Bike Question

Changing tires? Huh?

Why post this in COJ? This is a real question?

I would have responded with a more serious answer had this been under tri talk.  But since it's in COJ I couldn't resist

I didn't want to be too smart with him.  I might need a ride to the Tampa airport, so I must be nice

Most thought provoking... I'm trying to recall some of my physics and sadly I think that's all been replaced by Chapelle's Show re-runs.

Trying to work it out intuitively I would think that it would depend on the speed of the wind in relation to your speed.  Basically, if the tail wind is faster than you are going (say a 30 mph tailwind while cruising along at 20mph), then you would want to sit up.  But if that's not the case (i.e. a 10 mph tailwind while going 20) then you would want to remain aero, since you are still experiencing a net 10 mph headwind.  Since in the latter case it's not so much that the wind is pushing you as it is not hindering you.

bts

No.  When was the last time you were riding and didn't feel a headwind?  The tailwind would have to be significantly faster than your groundspeed to make up for the increased drag, and in those conditions you probably wouldn't want to be riding anyway.

Doh!

Last time I checked Janelle I do believe Stake knows a thing or two about wind, windspeed, drag, lift, apparent wind, combat kills, grenades, you know important stuff. 

He is yet another example of the endless column of exemplary men of BT of which none lives nears you or Renee. You girls are definitely geographically challenged. j/k so put down the pins and the overweight doll.

nbo10 - 2006-01-17 4:49 PM

runnergirl29 - 2006-01-17 3:08 PM I think we need a physicist to answer this question.

Hey that's me.

It's all yours.  If you just provide us with equations that'll be good

runnergirl29 - 2006-01-17 4:52 PM

nbo10 - 2006-01-17 4:49 PM

runnergirl29 - 2006-01-17 3:08 PM I think we need a physicist to answer this question.

Hey that's me.

It's all yours.  If you just provide us with equations that'll be good

Knock yourself out.  First you'll need to figure out the actual headwind component given the tailwind and groundspeed (for this exercise we'll ignore parasitic drag).  Read this article for background (you'll find the equations inside), plug in the numbers, and let us know which position is more efficient:

Aerodynamics of Cycling
                        

Jim Martin



I. What is aerodynamic drag? Put your hand out the car window and the

force you feel is the aerodynamic drag of your hand in the air stream.

Aerodynamic drag of bikes and riders is measured in the wind tunnels by

mounting the bike on a balance and blowing air over it typically at 30

mph, and results are expressed in pounds of drag at 30 mph. The

aerodynamic drag is related to the density and velocity of the air and to

the frontal area and shape of the object in the wind stream by the

following equation:



Drag force = 1/2rhoCdAVT^2



Where, rho is air density, CdA is the product of coefficient of drag and

frontal area, VT is air velocity in the wind tunnel. If we divide the

measured drag force by VT^2 to get 1/2rCdA, we can calculate drag at any

speed. Also, we can take it one step farther. Power is force times

velocity, so the power to push you and your bike through the air at any

given velocity is:



Aerodynamic power = 1/2rhoCdAVA^3



VA is air speed (i.e.; ground velocity + head wind velocity).



II. Aerodynamic drag represents the largest resistance while riding over

level ground, however, the total power required to ride a bike is a

little more complicated, and can be divided into 5 components:



1. Aerodynamic power to push you and your bike through the air

(1/2rhoCdAVA^3) (70 - 95%).

2. Rolling resistance power (CRRWTVG) (5-15%)

3. Power to rotate wheels (FwVG^3) (~1%)

4. Power to overcome gravity on a hill (WTVGsin(arctan(Road Grade))

(varies greatly)

5. Friction losses in the drive and bearings (small except for chain line

cross over) (~1%)



Additionally, if the power you the produce does not match the power

required you will accelerate or decelerate. Putting all the factors

together yield the equation for cycling power:



Power required = 1/2rhoCdAVA^3 + CRRWTVG + FwVG^3 + WTVGSin(Arctan(Road

Grade)



Where CRR  is the coefficient of rolling resistance (about 0.0024 for

clinchers on asphalt) WT  is total weight of bike and rider (Newtons), VG

is ground velocity, FW is factor related to the power to rotate the

wheels (Estimates of this number vary widely, and I have have not

measured them myself. I have used 0.0027 for a set of aero wheels, and

0.0044 for regular round-spoked wheels).



Of course this equation just represents a mathematical model which may or

may not represent real world. To test it's validity I performed a study

in which we measured drag in the wind tunnel of seven riders, then had

them ride at three steady state velocities while we measured power with

an SRM crank and wind conditions with an anemometer. The results indicate

that our predicted power matched our measured power with a standard error

of 5 watts, and demonstrate that this is a valid model for power during

real world cycling.



