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Weight Weenies on the Downhill Slide

Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

Weight Weenies on the Downhill Slide

Old 02-24-06, 10:14 AM
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Weight Weenies on the Downhill Slide

I always read posts about how losing 100 grams of weight gives you blank extra speed on hill climbs, but I never hear anyone mention what effect the lose of weight has on the other side of the hill. Does any one have any calculations that compare the gain going uphill to the lose going downhill?
Being a Clyde, I frequently get caught going up hills, but I generally fly past the skinny guys on the way down.
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Old 02-24-06, 10:56 AM
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Don't have a calculation, but it doesn't begin to make up the difference. The reason being is that wind resistence is a squared function of speed. thus to go from say 30 miles an hour to 31 mph takes a heck of a lot more energy than to go from 5mph to 6mph. Thus all that stored kinetic energy is expended on the downhill while hardly raising the bike speed.
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Old 02-24-06, 12:28 PM
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Originally Posted by merlinextraligh
Don't have a calculation, but it doesn't begin to make up the difference. The reason being is that wind resistence is a squared function of speed. thus to go from say 30 miles an hour to 31 mph takes a heck of a lot more energy than to go from 5mph to 6mph. Thus all that stored kinetic energy is expended on the downhill while hardly raising the bike speed.
It actually worse than that, you have to go proportionally faster on the downhill to catch up with what you lost in the uphill. So if rider A rides climbs at 6mph and decends at 30mph, rider B climbing at 5mph would have to decend at (6/5)*30 = 36mph to catch up at the bottom of the hill.
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Old 02-24-06, 12:47 PM
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Originally Posted by Pico
It actually worse than that, you have to go proportionally faster on the downhill to catch up with what you lost in the uphill. So if rider A rides climbs at 6mph and decends at 30mph, rider B climbing at 5mph would have to decend at (6/5)*30 = 36mph to catch up at the bottom of the hill.
Actually, it's even WORSE than that.

If the weight weenie goes up a one mile hill at 6mph, it takes him 10 mins. You go up at 5 mph, and take 12 mins.

Turn around at the top of this very steep hill.

Coming down you're both going to be going a lot faster. If he comes down at 30mph, it will take him 2 mins. He will be at the bottom of the hill as you turn around. Impossible to catch him by adding speed on the downhill.

-Greg
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Old 02-24-06, 01:27 PM
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Well.. you guys didn't watch the 05 giro d'italia. Salvodelli made up minutes over the climbers using his fast descent skills. That math assumes a lot.
 
Old 02-24-06, 01:41 PM
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Originally Posted by DocRay
Well.. you guys didn't watch the 05 giro d'italia. Salvodelli made up minutes over the climbers using his fast descent skills. That math assumes a lot.
You forget that the reason Salvodelli is such a good descender is that he can change his mass at will. Light going up, heavy going down. It's a Tai Chi thing, I think.
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Old 02-24-06, 01:44 PM
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Originally Posted by DocRay
Well.. you guys didn't watch the 05 giro d'italia. Salvodelli made up minutes over the climbers using his fast descent skills. That math assumes a lot.
It doesn't mean that one can't be a better descender. What the math does is to show that there is a limit to what can be made up going downhill, and the weight carried up a hill isn't going to pay for itself on the way down.
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Old 02-24-06, 03:33 PM
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That math merely offers a theoretical limit. The real difference is generally in the handling techniques at speed. Any idiot can go fast in a straight line.
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Old 02-24-06, 03:36 PM
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Originally Posted by CastIron
That math merely offers a theoretical limit. The real difference is generally in the handling techniques at speed. Any idiot can go fast in a straight line.
I think we started off talking about the effect of a 100 gram change in bike weight. i don't think that's going to change handling to the point of affecting descending speed
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Old 02-24-06, 03:38 PM
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True. Just following the derail.
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Old 02-24-06, 03:41 PM
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Originally Posted by merlinextraligh
I think we started off talking about the effect of a 100 gram change in bike weight.
zero. or close to it. Might as well calculate thrust from farting while climbing.
 
Old 02-24-06, 03:44 PM
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Originally Posted by DocRay
zero. or close to it. Might as well calculate thrust from farting while climbing.
It's a zero net for in the saddle. +8 grams for out.
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Old 02-24-06, 04:14 PM
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Sorry if my example obscured my point.

Even if you can make up the same PROPORTION of your time decending as you give up climbing, you still lose, because unless the overall course is downhill, you spend a LOT more time climbing.

If I'm twice as fast as you climbing, and you're twice as fast as I am descending, I'll still win, as climbs take a lot longer than descents.

