View Full Version : Weight Makes You A Better Descender, Huh?
Mon Ami
06-09-2006, 01:21 PM
Listening to the British Eurosport commentators on the Dauphine Libere, they kept saying that heavier riders had an advantage as they descend faster. Duffield even mentioned that some lightweight rider from the past used to be given a lead filled bidon at the top of climbs so he would descend faster. And there was I thinking that that Galileo sort of cleared up this question a few hundred years ago!
So, the question for the engineers and physicists on the forum, does your mass affect your descending speed?
Mon Ami (slightly confused)
dave thompson
06-09-2006, 01:29 PM
Listening to the British Eurosport commentators on the Dauphine Libere, they kept saying that heavier riders had an advantage as they descend faster. Duffield even mentioned that some lightweight rider from the past used to be given a lead filled bidon at the top of climbs so he would descend faster. And there was I thinking that that Galileo sort of cleared up this question a few hundred years ago!
So, the question for the engineers and physicists on the forum, does your mass affect your descending speed?
Mon Ami (slightly confused)
Yes, a heavier object will accelerate faster than a light object until that object reaches it's 'terminal velocity'. http://en.wikipedia.org/wiki/Terminal_velocity
Bruce K
06-09-2006, 01:34 PM
Having barely scraped by in Physics, I think you will find that falling, you would be correct, there would be no advantage to being heavier, especially in a vacuum.
But when you introduce a plane under the object (the road) in question you then get into vector analysis and determining how much of the mass is going down into the road vs. forward motion.
The resulting diagonal (downhill) component of the forces I believe would give the heavier rider an advantage, all other things being equal.
If you wanted to get nit-picky, you would have to analyze rolling resistance of the tires, rolling resistance of the wheel bearings, contact patch size, wind direction, frontal area of rider and bike, etc., etc. etc. to really figure it all out.
You also have to look at potential energy (stored by raising the mass to the top of the climb) vs kinetic energy turning the cranks and getting into motion.
I have a headache now. :rolleyes:
BK
Blastinbob
06-09-2006, 01:38 PM
I coast by my skinny pedaling buddies on long downhills all the time. :D
F=ma. a= gravity, so for both guys it's equal. So the heavier guy gets more force. Subtract the roughly equal mechanical resistance for rolling tires and bearing friction and the heavier guy gets more force applied down the hill anyway, that part's noise I included because I'm confused and I want you to be too. Aerodynamic drag is a function of the speed squared times the surface area. Or vice versa. Anyway, the larger person has more surface area so there's more drag. Subtract that from the force. So.... if the heavier guy has more mass per surface area unit he ought to subtract proportionately less from the force he gets. So he goes faster. Or not. What the hell do I know?
swoop
06-09-2006, 01:59 PM
the big dudes get down the hill better than the little ones. that's why it helps to be able to tuck in off the saddle and on your top tube. it's amazing how much speed you can gain by reducing your frontal area and relaxing too.
get your chin on top of the bars, your *** under the saddle, your pedals parallel to the ground and your hands next to your chin on the tops, get your crotch on the top tube (this is something that you only need do if you have the skilz to do so). this is a little nice thing about sloping top tubes bikes...
atmo. you can also gain some speed out of turns... but again.. you gotta practice cornering drills.
wheels make a big difference too.
flydhest
06-09-2006, 02:03 PM
the primary effect is wind resistance. In a vacuum, the two would both be accelereated at 9.8 m/s^2. F=ma. m is different, so F is different. The wind resistance (force) is approximately the same, so . . . you go downhill faster if you're fat like me.
dgauthier
06-09-2006, 02:06 PM
Yes, being heavier makes you descend faster. (Ask me how I know. :) )
However, like most things in cycling, technique is more important than anything else. Swoop speaks the truth: Quite a few times, while descending in the drops, I've been passed by guys who (from the look of them) are lighter than me by at least 20 pounds. They're doing that weird "chin on the stem" descending crouch thingy. I'm keeping pace with automobiles, yet these guys pass me like I'm standing still.
