Dan [editor: actually Rick Ashburn] does it again, thanks

Triathlon Forum
21

Frank,
I think you may want to look up the definition of the “Cd” part of “CdA” :wink:

The term “CdA” is actually a multiplication of 2 values: “Cd”, or “drag coefficient” which is a dimensionless value and represents how “well” air flows around an object, and “A”, or “effective frontal area” which is measured in units of area and basically “scales” the effect of the aerodynamic drag. Two objects of vastly different size can have the same Cd (for example, a billiards ball and a bowling ball have the same Cd), but the CdA’s of the objects will be different due to size.

Your made up term, “effective CdA” is basically meaningless…

I’m thinking your going to want to review the physics of going up and down hills vs. flat and the effect on speed as well. You might want to start out by running some “ballpark” numbers to see if your speculations hold water :slight_smile:

22

What is important regarding aerodynamics is effective CdA not actual CdA. Fairings actually increase the actual CdA but decrease the effective Cda because of how they improve air flow around the object. Pinning your number down on your back in a manner that it will not flap does not change the actual CdA at all but reduces the effective CdA.

Wow…

You are, indeed, a scientist…

I’ll be honest: I didn’t read another word after this statement…

23

Frank,
I think you may want to look up the definition of the “Cd” part of “CdA” :wink:

The term “CdA” is actually a multiplication of 2 values: “Cd”, or “drag coefficient” which is a dimensionless value and represents how “well” air flows around an object, and “A”, or “effective frontal area” which is measured in units of area and basically “scales” the effect of the aerodynamic drag. Two objects of vastly different size can have the same Cd (for example, a billiards ball and a bowling ball have the same Cd), but the CdA’s of the objects will be different due to size.

Your made up term, “effective CdA” is basically meaningless…

I’m thinking your going to want to review the physics of going up and down hills vs. flat and the effect on speed as well. You might want to start out by running some “ballpark” numbers to see if your speculations hold water :slight_smile:
Ugh, I have done the “ballpark numbers” and I think hills have little influence on overall times for a good cyclist as long as the descents are not too technical. Comparing the course records for the men at all of the IMNA events it appears they all are within 10 minutes of each other and IM Canada and IM Florida are only 2 minutes or so apart yet they are substantially different terrain wise.

You are correct that the definition of CdA is what I was referring to as “effective” CdA. In actual use though the terms are used poorly which is why I suspect I got confused here. One finds people talking only about frontal area when they are referring to CdA - that is the term used at analyticcycling.com (even though it looks like they are asking the user to determine “effective frontal area” in their calculations as they ask one to fill that in and leave “drag coefficient” alone). Most people don’t know either their drag coefficient nor their frontal area so such calculations are always a big fat guess.

Hence, it seems this term is frequently misused so is confusing to the average person. From Analyticcyling.com.
"

Speed is estimated based on rider power and the forces acting on a rider from wind resistance, rolling resistance, and gravity.
This form can be used to answer the following questions: At a given power level, how does speed change as the slope gets steeper? If Frontal Area can be reduced, how does speed change? How does speed change as power increases?
(See Power for details on the definitions of parameters.) Example, Speed For Given Power Speed For These Parameters 7.46 m/s Power 250 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 0.03 decimal

24

What is important regarding aerodynamics is effective CdA not actual CdA. Fairings actually increase the actual CdA but decrease the effective Cda because of how they improve air flow around the object. Pinning your number down on your back in a manner that it will not flap does not change the actual CdA at all but reduces the effective CdA.
Wow…

You are, indeed, a scientist…

I’ll be honest: I didn’t read another word after this statement…
I have gone back and reread the document and I retract my criticism of the aerodynamics portion of the document.

25

I think hills have little influence on overall times …

Wow. I know a cyclist or two – ok, say about a couple billion of them – who might disagree.

Tom challenged you to do the numbers. Go ahead – run the numbers with a CdA going up of, say, 0.33 and coming down at, say 0.26. You are using words to support a mathematical conclusion (conjecture, really). You need to do the math.

