Fastest Speed for a training session

[GettingFaster]

Weight is very much a factor in your speed descending (as it is when ascending.) You are correct that the acceleration is independent of the rider's mass as demonstrated by Gallileo many years ago But the acceleration is only relevant to how quickly you achieve your top speed. Once your speed has stabilised you are not accelerating any more and this terminal velocity is determined by the velocity at which the resistance force matches your weight (assuming you're not pedaling any more.) There is a small component of the resistive force which is weight-dependent (from the friction in the tyres) but the massive majority of your resistance comes from air resistance, which is dependent only on your frontal area. And as you correctly pointed out yourself, "Wind drag may vary slightly between a heavy person and a light one but will be affected more by position on the bike." So the greater your mass, the greater your weight and consequently the faster you go before the wind resistance (a strong function of velocity) matches this weight.

 

[Orange Fish]

80mph is 130kph, I only every reach that in my car nobody here would hit that on a bike.

Yeah I wonder what dream those two people were having when they hit 80mph.

 

[pod]

I think some people need to revisit their school text books on basic physics. Unless you are pedalling down a hill (or a tail wind blowing) the only force acting on you causing acceleration is gravity. Gravity is dependant on the mass of you+your bike+the earth. The mass of the earth is relatively so large that the mass of the rider is irrelevant. A light object accelerates at exactly the same rate as a heavy due to gravity.

Going up a hill you are effectively storing potential energy and this is dependent on the mass. More energy/more force is needed to lift a heavier object and this becomes more kinetic energy in the heavier rider when they roll back down the hill so they will potentially roll further on the flat at the bottom of the hill but they will not accelerate faster than the light rider going down the hill.

Galileo invented the telescope and was an important astronomer but it was not until much later that Isac Newton discovered gravity.

 

[dhk]

I think some people need to revisit their school text books on basic physics. Unless you are pedalling down a hill (or a tail wind blowing) the only force acting on you causing acceleration is gravity. Gravity is dependant on the mass of you+your bike+the earth. The mass of the earth is relatively so large that the mass of the rider is irrelevant. A light object accelerates at exactly the same rate as a heavy due to gravity.

Going up a hill you are effectively storing potential energy and this is dependent on the mass. More energy/more force is needed to lift a heavier object and this becomes more kinetic energy in the heavier rider when they roll back down the hill so they will potentially roll further on the flat at the bottom of the hill but they will not accelerate faster than the light rider going down the hill.

Galileo invented the telescope and was an important astronomer but it was not until much later that Isac Newton discovered gravity.

It's true that weight doesn't matter, if you're in a vacuum. A feather and a brick will have the same 32 ft/sec2 acceleration in a vacuum.

But, in the real world of air resistance, gravity only supplies a force equal to mass x 32 ft/sec2. When the aero drag on the falling body equals the force of gravity, acceleration stops. This is why a feather has a much lower terminal velocity than a skydiver or a brick.

Apparently you've never descended with riders of significantly different weights, because the effect of terminal velocity is pretty obvious.

 

[GettingFaster]

I think some people need to revisit their school text books on basic physics. QUOTE]

Dude I write physics textbooks, not use them to check stuff I learned long ago My post, along with Daniel's and DHK's, explain quite clearly why you are wrong - if there's something in them you don't understand then am happy to try and explain it to you though.

Newton formulated the mathematical theory of classical gravity but Galileo before him realised that all objects free-fall with the same acceleration regardless of their mass - cf the famous (but probably apocryphal) experiment with dropping masses off the leaning tower of Pisa.

 

[Fixey]

I think some people need to revisit their school text books on basic physics. Unless you are pedalling down a hill (or a tail wind blowing) the only force acting on you causing acceleration is gravity. Gravity is dependant on the mass of you+your bike+the earth. The mass of the earth is relatively so large that the mass of the rider is irrelevant. A light object accelerates at exactly the same rate as a heavy due to gravity.

Going up a hill you are effectively storing potential energy and this is dependent on the mass. More energy/more force is needed to lift a heavier object and this becomes more kinetic energy in the heavier rider when they roll back down the hill so they will potentially roll further on the flat at the bottom of the hill but they will not accelerate faster than the light rider going down the hill.