Nevermind the fact that it's a whole heck of a lot easier to be 20% faster climbing than it is to be 20% faster descending, due to the whole exponent thing in the aerodynamic drag equation.

That being said, there's obviously a point where becoming a better climber has diminishing returns. The microlight shorty climber types give up speed not only in descents, but on the flats, too...

-Greg
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Old 02-24-06, 05:12 PM
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If I can remember Newton's laws from my "Physics for Non Majors" class many, many years ago, mass doesn't have anything to do with rate of descent. Wind resistance certainly does. I remember the professor putting a feather and a quarter in a glass tube and evacuating the tube. When he flipped the tube, the quarter and the feather reached the opposite end at the same time. It was only when you add in air, does the quarter beat the feather as they fall.

Yes gravity will have more pull on a more massive object but given the difference between two humans against the gravitational attraction of the planet, the effect is so close to zero as to be zero for all practical purposes.

So a clydesdale will actually descend more slowly than he would if lost weight because his smaller, lighter body would be affected less by the wind resistance. You can ignore gravity. To say that additional weight makes one descend faster on a bicycle is false. It is actually quite the opposite.
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Old 02-24-06, 05:36 PM
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Originally Posted by fmw
If I can remember Newton's laws from my "Physics for Non Majors" class many, many years ago, mass doesn't have anything to do with rate of descent. Wind resistance certainly does. I remember the professor putting a feather and a quarter in a glass tube and evacuating the tube. When he flipped the tube, the quarter and the feather reached the opposite end at the same time. It was only when you add in air, does the quarter beat the feather as they fall.

Yes gravity will have more pull on a more massive object but given the difference between two humans against the gravitational attraction of the planet, the effect is so close to zero as to be zero for all practical purposes.

So a clydesdale will actually descend more slowly than he would if lost weight because his smaller, lighter body would be affected less by the wind resistance. You can ignore gravity. To say that additional weight makes one descend faster on a bicycle is false. It is actually quite the opposite.

Nah, if you keep wind resistance constant, a heavier bike/rider will descend faster than a light one. On my fully loaded touring bike, I decend much faster with weight (even with increased wind drag) than without. If an identical bike and rider were made of basal wood, the basal wood rig would descend a lot slower. The heavier object can more easily overcome the wind resistance/road friction with a higher kinetic energy.
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Old 02-24-06, 06:05 PM
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Originally Posted by fmw
You can ignore gravity. To say that additional weight makes one descend faster on a bicycle is false. It is actually quite the opposite.
Sorry fmv, but you must not get much opportunity to go down hills with riders that are heavier than you are. I'm over 200 pounds and I'm always amazed at how much affect gravity has when descending. I've tested it many times with my wife and with friends. My wife is much lighter and has less wind resisitance and when we start out side by side and coast down a hill, I leave her in the dust, and likewise, I have a friend that is 50 pounds heavier than I am with about the same wind resistance and doing the same test, he leaves me in the dust (and he and my wife and I all have the same bikes - LeMond Zurich's).

Another example is a couple years ago I was riding along on a fully loaded bike tour in New Mexico (panniers front and back with about 40 pounds of load) when I was overtaken by a Cat 3 Race (Tour of the Gila) as we were starting down a hill. I moved out to the center line as we were heading into a right hand turn so I would not interfere with the race and then we hit a steep section of this downhill. I was coasting and they were coasting (all light weight guys in a full aerodynamic tuck) and all of a sudden, I started passing them. I had to brake so the rest of them could get by me safely. I think extra weight definitely makes you descend faster.
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Old 02-24-06, 06:10 PM
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Oh, and as to the original post, the advantage of extra weight going downhill does not make up for the disadvantage of that weight going up the hill. I agree with the above posts.
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Old 02-24-06, 07:43 PM
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I would submit that if you spent 1/2 of your time going uphill and 1/2 your time going downhill, the weight difference would about cancel itself out. The problem is that the distances up and down hill might be the same but you go up at 10 mph and down at 30 mph, you are only going to be descending 1/3 as long as ascending.
If Newton actually believed that weight doesn't assist you when descending, he wasn't a Cub Scout. Every Cub Scout knows that in the Pinewood Derby, if you sneak more weight in the car you will go faster.
I am not a physicist, that is why I asked if there is a way of scientifically answering this question.
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Old 02-24-06, 10:32 PM
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My friend won a Pinewood Derby. Then he got disqualified for being past the weight limit...
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Old 02-24-06, 11:52 PM
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I don't think clydes have a huge increase in air resistance over us lightweights. Its a surface to volume/weight ratio. I ride with a clyde that has a good 70 lbs on me right now

150 vs 230 almost 2/3 on Volume/weight
I don't think there is a 1/3 more frontal area on him.
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