Fastest Speed for a training session

[wilmar13]

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.

I don't know anyone that rides in a vacuum, do you?

And if frontal area increases proportionally with weight increase then terminal velocity/max speed is also the same.

You keep missing the point that other posters have made, in that frontal area will not increase proportionally with weight.


Even so can't we all just agree that someone who is more "dense" will descend faster

 

[frenchyge]

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.

It's true that different weight riders should *accelerate* at the same rate, but that has no bearing on the terminal speed at which they descend. Since the gravitational force is greater on the heavier rider by a linear factor, and the resisting wind force is only greater by a less than linear factor, the heavier rider will reach a higher terminal velocity (see below).

And if frontal area increases proportionally with weight increase

But that's not the case. Mass is a function of volume, which is a cubic function of dimensions; while frontal (cross-section) area is a squared function of dimensions. Increasing mass only increases cross-section area by a factor to the 2/3 power, which is less than linear. In other words, much of the added mass adds thickness, which has no effect on frontal area or wind resistance, but does add to the gravitational force.

then terminal velocity/max speed is also the same.

 

[wilmar13]

It's true that different weight riders should *accelerate* at the same rate, but that has no bearing on the terminal speed at which they descend.

Sorry but the same reasons the heavier rider has a higher terminal velocity are also the same reasons they will *accelerate* down a hill faster. As speeds increase the difference in acceleration becomes greater.

But that's not the case. Mass is a function of volume, which is a cubic function of dimensions; while frontal (cross-section) area is a squared function of dimensions. Increasing mass only increases cross-section area by a factor to the 2/3 power, which is less than linear. In other words, much of the added mass adds thickness, which has no effect on frontal area or wind resistance, but does add to the gravitational force.

Your verbiage is muddy, but your point is totally correct. Compare two guys of the same weight, with one being smaller (because of denser bones/muscle etc.) who punches a smaller hole in the wind and he will be much faster descending (I think intuitively everyone agrees with this). But also take the same guy and let him gain 20 lbs, and he will end up descending even faster (even though his belly is dragging in the wind).

 

[frenchyge]

Sorry but the same reasons the heavier rider has a higher terminal velocity are also the same reasons they will *accelerate* down a hill faster. As speeds increase the difference in acceleration becomes greater.

No, actually Pod is right about that. The downward force is greater on the larger rider, but it takes a greater force to produce the same acceleration for him (F=ma). The initial acceleration for the 2 riders will be the same, and as speeds increase, the lighter rider's speed will start to steady out while the heavier rider just keeps on accelerating to a higher terminal velocity. In fact, at very low starting speeds (where the wind component is very small, but friction and tire losses aren't) the lighter rider may even accelerate faster at first. But that's really splitting hairs.

Your verbiage is muddy, but your point is totally correct. Compare two guys of the same weight, with one being smaller (because of denser bones/muscle etc.) who punches a smaller hole in the wind and he will be much faster descending (I think intuitively everyone agrees with this). But also take the same guy and let him gain 20 lbs, and he will end up descending even faster (even though his belly is dragging in the wind).

Muddy or not, it explains why even a larger rider of the same density (or maybe even lower) will still descend faster. You're right that a smaller (denser) person of the same weight will descend faster, and probably no one would dispute that. What they may have trouble understanding is why a larger, apparently fatter (ie, less dense) person is passing them downhill. The answer is that a lot of that extra size is thickness, not cross-sectional area.

 

[wilmar13]

Acceleration will only be equal in a vacuum. Regarding initial acceleration being equal I agree but we aren't talking about racing Pine Derby cars here... at any speed over 10mph rolling resistance takes a back seat. Since acceleration can be defined as a change in the speed over time, you yourself even state as much here:

No, actually Pod is right about that. The downward force is greater on the larger rider, but it takes a greater force to produce the same acceleration for him (F=ma). The initial acceleration for the 2 riders will be the same, and as speeds increase, the lighter rider's speed will start to steady out while the heavier rider just keeps on accelerating to a higher terminal velocity.

I don't understand how you can think the heavier rider reaches a higher speed but yet that he accelerated at the same rate as the lighter guy...if this were true they would both have the same terminal velocity. Maybe you are thinking in terms of an interval to terminal velocity whereby the heavier rider "accelerates" at the same rate from say 40-45mph as the lighter guy goes from 38-43mph? Is this your thought process? If so in this rough example the heavier guy accelerated from 38-43mph faster than the lighter guy.

