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# Help from the physicists!!

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# Help from the physicists!!

04-03-08, 06:38 PM
#1
DrWJODonnell
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Help from the physicists!!

Ok, I have taken physics, but that does not mean I know anything. I figured you engineering types with your TI-88 (or whatever calculus calculators are out there now) could explain this to me. I might be right, i might be wrong. What I want to know is the truth.

My coach has asked me for this saturday's TT to go harder into the headwind even if it means positive splitting and not doing as well wattage-wise in the tailwind section.

My feeling is that negative splitting is better from a physiological standpoint with the exception of hills. As such, regardless of the wind, I am thinking to go out at a determined wattage and come back at a slightly higher determined wattage.

I do understand that of course a headwind will make me slower and thus if I was splitting according to distance (versus time) I would have a longer 'out' and shorter 'back'. But if I split according to time, this seems to me to be irrelevant.

With hills, I understand zooming on the uphills a bit hot and "recovering" on the downhills. I guess I assumed this was due to gravity causing most of the resistance and taking aerodynamics out of the equation on uphills, yet aerodynamics plays a very important role on the downhills. With a headwind/tailwind, won't aerodynamics play an equal role both ways? In other words, even though you may go 5 mph faster with a tail wind, you still have the same equilibrium point(frontal resistance), as most of the wattage you use (and thus the max speed you can attain) is sucked up by wind resistance. Am I wrong? Should I consider a headwind/tailwind to be the same as an uphill/downhill situation?

Thanks for any help.
04-03-08, 06:42 PM
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kukusz
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When I was reading the last TT pacing discussion I was wondering the same thing. Enthalpic or Agnelle will know.

edit: some of the studies cited in this blog entry might help, trying to find the originals http://www2.trainingbible.com/joesbl...on-pacing.html

Ok, trying to work it out...

Power to Overcome Wind Resistance = ½*(air_density)*CdA*V_wind^2*V_road.

Last edited by kukusz; 04-03-08 at 06:54 PM.
04-03-08, 06:48 PM
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Can't you shut the F up and just come out and crush everyone's soul?
04-03-08, 07:08 PM
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ElJamoquio
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As most of your resistance is from the aerodynamics, and rolling resistance is roughly linear with speed, you gain a smaller amount from attempting to 'cheat' the wind - going harder into the headwind - when compared to cheating hills.

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04-03-08, 07:11 PM
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DrWJODonnell
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04-03-08, 07:13 PM
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VosBike
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The same effect as on hills will exist, but to a much smaller degree.

So yes, you should push slightly higher wattage into the wind and slightly lower wattage with a tailwind. However, short of hurricane force winds the effect will pale in comparison with the amount of time to be saved by going over threshold on uphills.
04-03-08, 07:14 PM
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"Dr O'Donnell's CdA just went negative!"

04-03-08, 07:15 PM
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*not a physics geek by a drunken longshot* but, if aerodynamic drag increases exponentially with airspeed, then I'm thinking the gains as tailwind have a much shallower curve.

Frankly, I'd love to some hopped up graph on the concept. Where's our local mad scientist WaterRockets?
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04-03-08, 07:21 PM
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Originally Posted by DrWJODonnell
At least it will be pouring rain Sat AM. So you have that going for you.

Learn to look at the Sunny Side. Oh wait, I guess that means there is no Sunny Side. Which would typically segway into me telling you that you are F'd, but this is a bicycle race. That means your opponents are F'd.

I am not one of them. I will cheer you on with the other old farts.
04-03-08, 07:21 PM
#10
ElJamoquio
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Originally Posted by DrWJODonnell
Cough. I don't think you're going to see a 20 MPH headwind, for reasons I've argued in the past.

I was trying a 6 MPH headwind and saw no (real) advantage going from a 350 watts there and back versus 370 out and 320 back (normalized power for those assumptions = 350). You have a really good CdA. I'll try again 10 MPH headwind.
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04-03-08, 07:26 PM
#11
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I'm an engineer, but I don't think this is really an engineering problem. It isn't all charts and graphs and watts/kg. A huge amount of this just "will" and pain management. There is a deeper thing going on deep inside that science and engineering doesn't understand.

Lot's of guys have recently posted that they put out a lot more watts going uphill than they ever could on flat ground. Do you really think that there is some sort of inclinometer inside those guys that turns the "governor" off when going up hill? I don't think so. I believe that they associate hill climbing with pain and have learned to cope with the pain because they expect it, but haven't learned how to cope with the pain when riding on flat ground where it is so easy to turn the pain off by just riding a bit slower.

For me headwinds are like hills. I expect pain when riding into them and somehow I can summon up extra watts when riding into them. Every month or so I "test" myself over 40 K. My best times are always when I start into a headwind and then return with a tailwind. The first half of the ride is always pure Hell. The return is just hell (lower case). For some reason I can put out a lot more watts into a headwind then I can with a calm wind or with a tailwind. Perhaps I have learned to embrace that kind of pain and make it my ally. Maybe your coach has sensed that you are one of those guys that can deal with the pain better when there is a headwind and is using it to get the optimal time out of you.

Of course this could all be BS. There is no science here.
04-03-08, 07:26 PM
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The last time the Sandy Hook Seashore had a 6mph headwind was before JFK's assassination. Yes, you can safely assume a 20mph headwind in that location. This is the Atlantic Ocean beach, not some Great Lakes Wussyfest. Dr. W. will go 22mph into it and 34mph coming back. Subtract 3-4mph from those numbers to get my speeds.
04-03-08, 07:27 PM
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Originally Posted by mollusk

Of course this could all be BS. There is no science here.
You think?
04-03-08, 07:29 PM
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ElJamoquio
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10 MPH show same - no advantage.