BdaGhisallo
06-09-2006, 02:10 PM
Hey Mon Ami,
Galileo did clear that up but it is only true in a complete vacuum. When a fluid, ie air, is introduced, the greatest force - in this case the weight of the rider, ceteris paribus - will prevail.
snipped
. . . you go downhill faster if you're fat like me.
I was informed that you mentioned something along those lines to Zip as you went by her on a descent at Nottrott '06. She usually descends well in that aero tuck.
MikeM
06-09-2006, 02:58 PM
Surely the key thing to descending quickly is density - i.e. give a small guy a lead bidon and he'll descend quicker than a guy who is larger but wieghs the same amount as the small guy with the lead bidon. Gravity is what is driving the rider down the hill with wind being the major factor slowing one down.
Mike
William
06-09-2006, 03:48 PM
Take it from a big guy, you do decend faster. When in a group, I always have to pull out from the group. Or else I have to ride my brakes the whole way down to keep from crawling up peoples back sides. On top of that, I love decending anyway so I'll be spinning out my 53/11 or 12, laying on the TT, and carving through the corners.
YEEEE HAWWW!
William
David Kirk
06-09-2006, 03:51 PM
It's all about the weight/frontal area ratio. If you want to go fast down hill be a heavy short guy and you the man.
Dave
dirtdigger88
06-09-2006, 03:52 PM
It's all about the weight/frontal area ratio. If you want to go fast down hill be a heavy short guy and you the man.
Dave
then you must be slow as h*ll going down hill :p
Jason
flydhest
06-09-2006, 03:59 PM
snipped
I was informed that you mentioned something along those lines to Zip as you went by her on a descent at Nottrott '06. She usually descends well in that aero tuck.
Indeed. She was fast. I was heavy. :)
mike p
06-09-2006, 04:11 PM
Balls that hang past your top tube.
Mike
Mon Ami
06-09-2006, 04:13 PM
...that part's noise I included because I'm confused and I want you to be too
You've succeeded!
How's about the question "Would the same rider, on exactly the same bike, with exactly the same tyres, riding in exactly the same position, on the same course, on the same day (etc etc etc) go downhill faster if he took on extra weight or not (in a lead filled bidon of exactly the same size and wind resistance as the more common empty bidon I usually have at the top of a climb.)
This is a really important question for me. I was always taught that Galilieo disapproved the Aristotolean theory that heavier bodies fell faster by dropping two cannonballs of different weights off the top of the Leaning Tower of Pisa (which was his local tower, by the way) and they hit the ground together! So when the commentator mentioned the advantage of weight, I laughed out loud. But on reading the comments, well, next thing people will be claiming there's no Father Christmas!
Mon Ami
BdaGhisallo
06-09-2006, 05:03 PM
Mon Ami,
In your example the one with the lead bidon will descend faster.
Think of it in terms of forces. Air resistance, drag, bearing friction act to retard the speed of a rider. The propelling force of the rider, in physics terms mass x the force of gravity, will act against those retarding forces and propel the rider down the hill. Now if, holding all else equal as you say, you increase the mass of one rider/bike combo by putting lead in his bidon, you will increase the force propelling the rider down the hill while the retarding forces don't change. The leaded rider will go faster.
As I said earlier, Galileos experiment/theory pertained to results in vacuum. I saw it in physics class in hs and didn't believe my eyes when I saw a heavy ball bearing fall at the same rate as a feather in that big vacuum tube. In the vacuum there are no retarding forces on the items. Outside a vacuum air resistance comes into play and the force balance between gravity, mass and air resistance will dictate that the bearing always hits the ground first.
JohnS
06-09-2006, 05:16 PM
You've succeeded!
This is a really important question for me. I was always taught that Galilieo disapproved the Aristotolean theory that heavier bodies fell faster by dropping two cannonballs of different weights off the top of the Leaning Tower of Pisa (which was his local tower, by the way) and they hit the ground together! So when the commentator mentioned the advantage of weight, I laughed out loud. But on reading the comments, well, next thing people will be claiming there's no Father Christmas!