Galileo invented the telescope and was an important astronomer but it was not until much later that Isac Newton discovered gravity.

I never studied Physics, not my cup of tea, but I tell you this, I descend alot faster, in a straight line, than skinny people, and they ascend much faster than me, curse them all! as for my position....I dont actually use that position if there is a likely hood of me having to break fast.....I know exactly what would happen if I did and it would not be pleasant....... BUT for shear speed you cant beat it!

 

[VeloFlash]

I never studied Physics, not my cup of tea, but I tell you this, I descend alot faster, in a straight line, than skinny people, and they ascend much faster than me, curse them all! as for my position....I dont actually use that position if there is a likely hood of me having to break fast.....I know exactly what would happen if I did and it would not be pleasant....... BUT for shear speed you cant beat it!

I would agree with you it would be difficult to have breakfast in that downhill position you adopt!

"Shear speed"? Has that got something to do with the love affair you New Zealand men have with sheep?

 

[dhk]

I never studied Physics, not my cup of tea, but I tell you this, I descend alot faster, in a straight line, than skinny people, and they ascend much faster than me, curse them all! as for my position....I dont actually use that position if there is a likely hood of me having to break fast.....I know exactly what would happen if I did and it would not be pleasant....... BUT for shear speed you cant beat it!

I don't worry about braking on long steady descents, so it's fun to play with the aero position. Riding with someone of similar weight, you can experiment to find your lowest drag position. The difference between sitting up on the hoods and a good aero tuck is very obvious once you and a buddy are at terminal velocity.

It's also obvious that position on the bike makes more difference than any of the equipment variables like wheels, tires, or frame profiles.

One note of caution: don't forget to look up every few seconds. Once you get in a good tuck at 30 mph plus, you can rapidly gain on riders ahead.

 

[pod]

As far as I know, for two riders with exactly the same wind drag, the heavier one will descend faster. Your terminal velocity, which is the speed you can reach while descending, is determined by your weight.

If you want the physics it is basically this:

When you hit your terminal velocity, your downwards force, dependent on mass, is equal to your drag, which depends on your speed squared. The other dependencies are constant and include acceleration due to gravity, frontal surface area, drag coefficient and air density.

So we can basically write this as: speed squared depends on mass.

Or: speed depends on the square root of mass

Or just have a look at http://analyticcycling.com/DiffEqMotionTerminalVelocitySlope_Page.html

A heavier rider does not have the same wind drag as a lighter/smaller rider. As I pointerd out, the relative wind drag will vary a bit but the position on the bike will affect wind drag relativities more. Have a look at the example below from Analytical Cycling. If you increase the weight and cross sectional area both by 10% (leaving all other factors the same) then the speed reached is almost exactly the same. Use their terminal velocity page that you referenced and adjust both the cross sectional area and weight by the same percentage and you will get exactly the same speed v time curve. Adjust the CwD on either and you will get a very different curve.

I know the cross sectional area will not change by exactly the same percentage as weight and this is what I was referring to when stating the wind drag will be slightly different relatively but this difference is much less than position on the bike.

I'm a heavy rider and do not decend any faster than my lighter riding partners and yes they do have a distinct advantage going up the hills. It's just not fair.

Speed For Given Power
Speed For These Parameters 21.54 m/s
Power 10 watts
Frontal Area 0.5 m2
Coefficient Wind Drag 0.5 Dimensionless
Air Density 1.226 kg/m3
Weight Rider & Bike 75 kg
Coefficient of Rolling 0.004 Dimensionless
Slope of Hill -0.1 decimal

Speed For Given Power
Speed For These Parameters 21.53 m/s
Power 10 watts
Frontal Area 0.55 m2
Coefficient Wind Drag 0.5 Dimensionless
Air Density 1.226 kg/m3
Weight Rider & Bike 82.5 kg
Coefficient of Rolling 0.004 Dimensionless
Slope of Hill -0.1 decimal

 

[dhk]

Interesting calculations, but have to disagree with your example because you've scaled up both frontal area and weight by 10%. IMO, this isn't realistic, because human weight is related to volume, which is a cubic factor of dimensions. IAW, a person that's 10% wider (and taller) will generally weigh on the order of 33% more.