 

[frenchyge]

Acceleration will only be equal in a vacuum.

Or in a case where the air resistance coefficient is essentially the same for both bodies, which is pretty much the case in our situation (especially in your example of 2 riders of equal size but differing densities/masses).

I don't understand how you can think the heavier rider reaches a higher speed but yet that he accelerated at the same rate as the lighter guy....if this were true they would both have the same terminal velocity.

because it takes a *longer time* before the velocity driven wind resistance increases to match the greater gravitational downward force on the heavier guy. He doesn't accelerate at a greater rate, but rather for a longer period of time and that's how he ends up with a higher velocity and shoots by the light guy at the bottom of the hill.

Maybe you are thinking in terms of an interval to terminal velocity whereby the heavier rider "accelerates" at the same rate from say 40-45mph as the lighter guy goes from 38-43mph? Is this your thought process? If so in this rough example the heavier guy accelerated from 38-43mph faster than the lighter guy.

I think we're thinking along the same lines here, and just disagreeing on a point of semantics. Certainly the heavy rider maintains his acceleration for a longer period of time, while the lighter rider's acceleration tapers off to zero (at terminal velocity) sooner. In that sense, yes, the heavy rider does have a greater acceleration beyond the point that the lighter rider has stopped accelerating. In the classical sense, acceleration is unaffected by differences in weight, although a bowling ball does have a greater terminal velocity than a volleyball.

 

[wilmar13]

Or in a case where the air resistance coefficient is essentially the same for both bodies, which is pretty much the case in our situation (especially in your example of 2 riders of equal size but differing densities/masses).

If the drag is the same and one rider has more mass he will accelerate faster down the hill. I don't understand how you can debate that??? It is only in the absence of air resistance where acceleration rates are the same for objects of differing masses.

because it takes a *longer time* before the velocity driven wind resistance increases to match the greater gravitational downward force on the heavier guy. He doesn't accelerate at a greater rate, but rather for a longer period of time and that's how he ends up with a higher velocity and shoots by the light guy at the bottom of the hill.

What you are saying here is that the two riders with different masses and similar drag would be side by side accelerating down the hill and when the lighter rider reaches terminal velocity it is as if he hits his brakes with the heavier rider continuing on.Take your bowling ball and your volley ball and let them roll down a hill, the bowling ball will pass the volleyball long before the volleyball has reached its terminal velocity. The acceleration dynamically changes as the force of gravity and resisting forces (air drag) approach equilibrium.

The acceleration rate of the heavier object is greater at all speeds when compared to the lighter object (except at 0 where it is equal) when the air resistance coefficient is equal for both objects.

 

[frenchyge]

If the drag is the same and one rider has more mass he will accelerate faster down the hill. I don't understand how you can debate that??? It is only in the absence of air resistance where acceleration rates are the same for objects of differing masses.

Arrrgghh. I had to drag out the equations and scratch paper to prove it to myself, but you are right.

Man, I've got to get out of this e-mail and paperwork job and get back to some engineering. I can tell I'm losing brain cells by the day having to deal with management.

I can only hope they're being transferred down into my legs....

 

[wilmar13]

Arrrgghh. I had to drag out the equations and scratch paper to prove it to myself, but you are right.

Man, I've got to get out of this e-mail and paperwork job and get back to some engineering. I can tell I'm losing brain cells by the day having to deal with management.

I can only hope they're being transferred down into my legs....

Nah it is easy to fall into a certain line of thought, and then find ways of rationalizing it... we ALL do it. But I do agree as we get more and more converted into corporate tools and further away from fundamentals, it gets easier and easier to follow into these thought traps.

At least you are one of the few that are capable of admiting when they are wrong, I get plenty of practice with this as well

 

[pod]

We're really arguing at cross purposes here. You're correctly arguing that a heavier rider will reach a higher speed decending, with all other things being equal and I'm correctly arguing that all other things are not necessarily equal and the effect of the weight difference is relatively small and the aerodynamics of position on the bike are more important. I can support this argument with evidence.

Firstly, I'm not talking about comparisons between a child and an adult or a brick and feather for that matter but two adult bike riders on similar bikes, one 10% heavier than the other going down a 10% grade and both only producing a nominal power output of say 10 watts.

