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aarontoy's 3-mile test results demistyfied...(32 posts)

aarontoy's 3-mile test results
Jul 31, 2003 10:43 AM
Air resistance does not increase linearly with speed, which means, that if you want to double your speed, you'll have increase your power input by much more than factor of 2.
Also, pedaling at 30mph with 10mph tailwind, is essentially the same (power-wise) as riding at 20 mph!
I went to, and did some calculations. It turns out, that at 9m/s (approx 20mph) a hypothetical rider (I used their default settings) needs to produce 337 watts, while at 13m/s (approx 30mph) the power output has to almost double: 661 watt!
Hence, aarontoy's results shouldn't surprise anyone. Whatever tailwind he had could be just enough.
re: aarontoy's 3-mile test results demistyfied...James OCLV
Jul 31, 2003 11:00 AM
What did you put in for the slope? I can tell you, an average-framed cyclist can easily ride at 20 mph on a flat road way < 337 watts... Going up-hill is a different story, and 337 probably seems reasonable.

Besides, pedaling 30mph with a 10mph tailwind is not the same (power-wise) as riding at 20mph... (where did you come up with this?)

Once you exceed 20mph, the wind resistance created by the cyclist him/her self accounts for the obsticle in gaining more speed. The difference between riding 23mph and 20mph is HUGE!
re: aarontoy's 3-mile test results
Jul 31, 2003 11:07 AM
I meant to say at 20 mph with NO wind at all.
All things are relative, right? Therefore, riding at 30 mph with no wind whatsowver you are facing same air resistance as if standing still with 30 mph wind blowing into your face. With 10mph tailwind helping you, you only have to face 20mph blowing at you...
Actually, it's not all relative...James OCLV
Jul 31, 2003 11:23 AM
You have to consider the headwind created by the cyclist riding at 30 mph... Think about this for a minute. You can't ride 30 mph with no wind whatsoever, even on a calm day. If you could, there would be no need for aero equipment...

If you're riding 30 mph, you are creating a headwind.
How much wind? This depends on the frontal area of the rider.

With a tail wind, at some point (again this depends on the frontal area of the rider) you overcome the tail wind and start to creat a head wind (even with a tail wind of 20 mph)...
And your point is ...Humma Hah
Jul 31, 2003 11:34 AM
Yeah, you should always feel the wind in your face when moving, even with a tailwind. So?

I do this stuff for a living. The dynamic pressure on a pitot tube facing the wind = ro v^2. ro is air density, typically in kg/cubic meter. If v is in meters/sec, the dynamic pressure works out in Pascals. Multiply dynamic pressure by effective area in square meters, and you get force in Newtons.

Work equals force times distance, so multiply that force calculated above by distance traveled to get work (in energy units). Work per unit time is power, so if you multiply the force of the air resistance by velocity in meters per second, you get power required in watts.

The point is not "did the rider feel a headwind", it is "how much headwind did the rider feel. A rider with a 10 mph tailwind can take it one of two ways:

Ride easier.

Ride faster.

A rider who can comfortably do 20 mph in still air should be able to push almost the same airspeed with a tailwind. With a tailwind of 10 mph, they should be able to do around 30 mph.
My point is this...James OCLV
Jul 31, 2003 11:49 AM
That if you're riding at 20 mph, a 10 mph tailwind will not push you to 30 mph. The poster above was trying to use a tailwind to explain how a poster below was able to record a 5k TT in ~6 minutes (average speed ~30 mph). My argument is that the tailwind alone cannot account for this speed.

Besides, if the air is "still", wouldn't riding 20mph in any direction create a head wind for the rider?
My point is
Jul 31, 2003 11:58 AM
Tailwind = wind blowing in the same direction as you are riding. If you are riding north, then wind blowing from the south is tailwind for you. The same wind will be considered "headwind" for someone riding south.
Tailwind won't "push" you to 30 mph. However, it will DECREASE air resistance sufficiently for you to be able to go 30mph while pedaling only as hard as you would at 20mph in still air.
Oh, I guess I was confused. Sorry...James OCLV
Jul 31, 2003 12:04 PM
So THAT's what a tailwind is... Thanks for explaining!

If you are standing *still* with no wind and begin to travel North at 20mph, you are in effect creating a HEADWIND from the North...