For your example, assume the smaller rider is 66 kg on a 9 kg bike. I'd estimate the 10% larger rider then would weigh in at 88 kg, for a total weight of 97 kg. Obviously, this weight will produce a much different result in the calculation.

Experience for me is that I always coast faster than lighter riders, when in the same relative position on the bike. And, the heavier guys consistently beat me downhill. If much lighter guys are going by you on the descents, I'd venture that something just isn't right with your position or equipment.

 

[pod]

Interesting calculations, but have to disagree with your example because you've scaled up both frontal area and weight by 10%. IMO, this isn't realistic, because human weight is related to volume, which is a cubic factor of dimensions. IAW, a person that's 10% wider (and taller) will generally weigh on the order of 33% more. .

No my example added 10% to the weight and cross sectional area not 10% wider and taller. Your example (using a cylindar shape) would make the cross section 21% more and you would still have to add more frontal area than this because of the extra height. Taller riders need a bigger bike and most riders don't get perfectly horizontal so the extra body length adds significantly to frontal area. 33% extra weight from your example will end up close to 33% extra frontal area. In addition to frontal area there is additional surface area which adds to wind drag. If two riders are the same height then the crossection area and weight differences are exactly the same in percentage terms.

For your example, assume the smaller rider is 66 kg on a 9 kg bike. I'd estimate the 10% larger rider then would weigh in at 88 kg, for a total weight of 97 kg. Obviously, this weight will produce a much different result in the calculation. .

No the 88kg rider will have a 21% greater cross section, be on a bigger bike, will have head and shoulders higher and will have more surface area increasing his coefficient of wind drag. If he can get completely horizontal with his body then he will have a slight advantage. Like I've said all along the relativities in wind drage change slightly but position on the bike is more important than weight... and of course most decent hills have lots of sharp corners/switch backs so technique and braking become the most important factors and bigger riders don't have any advantage here.

Experience for me is that I always coast faster than lighter riders, when in the same relative position on the bike. And, the heavier guys consistently beat me downhill. If much lighter guys are going by you on the descents, I'd venture that something just isn't right with your position or equipment.

There are so many variables such as speed at the top, aero position, clothing, helmet even and drafting effects so it is very hard to say just from personal experience. I know my lighter brother gets in an extreme aero position and leaves me behind downhill even though I am in a fairly aero position myself. This is exactly my point. The position on the bike is more important than the weight difference. My ride cost around $5k and his is a 20 year old piece of junk so it not the equipment.

 

[dhk]

No my example added 10% to the weight and cross sectional area not 10% wider and taller. Your example (using a cylindar shape) would make the cross section 21% more and you would still have to add more frontal area than this because of the extra height. Taller riders need a bigger bike and most riders don't get perfectly horizontal so the extra body length adds significantly to frontal area. 33% extra weight from your example will end up close to 33% extra frontal area. In addition to frontal area there is additional surface area which adds to wind drag. If two riders are the same height then the crossection area and weight differences are exactly the same in percentage terms.


No the 88kg rider will have a 21% greater cross section, be on a bigger bike, will have head and shoulders higher and will have more surface area increasing his coefficient of wind drag. If he can get completely horizontal with his body then he will have a slight advantage. Like I've said all along the relativities in wind drage change slightly but position on the bike is more important than weight... and of course most decent hills have lots of sharp corners/switch backs so technique and braking become the most important factors and bigger riders don't have any advantage here.


There are so many variables such as speed at the top, aero position, clothing, helmet even and drafting effects so it is very hard to say just from personal experience. I know my lighter brother gets in an extreme aero position and leaves me behind downhill even though I am in a fairly aero position myself. This is exactly my point. The position on the bike is more important than the weight difference. My ride cost around $5k and his is a 20 year old piece of junk so it not the equipment.

Agree on the bike position being most important, but we disagree on the bigger riders having the same ratio of frontal area to weight as the smaller riders.

Even if the 21% increase in area is true for a 10% "bigger" (in every dimension) rider, he'll still weigh roughly 33% more, given the same relative body fat %.

The taller, heavier rider will have the advantage since torso length adds nothing to frontal area and very little to overall drag. Think tandem, which descends a lot faster that the same riders on solo bikes do.