Let's put it this way - against the headwind, you're actually going faster, aerodynamically speaking, than with a tailwind. So, to go faster, you have to fight the V^3 termed - which is counter-productive.
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04-03-08, 07:29 PM
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DrWJODonnell
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You are right. they changed the forecast in the last day. More rain, less wind. Now they say 13 mph with 22mph gusts. that is down from yesterday when they said 24mph wind and 38 gusts.

Oh, did I mention this is essentially at sea level, exposed in the atlantic ocean? Yes, it gets windy.
04-03-08, 07:30 PM
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ElJamoquio
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Originally Posted by mollusk
Do you really think that there is some sort of inclinometer inside those guys that turns the "governor" off when going up hill?
Part of it is psychological, part of it is physiological. I see a ~5-10% drop in power from the hoods to the aero-bar.
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04-03-08, 07:30 PM
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mollusk
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You think?
Yeah. Not a whole lot of Nobel Laureates are winning TdF stages, though.
04-03-08, 07:34 PM
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Originally Posted by DrWJODonnell
You are right. they changed the forecast in the last day. More rain, less wind. Now they say 13 mph with 22mph gusts. that is down from yesterday when they said 24mph wind and 38 gusts.

Oh, did I mention this is essentially at sea level, exposed in the atlantic ocean?
Yes, it gets windy.
Understatement of the Month.
04-03-08, 07:35 PM
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Racer Ex
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I think the question needs a constant to really be hashed out, which would be the actual wind speed. If you consider the wind speed to be a factor in the actual measure of the course in distance, the higher the wind, the longer in effective distance the headwind leg is, so if you apply more watts there, you should have a faster overall time, because you're applying it for a longer period to the turn around.

Say 20 minutes out and 10 minutes in, your net applied wattage, assuming your CDA is the same, would be greater hammering the front leg than the back.

Plus if it's howling, from a pure "dude on the bike" POV, you're going to be spun out coming back and I find it much harder to stay on top of the gear.
04-03-08, 07:37 PM
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ElJamoquio
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Originally Posted by Racer Ex
Say 20 minutes out and 10 minutes in, your net applied wattage, assuming your CDA is the same, would be greater hammering the front leg than the back.
I took that into account when calculating an equivalent normalized power (370 out, 320 back, instead of 350).

Is there any sort of ground cover/trees/buildings at this event... or is it right on the oceanfront/etc?
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04-03-08, 07:38 PM
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ElJamoquio
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Originally Posted by DrWJODonnell
With a headwind/tailwind, won't aerodynamics play an equal role both ways?
Roughly. For the intent at hand, the answer is yes.
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04-03-08, 07:44 PM
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DrWJODonnell
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Think maybe some trees, but not enough to protect anything. The whole place is a 4 feet above sea level and exposed on all sides.
04-03-08, 08:12 PM
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Racer_Ex has a good point about not being able to go hard on the tailwind coming back.

There are also the physiological and psychological points about pushing hard against intense resistance, just like a hill -- it's easier to keep the watts high.

The physics, which a couple of you have hit has a large impact too.

Extreme case here (to obviate the differences):

If you're fighting a 30mph headwind, doing 10mph, you're in a 42mph apparent wind. If you go harder and get 1 mph out of it, you're going 10% faster (!!!), but you've only increased your apparent wind by 2.4%. The cube of the wind increase is the power requirement, so that would require a 13.8% power increase.

Compare that to still air, going 30mph (this is DrWJO). To go 1mph faster, it's only 3.3% faster, but it increases the apparent wind by the same amount. The wind speed takes the cube of the increase in power, so that would require 36% more power!

So, in the headwind, 13% more power gets you 10% more speed. In the still air, 36% more power only gets you 3.3% more speed.

Of course, we're talking about a 5% variance, so the real-world numbers won't be quite so dramatic.

Ok, so where'd I mess up?
04-03-08, 08:29 PM
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TI-88? Old school... TI-89 is where it's at.

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04-03-08, 08:44 PM
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DrWJODonnell
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Originally Posted by waterrockets
Racer_Ex has a good point about not being able to go hard on the tailwind coming back.

There are also the physiological and psychological points about pushing hard against intense resistance, just like a hill -- it's easier to keep the watts high.

The physics, which a couple of you have hit has a large impact too.

Extreme case here (to obviate the differences):

If you're fighting a 30mph headwind, doing 10mph, you're in a 42mph apparent wind. If you go harder and get 1 mph out of it, you're going 10% faster (!!!), but you've only increased your apparent wind by 2.4%. The cube of the wind increase is the power requirement, so that would require a 13.8% power increase.

Compare that to still air, going 30mph (this is DrWJO). To go 1mph faster, it's only 3.3% faster, but it increases the apparent wind by the same amount. The wind speed takes the cube of the increase in power, so that would require 36% more power!

So, in the headwind, 13% more power gets you 10% more speed. In the still air, 36% more power only gets you 3.3% more speed.

Of course, we're talking about a 5% variance, so the real-world numbers won't be quite so dramatic.

Ok, so where'd I mess up?

So I understand the headwind and from that standpoint and it makes sense to power ahead to get more speed. and so just to make sure, the opposite applies with a tailwind? so essentially yes the same as the uphill downhill phenomenon? So I should positive split with the headwind?