A couple of people have referred to the frontal area only increasing by two thirds the weight increase. This is the relative increase in cross sectional area for a cylindar if the height increases in the same proportion as other dimensions, however, this assumption of an increased height is not necessarily correct. eg I'm about 18kg heavier than Daniels and 150mm shorter. I can find no evidence to support the 2/3rd claim but I do accept that there are some scaling differences with heavier riders having an advantage and I refered to these differences in relativities in my earlier posts. For the sake of the exercise lets assume the 2/3rds ratio is correct and plug the numbers into Analytical Cycling's calculator.

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 These Parameters 21.86 m/s 1.49% increase
Power 10 watts
Frontal Area 0.5333 m2 2/3rds of 10% increase
Coefficient Wind Drag 0.5 Dimensionless
Air Density 1.226 kg/m3
Weight Rider & Bike 82.5 kg 10% increase
Coefficient of Rolling 0.004 Dimensionless
Slope of Hill -0.1 decimal

The 10% heavier guy gets to 0.32 kph (or 1.49%) faster. The difference is tiny and we havent taken into account his larger surface area which will increas his CwD. A slight change of position on the bike adding just 3% to the CwD will reverse the result and make the terminal velocity the same for both riders.

Speed For These Parameters 21.54 m/s
Power 10 watts
Frontal Area 0.5333 m2
Coefficient Wind Drag 0.515 Dimensionless 3% increase
Air Density 1.226 kg/m3
Weight Rider & Bike 82.5 kg
Coefficient of Rolling 0.004 Dimensionless
Slope of Hill -0.1 decimal


The CdW is a very sensitive variable in that small positional/shape changes can make big differences and there is lots of evidence available to support this. NASA have some sample values for different shapes on this site http://www.grc.nasa.gov/WWW/K-12/airplane/shaped.html

So I agree with your arguments but some of you are missing my point that the position on the bike can be more important.

 

[wilmar13]

We're really arguing at cross purposes here. You're correctly arguing that a heavier rider will reach a higher speed decending, with all other things being equal and I'm correctly arguing that all other things are not necessarily equal and the effect of the weight difference is relatively small and the aerodynamics of position on the bike are more important.

No argument from me on this, I took issue with the 10% more weight =10% more area statement. For different riders this is not true (but you may not have been saying it was), and for the same rider, it is so tough to determine as we are not made of simple shapes of uniform density.

A couple of people have referred to the frontal area only increasing by two thirds the weight increase. This is the relative increase in cross sectional area for a cylindar if the height increases in the same proportion as other dimensions, however, this assumption of an increased height is not necessarily correct. eg I'm about 18kg heavier than Daniels and 150mm shorter. I can find no evidence to support the 2/3rd claim but I do accept that there are some scaling differences with heavier riders having an advantage and I refered to these differences in relativities in my earlier posts.

I think part of the problem is that it is very complex to actually calculate the frontal area from a change in volume with something like a cyclist, add to that how to you take into account the distribution of mass that isn't uniform? Then some of us are talking about larger riders vs. smaller riders while others are talking about giving the same rider more weight (which are totally different).

FWIW: The 2/3 thing only makes sense to me for legs, and even there it is a bit of a stretch. The upper body is more of a rectangular box with the smallest profile into the wind (at least with good form). In a really simple model I would tend to throw out the legs, and add the 10% mass uniformly distributed to the rectangular box (holding the length the same for the same rider since they don't get taller) resulting in a 1:1 relationship of area to mass (only for the same rider), so in a way I agree with what you are saying, but only thinking that it is a really grossly inaccurate and possibly misleading way to look at it, as well as meaningless to perform calculations with.

The CdW is a very sensitive variable in that small positional/shape changes can make big differences and there is lots of evidence available to support this. NASA have some sample values for different shapes on this site http://www.grc.nasa.gov/WWW/K-12/airplane/shaped.html

Perhaps stating the obvious but...CdW does not contribute to the drag on the same scale as frontal area which is a squared component. This is like everyone getting hung up on the Cd of a car... a car with a terrible Cd but half the frontal area will have a much higher top speed with the same power requirement than one that slips through the wind with a tiny Cd, but is larger. Of course once you have minimized your frontal area as much as possible, CdW is the next thing to target in the Pareto and it certainly does make a large difference.

So I agree with your arguments but some of you are missing my point that the position on the bike can be more important.