Mon AmiThat's true, but two cannonballs have the same mass/ frontal area ratio since they are both the same shape and both made out of the same homogenous material.
flydhest
06-09-2006, 05:27 PM
mon ami,
drop a large cardboard box and a match box car from a second floor window. The carboard might actually weigh more. The match box car will hit the ground first for exactly the same reason. Drop a packing peanut and a paper clip (about the same weight).
The "two bodies fall at the same rate" only works in a vacuum or under the special conditions that JohnS pointed out.
JohnS
06-09-2006, 05:30 PM
mon ami,
The "two bodies fall at the same rate" only works in a vacuum or under the special conditions that JohnS pointed out.Geez, you're pretty intelligent and insightful! :D
You've succeeded!
How's about the question "Would the same rider, on exactly the same bike, with exactly the same tyres, riding in exactly the same position, on the same course, on the same day (etc etc etc) go downhill faster if he took on extra weight or not (in a lead filled bidon of exactly the same size and wind resistance as the more common empty bidon I usually have at the top of a climb.)
This is a really important question for me. I was always taught that Galilieo disapproved the Aristotolean theory that heavier bodies fell faster by dropping two cannonballs of different weights off the top of the Leaning Tower of Pisa (which was his local tower, by the way) and they hit the ground together! So when the commentator mentioned the advantage of weight, I laughed out loud. But on reading the comments, well, next thing people will be claiming there's no Father Christmas!
Mon Ami
There are a lot of forces acting on a rotating body, so it's difficult to comprehend.
but if you put all of that business aside, and think of it in terms of momentum, I think you can see where the heavier person has an advantage.
two objects falling both hit the ground at the same time (in the absense of air), but the heavier object still hits the ground harder. when descending a hill, the heavier rider will have more momentum and will keep his speed after the hill is gone.
-j
ada@prorider.or
06-09-2006, 05:55 PM
that heavier riders had an advantage as they descend faster. Duffield even mentioned that some lightweight rider from the past used to be given a lead filled bidon at the top of climbs so he would descend faster. (slightly confused)´
well this is what we called ´´omgekeerd evenredig```
turn the question around and you have the answer
with the same force and rider who climbs faster
the light one or the heavy one
see the answer is simple
its the same as 12 =3x4
so 12/3=4
or 12/4=3
David Kirk
06-09-2006, 06:04 PM
´
well this is what we called ´´omgekeerd evenredig```
turn the question around and you have the answer
with the same force and rider who climbs faster
the light one or the heavy one
see the answer is simple
its the same as 12 =3x4
so 12/3=4
or 12/4=3
well put!
Dave
Louis
06-09-2006, 06:42 PM
Keep in mind that while the force of gravity is linear with mass (F=ma) the aero drag forces are non-linear with speed (proportional to speed squared). Going up the drag force is very small, since both riders are going relatively slowly. However, going down it is important.
The net effect is that most other things being equal, (i.e. power output of the riders, total vs output per unit mass) when you consider both going up and going down, the heavier rider is penalized more because s/he looses more energy due to the greater drag. Even though he does descend faster, due to the increased drag at the higher speed he does not descend fast enough to recoup the differences incurred during the climb.
Louis
shaq-d
06-09-2006, 06:58 PM
edit:
i had a big post. but i think i'm just talking outta my butt
sd
William
06-09-2006, 07:07 PM
Me heavier, me go faster, hill yes. Hmmm.
William ;)
woolly
06-09-2006, 09:06 PM
Fat Falls Fast. That's my story, & I'm stickin' to it.
CalfeeFly
06-09-2006, 10:23 PM
I coast by my skinny pedaling buddies on long downhills all the time. :D
Me too! bike:
Ti Designs
06-09-2006, 10:43 PM
Duffield even mentioned that some lightweight rider from the past used to be given a lead filled bidon at the top of climbs so he would descend faster.
What's a bidon? I googled it and came up with two answers. On is what you do on ebay - bidon, as in you bidon this, you bidon that... makes no sense. The second is a bottle or can. They can't be talking about the water bottle - they would just say bottle, right??? It's bad enough I can't understand 80% of what ATMO writes, do we need to start sneaking in french terms???
