Cycling downhill

BHSpeedrom7

Do heavier cyclists go faster downhill than lighter cyclists?

fatbhoy

BHSpeedrom7 wrote: »

Do heavier cyclists go faster downhill than lighter cyclists?

Yes. Big time.

I go mountainbiking with a mate who's almost twice my weight: he goes up like a fu*king-mother and comes down like a mother-f*ucker.

smacl

If both are freewheeling, have similar wind resistance, similar rolling resistance, don't touch the brakes, etc.. yes. In reality, I reckon the better descender descends faster, and practice, skill and confidence play a huge part.

Joxer_S

The most entertaining hour I ever had on the bike was two years ago at the etape in Clare, I was doing it at a leisurely enough pace with my uncle, who wasn't huge but was carrying a good few extra pounds. About 20km from the finish I seen an epic battle of will unfold between him and a really heavy guy who we were going about roughly the same pace as. A pattern emerged where the other guy would gain a bit of time on the downhill, and we'd overtake him again going up the hill, with my uncle constantly muttering, "I don't care if I die at the finish as long as that f**ker doesn't beat me there".

In short, yes.

Wishbone Ash

No.

One of the basic laws of physics is that objects of different weight fall at the same speed (all else being equal).

If you threw a bag of coal and a lump of coal off the top of Liberty Hall, both would hit the ground at the same time.

Joxer_S

Wishbone Ash wrote: »

No.

One of the basic laws of physics is that objects of different weight fall at the same speed (all else being equal).

If you threw a bag of coal and a lump of coal off the top of Liberty Hall, both would hit the ground at the same time.

They are not falling, they are accelerating, which is affected by mass, so yes.

Edit: A falling object accelerates too, the following post beat me to it.

sconhome

Wishbone Ash wrote: »

No.

One of the basic laws of physics is that objects of different weight fall at the same speed (all else being equal).

If you threw a bag of coal and a lump of coal off the top of Liberty Hall, both would hit the ground at the same time.

If I remember correctly this only applies in a vaccum where the only force involved is gravitational attraction. Air resistance and the mass of the object has a big part to play in it.

Two different weighted cyclists would accelerate downhill (freewheeling) at different rates until they achieve terminal velocity which would be identical.

The science needs to be checked on this, as Mythbusters is not always 100% reliable

Al Wright

Remember elementary physics, Gallileo's experiment in dropping items of different weight (term 'mass' is more correct) from leaning tower of Pisa. He proved that acceleration due to gravity is constant. items fell at the same rate when corrected for the effects of air resistance.

So in the case of two cyclists of different weights, starting and freewheeling downhill under identical conditions, all losses being identical, both shall decend at the same rate. However, the heavier of the two shall continue a bit farther when they reach the level. He started with greater potential energy due to his extra weight.

Wishbone Ash

Joxer_S wrote: »

They are not falling, they are accelerating, which is affected by mass, so yes.

I presumed the OP was referring to freewheeling down a descent.

You can't compare acceleration and weight as all else is not equal.

Inquitus

Wishbone Ash wrote: »

I presumed the OP was referring to freewheeling down a descent.

You can't compare acceleration and weight as all else is not equal.

The heavier person will freewheel faster down a descent ignoring differences in surface area for wind resistance. They will accelerate faster to the shared terminal velocity, thereby reaching the bottom first.

Kat1170

Wishbone Ash wrote: »

No.

One of the basic laws of physics is that objects of different weight fall at the same speed (all else being equal).

If you threw a bag of coal and a lump of coal off the top of Liberty Hall, both would hit the ground at the same time.

Long time since I left school, but is that not only in a vacuum.

el Bastardo

Aerodynamics and bike handling skills matter just as much as a rider's weight, in my (limited) experience. How much heavier are we talking anyway?

Joxer_S

Wishbone Ash wrote: »

I presumed the OP was referring to freewheeling down a descent.

You can't compare acceleration and weight as all else is not equal.

I didn't mean that they were accelerating by pedalling, I meant that their speed increased at a faster rate as they were descending, assuming neither is pedalling and all other variables are constant.