Totally true! Again my only issue is that I don't believe you can easily establish a relationship between increase in frontal area, and increase in mass and this is needed to establish a difference in realized speeds...my intuition and experience tells me that I descend faster with 215 lbs than my now svelte 198 lbs, but the amount of effort to climb prior to experiencing that slightly greater speed is not worth it (no one will debate that I predict).

 

[bikeguy]

Wilmar 13 wrote:

Perhaps stating the obvious but...CdW does not contribute to the drag on the same scale as frontal area which is a squared component. This is like everyone getting hung up on the Cd of a car... a car with a terrible Cd but half the frontal area will have a much higher top speed with the same power requirement than one that slips through the wind with a tiny Cd, but is larger. Of course once you have minimized your frontal area as much as possible, CdW is the next thing to target in the Pareto and it certainly does make a large difference.

I don't think so, halving the frontal area or halving the drag coefficient will result in the same effective drag area. In fact I think it's possible to have a drag co-efficient greater than 1, for example an inverted cone with the wide part facing out in the direction your going, the air piles up and has nowhere to go except deeper into the cone, and then back out.

The really fast HPV's get so by having ridiculously low coefficients of drag which results in very low CdA. Aerobars work in the same way, I don't get anywhere near the speed riding nose to handlebars (actually very little difference) as I get from riding even with a little higher trunk position on shitty aerobars. Yes, I know the arms are moved in front of the body thus decreasing frontal area, but a big boost is from the arms being in front like a v-shape cutting through the air, which means less air striking the blunt area of the chest and shoulders, meaning reduced drag coefficient.

Also, riding on aerobars seems to reduce the air striking the abdomen area, which has the worst drag coefficient of any area because it somewhat resembles the inverted cup I mentioned above, the air really piles up here and produces a lot of drag.

Another weird thing is that below and above a certain speed, the air can change from turbulent to markedly less so depending on the shape of the object. i.e, Cd can change depending on speed.

The equation for power needed to overcome air resistance is 0.5*air density (kg/m^3) *Vg*Va*Va*frontal area (in metres^2)*drag coefficient. Vg is ground velocity in m/s, Va is air velocity in m/s. drag coefficent is dimensionless. As you can see from the equation, drag coefficient (Cd) has just as large an effect on CdA as A (frontal area) has.

Wilmar 13 wrote:
FWIW: The 2/3 thing only makes sense to me for legs, and even there it is a bit of a stretch. The upper body is more of a rectangular box with the smallest profile into the wind (at least with good form). In a really simple model I would tend to throw out the legs, and add the 10% mass uniformly distributed to the rectangular box (holding the length the same for the same rider since they don't get taller) resulting in a 1:1 relationship of area to mass (only for the same rider), so in a way I agree with what you are saying, but only thinking that it is a really grossly inaccurate and possibly misleading way to look at it, as well as meaningless to perform calculations with.

Yes, the closer the rider is to horizontal the closer Fa of the upper body will scale directly with a mass increase. However with the legs being pointed (always) down, never present the "depth" component as frontal area, except when the leg is up. Bigger arms would have little effect on frontal area if you ride with them in front of your body.

Wow, I think we've about dissected this enough, no?

 

[wilmar13]

I don't think so, halving the frontal area or halving the drag coefficient will result in the same effective drag area. In fact I think it's possible to have a drag co-efficient greater than 1, for example an inverted cone with the wide part facing out in the direction your going, the air piles up and has nowhere to go except deeper into the cone, and then back out.

The really fast HPV's get so by having ridiculously low coefficients of drag which results in very low CdA. Aerobars work in the same way, I don't get anywhere near the speed riding nose to handlebars (actually very little difference) as I get from riding even with a little higher trunk position on shitty aerobars. Yes, I know the arms are moved in front of the body thus decreasing frontal area, but a big boost is from the arms being in front like a v-shape cutting through the air, which means less air striking the blunt area of the chest and shoulders, meaning reduced drag coefficient.

Also, riding on aerobars seems to reduce the air striking the abdomen area, which has the worst drag coefficient of any area because it somewhat resembles the inverted cup I mentioned above, the air really piles up here and produces a lot of drag.

Another weird thing is that below and above a certain speed, the air can change from turbulent to markedly less so depending on the shape of the object. i.e, Cd can change depending on speed.

Much of what you wrote is erroneous...I think you may have confused some terms and concepts.

