How/Why did this happen?
#1
Senior Member
Thread Starter
Join Date: Aug 2005
Location: Southern Oregon
Posts: 208
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
How/Why did this happen?
I went riding with four friends today. I am 6'1" 200# and was riding my touring bike with 700X28 tires at 100 PSI and Deore hubs. Two of the other riders were my size and the other two were quite a bit smaller. One was on a cross bike and the other three were on road bikes. We climbed a very steep hill and I was able to stay with them without any trouble at all. When we got to the top of the hill we stopped and all started at the same time. We coasted downhill about 2 miles and none of us pedalled during the downhill run. My bike is no doubt heavier than the others. All of them passed me on the downhill and walked away from me as we were all just coasting. I would have thought that because of my weight and the weight of the bike that I would have been the one going the fastest. Could someone please explain this to me. Were their wheels that much better? What am I missing here.
#2
Senior Member
Join Date: Apr 2004
Location: SPS, Texas
Posts: 478
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 2 Times
in
2 Posts
Probably a few variables in there. More than likely the one with the smoothest hubs wins.
__________________
'84 Trek 850--spinbackle-built, '85 Trek 670 Campy Nuovo Record--project, '87 Trek 560 SS/Fixed--project, '87 Specialized Stumpjumper Comp w/ Deore XT--Specialized-built, '87 Rossin Record, '03 LeMond Wayzata--commuter,
'?? TST Mtn Bike frame--project, '07 Tsunami Tandem--home-built
'84 Trek 850--spinbackle-built, '85 Trek 670 Campy Nuovo Record--project, '87 Trek 560 SS/Fixed--project, '87 Specialized Stumpjumper Comp w/ Deore XT--Specialized-built, '87 Rossin Record, '03 LeMond Wayzata--commuter,
'?? TST Mtn Bike frame--project, '07 Tsunami Tandem--home-built
#4
Senior Member
Join Date: May 2004
Location: Southern California
Posts: 2,191
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 114 Post(s)
Liked 119 Times
in
92 Posts
Over a relatively long distance like two miles, many factors could explain your slower speed. I believe one of the biggest factors is the aerodynamics. I doubt all the riders equally tucked and offered the same drag. Head, arm, leg positions all factor into the overall drag. Berarings and mechanical drag are probably a lesser contributor.
#6
Senior Member
Yeah, but total drag from rolling resistance is minimal, probably less than 5% of the energy pushing those bikes on the downhill. It's most likely all aerodynamics. Was your face 1" away from the stem with a flat? Were your elbows tucked tightly against your side? Just changes in position makes a 15-40% difference in speed on most of the downhills I do around here.
#7
Can't ride enough!
Join Date: Aug 2002
Location: south Louisiana
Posts: 1,235
Bikes: IFab Crown Jewel, Giant Defy, Hardtail MTB, Fuji finest, Bianchi FG conversion
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 2 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
OK, I run 6'3" and 220 pounds. My bikes run loose bearing hubs with high grade bearings and high quality grease. tires are 700 x 25 at 95 psi on my main road bike. And I run a rather aggresive riding position, with a 4" drop from the saddle to the tops of the bars.
I always walk away from lighter riders on downhills.
I always walk away from lighter riders on downhills.
#8
Senior Member
Join Date: Sep 2004
Location: North Central Massachusetts
Posts: 1,281
Bikes: Cannondale R600
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
My first guess would be tire pressure. I don't know about the physics, but seems that could make a bigger difference than aerodynamics.
Otherwise, weight is huge factor - my wife and I have similar bikes, tires, etc. and when we go downhill, I go a lot faster all other things being equal.
Otherwise, weight is huge factor - my wife and I have similar bikes, tires, etc. and when we go downhill, I go a lot faster all other things being equal.
#9
BOATS AND HOES!
Join Date: Jun 2005
Location: Gilbert, AZ
Posts: 666
Bikes: Madone
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
Funny you ask, I had a similar question a few days ago.
https://www.bikeforums.net/road-cycling/135802-difference-rolling-resistance.html
https://www.bikeforums.net/road-cycling/135802-difference-rolling-resistance.html
#10
Senior Member
Join Date: Aug 2005
Location: Pittsburgh, PA
Posts: 33,656
Bikes: '96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '20 Surly Midnight Special, All are 3x10. It is hilly around here!
Mentioned: 39 Post(s)
Tagged: 0 Thread(s)
Quoted: 2026 Post(s)
Likes: 0
Liked 1,096 Times
in
742 Posts
I agree that tire pressure, hub bearings, etc. are very minor components in this case.
Your aerodynamics were bad enough that they outweighed (pun intended) your weight advantage.
I have a friend who outweighs me by 25 pounds and we ride nearly identical bikes with the same tires, tire pressures and hubs that I overhaul and adjust for both bikes. I always beat him down long hills because he doesn't tuck-in as much as I do.
Your aerodynamics were bad enough that they outweighed (pun intended) your weight advantage.