You can’t just make stuff up.

Most people don’t know either their drag coefficient nor their frontal area so such calculations are always a big fat guess…

Re-read the part about how it isn’t necessary to separate the terms. Nobody does, even in wind tunnel tests. Those figures are anything but a “big fat guess.”

“Frank (can’t be a scientist …”

Well, that about sums it up.

26

Fair enough. Good for you-seriously.

27

I think hills have little influence on overall times …

Wow. I know a cyclist or two – ok, say about a couple billion of them – who might disagree.

Tom challenged you to do the numbers. Go ahead – run the numbers with a CdA going up of, say, 0.33 and coming down at, say 0.26. You are using words to support a mathematical conclusion (conjecture, really). You need to do the math.

You can’t just make stuff up.
The question is not are times generally slower on a “hilly” course, but why. It is unlikely that is it due to the aerodynamics of racing. It is more likely due to many other factors including to what you alluded to, being “timid” on the downslope, being afraid of high speeds or the need to brake on curves when going fast. If one could go faster on the downslope but doesn’t then that will slow them down. That “slowing” is not due to physics. Or, if one has to brake on corners because of speed on the downslope that will also slow them down but that is a technical descent issue, not a hill issue per se.

As I noted, the bike course records for a wide variety of courses are almost identical. Steve Larsen was 10 minutes faster setting the bike course record at IM Florida in 2003 than he was setting the Bike course record at IM LP in 2001 which might say something except LP was Steve’s first IM ever, and only third run as long as a 1/2 marathon (and first marathon) so I suspect he was holding back a bit on the bike at LP (plus his seat post collapsed on his borrowed bike so he rode most of the course in a sub-optimal position) compared to Florida where he was trying to make a statement after a dismal performance in Kona.

Most people don’t know either their drag coefficient nor their frontal area so such calculations are always a big fat guess…

Re-read the part about how it isn’t necessary to separate the terms. Nobody does, even in wind tunnel tests. Those figures are anything but a “big fat guess.”

“Frank (can’t be a scientist …”

Well, that about sums it up.

The numbers people put in at analyticcycling.com are big fat guesses as few have ever been in a wind tunnel.

I have already retracted my criticism of this portion of the document.

28

I think hills have little influence on overall times for a good cyclist …

Taking a different tack in addressing this…I need to keep in mind that my replies here are read by people other than Frank. For the rest of you…

My explanation of the interaction of gravity, power and bike speed in the article was not intended to address strategy. Sure, people will ride harder or easier and some are more fit than others. The power-gravity-speed equation still holds for those people.

It’s a silly thing to try to upend this apple cart by pointing out that some people will just ride harder and not slow down. In the poetic words of Bart Simpson – “Well, duh.” If Chris Lieto hits the hills and amps it up to 350 watts for a while – go ahead and ride next to him. Be my guest. Have a nice 26 mile walk after the bike leg. We’ll bring you a glow stick and a jacket for when it gets cold.

What you, the reader, should be concerned about is the effect of hills on you – at your own personal fitness level.

Pay no attention to the guy in the armchair telling you that hills don’t slow you down. Reading Frank’s insinuation that the laws of physics somehow don’t apply to some cyclists – well, you can go with that if you want.

29

It is unlikely that is it due to the aerodynamics of racing. It is more likely due to many other factors including to what you alluded to, being “timid” on the downslope, being afraid of high speeds or the need to brake on curves when going fast.

I would agree with you, but you’re wrong. Completely, utterly and irretrievably wrong. I know you’re going to dig in on this issue now that you’ve got your teeth in it. Like a dog with a Barbie doll.

But you are wrong. Spherically wrong.

Wrong no matter which way you turn it around and attack it.

Once again – to the lurkers and readers – please ignore these remarks on this topic. There are plenty of quasi-interesting nuances and refinements to be discussed about the power model, and I’m happy to do so (for example, I’ve left out the “delta-KE” term from the model. That’s of interest to kilo racers and crit specialists, but not so much to triathletes and TT racers).