Lumen

Mice bounce, dogs thud, horses splash. Or something.

Joxer_S

Lumen wrote: »

Mice bounce, dogs thud, horses splash. Or something.

What about bags of coal?

Inquitus

With enough EPO big lumps like Bjarne can go uphill faster too!

Squidgy Black

It's not the acceleration you're taking into account when freewheeling downhill though, it's the momentum of the riders being carried down the hill. Momentum is mass x velocity (take the velocity as being constant for both riders).

So of both riders were travelling technically at the same speed before freewheeling, say a constant velocity of 2.5m/s/s, the heavy rider has a much larger momentum pulling him down the hill.

Acceleration due to gravity only really comes into effect when it's more or less a straight vertical drop as opposed to a decline.

sconhome

Lumen wrote: »

Mice bounce, dogs thud, horses splash. Or something.

Interestingly enough this is to do with the density of the falling object. The light mouse will fall slower due to bigger air resistance slowing its rate of fall so it lands gently. This is why a feather falls and lands , like a feather whereas a lump of coal will probably shatter as it has more energy released on impact.

bogmanfan

In my experience, confidence is the most important factor. I'm 95kg and when purely freewheeling i will go faster downhill than one of the smaller guys. However I have no nerve, and start braking once I go over 50km/h. Therefore I'm usually one of the last to the bottom.

sconhome

Joxer_S wrote: »

What about bags of coal?

become bags of dust.

Leroy42

but wind resistance plays a massive part, so wouldn't the lighter rider in many cases have a smaller frontal area and therefore less resistance. would this outweigh (see what I did there!) the benefits gained from the extra weight.

Mycroft H

Wishbone Ash wrote: »

No.

One of the basic laws of physics is that objects of different weight fall at the same speed (all else being equal).

If you threw a bag of coal and a lump of coal off the top of Liberty Hall, both would hit the ground at the same time.

Air resistance is the big player here. Given an identical bike a 90kg rider will descend faster then a 60kg rider. This nothing to do with the greater power but rather then the surface area of the rider. A 90kg does not have a 50% greater frontal surface area compared to the 60kg rider. Surface area is not scalar to mass in the human body.

Decending by gravity alone, the heavier rider will be faster

irishmotorist

A lot of the different reasoning above sounds plausible and reasonable to me, but I can't remember all of that science. In my own experience heavier descends faster. I'm a bit heavier than others (not 10 stone heavier or anything) but at the likes of the ROK when descending from the top of Coomakiste (food stop and standing start for many people), I have tended to freewheel past somebody who has started descending before me. Others pedal past me but I've put it down to my weight.

MrCreosote

Inquitus wrote: »

The heavier person will freewheel faster down a descent ignoring differences in surface area for wind resistance. They will accelerate faster to the shared terminal velocity, thereby reaching the bottom first.

Why will they accelerate faster if acceleration due to force of gravity is a constant? Correcting for wind resistance etc, and take a stationary cyclist who then freewheels down a slope, the acceleration will be the same regardless of the rider's mass. And if acceleration and starting velocity are the same, it follows that velocity will be the same at every point on the descent.

My own opinion- you see a big cyclist descending faster than you, first thing they stand out more and secondly it's easier to say "they're faster because they're heavier" rather than they have better bike handling or bigger cojones or whatever.

FISMA

I found this equation

s = (-giM/kaA)1/2

Here.

g, i, and ka are all constants. Thus

s α (M/A)1/2

Where "α" is the proportionality symbol.

Since it is unlikely that doubling your mass will double your surface area, the ratio M/A will likely be greater than one. Thus, larger riders have the downhill advantage.

However, the winners in the hills will continue to be smaller. That's because you spend far more time climbing than descending.

serendip

Wishbone Ash wrote: »

No.

One of the basic laws of physics is that objects of different weight fall at the same speed (all else being equal).

If you threw a bag of coal and a lump of coal off the top of Liberty Hall, both would hit the ground at the same time.

Yes (ignoring drag, for the moment).

However, F=MA. Since their accelerations are the same and their masses are different, the forces being applied (due to gravity) must be different too. The force is larger for the heavier object.