III. Knowing the power required for a given riding velocity may be

meaningless if you don't know how much power you can produce. If you, as

a Triathlete or Duathlete, are equally well trained at cycling and

running, and have average running economy (1.6 kcal/kg/mile) and average

cycling efficiency (19% gross cycling efficiency) your sustainable power

output can be estimated from this simple equation:



Power (watts) = 60 * Body weight (lb.) /10k run time (minutes).



Based on this equation, Table 1 presents the estimated power output for 4

categories of triathletes/duathletes. Keep in mind that if you are

estimating you power in a multi-sport event, you should use your

'multi-sport run' time, whereas if you are estimating your cycling time

trial performance, use your 'run only' time. These estimated power

outputs will be used to illustrate the effects of aerodynamics on a

variety of riders.



Table 1. Estimated cycling power output for a 70 kg person based on 10k

multi-sport running time:



Elite Well Trained Trained      Recreational

10k Time  35 min 40 min 48 min         60 min

Power   264 watts 231 watts 192 watts 154 watts



IV. Although much attention is focused on the aerodynamics of equipment,

the most important aerodynamic consideration for a bike and rider

combination is the rider. A  typical 70 kg rider on a regular bike with

standard wheels will have a drag of about 8 lb., a better position will

reduce drag to about 7 lb., and an excellent position will yield a drag

of 6 lb.. Based on these drag numbers, and the power outputs estimated

above, equation 1 can be used to predict the effects of these positions

on cycling performance on a flat course with no wind shown in Table 2.

The differences in performance with no change on power are remarkable,

ranging to about 6 minutes when changing from a typical to an excellent

position.



Table 2:  Predicted 40k time, flat course, calm conditions, 3 body

positions, standard wheels.

Position    Drag @30 mph  Elite Well Trained Trained  Recreational

Typical       8.0    62:49     65:51        70:16       76:01

Good       7.0    60:14     63:07        67:22       72:57

Excellent     6.0    57:23      60:10       64:07       69:47



The key elements of a good aero position are:



1. Horizontal torso. Defined by having your chest, or better yet, your

back parallel to the ground, this is absolutely the most important

element, as it can result in large magnitude changes in aerodynamic drag.

Unfortunately, it may be the most difficult to achieve, because as you

approach this position, your thighs start to hit your torso. This

interference imposes limits on your body's aerodynamic position, but is

due to traditional bike geometry (i.e.; seat tube angles of  73 to 75

degrees). The way to overcome this limitation is to go to a more forward

position, which will allow you to roll your whole body forward. Note of

caution: a forward position seat post and long steeply-dropped stem may

allow you to assume a good aero position, but will result in a bike that

is not well balanced, and my be dangerous to ride. A much better approach

is to buy a frame that is designed to be ridden in a forward position.

These positions are uncomfortable in two ways. First and foremost, by

rotating your hips forward to get your torso horizontal, you are rotating

your weight right on to your soft and tender parts. Specifically, riding

in this position may exacerbate the condition of prostatitis that is

common among cyclists. Extra seat padding helps but does not eliminate

the problem. A truly anatomical saddle that distributes your body weight

over the whole seat might really help. Some riders try to alleviate this

problem by tilting the nose of the saddle down, but this only results in

a tendency to slide off the saddle and to strain your shoulder and arm

muscles. Secondly, and to a much lesser degree, you tend to get a sore

neck the first few times you ride, the discomfort lessens with time and

can be minimized with stretching and massage. These draw backs are

minimal because you don't have to ride the forward position daily to go

fast on it. My experience with Team EDS, as well as my own bike is that

you only need to ride it once a week (or less) to stay adapted to the

position.



2. Narrowly spaced elbow pads. Narrow elbows are an essential detail of

an aero position. However, the magnitude of improvement is much less than

what is achieved by adopting a horizontal torso position. Research

conducted by Boone Lennon has shown that subtle changes in elbow width

and aero bar angle may have significant effects on drag. This research

was performed on traditional geometry bikes, with the torso adopting the

characteristic cupped shape, and probably illustrates the need to block

air flow out of the torso area. More recent data on riders in a

horizontal torso position shows much less effect from these variables. I

do not believe these two findings are contradictory, rather, they

indicate that once the torso is horizontal there is little you can do to

improve or impair aerodynamic drag.



3. Knee width can change aerodynamic drag by up to half a pound. Pedaling

with your knees close to the top tube is an essential part of good

aerodynamics.