The equation for power needed to overcome air resistance is 0.5*air density (kg/m^3) *Vg*Va*Va*frontal area (in metres^2)*drag coefficient. Vg is ground velocity in m/s, Va is air velocity in m/s. drag coefficent is dimensionless. As you can see from the equation, drag coefficient (Cd) has just as large an effect on CdA as A (frontal area) has.

Double the area what happens: Drag increases by 4, double the Cd, what happens: drag increases by 2, just look at the equation you wrote.

 

[bikeguy]

Wilmar 13 wroteouble the area what happens: Drag increases by 4, double the Cd, what happens: drag increases by 2, just look at the equation you wrote.

Wilmar13, you're going to have a hard time explaining how heavier riders with frontal area scaling as BM^2/3 reach a higher terminal velocity down a hill if drag increases by a squared factor of frontal area, especially considering that you just spent a few days arguing that heavier riders do descend faster. (yes, heavier riders do generally reach higher terminal velocities). In the equation I wrote, you can see that frontal area (A) and drag-coefficient Cd are raised to the exponent 1, which means that a x% increase in either increases (decreases, if x is negative) total drag by x%. Where do you see a squared term? How would displacing twice as much air quadruple drag?

Pop question: if velocity doubles, how much more power is needed to overcome air drag?

If it's not clear, the squared term I wrote means square meters (frontal area), not the frontal area squared.

Wilmar 13 wrote:Much of what you wrote is erroneous...I think you may have confused some terms and concepts.

Please enlighten me. After taking 2nd level calculus/physics courses and doing just fine, I have this feeling I can plug values into a simple equation and see what effect doubling some value has.

If you want, go to analyticcyling.com and double the frontal area for some example. You won't get quadruple the power needed (even neglecting rolling resistance).

-Bikeguy

 

[wilmar13]

Wilmar13, you're going to have a hard time explaining how heavier riders with frontal area scaling as BM^2/3 reach a higher terminal velocity down a hill if drag increases by a squared factor of frontal area, especially considering that you just spent a few days arguing that heavier riders do descend faster. (yes, heavier riders do generally reach higher terminal velocities).

Not really... read what I wrote and you will see I actually argued there is no such relationship between gain in mass and frontal area.

In the equation I wrote, you can see that frontal area (A) and drag-coefficient Cd are raised to the exponent 1, which means that a x% increase in either increases (decreases, if x is negative) total drag by x%. Where do you see a squared term? How would displacing twice as much air quadruple drag?

Yeah, I misread your equation, assuming your equation is correct Cd and A have the same contribution in terms of coefficients ...I will say that I am wrong not only because I misread your equation, but also because I really thought that area has a much larger contribution to drag than Cd (of course I am 10 years out of school and am suffering the same fate as frenchyge, you have to give us old guys a break for being cranky and senile).

Perhaps this thought originated in the fact that A is much easier to change than Cd. If I go into a tuck and reduce my frontal area by 30% and my Cd gets 10% worse do I care because I am still faster. FWIW at least it is possible albeit very difficult to calculate frontal area, Cd needs to be experimentally established. Anecdotally it is really hard to reduce or increase Cd in large %'s... area is pretty easy. I can almost guarantee you that with your HPV example they try to maintain as small a frontal area as possible 1st, then worry about optimizing the shape and surface to decrease Cd. Agree?

Pop question: if velocity doubles, how much more power is needed to overcome air drag?

Ummmmm 8 times.

Please enlighten me. After taking 2nd level calculus/physics courses and doing just fine, I have this feeling I can plug values into a simple equation and see what effect doubling some value has.

If I remember correctly you were saying reducing Cd or reducing area reduced "drag area" the same amount or something like that which made no sense to me. Sorry didn't mean to sound condescending if that is how it came out.

 

[bikeguy]

Wilmar 13: Ok, I actually think I'm a bit cranky today... yeah, the equation is correct and it's straight from High Performance cycling edited by Asker Jeukendrup. I am quite sure the equation is correct, and even went through the first principles as to how it's derived.

It's been a while since I've been in university, and my skills are also rusty.. I was studying for a BSc in chemistry but failed my chemistry courses, the calculus and physics went ok although I wasn't getting A's or anything..
If you want some fun try determining what the chemical formula of an unknown substance is by analyzing it with mass spectroscopy. That piece of equipment was so expensive they wouldn't even let us see it..