I have a friend who outweighs me by 25 pounds and we ride nearly identical bikes with the same tires, tire pressures and hubs that I overhaul and adjust for both bikes. I always beat him down long hills because he doesn't tuck-in as much as I do.
#11
Junior Member
Join Date: Sep 2005
Location: Pittsylvania
Posts: 21
Bikes: Specialized
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
Weight will make you go down hill faster? That's a whole new law of physics. Galileo may have to rise from his grave on this news!
Remember from high school science: Velocity of a falling object = acceleration of gravity X time. There is no mass (weight) in this calculation.
Weight gives you momentum; not acceleration!!! At the bottom of a 2 mile hill, a 200# rider will have a bigger impact on a brick wall than a 75# rider; but, if they are on comparable bikes with equal consideration for wind resistance, they will hit the wall at the same time!!!!!
Remember from high school science: Velocity of a falling object = acceleration of gravity X time. There is no mass (weight) in this calculation.
Weight gives you momentum; not acceleration!!! At the bottom of a 2 mile hill, a 200# rider will have a bigger impact on a brick wall than a 75# rider; but, if they are on comparable bikes with equal consideration for wind resistance, they will hit the wall at the same time!!!!!
#12
You need a new bike
Join Date: Aug 2002
Posts: 5,433
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 3 Post(s)
Likes: 0
Liked 4 Times
in
3 Posts
Originally Posted by JKfromPBurgh
Weight will make you go down hill faster? That's a whole new law of physics. Galileo may have to rise from his grave on this news!
Remember from high school science: Velocity of a falling object = acceleration of gravity X time. There is no mass (weight) in this calculation.
Weight gives you momentum; not acceleration!!! At the bottom of a 2 mile hill, a 200# rider will have a bigger impact on a brick wall than a 75# rider; but, if they are on comparable bikes with equal consideration for wind resistance, they will hit the wall at the same time!!!!!
Remember from high school science: Velocity of a falling object = acceleration of gravity X time. There is no mass (weight) in this calculation.
Weight gives you momentum; not acceleration!!! At the bottom of a 2 mile hill, a 200# rider will have a bigger impact on a brick wall than a 75# rider; but, if they are on comparable bikes with equal consideration for wind resistance, they will hit the wall at the same time!!!!!
However, all things are generally not equal and in the case above, the lighter riders were probably more aerodynamic for some reason than the OP, resulting in a lower air resistance force and a higher resulting speed.
#13
Senior Member
Join Date: Jul 2005
Posts: 49
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
If you are heavier, you should go down hill faster despite the fact you are a bigger 'wind sheild', but many factors can come in to play which will tip the scale the other way, i will just list a few so you get the picture.
If they are in a very aero position e.g. in the drops and you are leaning back. If you are wearing baggy clothes and htye are wearing lycra. Low presure tires. Breaks rubbing. Extra friction fomr seized up bearings in the hubs. Any of these could be part of the problem and there are lots of other possibilities, you could have even been zig-zaging slightly which would slow you down (by this i dont mean cutting from side to side i just mean a bit of minute movment of the wheel, could even be due to a loose headset)
If they are in a very aero position e.g. in the drops and you are leaning back. If you are wearing baggy clothes and htye are wearing lycra. Low presure tires. Breaks rubbing. Extra friction fomr seized up bearings in the hubs. Any of these could be part of the problem and there are lots of other possibilities, you could have even been zig-zaging slightly which would slow you down (by this i dont mean cutting from side to side i just mean a bit of minute movment of the wheel, could even be due to a loose headset)
#14
Senior Member
yeah, the frontal surface-area increases with the square function of size (^2), but weight goes up by the cube function (^3). So if you take a small guy and double his physical dimensions (x2) in all directions, he's going to block four times (2x2=4) as much wind as before, but will weight 8x heavier (2x2x2=8). So the bigger guy will block 4x as much wind, but have a lot more weight to push that wind aside, so he'll go faster than the smaller guy on downhills.
Position on the bike really makes a huge difference as well. Tucking in the knees together reduces turbulence and help stabilize the bike. I always try to get as low as possible with my back perfectly flat and horizontal. That way, my entire upper-body only blocks as much wind as my shoulders. Then having one arm behind my back also reduces drag a bit as well, typically gives me an additional 1-2mph at 50mph. Rubberized skinsuits are about 1-2mph faster than normal skinsuits @ 50mph. Normal skinsuits are about 1-3mph faster than separate jersey/t-shirt & shorts depending upon how tight-fitting they are and the rear-pocket configuration.
Position on the bike really makes a huge difference as well. Tucking in the knees together reduces turbulence and help stabilize the bike. I always try to get as low as possible with my back perfectly flat and horizontal. That way, my entire upper-body only blocks as much wind as my shoulders. Then having one arm behind my back also reduces drag a bit as well, typically gives me an additional 1-2mph at 50mph. Rubberized skinsuits are about 1-2mph faster than normal skinsuits @ 50mph. Normal skinsuits are about 1-3mph faster than separate jersey/t-shirt & shorts depending upon how tight-fitting they are and the rear-pocket configuration.