Amesja

+1

mikeinroch

Gravity is a force, with a constant rate of acceleration working on the mass of the rider + bike (F=ma). When mass is larger, the force is larger, and acceleration won't reach zero until that larger force is balanced out by a larger force of friction of the air (wind resistance) and the bearings. So terminal velocity is higher, and up to that point acceleration remains higher for the heavier rider (since wind resistance is going to be pretty similar between any two riders on road bikes at the same speed).

JohnDThompson 

Galileo proved many centuries ago that the acceleration of gravity is independent of mass.

CardiacKid

Theoretically, if the ramp was short enough, that might be true. Practically speaking, the reason why a steel bike is faster than a carbon bike on a descent is because it weighs more. There is another reason that sometimes applies that was not given by the OP. When I am climbing, I try and conserve energy and use it on the descent. Skinny people try and blast up the hill and rest on the descent. We are both playing to our strengths, which brings us back to the first point.

aixaix 

Only in a vacuum. Factor in wind resistance (as other respondents have) and the situation changes. What's confusing is that gravity is pulling heavy objects down with more force than light ones, but the extra pull is exactly counteracted by the greater inertia of a heavier object so acceleration is the same.

In a vacuum. Only in a vacuum. Add air, and the heavier object's greater momentum will accelerate it faster than a light one.

In the atmosphere, a heavier man will accelerate faster down a hill and have a higher terminal velocity than a light one. Frontal area grows more slowly than the volume for a given shape, so it doesn't slow the heavier man down as much as his extra mass speeds him up.

Gravity hasn't changed: only the presence of an atmosphere makes a heavy rider faster than a light one.

I kind of like the steel bike/magnetism theory.

gregf83 

True, but your acceleration down the hill is due to the net force on the rider which is not independent of mass.

Inertia has no effect on the force due to weight.

Wind resistance and rolling resistance matter - but unless you have something really wrong they're not going to vary a great deal between one person and another.[/QUOTE]Sort of true but if the wind resistance doesn't increase with a heavier rider he will reach a higher terminal velocity before the forces due to gravity are balanced with wind resistance.

This.

noglider 

As for a bike with a theoretically non-existent rider, I believe tires make more of a difference than anything else in downhill speed. So don't worry about the frame.

Puget Pounder

Whip out the vector diagrams!

spunkyj

Yep, gravity is accelerating all riders at a rate of g*sin(theta), where theta is the angle of elevation you are descending.

However, wind resistance will contribute a negative acceleration (i.e. a deceleration) that does depend on mass. Deceleration due to wind drag is proportional to 1/mass, which means it will slow down heavy riders less.

So, gravity accelerates all equally, but heavy riders are slowed down less by the wind, and therefore have a larger net acceleration downhill.

repechage

But my ringer pinewood derby car at 4.98 oz. (I actually used a gram scale to walk as close to the max as possible) will GO, especially due to the dressed round wheels and polished nails for axles... there are a few other tricks too besides an aero body. No magnets either.

spunkyj

Actually, I think rolling resistance is independent of the rider mass, just like the gravitational acceleration. That is to say that in a vacuum chamber two riders with identical bikes but different masses would accelerate down an incline at the same rate.

Creme Brulee

the answer is yes

OldsCOOL

What about my Technium PRE with Alcoa 6061 aluminum and Tange steel stays?

fietsbob

See : https://en.wikipedia.org/wiki/File:Ap...ammer_drop.ogg

DiabloScott

You don't **really** believe we ever went to the moon... right? That's obviously a studio in Burbank.

gaucho777 

+1!!! The acceleration of gravity is the same regardless of the mass of the object. (Actually, there are very slight difference depending on the proximity of the center of gravity of the two objects--yes, you do pull on the earth as well. A rider in Death Valley will weigh slightly more than if that same rider & bike were at the top of Mt. Everest, but let's stick to Newtonian physics for this discussion.) As many have said, it's the air resistance that causes a heavy rider to descent more quickly. More precisely, it is the fact that the heavy rider has a greater ratio of weight-to-surface area (read:air resistance) than the lighter rider.