I can accept that my 6'3", 225 lb buddy descends faster than me, but the problem is he can climb faster as well. Could be the fact that he's in his 20's, and on a college cycling team.

 

[dhk]

Just a footnote to the discussion. Watching OLN coverage of the Giro today, Paulo Salvodelli lost big time on the Fenestre climb to Simoni. Bob Roll mentioned matter-of-factly that Salvodelli had about 25 lbs mass on Simoni, and would surely make up many seconds on the long, straight sections of the descent as a result. Neither Liggett or Sherwin questioned this assertion at all.

In fact, "Il Falco" did just that, saving his Maglia Rosa for the day and likely the race.

 

[daniels]

I think some people need to revisit their school text books on basic physics. Unless you are pedalling down a hill (or a tail wind blowing) the only force acting on you causing acceleration is gravity. Gravity is dependant on the mass of you+your bike+the earth. The mass of the earth is relatively so large that the mass of the rider is irrelevant. A light object accelerates at exactly the same rate as a heavy due to gravity.

Going up a hill you are effectively storing potential energy and this is dependent on the mass. More energy/more force is needed to lift a heavier object and this becomes more kinetic energy in the heavier rider when they roll back down the hill so they will potentially roll further on the flat at the bottom of the hill but they will not accelerate faster than the light rider going down the hill.

YES! The FORCE due to gravity is equal to the mass of the Earth MULTIPLIED by the mass of the rider+bike. So a 10% increase in mass results in a 10% increase in FORCE! Since your terminal velocity depends on the FORCE (due to aerodynamic drag) your velocity is dependent on your mass!

 

[bikeguy]

Daniels... hold on to your horses. If aero drag increases 1:1 with the mass increase, then acceleration and terminal velocity coasting down a hill will be the same. However, for heavier riders aero drag usually does not scale 1:1 with body weight increases but more like BodyMass to the power of 2/3. A person with very high bodyfat percentage may have the same aero drag as a lighter, leaner person though as bodyfat takes up more space than muscle.

-Bikeguy

 

[daniels]

Yes, i agree. I'm mainly just trying to dispel what was said earlier about descending speed being independent of mass, and somehow being related to acceleration. I'm a tall guy with a relatively low mass (6'4"/194cm and 72kg) and I get outrun on descents by heavier people all the time so i know inherently (and scientifically) that it ain't true!

 

[MichaelB]

YES! The FORCE due to gravity is equal to the mass of the Earth MULTIPLIED by the mass of the rider+bike. So a 10% increase in mass results in a 10% increase in FORCE! Since your terminal velocity depends on the FORCE (due to aerodynamic drag) your velocity is dependent on your mass!

But dont forget if you are more massive you have a larger surface area and more drag! Probably not by much though.

 

[MichaelB]

I cant help but notice that there is no category for 70 to 80!

 

[pod]

YES! The FORCE due to gravity is equal to the mass of the Earth MULTIPLIED by the mass of the rider+bike. So a 10% increase in mass results in a 10% increase in FORCE! Since your terminal velocity depends on the FORCE (due to aerodynamic drag) your velocity is dependent on your mass!

Yes and you need a 10% increase in force to accelerate the 10% more mass to the same speed. ie weight has no affect on acceleration due to gravity.

And if frontal area increases proportionally with weight increase then terminal velocity/max speed is also the same. I don't dispute that body shape has an effect on aerodynamics either down hill or on the flats but if you put on 10kg in weight you would not decend faster. You wouldn't be any taller so your girth would increase in proportion to your weight increase and you would decend at the same speed. Don't believe me, put the numbers into Analytical Cycling yourself.

I stick by my original statement, weight has nothing to do with it and that the relativities of the aerodynamics change slightly with different sized people and this will have an effect, but position on the bike is more important. It's not the weight but the relative aerodynamics that make the difference and as an extreme example if you went from lean and tall to 25kg fatter with most of the excess around your middle then your frontal area will increase by more than your weight in percentage terms and you will decend slower.

 

[larryb]

weight increases coasting speed on downhills.

if you don't believe that, get on a tandem and get ready to clean out your pants.

cu,
larryb