But, this particular point about simple up-and-down-hills is a dead end.

30

Steve Larsen was 10 minutes faster setting the bike course record at IM Florida in 2003 than he was setting the Bike course record at IM LP in 2001…

Two different races, more than two years apart, for which you have zero power data in hand and for which you have no course elevation model, and on which you have run exactly zero numbers…

And you’re going to make a conclusion about the physics of gravity, aerodynamics and cycling from that data set?

Keep going…this is fun to watch!

31

I think hills have little influence on overall times for a good cyclist …

Taking a different tack in addressing this…I need to keep in mind that my replies here are read by people other than Frank. For the rest of you…

My explanation of the interaction of gravity, power and bike speed in the article was not intended to address strategy. Sure, people will ride harder or easier and some are more fit than others. The power-gravity-speed equation still holds for those people.

It’s a silly thing to try to upend this apple cart by pointing out that some people will just ride harder and not slow down. In the poetic words of Bart Simpson – “Well, duh.” If Chris Lieto hits the hills and amps it up to 350 watts for a while – go ahead and ride next to him. Be my guest. Have a nice 26 mile walk after the bike leg. We’ll bring you a glow stick and a jacket for when it gets cold.

What you, the reader, should be concerned about is the effect of hills on you – at your own personal fitness level.

Pay no attention to the guy in the armchair telling you that hills don’t slow you down. Reading Frank’s insinuation that the laws of physics somehow don’t apply to some cyclists – well, you can go with that if you want.
Here is what I wrote in my original post:

Hills will only have a significant effect on speed if the descents are technical (or the rider is “timid”) and the brakes are used on the down hill or there is a lot of turns (it takes a lot of energy to turn at high speed, which slows the average speed down a lot at high speeds, not so much at slow). Another reason “hilly courses” slow riders down is they go beyond their capabilities on the uphill portions, causing them to fail later on in the race. I suspect this is the most common reason to explain why riders go substantially slower on most “hilly” triathlon courses.
My criticism was based upon the fact you failed to address strategy. I doubt there isn’t a single reader here who doesn’t change their manner of riding when going uphill compared to going down in order to maximize race benefit. That was the point. While, if we were automatons riding in the same position and at the same power for the entirety of the race hills might have a significant slowing effect we are not. Most will ride in a more open position climbing in order to maximize power output when the aerodynamic penalty for doing so is small and also try to maximize the aerodynamics when power output is low and speed high on the descents. It is how people ride and any assessment of the “physics” of the ride should account for this

As I have tried to say I believe the “slowing” effects of hills (which is very real for most) are mostly due to factors other than the “physics” up and down nature of the ride.

32

I have asked MANY times…

These folks want a “stiffer” crank

How much do we leave on the table with sock compression? How about compression of the flesh on the bottom of my foot? I am thinking that we need a direct interface from our pedals to the bones in our feet. I think that you Rroof and a dentist could come up with a system.

33

It is unlikely that is it due to the aerodynamics of racing. It is more likely due to many other factors including to what you alluded to, being “timid” on the downslope, being afraid of high speeds or the need to brake on curves when going fast.

I would agree with you, but you’re wrong. Completely, utterly and irretrievably wrong. I know you’re going to dig in on this issue now that you’ve got your teeth in it. Like a dog with a Barbie doll.

But you are wrong. Spherically wrong.

Wrong no matter which way you turn it around and attack it.

Once again – to the lurkers and readers – please ignore these remarks on this topic. There are plenty of quasi-interesting nuances and refinements to be discussed about the power model, and I’m happy to do so (for example, I’ve left out the “delta-KE” term from the model. That’s of interest to kilo racers and crit specialists, but not so much to triathletes and TT racers).

But, this particular point about simple up-and-down-hills is a dead end.
If you say so. . .:slight_smile: I’ve made my point.

Ugh, what does the delta-KE term have to say about riding a straight line?