Eventually, they reach terminal velocity. At that point, the drag (due mainly to air resistance) exactly balances the gravitational force. So, since the forces applied to the objects are different, the drags at terminal velocity must be different too.

Therefore, given two objects with exactly the same size and shape but different masses, the heavier one will have a higher terminal velocity.

For cyclists, a heavier cyclist is slightly bigger than a lighter cyclist. Size contributes somewhat to increased drag. That's why feathers fall slowly. However, the increased drag due to increased size is less than the increased force due to increased mass. So the heavier cyclist has a higher terminal velocity.

Briain O Loinsigh

I'm pretty heavy and got just shy of 70KMH on knockmaroon hill last friday. Scary stuff hitting a speed ramp at that speed with oncoming traffic:D

velopeloton

Fast descending is all about technique, good aero position and the skill to carry the speed through the corners.
In my experience big (fat) riders are usually very bad descenders, fitter lighter riders have better handling at high speed and can corner faster. Fitter riders ride more and are more skilful and confident on the bike.

Holyboy

serendip wrote: »

Yes (ignoring drag, for the moment).

However, F=MA. Since their accelerations are the same and their masses are different, the forces being applied (due to gravity) must be different too. The force is larger for the heavier object.

Eventually, they reach terminal velocity. At that point, the drag (due mainly to air resistance) exactly balances the gravitational force. So, since the forces applied to the objects are different, the drags at terminal velocity must be different too.

Therefore, given two objects with exactly the same size and shape but different masses, the heavier one will have a higher terminal velocity.

For cyclists, a heavier cyclist is slightly bigger than a lighter cyclist. Size contributes somewhat to increased drag. That's why feathers fall slowly. However, the increased drag due to increased size is less than the increased force due to increased mass. So the heavier cyclist has a higher terminal velocity.

I thought terminal velocity was constant?!
Edit: Hmm it seems it is not constant, so does this mean the higher the altitude of the rider it also means a slightly faster decent because the air is less dense?

J Madone

BHSpeedrom7 wrote: »

Do heavier cyclists go faster downhill than lighter cyclists?

Probably

But they would be so far behind the lighter guy going up the hill they would still be slower

Shadow78

If you are in doubt about the effects of weight on hills get a rucksack and put 10 kilos of sugar in it. Then cycle up a hill. Free wheel back down in a fairly upright position and note your time. Then cycle back up without rucksack and freewheel back down in the same position and record your time.

Cork24

Being 96kg and some of the lads who are around 70kg can come down the hill faster then me.

1. Type of wheels they have

2. Weight of bike

3. Weight of person

4. Position of person on bike while downhill

Are all the effect you need to get right to be a fast downhill rider. I learned from watching YouTube other cyclist and how they use their body while downhilling

So I now turn my knees into the frame place my hands on the end part of the handle bars feet pointed down. Feet level to each other so my body is now fully lunge down by the hangle bars

By doing this I have now notice less wind/ more speed

serendip

Holyboy wrote: »

I thought terminal velocity was constant?!
Edit: Hmm it seems it is not constant, so does this mean the higher the altitude of the rider it also means a slightly faster decent because the air is less dense?

Yes.

dave2pvd

stetyrrell wrote: »

Acceleration due to gravity only really comes into effect when it's more or less a straight vertical drop as opposed to a decline.

This is not true.

The heavier rider should arrive at the bottom sooner (ignoring skill levels, flapping jerseys or stray sheep).

The heavy chap is being accelerated faster down the hill by gravity. He'll reach terminal velocity eventually. However, his TV may be a lower speed than skinny chap's TV, as frontal area may be larger - giving skinny chap a chance of catching up again.

Except heavy rider is on Zipp 404s and has his knees tucked in, with his chin almost on the bars. Skinny chap has a riser stem. He hasn't a hope.

recedite

Leroy42 wrote: »

but wind resistance plays a massive part, so wouldn't the lighter rider in many cases have a smaller frontal area and therefore less resistance.

Less overall wind resistance, but more per kilo. Like comparing a bag of feathers V a slightly larger bag of coal. So...............

serendip wrote: »

Therefore, given two objects with exactly the same size and shape but different masses, the heavier one will have a higher terminal velocity.

