Yesterday I was talking to Lindsay why I need to paddle downhill while some other cyclists don't and still go faster than me? I was wondering whether there's some problem with my bike.
Lindsay replied that it is because I am lighter than other riders. After giving it a further thought, it didn't make sense.
if the gradient it x degrees, and the gravity is 'g', all the riders will experience an acceleration of g cos (x)
So irrespective of more mass or less mass, all the riders should have same acceleration and hence the same speed downhill. Then how the hell a tandem goes faster than a road bike downhill? Any thoughts?
Why heavier bikes/riders are faster downhill?
http://www.thenakedscientists.com/HTML/ ... tion/1843/
Differential aerodynamics. Try getting into a tighter tuck.
Differential aerodynamics. Try getting into a tighter tuck.
You need to remember, that Galileo, who proved that weight makes no difference is refering to a vacuum.
I had to sit down and work this out, I didn't believe it at first. What it basically comes down to, is that a certain amount of the resistance when descending comes from rolling resistance, a heavier rider has more force (F=ma, Force = mass x acceleration) to overcome this resistance.
While Weiyun is correct, I think that you will find that if you tuck down a hill, and then put a bidon filled with lead shot on your bike, and do the same descent, you will descend quicker.
Take an extreme with Galileo's experiment, drop an apple and a feather, the feather will fall slower, because it has higher resistance, but if you then add extra weight to the feather, it will fall quicker.
In relation to the tandem, they have roughly the same resistance as a single rider, same wind resistance, and only slightly higher rolling resistance. But they have roughly twice the weight to overcome that resistance. Thus they will descend quicker.
James
I had to sit down and work this out, I didn't believe it at first. What it basically comes down to, is that a certain amount of the resistance when descending comes from rolling resistance, a heavier rider has more force (F=ma, Force = mass x acceleration) to overcome this resistance.
While Weiyun is correct, I think that you will find that if you tuck down a hill, and then put a bidon filled with lead shot on your bike, and do the same descent, you will descend quicker.
Take an extreme with Galileo's experiment, drop an apple and a feather, the feather will fall slower, because it has higher resistance, but if you then add extra weight to the feather, it will fall quicker.
In relation to the tandem, they have roughly the same resistance as a single rider, same wind resistance, and only slightly higher rolling resistance. But they have roughly twice the weight to overcome that resistance. Thus they will descend quicker.
James
i dont know how it works, but weight does make you faster down hill 
and it seems that into the wind, in a sprint at heffron, if your bigger, once you can get your self moving in the sprint at heffron, you move through the wind better, and yeah, ive lost too much weight!!
the tandoms down hill have two times the power and only one person worth of wind resistance, they kill it down the hill, but yeah, in that case the weight is helping them too, but i dont know how it works
and it seems that into the wind, in a sprint at heffron, if your bigger, once you can get your self moving in the sprint at heffron, you move through the wind better, and yeah, ive lost too much weight!!the tandoms down hill have two times the power and only one person worth of wind resistance, they kill it down the hill, but yeah, in that case the weight is helping them too, but i dont know how it works
as far as a heavier rider moving through the wind better, that is probably because of momentum
p=mv
momentum = mass x velocity
once you are moving, it is harder to stop. But, a bigger rider would present a larger profile to the wind, and wind resistance is measured by
a=kvv (if I remember my 4 unit maths properly)
acceleration = wind coefficient x velocity x velocity (or velocity squared)
so a physically larger rider would have a higher wind coefficient compared to a smaller rider. This is also why the faster you go, the higher the wind resistance, and you have a terminal velocity, when in free fall, you are affected by gravity for acceleration (9.81m/s/s), but the faster you go, the more wind resistance you get. So your effective acceleration will then become lower, eventually, the wind resistance will apply 9.81m/s/s, so you will not increase in speed any more.
And then you get into relativity, where, the faster you travel, the more you weigh, so you are able to then fall quicker. Which means that you get heavier, and can fall quicker. Actually, this is starting to sound like one of Zeno's motion paradoxes.
and before someones thinks that it should be deceleration rather than acceleration, deceleration is simply a negative value of acceleration (or acceleration in the opposite direction)
James
p=mv
momentum = mass x velocity
once you are moving, it is harder to stop. But, a bigger rider would present a larger profile to the wind, and wind resistance is measured by
a=kvv (if I remember my 4 unit maths properly)
acceleration = wind coefficient x velocity x velocity (or velocity squared)
so a physically larger rider would have a higher wind coefficient compared to a smaller rider. This is also why the faster you go, the higher the wind resistance, and you have a terminal velocity, when in free fall, you are affected by gravity for acceleration (9.81m/s/s), but the faster you go, the more wind resistance you get. So your effective acceleration will then become lower, eventually, the wind resistance will apply 9.81m/s/s, so you will not increase in speed any more.
And then you get into relativity, where, the faster you travel, the more you weigh, so you are able to then fall quicker. Which means that you get heavier, and can fall quicker. Actually, this is starting to sound like one of Zeno's motion paradoxes.
and before someones thinks that it should be deceleration rather than acceleration, deceleration is simply a negative value of acceleration (or acceleration in the opposite direction)
James
The other consideration for "heavy" riders relates to their greater muscle bulk (typically for fit bike riders). On the flat, weight is less of an issue than the absolute power produceable by the said rider. More muscle (more weight) = More power.
If there are significant number of turns, then the other consideration may relate to your bike handling skills given your motorbike experience.Absolutely I'm faster downhill than Jo - it's got to be because I'm way heavier. And also way slower up hill.
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mikesbytes
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Yip, its all about wind resistance to weight ratio.
A good aero position will make a significant difference and in your case a full body waxing may help
A good aero position will make a significant difference and in your case a full body waxing may help
This makes sense! Good explaination Jamesas far as a heavier rider moving through the wind better, that is probably because of momentum
p=mv
momentum = mass x velocity
once you are moving, it is harder to stop. But, a bigger rider would present a larger profile to the wind, and wind resistance is measured by
a=kvv (if I remember my 4 unit maths properly)
acceleration = wind coefficient x velocity x velocity (or velocity squared)
so a physically larger rider would have a higher wind coefficient compared to a smaller rider. This is also why the faster you go, the higher the wind resistance, and you have a terminal velocity, when in free fall, you are affected by gravity for acceleration (9.81m/s/s), but the faster you go, the more wind resistance you get. So your effective acceleration will then become lower, eventually, the wind resistance will apply 9.81m/s/s, so you will not increase in speed any more.
And then you get into relativity, where, the faster you travel, the more you weigh, so you are able to then fall quicker. Which means that you get heavier, and can fall quicker. Actually, this is starting to sound like one of Zeno's motion paradoxes.
and before someones thinks that it should be deceleration rather than acceleration, deceleration is simply a negative value of acceleration (or acceleration in the opposite direction)
James
I read something in a cycling magazine years ago about this. The answer was that the increase in mass isn't proportional to the increase in surface area.
So I love downhills especially on the tandem!
If Marian sits up at all when we are descending a decent hill or max velocity will be about 70kmh and it doesn't matter what sort of aero tuck i get into. Luckily she's a trusting soul and our max speed so far has been 94kmh
So I love downhills especially on the tandem!
If Marian sits up at all when we are descending a decent hill or max velocity will be about 70kmh and it doesn't matter what sort of aero tuck i get into. Luckily she's a trusting soul and our max speed so far has been 94kmh
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