andr0id

You're looking at a rolling total weight differences between 184 and 200 or around an 8% difference.

If your positions are similar, that should result in a extra 3 or 4 mph for you heavier bikes on a 40mph downhill.

Otherwise it's something else. Except for insane bikes made of lead of something*, the human on the bike is by far the largest percentage of the rolling weight.



*if I don't put this in, some pedant will bring up some crazy 200lb bike to refute my claim, hence the disclaimer.

andr0id

So this conversation started as a hypothetical physics question.

Why are you bringing up braking and bike handling skills? That has nothing to do with the original question.

fietsbob

Gravity works for you , pulling you down a hill, but it works against you by retaining the atmosphere, which, thru air resistance, slows you down.

AlmostTrick

Discussion board and fun?

The very first reply answered the hypothetical physics question. I guess the thread could have ended there?

Yes, all other factors being equal, a heavier rider will descend faster. (or brake more)

I weigh 135. Maybe I need to load my bike down with weight and head out to CO to test this "advantage" out for myself!

FBinNY 

Sorry but off the mark.

Everything and everybody on the same slope has the same acceleration regardless of size and weight. That's before factoring wind drag and frictional resistance. So, both riders will start out OK and the heavier rider with more mass compared to the drag factors will slowly pull away as he opens up the speed differential.

Keep in mind that riders can change their aero profile, so I'll say the unsaid (all other things being equal).

One thing to keep in mind is that humans aren't simple projectiles, so one common scenario is for the "slower" rider to fall in behind the faster one. The slipstream reduces the air drag so he can now catch up, and if he times things right slingshot ahead of the one who is innately faster. Timing is important because once he swings out the higher air drag will slow him back to the equilibrium state of his slower terminal velocity.

Because gravity and wind drag are large forces compared to the various sources of friction, the effects of weight will overshadow other factors like bearing and tire drag, especially on decent slopes where terminal velocities are higher.

BlazingPedals

yeah, but what about power assist? That's gotta affect downhill speeds, too?

wphamilton

Everything always is unless you put an equation to it.
Depends on the individual's drag, his mass, which is faster:

a=g sin(theta) - D/m
bike's acceleration = g sin(slope angle) - Drag/mass


D= 1/2*d*v^2*Cd*A
drag is proportional to velocity squared and Area

A = (1/d)* M^2/3
Area is (the cube root of Mass) squared, divided by density

--Oops, that isn't true. OK for a cube or a sphere, but "A" is the area presented to the wind, not the entire surface.
Short round guy who looks like (___) from the top vs bean-pole (__) is presenting more area to the front than the above simplification. When they're both leaning forward. The square/cube does not apply. We cannot even say for certain that M1/M2 > A1/A2

BlazingPedals

It's probably accurate enough for the OP to say the the heavier rider is usually, but not always, going to be faster on a big downhill. I leave it to the duelling pencils(tm) to figure out where the crossover point is that would make the heavier rider slower. Some assumptions will have to be made, though. Does the heavier rider carry the extra weight in the hips or belly? How much is frontal area increased? Does the extra weight cause the heavy rider's clothing to be tighter and flap less? And of course the slope of the downhill affects how much energy is available to overcome the air resistance.

FBinNY 

All the above is true, but not related to the correction I posted. You posited that the advantage switches from one rider to the other depending on speed, That's what is wrong.

The same factors related to the ratio of mass, slope and wind drag that govern terminal velocity govern at all speeds.

So, when speeds are low, both are on roughly equal terms, subject only to gravitational acceleration (unrelated to mass) almost as if falling in a vacuum. Then as speeds increase, the negative effects of wind become more important, and the lighter rider's rate of acceleration will slow progressively until he reaches terminal velocity. The heavier rider also sees the rate of acceleration slow, but the process is slower and the terminal velocity higher. The point is that while the amount of advantage added mass confers varies from near zero to whatever it ends up being at terminal velocity, but it's always on the same rider.

Again, for those raising other factors, ie tuck, shape, tire drag etc. the above assumes other factors are kept equal or close.

mrodgers

80 lb of bike and gear and 230 lb me 400 miles across NY, I had to wait for the others to start the downhills before I went. I was like a run-away freight train.

On a half mile 10% downhill, they could be halfway down when I start coasting from the top and I'd be barreling past them before the bottom.

I ride with a coworker after work sometimes. When he was on his road bike, he would be tucked as much as possible on the downhills while I sat right beside him sitting upright on my hybrid no hands basking in the breeze. As soon as I would lean down even a little bit to grab the handlebars, I blasted past him like a rocket.

wphamilton

pfft. As a self-respecting nerd/engineer, any mistake I may have made is a trivial oversight whereas anywhere I'm right has overwhelming importance. You should know that.

The "oops" part is therefore the salient feature.

berner

I often wonder, (especially lately), how democracy could possibly work.

Sy Reene

Do all the hills in CO only go downhill? The 'real world' you generally have to go up about as often as going down. So being faster downhill due to weight is somewhat meaningless, isn't it?

BTW, this guy would have a serious disadvantage going downhill I presume.

pickettt

Regardless, I'd advise against gaining 100lbs. just so you can go downhill faster. I assure you your health will go downhill faster than either rider.

Camilo

Some of my heavier friends go faster downhill than me, some slower because they have more frontal area to increase drag - especially if they're tall and relatively thin - but still heavier than short-ish, compact-ish me. Tall bike, long torso, etc, they have to be aware of and work on being aero, where it's pretty easy for me on my smaller bike.

I do believe that all things equal the heavier rider will go faster, but wind drag is much more important. I out coast a lot of taller/heavier riders regularly. But they're fun to draft on the flats!

FBinNY 

I try to answer the question as asked.

The OP posited the question about which would descend faster based on weight. Had he asked the same question based on size, position or tuck then I would have answered that with weight being the one of those other things being equal.

It's not really possible to answer based on two or more variables, because we get into the quicksand of how much difference in wind drag coefficient vs. how many pounds.

stevoo

A nice example for the OP would be to follow me on my tandem down a hill.

About the same, maybe tad worse aero than single but due to the extra weight of the second person the speed differene is very noticeable.

There is a fairly short 11%-12% grade by my house good for 70mph on the tandem. Cannot quite get there on my single. Too much wind resistance to weight ratio.

Anyway, it is a good real world physics lesson.

AlmostTrick

If two democracies start going downhill at the same time, and one is twice as large as the other...

pickettt

Garmin screenshot or it didn't happen.

36Oly_Rider

Being a lighter rider (140+), riding with heavier riders seems more of a chore on a 15 lb bike. I very rarely break the 50mph mark where someone weighing 10-20 lbs more than me crush it.