^ (referring to air, not vacuum)

mcgratheoin

Jeez guys, lots of spurious science here. Why are people "correcting for air resistance" when the question posed is hugely affected by air resistance? Assuming equal abilities, then the following applies.

1. In a vacuum, both light and heavy riders will descend at the same speed
2. We don't cycle in a vacuum, therefore wind resistance is the major altering factor.
3. Gravity has a huge effect on a decline - that's why you go faster down steeper hills - stetyrrell - hang your head in shame for suggesting that momentum is the major factor
4. terminal velocities will not be equal - serendip explains it well.
5. TV depends on the surface area exposed to the wind - as mentioned, this is not linearly related to mass, therefore a guy that's 20 stone will not present a surface area that's twice the size. BX 19 has it right.
6. TV at the top of a climb is higher than at the bottom due to lower air pressure

As pointed out by Cork24, aero position and rider ability are much bigger factors - I regularly cyle with a guy about 20kg heavier than me but leave him for dead on any sort of steep descents.

ednwireland

easy,
brain in neutral
dont use the brakes unless you absolutely have to
careless disregard for the consequences

you'll be as fast as anyone then

hardCopy

But how does he pedal backwards?

NeedMoreGears

Consider the forces involved

Gravity - proportional to mass for any given slope ; say k1.m

Rolling - directly proportional to mass ; say k2.m

Air - directly proportional to velocity squared ; say k3.v.v

Hence at TV k1.m = k2.m + k3.v.v

Rearrange k3.v.v = k1.m - k2m

Hence the v squared is directly proportional to mass ; i.e. Heavier folk should descend faster.

K1 is constant for all masses for a given slope. K2 is the same for all masses for the same bike, road surface etc

K3 depends on frontal surface area etc

In realility there are two further factors - C & S.

C is cowardice where I score quite highly and S is skill where I hardly score at all.

So you get Vsquared is proportional to mass less Cowardice plus Skill

FISMA

mcgratheoin wrote: »

1. stetyrrell - hang your head in shame for suggesting that momentum is the major factor

?


Gravity accelerates both the greater and lesser at the same rate, albeit dilluted from free fall. Since it is constant, factor it out.



The major factors here are mass and surface area which determine your speed.


Fundamentally, this is a Newton's Second Law problem.



Newton never wrote F = ma, rather he formulated his second law in terms of a change in momentum.


F = ma = mdv/dt = d(mv)/dt = dp/dt. So...


Fdt = dp - Impulse. A force applied over time causes a change in momentum.


Sum the forces on the riders and we'll tell you who had the greater change in momentum.


Sounds exactly like what the OP wanted to know.

07Lapierre

So does all this "science" go out the window if you PEDAL while cycling downhill?

rab!dmonkey

Lumen

A decent bike is important.

A certain tall and powerfully-built cycling forum mod was recently seen riding a tiny little yellow Litespeed up and down hills in Spain after his own bike broke. It is fair to say that a one-legged grandmother could descend faster. The thing would visibly flex round corners like it was about to snap.

There was a good comparison of old steel vs new carbon bikes published a couple of years ago in a cycling magazine. The testers universally hated the old bikes, citing descending performance as the most significant disadvantage (due to lack of stiffness in the fork/headtube I think).

mcgratheoin

FISMA wrote: »

Sum the forces on the riders and we'll tell you who had the greater change in momentum

Sounds exactly like what the OP wanted to know.

Not arguing with your physics, but that's not what stetryell said, and the change in momentum is not what the OP wanted to know - he wanted the change in velocity, so you'll have to correct your change in momentum for the respective masses of each rider.

stetyrell wrote:

So of both riders were travelling technically at the same speed before freewheeling, say a constant velocity of 2.5m/s/s, the heavy rider has a much larger momentum pulling him down the hill.

Acceleration due to gravity only really comes into effect when it's more or less a straight vertical drop as opposed to a decline.

rab!dmonkey has it right - the question posed (3 pages ago) is essentially who has the higher terminal velocity - and it's the fat guy...