34 
My criticism was based upon the fact you failed to address strategy. I doubt there isn't a single reader here who doesn't change their manner of riding when going uphill compared to going down in order to maximize race benefit. That was the point. While, if we were automatons riding in the same position and at the same power for the entirety of the race hills might have a significant slowing effect we are not. Most will ride in a more open position climbing in order to maximize power output when the aerodynamic penalty for doing so is small and also try to maximize the aerodynamics when power output is low and speed high on the descents. It is how people ride and any assessment of the "physics" of the ride should account for this

As I have tried to say I believe the “slowing” effects of hills (which is very real for most) are mostly due to factors other than the “physics” up and down nature of the ride.

Here’s a fun project for you. Let’s take the “how aggressive of a descender are you” out of the equation and assume that the descent is perfectly straight. In fact, the bike is geared high enough that you can even put out almost as much power to the wheels as when you are climbing, which is actually a “better case” for a higher speed than what you’ve been assuming.

So…take a look at a simple comparison: two 10 mile courses that are perfectly straight. One course is perfectly flat, the other course goes up for 5 miles at 6% and then down for 5 miles at -6%. For the flat course and the downhill, assume a CdA of 0.26 m^2. For the uphill, increase that to 0.31 m^2 if you prefer (or don’t think you can hold an aero position on a 6% grade). Keep the same average power of 250W for both courses.

Anyway…start with the simple case of same average power for both cases…and then report back which rider would complete 10 miles first. Then, start playing around with any reasonable assumptions you want to make at attacking the 2 courses…and tell us if the “hill” rider EVER finishes in the same time as the flat rider.

Here’s something for you to think about before starting this exercise: although the potential energy gained in climbing the hill is all returned as kinetic energy on the downhill, the aerodynamic drag is not linear with speed. In fact, as Rick explained, the drag force increases with the square of the velocity and the power to overcome that drag increases with the cube. Think about the implications of that in your exercise…

Oh yeah, please show your work :wink:

edit: oops…I didn’t have the grades consistent…all fixed

35

I didn’t say there was no effect. I have simply said it is a small effect if one races “properly”. Especially when most “hilly courses” have large portions that are essentially flat. But, let’s assume the worst case, a 10 km course that is 5 km straight up followed by a straight down and compare it to a flat course. I went to analyticcycling.com and made several assumptions. In each assumption the rider “averages” 250 watts for the case.

Here are the scenarios. 10k flat course with the standard frontal area of 0.5 and sustained power of 250 watts.
5 k 6% up followed by 5 k 6% down with the rider

  1. maintaining a steady power of 250 watts on both the up and the down and maintaining the frontal area of 0.5 on both the up and down.
  2. 400 watts on the up and 100 watts on the down with a frontal area of 0.55 on the up and 0.35 on the down
  3. 500 watts on the up and 0 watts on the down with a frontal area of 0.55 on the up and 0.35 on the down.

Lets see how they compare. First, what does the rider on the flat do?

He will average 11.23 m/s and complete the 10k course in 890 minutes. (14.83 minutes)

What do the hills do to the times. I will rank them slowest to fastest.

Slowest. 250 watts both climbing and descending. He will climb at 4.92 m/s and descend at 18.85 m/s for a total time of 1281 seconds (21.35 minutes)
Next slowest. 400 watts climbing, 100 watts descending. He will climb a 7.17 m/s and descend at 20.71 m/s for a time of 938 seconds (15.63 minutes)
Fastest. 500 watts climbing, 0 watts descending. He will climb at 8.46 m/s and descend at 19.59 m/s for a time of 846 seconds (14.1 minutes)

So, it would appear that how the hills affect your time appears to be determined by how you ride them. It appears that if you attack the ups and rest on the descents, getting ready for the next attack the hills can actually make one faster. But, what do I know?

Now, I will admit it is not quite that simple as the average wattage of the various riders is not the same but it is unfair to expect them to be as most people can generate more power and sustain more power when opened in a climbing position and at a lower climbing cadence than they can when trying to ride aero at higher speed and cadence.