The point is that while a tailwind will decrease air resistance, it's not as simple as you are making it out to be.
Precisely!Humma Hah
Jul 31, 2003 12:33 PM
If the air is still, a rider doing 20 mph will feel a 20 mph headwind, which will create almost all of the force that resists the rider's progress. Tire resistance, etc, are negligible by comparison. That's a pretty decent pace for a casual rider or someone on a century.

If that same rider, sitting still, has a 10 mph wind coming from the west, and then heads east, if that rider does 20 mph, they'll only feel a feeble 10 mph wind in their face. That's something like one a quarter of the force they felt before, and they're using something a little over 1/8th as much energy as before. So they shift up to the big ring and pedal until they feel the customary amount of wind in their face, which means they need a groundspeed of 30 mph. At that point, the wind drag is what they're used to in still air. Energy usage is slightly higher because you figure it for distance traveled over ground, not air, but that's a quibble for this example.
Right, but...James OCLV
Jul 31, 2003 12:41 PM
For some reason (and I'm not a physicist), 30 mph doesn't seem that easy... I can ride and sustain my speed at 20 mph no problem, but 23 is a different story... Even with a 10mph tail wind, I can't imagine sustaining 30 mph for 3 miles! I'm not saying it can't be done (obviously, it can), but for some reason it doesn't seem that simple...
Agree with James.Ian
Jul 31, 2003 3:36 PM
There are to many other factors to be taken into consideration. It is not as simple as, "Whatever tailwind you have will increase your speed by that much." A 10 mph tailwind does not increase ones speed to 30 mph at the same power output, just as a 10 mph headwind does not decrease a riders speed to 10 mph.

I won't argue your equations, I am not an engineer, but a think anyone who has ridden a bike for an amount of time would agree that a tailwind / headwind does not make the difference that you propose.

Humma, I know you have been riding for a while, I have seen your name one the board for a long time. I am surprised by what you have said.

Agree with Hummayeah right
Jul 31, 2003 4:05 PM
Humma's argument is correct, you can't argue it as long as long as you neglect the increased friction in the bearings from the wheels spinning faster (which is negligable), and assume the tail-wind blows straight from behind in a steady state manner such that the coefficient of drag stays the same in all cases. If you can beat physics I'll be impressed. With a good tailwind keeping 30 isn't hard, I've had a harder time keeping upright from headwinds.

When I work on aircraft designs, doing performance estimation and so forth, wind speed makes a huge impact, because that's what you're travelling against, and the same is true with bikes. You measure against the road, but you ride against the air.
I'd have trouble at 30 mph on the cruiser ...Humma Hah
Jul 31, 2003 4:42 PM
... but only because I'd be pretty spun-out in the only gear. I'm assuming you roadies typically have a good downhill gear that'll allow 30 mph at a cadence that won't blow you up in 15 minutes.

The world record for a bicycle drafting a motor vehicle now stands somewhere up in the 165 mph range, set on salt flats behind a pickup truck with a special windbreak. The bike, a fixed-gear, had a chainring the size of a trash can lid. That's the speed range that can be hit when wind resistance drops to zero.

Since the cruiser is faster coasting downhill than most roadbikes, I'm convinced tire and bearing drag are absolutely trivial at high speed. Its all aerodynamics at those speeds. I can compensate for all the cruiser's mechanical drag by crouching in tighter than most roadies are willing to get.
Keeping 30 isn't hard?James OCLV
Jul 31, 2003 4:48 PM
That's exactly my point - keeping 30 is hard, no matter what the wind conditions are.

If you're saying it isn't, then either I'm hanging with the wrong group, or you guys should be hanging out in the TDF peleton...
Based in theory, not the real world.Ian
Jul 31, 2003 5:28 PM
Yeah Right, I am sorry, but you are crazy. There is absolutely no way that a 10 mph tailwind increases speed by 10 mph, on a bicycle. In aeronautics, it may very well be the case, but a bicycle? No.

If you do believe it is so, then I suggest the the next time there is a good wind where you live, you go out and test your theory. Lets assume you are a fairly fit rider who can average 20 mph over a 40K TT. You believe that with a 10 mph tailwind you can average 30 mph? And were you to then turn around, you would only average 10 mph. Also, lets take this another step. Lets throw in a 20 mph wind. You could then average 40 mph on the way out, and not even be able to ride into the wind?

I do not know where you live, but I live in FL where we ride in the wind the majority of the time. It is the exception to have a calm day. My real world experience just tells me what you are saying is not true.

I do not want to be an @$$ or start a fight, but I just can't accept what you are saying.