Planning on going back to study and so I figure sharpening my skills by analyzing things bicycle might be a good thing.

As for mass and frontal area scaling, I think the easiest real world example is when was the last time a little kid outran you just coasting down a hill... I think this makes it quite clear that heavier riders will descend faster when air resistance is a factor.


Wilmar13 wrote:Not really... read what I wrote and you will see I actually argued there is no such relationship between gain in mass and frontal area.

Yeah, in your last post. My point was more that IF (it doesn't!) drag went as frontal area squared, that the heavier rider would descend slower.. and the bigger they got, the slower ...

wilmar13 wrote: If I go into a tuck and reduce my frontal area by 30% and my Cd gets 10% worse

The tuck will reduce Fa and Cd. Aerobars really improve Cd. Here's a cool thing I've noticed if in an aero position and make a strong effort to depress my shoulder girdle, the more v shape of my upper back makes a noticeable difference in speed.. you can experiment in a pool with water jets too and see what position (arms wide, arms narrow, bent arms, curved back vs. flat back etc) pushes you back the least.

Wilmar 13 wrote:

Anecdotally it is really hard to reduce or increase Cd in large %'s... area is pretty easy. I can almost guarantee you that with your HPV example they try to maintain as small a frontal area as possible 1st, then worry about optimizing the shape and surface to decrease Cd. Agree?

No I don't agree for HPV's, the thing is like an knife cutting through the air, that is the fairing is very long and tapered relative to the width. This cuts Cd to like 0.10 or even less, while for a typical racing cyclist it's like 0.5-0.6.
Cutting frontal area by a factor of 6 (or even 2) over a UCI legal bike and position is impossible.


As for worsening Cd, make the angle of surfaces striking the wind perpendicular to the wind direction, and you will increase Cd a lot. Anything that causes air to pile up (without a doubt) or constrict or produces turbulence (note: there are exceptions, some kinds of turbulence may improve Cd, note the use of pockmark type indentations in Zipp's latest disk wheel, and why that is, is beyond my depth) will worsen Cd. I'm not an aeronautics engineer and I didn't study fluid dynamics,but these are the general guidelines.

-Bikeguy

 

[wilmar13]

The tuck will reduce Fa and Cd. Aerobars really improve Cd. Here's a cool thing I've noticed if in an aero position and make a strong effort to depress my shoulder girdle, the more v shape of my upper back makes a noticeable difference in speed.. you can experiment in a pool with water jets too and see what position (arms wide, arms narrow, bent arms, curved back vs. flat back etc) pushes you back the least.

Not necessarily, Cd is is where all the dependancies you can't calculate end up and you have to determine it experimentally. In both of these examples you are talking about reducing CdA, not Cd see the difference? Maybe you did reduce Cd, maybe you reduced A and Cd went up, the only thing you know for sure is that CdA is less...

No I don't agree for HPV's, the thing is like an knife cutting through the air, that is the fairing is very long and tapered relative to the width. This cuts Cd to like 0.10 or even less, while for a typical racing cyclist it's like 0.5-0.6.
Cutting frontal area by a factor of 6 (or even 2) over a UCI legal bike and position is impossible.

Exactly! How much frontal area does the sharp edge of a knife have? You are not wrong, but keep in mind you can take two things of the same shape and material with one 5 times larger and you will have a similar Cd but your CdA will be 5 times as much. Agree it would be tough to cut by a factor of 6 over a UCI legal bike, but have you seen how low and slim those things are!... take a rider well over 6 ft like me and lay me down flat between the wheels, a factor of 4 seems like a low estimate IMO. Again, all I am saying is the the area reduction is easy, reducing the Cd is tougher...and knowing you did is impossible without testing.

Don't forget really all we want is to get faster on a UCI legal bike anyway right? So yeah do whatever you can to reduce A and whatever you are "pretty sure" reduces your effective Cd so that your CdA is lower... I'll be waiting for you at the bottom of the hill

As for worsening Cd, make the angle of surfaces striking the wind perpendicular to the wind direction, and you will increase Cd a lot. Anything that causes air to pile up (without a doubt) or constrict or produces turbulence -Bikeguy

Yes... Cd is not total drag as many people think and that really was what trigered my knee-jerk reaction and underemphasing of its importance.

 

[bikeguy]

Wilmar 13 wrote: Exactly! How much frontal area does the sharp edge of a knife have?