Brons2
06-09-2006, 11:24 PM
I'm 6'7" and 260lbs. I fly downhill pretty fast. However, I find I can only reach terminal velocity (lol) by doing one of those "aero-tuck thingies".
If I sit up real high on the handlebars, the wind resistance will actually slow me down on a moderate downhill. On a steep one though, forget it :D
ada@prorider.or
06-10-2006, 04:50 AM
well the bidon filled with lead is pratice is very well know in europa,
meaby some remember the time trail in st etienne
(1997) we did about the same thing on the top of the climb of the mountain switched from a very light climbing bike to a muche heavier deceding bike
like a formula one change
this was all done by RIIS when he and JAN where riding my stuff
fun to see also the comotion then when changing bike´s
William
06-10-2006, 06:53 AM
I don't remember who made them, but you could buy water bottles filled with lead for training. Certainly wouldn't hurt on the down hills.
William
Grant McLean
06-10-2006, 08:28 AM
Take it from a big guy, you do decend faster. When in a group, I always have to pull out from the group. Or else I have to ride my brakes the whole way down to keep from crawling up peoples back sides.
William
yes, but the same is true for any rider back in the group,
because of the aero drag, any size rider behind is at an advantage.
I'm small and light, and never get dropped by a bigger rider on
the decents. With skill, I can stay on their wheel. So for racing,
this is why it's an advantage to be light, as the heavy rider is
at a major disadvantage up the climbs, and the lighter riders
can sit on the wheels of the bigger guys on the down hill.
g
Brons2
06-10-2006, 09:58 PM
I'm small and light, and never get dropped by a bigger rider on
the decents.
It doesn't make for much of a comparison though when you're pedaling and I'm not :D
Which, is pretty normal for me on the descents vis a vis the other riders.
obtuse
06-10-2006, 10:03 PM
big guys descend better....but guys with big balls descend the best.
obtuse
Fat Robert
06-10-2006, 10:07 PM
second race in 1994...after taking some years off...was chasin an obtuse-sized dude on a descent, both of us trying to catch back on to a mp 3 field...this guy was trying to pull a pantani (chest on the seat aero tuck)...he bit it big time and put his hip into a few pieces....
fat guys with big balls are the fastest downhill
obtuse
06-10-2006, 10:08 PM
second race in 1994...after taking some years off...was chasin an obtuse-sized dude on a descent, both of us trying to catch back on to a mp 3 field...this guy was trying to pull a pantani (chest on the seat aero tuck)...he bit it big time and put his hip into a few pieces....
fat guys with big balls are the fastest downhill
just what do you mean by "obtuse" sized?
obtuse
manet
06-10-2006, 10:09 PM
I don't remember who made them, but you could buy water bottles filled with lead for training. Certainly wouldn't hurt on the down hills.
William
http://images.google.com/imgres?imgurl=http://www.pccoach.com/images/piggcase.jpg&imgrefurl=http://www.pccoach.com/products/software/plan_pigg.htm&h=198&w=200&sz=7&hl=en&start=3&tbnid=8cZ5gV0fk35FJM:&tbnh=98&tbnw=99&prev=/images%3Fq%3Dmike%2Bpigg%26svnum%3D10%26hl%3Den%26 lr%3D%26sa%3DG
catulle
06-10-2006, 10:15 PM
Lobotomy made me a better descender is what Scarlett told me, atmo.
Fat Robert
06-10-2006, 10:19 PM
just what do you mean by "obtuse" sized?
obtuse
tall, elegant, chisled from imported italian marble, and frequently mistaken for a hyperthyroid chimp on hgh
or maybe just a six two guy with really long arms
obtuse
06-10-2006, 10:24 PM
tall, elegant, chisled from imported italian marble, and frequently mistaken for a hyperthyroid chimp on hgh
or maybe just a six two guy with really long arms
oh right...i'll see you at the first booth at redlobster.
obtuse
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