first part is right -- you can give the FORCE of gravity two vectors. Fine. And the FORCE vector for a heavier object will be longer, yes. HOWEVER, and this actually does matter, the ACCELERATION vector is the force divided by the mass. The ACCELERATION vectors down the hill, from the force of gravity will be the SAME.

What's not the same is the acceleration, because when you find acceleration = (F_g - F_r)/m -- where F_g is the force of gravity, and F_r is the force of everything resisting the forward motion (rolling resistance, air resistance, the bugs hitting you in the face, etc) you see that F_r/m is smaller for a heavier mass, thus the part being subtracted away is less in the case of a heavier rider and the overall acceleration will be greater.

I've gotten my physics wrong on this board before, and I'm not perfect, but I'm pretty damn sure the stuff above is dead on.
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Old 04-23-08, 01:02 PM
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It's the weight.
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Old 04-23-08, 01:09 PM
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Originally Posted by grolby View Post
You're missing the part where, when an object is on an incline, you can give the force of gravity two vectors - one straight down onto the slope, one parallel to the slope (the direction in which the force gravity causes the object to accelerate). For a heavier object, the vector for acceleration is longer. Drag and rolling resistance have nothing to do with it. More force of gravity on an object on an incline = higher speed.

drag and rolling resistance have quite a bit to do with it. make a 1 pound styrofoam ball and a 1 pound steel ball and then roll them down a hill. You will find that the 1 pound steel ball will roll down faster due to significantly less drag and rolling resistance. You have to account for every force acting on an object, not merely gravity. Your way of thinking does quite well if we account for identical spheres of varying density, but doesn't do so well in the real world.

Last time i checked, the force of gravity is only one vector straight down, but you can show the force acted on the object with a vector down the slope. That whole f=ma thing. The force of gravity will always utilize the same acceleration (9.8m/s^2 or 32.2ft/s^2) and always acts vertically. The vector you are talking about parallel to the slope is the result of all the forces acting on the object. The vertical force of gravity is broken down into its component forces in the x and y direction (in relation to the flat surface x is parallel, and y is into the surface) . In a vacuum assuming no friction and no rotational forces, the only other force would be the force the surface on tha ball (perpendicular to the surface), which causes the net force to be in the x direction. Outside of the above situation, we would have drag, friction, rotating forces, etc.

The net force above is the x component of gravity. Now, that is divided by mass again f/m = a. Thus the mass is negated, all else considered. consider the case of a 1kg object and a 2kg object . To calculate the x force on a 45 degree plane would be 1kg*sin(45)*9.8 and 2kg*sin(45)*9.8. So, as the sin (45) is in both, we can forget about it at the moment for the sake of keeping easy numbers (1 and 2) since your brain seems small anyway (and I don't have a calculator, so that is a nice easy number vs. rounding). now we have 2 force 9.8 and 18.6kg. Using the above f/m=a we divde those by 1 and 2 respectively. And end up sin(45)*9.8m/s^2 acceleration for both objects (I brought back the sin(45) to make it the correct number).

To actually have a difference in acceleration, we have to look at drag, rolling resistance, etc. which all change the force in the x direction that is divided by the mass to give a. the difference you are percieving shows up in the density of the objects, not so much the actual weight. If you can get 2 different masses to have the same air resistance, the heavier object will win out.
That is likely the case here. Cyclists in good form can get the difference in air resistance low enough to be considered the same and thus the heavier weight can will win out, making it seem, to the feeble mind, that the gravity somehow changes for every object (which it does, but to such a tiny degree for human sized objects that it has no effect).
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