You're missing the point. With the same frontal area, but putting a fairing around but extending the length to make a sharp and gradual point in front reduces Cd by a huge amount.

Wilmar 13 wrote: You are not wrong, but keep in mind you can take two things of the same shape and material with one 5 times larger and you will have a similar Cd but your CdA will be 5 times as much.

Yes. But what does riding with a parachute behind you have to do with reducing Cd?




Wilmar 13 wrote: Agree it would be tough to cut by a factor of 6 over a UCI legal bike, but have you seen how low and slim those things are!... take a rider well over 6 ft like me and lay me down flat between the wheels, a factor of 4 seems like a low estimate IMO.

I'll go to the HPV website I was looking at and get the frontal area values for one of the best HPV's.


Wilmar 13 wrote: Again, all I am saying is the the area reduction is easy, reducing the Cd is tougher...and knowing you did is impossible without testing.

The frontal area and Cd for good HPV's is readily available. You're right in that the fairings and design for these HPV's cost a lot of money and effort, but the drag reduction from the fairings far exceeds that of being in a recumbent position. You could just have a regular recumbent with no fairing that would have less Fa than a faired recumbent, but you won't hit 60 mph on a flat road with no wind. The best faired recumbents have done over 80 mph.

Calculating Cd is of no interest to me, I don't have the time, to say do coast down tests and take pictures of my frontal area so I can caluclate Cd knowing Fa and how far it took to coast down in a certain position. I can make the generalization that anything that makes your body or parts thereof into more of a v-shape (if they don't increase frontal area) will almost undoubtedly cut Cd and reduce total drag.

Wilmar 13 wrote: I'll be waiting for you at the bottom of the hill.

Hmm.. I sense some boasting.. ok, I will play too.

Two days ago I hit 70 km/hr down about a 5-6% gradient going from 35 km/hr to 70 km/hr in about 400 meters with a slight tailwind and I was quite relaxed, and I wasn't on aerobars. I blasted by one guy who went fast with me down the hill.

I think my best (sprint) accomplishment though is hitting 44 km/hr and holding it for at least 6 seconds UP a 10% gradient in like a 53x13 gear (83 rpm) with maybe a 3-5 km/hr tailwind. That was sweet, although it made me sore after. I regularly climb this (the hill is about 140 m long with a 90 m section at 10%) hill at 35-40 km/hr.

-Bikeguy

 

[bikeguy]

Here is a link to HPV data for the Diablo faired recumbent ridden by Sam Whittingham for an hour record of over 83 km/hr.

http://www.legslarry.beerdrinkers.co.uk/tech/AeroDrag.htm

For the Diablo: Cd 0.110 Frontal area: 0.1830 m^2 CdA=0.02 m^2

Best frontal area for a 70 kiloish rider on a UCI bike+legal position is probably just over 0.4 m^2 so this recumbent manages to get only 43.5% of the frontal area, while Cd is no better than 0.5 for UCI bike+ legal position (roughly), so the Diablo's Cd is only 22% of that value.


-Bikeguy

 

[DaveLee71]

Not up to speed with all the physics but 10 years ago, when I was 23 and 140lbs I could average 35mph for 5 miles from my town to the next, on a charity ride I managed to hold 40mph for about 4 miles with the wind for help. I could cycle the trip to work(around 10 miles) in 21 mins. Managed to hit 65mph downhill and got stopped by the police twice in one week for doing 49 and 50mph in a thirty zone!
I used to ride about 250-300 per week just for fun.
Then things happened, met my wife etc and cycling went off the boil until now, bought a road bike again last september, started cycling again, after 3 weeks practice in 9 years I entered a ride and came 5th out of 23 riders. Shortly after that I broke my ankle badly and it's only in the last 5 weeks I've been able to ride again but looking at my current times they would put me in the top ten for my local club! After about a months practice of only maybe 50miles a week I can manage 4.1 miles in under 10mins, might not sound fast but considering the amount of practice I'm quite chuffed, I'm now going to train for the rest of the year and join the club next year and see how I do. I'm now around 168lbs so hills are more fun but when I cycle to work I have a marvelous to go up in both directions!
My resting heart rate is around 49-52bpm and I'll be honest, I've probably got the worst diet on these forums (which my wife is going to change for me!).
I'm 33 now and regretting not keeping up with cycling over the last ten years but I'm more motivated now that ever to do something over the next ten!!
Dave