Ian, some advice53T
Jul 31, 2003 5:47 PM
Whenever you find yourself resorting to, "anyone who has riden a bike can tell you physics does not apply in this case" you should slow down, think it over and start back-pedaling.

Physics applies in every case. My 5 year old daughter has ridden her bike a lot, with a couple of podium finishes, but she knows not to argue physics with engineers.

A tailwind of 10 mph, applied consistantly parallel with the riders direction of travel will give you a big drop in power for a given forward velocity. To a first order approximation, 10 mph tailwind equals the difference between riding 20 mph and riding 30 mph.

Getting a 10 mph tailwind to consitantly be parallel to your direction of travel is not very common, but it does happen.
Anecdotal examplevindicator
Jul 31, 2003 7:22 PM
I've only been riding a year, and 30+ mph speeds other than on downhills are pretty rare for me. In the local practice crits, my sprint will top out in the 31-32 range for a few seconds. Most of the time I cruise the flats solo anywhere from about 20.5 to 24 depending on wind, legs, temp, how "flat" the flats really are, etc. Usually average anywhere from 17-19 for a complete ride.

With that background, I can tell you that in a recent race one "leg" of the square 15-mile course was 4-5 miles long, mostly flat, VERY smooth road surface, and had an almost perfect tailwind (no idea how many mph). In a six-man paceline the first lap and a 3&4 man paceline the second (we picked a guy up mid-leg), we rode that leg doing 31 most of the way. It wasn't easy by any stretch, and we did have some help from the paceline, but I think the point is pretty well illustrated that "average" guys like me CAN maintain that speed for a few miles with a tailwind when otherwise it just doesn't happen. And to compensate for the paceline a bit, this wasn't a TT - we had to keep going when we turned on to the next leg and lost the tailwind. My average speed for the whole race was 21 so there was something about the wind on that leg...
Perfect! I love your numbers! A real example of truth...rwbadley
Jul 31, 2003 9:29 PM
on the board! I have to agree with every point here. Well done, good job. It's great to see numbers that reflect general reality.

I can attest to what wind (tail and head) will do.

I agree with Humma. A ten mph steady tail is considerable, and will add an easy 8-10 mph, all else being equal.
OK, fine. Who here has done it?Ian
Aug 1, 2003 7:51 AM
Who has gone out and been able to up their average by 10 mph with a tailwind of 10 mph? And if you want to talk about a distance of 3 miles, then I am guessing most club / Cat 5 / Cat 4 riders would be able to do that at an average of 23-25 mph. They should then be able to go 33-35 mph. Who has done it?

And my other question was ignored. With 10 mph headwind, would that TT speed drop to 13-15 mph?

What about the last TT in the Tour this year? The riders had a stiff wind coming off the water and blowing them straight towards the finish line. But they were unable to break the Tour TT speed record. And really didn't go that much faster than in years past.

I am not trying to say that physics doesn't matter. But I really think there are to many other variables that we either are not considering or do not know about. I have been road cycling for 7 years. Probably 5,000 - 6,000 miles per year. I can hold 20 mph quite comfortably. But I have never been able to hold 30 comfortably, not even with a stiff tailwind.

I suppose we will have to agree to disagree and this one.

And your point is ...elcameron
Jul 31, 2003 5:45 PM
It may make mathematical sense, but it doesn't pan out on the road. I have ridden a century, solo in 5 hrs. There is no way I could ride one in 3 hrs 20 min even with a 10 mph tailwind.
And your point is ...elcameron
Jul 31, 2003 6:01 PM
It also seems that a heavier rider would be affected more by gravity, i.e. ground resistance.
Well ...Humma Hah
Jul 31, 2003 6:13 PM
... we're talking about a 3-mile time trial, not a century. Centuries got turns, SAG stops, traffic, traffic lights, etc, that will keep a rider from maintaining a steady 30 mph. Traffic lights hurt faster riders disproportionately. But a stretch of US highway heading east across Kansas with a stiff tailwind and a bike with good gearing? WATCH OUT!

Weight of rider: hardly matters if no climbing is involved. If the rider is especially LARGE, then it increases wind resistance. Extra tire resistance? Very little and a little extra pressure should compensate.

Bearing drag, tire resistance ... all those things are designed to be trivial on a bike, and they're fairly constant. Only wind resistance increases with the square of velocity, and grows to monsterous proportions.
Jul 31, 2003 7:10 PM
3:20 is "other-worldly"!! Anything under 4 hours is inconceivable.