Btw, p=mv would tell you how much damage a heavy and a light rider would cause if they smashed into a wall at the bottom of a hill. It doesn't explain why the heavy rider would reach the wall at the bottom of the hill first.

P.s. It is worth noting that riders on steel, lugged bikes tend to have a greater mass-to-surface area ratio. So, yes, in the practical sense, steel lugged bikes go downhill faster.

AlbertaBeef

Why do people keep trying to prove their point by arguing things that only apply in a vacuum? We don't ride in a vacuum, people. In the world we live in, net force is greater in the heavier rider, the bike and rider position being equal, period.

RobbieTunes

I'm not sure, but I've seen lighter cars win the Pinewood Derby. Not often, but all things being equal, the ones at 4.9oz generally are the fastest.

RobbieTunes

Gravity is just invisible rubber bands, anyway.

Andycapp

So... am I faster going north on a steel bike (northern hemisphere of course)? Don't forget to ride in metric, that's faster too

gaucho777 

You may be closer to the truth than you realize (now that the Higgs boson has been detected).

2manybikes

For a number of years now, work has been proceeding to bring perfection to the crudely conceived idea of a machine that would not only supply inverse reactive current for use in unilateral phase detractors, but would also be capable of automatically synchronizing cardinal grammeters. Such a machine is the "turbo-encabulator." Basically, the only new principle involved is that instead of power being generated by the relative motion of conductors and fluxes, it is produced by the medial interaction of magneto-reluctance and capacitive directance.

The original machine had a base plate of prefabulated amulite, surmounted by a malleable logarithmic casing in such a way that the two spurving bearings were in direct line with the pentametric fan. The latter consisted simply of six hydrocoptic marzelvanes, so fitted to the ambifacient lunar waneshaft that side fumbline was effectively prevented. The main winding was of the normal lotus-0-delta type placed in panendermic semiboiloid slots in the stator, every seventh conductor being connected by a nonreversible tremie pipe to the differential gridlespring on the "up" end of the grammeters.

Forty-one manestically spaced grouting brushes were arranged to feed into the rotor slipstream a mixture of high S-value phenylhydrobenzamine and 5% remanative tetryliodohexamine. Both of these liquids have specific pericosities given by P=2.5Cn6.7 where n is the diathetical evolute of retrograde temperature phase disposition and C is Chlomondeley's annular grillage coefficient. Initially, n was measured with the aid of metaploar refractive pilfrometer (for a description of this ingenious instrument, see Reference 1), but up to the present, nothing has been found to equal the transcendental hopper dadoscope (2).

Electrical engineers will appreciate the difficulty of nubing together a regurgitative purwell and a supramitive wennelsprock. Indeed, this proved to be a stumbling block to further development until, in 1942, it was found that the use of anhydrous nangling pins enabled a kryptonastic boiling shim to the tankered.

The early attempts to construct a sufficiently robust spiral decommutator failed largely because of a lack of appreciation of the large quasi-piestic stresses in the gremlin studs; the latter were specially designed to hold the roffit bars to the spamshaft. When, however, it was discovered that wending could be prevented by a simple addition to the living sockets, almost perfect running was secured.

The operating point is maintained as near as possible to the h.f. rem peak by constantly fromaging the bitumogenous spandrels. This is a distinct advance on the standard nivel-sheave in that no dramcock oil is required after the phase detractors have been remissed.

Undoubtedly, the turbo-encabulator has now reached a very high level of technical development. It has been successfully used for operating nofer trunnions. In addition, whenever a barescent skor motion is required, it may be employed in conjunction with a drawn reciprocating dingle arm to reduce sinusoidal depleneration.

gaucho777 

^I have heard from reliable sources that work on the turbo-encabulator was thwarted by the powerful dramcock oil industry lobby.

oban_kobi

A bit late, but this was bothering me. Good instinct, but think it through a bit. There's air on Earth. Two objects with the same air resistance, but different masses, will go downhill at different speeds.

revchuck 

Ed - FWIW, this is standard procedure in a paceline when you find yourself getting too close to the rider ahead of you.

Also, getting that close to a cyclist whose ability and intentions you don't know is a good way to effect a face/ground interface. If your front wheel touches his/her rear wheel, you're most likely going down. Be careful out there!