Anyhow, my comments stand.

36

“Slowest. 250 watts both climbing and descending. He will climb at 4.92 m/s and descend at 18.85 m/s for a total time of 1281 seconds (21.35 minutes)
Next slowest. 400 watts climbing, 100 watts descending. He will climb a 7.17 m/s and descend at 20.71 m/s for a time of 938 seconds (15.63 minutes)
Fastest. 500 watts climbing, 0 watts descending. He will climb at 8.46 m/s and descend at 19.59 m/s for a time of 846 seconds (14.1 minutes)”

Uhhhhh…am I missing something?

Why does your hypothetical rider descend slower at 250W than at 0 or 100W ?

Haim

37

OMG! It’s still alive!

with the standard frontal area of 0.5

Nobody ever measures “A” alone. What is the Cd in your example? If you used the 0.5 entry in A-Cycling.com, go back and start over.

  1. maintaining a steady power of 250 watts on both the up and the down and maintaining the frontal area of 0.5 on both the up and down.
  2. 400 watts on the up and 100 watts on the down with a frontal area of 0.55 on the up and 0.35 on the down
  3. 500 watts on the up and 0 watts on the down with a frontal area of 0.55 on the up and 0.35 on the down.

If you plugged 0.35 as “A” into ACycling, you got a CdA of 0.175 – which no human on a real road racing bicycle has ever achieved. You clearly still do not understand CdA and how it works.

Once again, you have proved that you do not understand the sport whatsover.

Did you bother to calculate the normalized power of the three examples? People can’t just ride a course at whatever the hell power they feel like! They are human beings, capable of some things and not capable of others. You’ve given an example where the fittest athlete rode the fastest! Again, “Well, duh.” If we’re starting with an athlete that can only manage 250w for 10k at whatever race distance he’s involved in, then how is he supposed to go at 200% of that power on a 5k climb? Have you ever actually raced a bike? All you’ve shown is that guys who can ride at higher power than other guys can go faster.

Frank, you are only showing that, if you make up a hypothetical set of conditions that apply to no actual athlete on the actual planet of Earth, you can prove anything. You keep claiming to be trained in the principles of proof and evidence. Do you have the slightest concept of *ceteris paribus? *Go look that one up and do another example. Hold the fitness of the riders the same and have them ride the course again. I’ll do this tomorrow – my model isn’t on this computer. Give me an example of the same guy, riding a course up and down a hill in any way you want, so long as he is limited by his own fitness. I’ll give you the figures.

Anyhow, my comments stand.

Your words typed on the page will perhaps stay on the page, but your comments are getting increasingly ridiculous.

Like a dog with a Barbie doll. I knew you wouldn’t let go of this – keep it coming. I can do this for a long, long time.

38
  1. 400 watts on the up and 100 watts on the down with a frontal area of 0.55 on the up and 0.35 on the down
  2. 500 watts on the up and 0 watts on the down with a frontal area of 0.55 on the up and 0.35 on the down.

Hmmm…I guess you missed the word *reasonable *above…

Anyhow, my comments stand.

“Stand” what? They certainly don’t “stand” up to scrutiny.

39

I just wanted to post to see if my new sig worked. Hopefully it does. To Frank - you give me hope that no matter how assinine my ideas I can still attempt to pollute the earth with them, with a modicum of sucess. Please shut up. To Ashburn - this discussion is beneath you. Seriously, write more articles for the site, spread your knowledge and ignore this thread. People who know far less than you can respond, or simply let it die. Noone is being influenced here, just entertained. I gain far more from your initial writings than from your needless defense of them. Thanks. -Soulswimmer

40

“2. maintaining a steady power of 250 watts on both the up and the down and maintaining the frontal area of 0.5 on both the up and down.
3. 400 watts on the up and 100 watts on the down with a frontal area of 0.55 on the up and 0.35 on the down
4. 500 watts on the up and 0 watts on the down with a frontal area of 0.55 on the up and 0.35 on the down.”

Never mind…I see the “problem” now.

Haim