Although, my wife's parents live in Oklahoma, and you may have planted a seed.
At those speeds, its all airspeed ...Humma Hah
Jul 31, 2003 11:21 AM
... how fast the air is moving over the body is something like 99% of the total energy budget at roadrace speeds on flat ground. The resistance in force units increases as AIRSPEED squared. Power increases as airspeed squared times groundspeed.

Compare 30 mph in still air: 30 cubed is 27,000.
10 mph tailwind =20 mph airspeed at 30 mph: 12,000

That's 2.25x as much power needed in still air as with the tailwind! It still wouldn't let ME do Armstrong's times, but it would sure help.
Yes, my proof in the out-and-back TT...Spunout
Jul 31, 2003 11:02 AM
Our club 15k TT is on an out-and-back course. The prevailing wind is usually in your face coming back, unfortunate.

But, I ride at 40km/h for a 23min TT. My speeds may be from 34-50km/h at various places on the course. If I could pick the best 5K, I could log 48 km/h no problem.
How long have you been riding?James OCLV
Jul 31, 2003 11:17 AM
Are you a "newbie"; it doesn't sould like it... It's certainly possible to do 48km/h for 5k's... It's just hard to believe that a self-admitted "newbie" with little training can do it. Maybe he really did it... if so, great for him!

You must have a very strong Anaerobic Capacity to be able to do 48km/h in a 5k and only 40km/h in a 23min TT.

I've ridden at about 40.25km/h in a 15k TT, but sadly I had to struggle to do 45km/h in a 1k...
Back racing one year, but always fit...Spunout
Aug 1, 2003 4:10 AM
My intention was to note that the fastest portion of my 15km tt is around 48km/h(with the wind), then I hit the turnaround!

But, 4okm/h for a 15km TT puts me in the middle of the Senior 3/Masters in my club. The real hot rides come in at 19 to 20 minutes.

And yes, my speedwork helps build upper limits, although for shorter periods of time. 55 for 15 seconds, 50 for one minute, and do on.
I thought your "Uniballer" comment was funny! (n/t)stan_b
Aug 1, 2003 5:19 AM
According to my calculations....theweasonator
Jul 31, 2003 11:42 AM
my head is going to EXPLODE with too much physics... Can't we just ride and enjoy it?

My thought is it is not about how fast you can go for how long, it is about how long you can go for how fast? This is where experience and wisdom play into the picture and where you have to pace yourself.

Maybe this philosophical question will make the physicist's brains in here explode.

I'm not knocking the newbie by any means. I think it is fantastic you are proud of your accomplishment. Given the TT is pretty short, the course is relatively easy, and the weather conditions are right, these can all play to someones advantage. It sounds like stars were definitely in line for you when you did this or maybe you have raw talent which is what we all wish we could say. Great! Let's get on with life and everybody go pump those tires and ride instead of banging on computer keys and talking about it...
Wish I could...James OCLV
Jul 31, 2003 11:51 AM
Unfortunately, I'm stuck at work for at least another two hours... So, my only option right now is to bang on the computer keys and talk about it...
Same here, but tonight, everybody but MB1 ...Humma Hah
Jul 31, 2003 1:02 PM
... gets to do a 3 mi VO2-max interval, timed. Come across the finish line totally spent, HR at 95% of what your cardiologist thinks is your max.

MB1 can hold the stopwatch in his good arm.

What a time to have the Paramount laid up with a busted spoke. Gotta do it on the cruiser, fenders, lights, and all.
re: aarontoy's 3-mile test results demistyfied...aliensporebomb
Aug 1, 2003 8:05 AM
I'm looking at my riding logs over the past few years
and what I note for high average speeds and/or high
maximum speeds is when one of three things is involved:

(1) when a tailwind is involved, or (2) I feel "strong"
that day, (3) the presence of long downhills or (4)
in one case when I peaked at 36.8 mph when a motorist
was threatening me while I was on the road and I wanted
to get the heck out of there in a different direction the
motorist could not get to (my ride log says I was angry,
which would be a different reaction than if I was scared
I suppose).

I suppose it all depends on your physique, health,
equipment (fast road bike versus comfort bike, etc.),
gearing and emotional/intentional status.

Look at Lance - when he crashed due the the musette
bag snafu in the TdF recently he got mad as hell and
took off like an airplane and Jan was taken aback.

I know adrenalin can make performance improvements
occur but it would be nice if that could be controlled
on demand.