Is it normal for the heavier rider to out run other riders on a down hill just costing? I ride with one of my coworkers and I know I have the heavier bike and out weigh him by at least 25lbs, but I just run over him on down hills!

Yes, it is one of the few advantages of being a bit heavier.

But, you sure pay for it getting to the top!! (Personal experience tells me so)

Well.... weight does not actually affect your speed. Aerodynamics play a larger part in it and so does the drafting factor. If you pass him from behind you are in some way benifiting from the draft. The only way weight helps you is through momentum, that is, it will take more force to decelerate you than a rider who is lighter, but that soon runs out and then the reverse holds true on the accent. If you remember all objects fall at 9.8 m/s ^2, no matter their mass. Perpendicular vectors do not affect each other so the pitch of the road in no way determines your actual speed, it's gravity you are trying to overcome which....is parallel to the movement (up or down) you are trying to acheive on the hill.

it has to do with the interplay between aero drag and gravity

the heavy rider has approximately the same frontal area as the lighter rider (actually the heavier rider usually does have a little more frontal area), so the force due to aerodynamic friction (drag) is approximately equal. However, the horizontal component of gravity with respect to the road surface (the part of gravity that pulls you forward) is greater with the heavier rider than with the lighter rider - this is assuming that both riders are applying the same amount of power to the pedals.

To put it simply:

10 - 2 = 8 is greater than 8 - 2 = 6

In order to match the heavier rider on the descent, the lighter rider would need to apply enough additional power at the pedals to overcome the deficit between their two gravitational forces.

All this has me baffeled.:confused:

He rides a Trek Y Foil 66 time trial bike and I ride a Trek 2000 so he should have an aero advantage over me, although I think I have the tuck down pretty good. He told me twards the end of the ride "man I just can't catch you, I even tried to pedal to catch up and all I was doing is just burning my self up before the next roller". I wasn't even turning the cranks, untill my speed dropped below a certain level on the next hill.
As far as drafting goes. If I'm in back I have to feather my rear brake to keep from running him down. I have even set bolt up right in the saddle to catch as much air as I can and still blow by. If I'm in the front I just keep pulling away while he is spinning. This leaves me in better shape to tackle the next rise also, so it's not such a bad thing if the weight is what is causing me to go faster.

the force of gravity difference is much more significant than the difference in coefficient of drag.

let's say you're 100 kilos and he's 80 kilos. your weight in newtonws would be 980, his would be 800. now say you're going down a 45 degree grade. that would mean that half of gravity is pulling you down the slope, while the other half is working to keep you and your wheels stuck on the slope. That means gravity would be pulling your bike forward with 90N more force than your lightweight friend.

here's another example..... a big ping pong (high drag, low mass) will still travel more slowly than a small steel ball (low drag, high mass)

oops i meant a small ping pong (low drag, low mass) vs a large steel ball (high drag, high mass) LOL

I think several people are partially right in this discussion. Your weight does not affect your speed attained due to the force of gravity going down hill. Velocity = initial velocity + acceleration * time. Acceleration due to gravity is a constant here on Earth at 9.8 meters/s^2. There is no room for mass in this equation.

Now your acceleration do to other forces will be affected by your mass. The higher the mass the more difficult it will be to acheive the same acceleration. Force = mass * acceleration. So acceleration = force / mass. Apply the same force and the lighter
person will go faster.

Also the higher the mass the greater the momentum and the harder it is to slow you down. If a heavy person and a light person apply the same pressure to the brakes the light person is going to stop faster. The same wind is going to slow the lighter person more.

Given all of that I don't know why some people are faster down hills than others. I read something somewhere (maybe here or on Sheldon Brown's page) that said some people are just faster down hill because they are. It could be they don't hesitate at the top so their initial speed is faster. They may have a better Aero position. They may hit the brakes less. They just may not have the fear of going fast. The only thing I do know is it is not acceleration due to gravity. It might be mass and momentum so other forces slow them less but it is not gravity.

Steve

I second what deleriou5 said. The heavier rider has a lower surface-to-weight ratio that results in a greater ratio of gravitational thrust to air drag.

it IS acceleration due to gravity!! the only reason you roll down a slope is because of gravity!!!

when you're standing on a flat piece of ground, the gravity vector points straight down, which is equally and oppositely opposed by the normal force.

now, when you're standing on a slope, the gravity still points straight down toward the center of the earth, but if your frame of reference is with the road as being horizontal, then now your gravity vector has TWO components - one that pulls you forward in the downward direction of the slope, and another that pulls you vertically down into the asphalt itself - which is equally and oppositely opposed by this new, lighter normal force. in fact, the asphalt "feels" like you are lighter because it does not have to support your full weight - it only needs to support that component of your weight which is pushing you vertically into the asphalt. if you take the extreme of a "slope" which just goes vertically straight down, the asphalt doesn't provide ANY normal force at all, and 100% of the force of gravity is being used to pull you "forward" down the "slope".

You were correct that ACCELERATION due to gravity is constant, no matter what. But two things you neglect:

1. 9.8m/s^2 assumes that the object is free falling in a VACUUM, with no opportunity for viscous drag in the equation. obviously aero drag is VERY important in cycling, so there are other kinematic components to take into account when discussing the above problem. your reference to the acceleration equation while correct, refers to a specific problem - that is, an object in FREE fall with NEGLIGIBLE frictional forces. A bike going down a slope is far from free fall - and there is plenty of friction involved from the rolling resistance of the tires to the rolling resistance at the hubs to the aero resistance of the bike and rider.

2. althought acceleration is constant, the FORCE DUE TO GRAVITY depends upon the mass of the object. Gravity pulls a 200kg man down to the earth with twice the force that it pulls a 100kg man.

Sorry, we don't ride our bikes in a vacuum with no friction, rolling resistance and wind resistance.

Those components throws the formula all to heck.

All I know is I'm a big guy and I can coast down hills, even touching on the brakes now and then, and fly past lighter guys who are still pedalling madly. I have a friend who is heavier than me and I can't catch him on a downhill.

Sounds like gravity to me.

The best authority on this is analytic cycling

https://www.analyticcycling.com/Force...ight_Page.html

Less weight going up a slope means faster times. How much faster?

Suppose one is considering buying a new, light-weight frame or going on a diet. What is the benefit from having less weight? How much time would be saved over a given distance on a specified slope? How much more distance would be covered in a given time?

Two riders, identical except that one has less weight than the other, ride a given distance up a hill. The the calculation gives the distance and time between the riders as the lightest rider reaches the given distance.

The plot shows the benefit from less weight when riding up a hill. The range of the plot is from -10% to +10% grade and from 0 to 5 kg less weight. The table gives specific values for the given speed.

Note that large changes may produce mathematical results, but may not have real-world meaning. Keep changes small.

"Negative values for improvements" show that weight is an advantage going down a hill. The plot shows that weight is more of a penalty going up hill than an advantage going down.

reminds me of that far side cartoon: Two scientists are working on some complicated stuff and the dog is staring at the chalk board. One comments to the other: "He's so cute when he tries to understand quantum physics."

The short answer: Yes, heaver is faster down hill. I don't know the physics. But I've done a good deal of observation.

I stand corrected. Here is a link to an article that describes exactly why and how bigger is faster down hill. https://www.sportsci.org/encyc/cyclin....html#downhill

Steve