V. Is there a trade-off between position and power output? If done badly,

maybe, but if done well, no. Recently, Heil et al., (MSSE, May 1995) have

investigated this question, and the results tend to show that your

cardiovascular stress for a given power is increased by decreasing the

trunk to femur angle. Therefore, if you lower your elbow position, you

may need to move the saddle forward to maintain your trunk to femur angle

while getting a lower, more nearly horizontal torso position.



VI. The effects of aerodynamic wheels can be substantial. They can lower

the aerodynamic drag by about 0.4 lb. compared with standard wheels with

round-wire spokes and require about half the power to rotate. For the

following examples, I will use a Specialized 3 spoke front and a

lenticular rear disc. Table 3 shows the predicted effects these wheel

will have on 40k time trial performance.



Table 3:  Predicted 40k time, flat course, calm conditions, 3 body

positions, aero wheels.



Position    Drag @30 mph  Elite Well Trained Trained  Recreational

Typical        7.6 61:40      64:38         68:54       74:39

Good        6.6 58:58     61:47         65:55       71:23

Excellent      5.6 55:57     58:39         62:35       67:47



The difference made by aero wheels is about a minute and a half to two

minutes. When I was preparing this talk and I got to this part, I didn't

believe the model's prediction. So I recruited a friend and went out to a

fairly flat loop and rode at constant power with regular and aero wheels.

The results were almost exactly what the model predicts.  This study

needs to be repeated with better control such as wind and road grade

measurement, but it provides anecdotal evidence that the predicted

effects of wheels are realistic.



VII. Similarly, the effects of aerodynamic frames can be substantial. The

best frames can reduce drag an additional 0.3 lb. compared with round

frame tubes. The critical areas of a frame seem to be the leading edge

(fork, head tube, handlebars) and the area between the riders legs. The

frames that perform the best tend to have air foil shaped leading edges

and seat tubes (or no seat tubes). The effects of an aero frame are

estimated in Table 3.



Table 4:  Predicted 40k time, flat course, calm conditions, 3 body

positions, aero wheels, aero frame.



Position    Drag @30 mph Elite Well Trained   Trained  Recreational

Typical         7.3       60:53       63:47       68:04       73:40

Good 6.3   58:05      60:53        64:57       70:20

Excellent    5.3   54:59      57:39        61:30       66:38



The effects of an aero frame result in saving about an additional minute.



VIII. The effects of light weight components seem to be a topic of

interest for many triathletes/duathletes, however the effects of weight

on cycling performance may not be as significant as one expects.  To

illustrate the effects of weight I have modeled a very tough out and back

40k with a constant grade of 3% which results in about 2000 feet of

climbing/descending with aerodynamic bikes that weigh 22 lb. and 17 lb.,

and a slightly less aero bike/position that weighs 17 lb.



Table 5:  Predicted 40k time, 3% grade out and back course, calm

conditions, 2 body positions, aero wheels, 3 bikes.



Drag @30 mph  Elite Well Trained   Trained  Recreational

22 lb. 6.3 65:04    69:38          76:55      87:24

17 lb. 6.3 64:37   69:05           76:12       86:27

17 lb.    6.8 65:52   70:22           77:31       87:47



An extremely light bike on a very tough climbing course will only save

you about 30 seconds to 1:00, but if this lighter bike compromises your

aerodynamics even a little bit, you will be SLOWER.



IX. Till now, I've modeled everything in calm conditions, however, I

personally have rarely ridden in calm conditions. Wind effects can be

remarkable, largely because you spend a longer time in the head wind than

you do in the tailwind, and consequently, the slower head wind portion

has a greater effect on average velocity. Table 6 demonstrates the

effects of 5 and 10 mph winds on an out and back course, direct head wind

one way, tail wind the other.



Table 6:  Predicted 40k time, flat out and back course, windy conditions,

good body position, aero wheels, aero frame.



Drag @30 mph  Elite  Well Trained     Trained  Recreational

Calm 6.3 58:05        60:53           64:57       70:20

5 mph wind6.3 60:11        63:17           67:51       74:05

10 mph wind6.3 67:30        71:51           78:27       87:56

 

Yeah what he said .....

Seriously (BS in Aerospace here)....like Stake said the tailwind component would have to be so strong that you probably wouldn't be riding in it anyway. And the odds of a true tailwind on the same magnetic (or true) heading that you are traveling on is rare at best. The wind fluctuates 30 degrees at any given time so a true tailwind component is not something you are going to see very often.

 Zzzzzzzzzzzzzzz ....