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Monday, September 27, 2010

Aerodynamics in cycling and how to be faster with no additional effort

There are a number of factors which determine how fast you can ride.  The biggest, of course, is your power output, followed by your position, your riding equipment (bearing friction and tire resistance for example), and your weight.  Additionally the road grade, wind speed/direction, temperature and even elevation all play factors in determining your velocity on a bicycle.

Becoming as aerodynamic as possible is by far and away the best way to improve your cycling velocity with the least amount of effort.  The below chart (Figure 1) illustrates how a velocity is affected by a cyclist's power output and body position on the bike.  Hopefully we all know that the relationship between power and speed is not linear (straight line).  Rather, it takes progressively greater amounts of energy (watts) for each mile per hour gained. 

Figure 1:  Relationship between Power, Body Position, and Velocity

  Figure 2:  The Effect of Hand/Arm Position on Power and Velocity
Figure 2 illustrates the effects of aerodynamics due to body position.  Two important points: (1) again the graph lines are not linear (as mentioned above); and (2) the bike speeds start close together at lower speeds and progressively separate.   With each additional watt, the more aero-position (aerobars) becomes increasingly faster than the less aero-position (hoods).  

Does Equipment and Weight Matter?

Having aerodynamic equipment is typically far more important than having light equipment.  For example:  two kilograms of weight savings for me would only drop my 40K TT by 3.6 seconds on a flat course.  Yet just adding an aerodynamic fork vs using a standard fork can mean a decreased time of about 30 seconds or even more for an over-sized round fork - up to 50 seconds.

Weight does play an important role for climbing, and accelerating and for rotating parts such as wheels, shoes, pedals and cranks.  In a nutshell, lighter is better.  But keep in mind that once your equipment is up to speed its weight becomes significantly incredibly less important (including wheels).  Once you have broken the 10 mile per hour speed (16 kph) barrier (no wind) aerodynamics is again king for determining ultimate velocity.  At speed below 10 mph (16kph) aerodynamics are generally not in play. See Figure 3 (below) for an illustration of road grade, speed, position and aerodynamics. 
Figure 3:  road grade, position and aerodynamics v. speed
The funny thing about cycling and hills is that you can not recapture the loss of speed from climbing by going down the same  hill.  Let me give specifics to illustrate this:  I can ride 10 miles (16.1 km) on a flat road, in my drops, at an average speed of 26.18 mph (42.1 kph) in 22.92 minutes at 340 watts.  With the exact same effort and position, I would travel up a 5 mile (8 km) hill with a 6% grade at an average speed of 12.59 mph (20.26 kph) in 23.84 minutes.  Going downhill, I would average 41.94 mph (67.5kph) and complete the descent in just 7.15 minutes.  The combined incline and decline results would give an average speed of 19.36 mph (31.15 kph) and a travel time of 30.99 minutes.  In short, this hilly course (the same total distance as the flat course) would slow my average speed down by 6.82 mph (10.97 kph) and add 8.07 minutes total travel time compared to a flat course.  Incidentally,  if  I didn't pedal at all on the down hill section I would only lose another  0.76 mph (1.2 kph) average speed and add 1.25 minutes to the total travel time.  There is not a good performance return for pedaling down steep hills. 

Do Tires Matter?

Figures 4 and 5 (below) provide additional support, and are followed by  real treat for readers who are techno geeks like myself.  The figure 4 shows that narrower tires have greater rolling resistance, but yet are still faster because of aerodynamics!  Figure 5 shows that tubular tires have less rolling resistance than clinchers.  (So tubulars are lighter, faster, and corner better.... yes they cost more).
Figure 4:  Effect of Tire Width on Rolling Resistance
Figure 5:  Effect of Tire Width on Power Output
 Bike Calculator

And now the real treat (or at least I think it's the bomb) is a bike performance calculator.  It is massively cool for allowing you to see the effects of wind, weight, power, temperature, elevation, body position, and even tires on cycling performance.  After inputting a few values, this handy calculator will determine your velocity, time, calories, and weight loss.  From my real world experiences, I have found it to be amazingly reliable, but I should point out that it is only a model and is not without some degree of flaw.  But judge for yourself.  

Figure 6 (below) is an photo image of the calculator , and if you click on the title you will be linked to the site that hosts it.  I have also added it to my sidebar as a link titled "Bike Calculator" under "Links to people and things that I like".  
Figure 6:  Bike Calculator

Drafting Aerodynamics

The effects of aerodynamics is HUGE in road racing, time trials, criteriums, and even sprinting.  For example, drafting can reduce oxygen costs by 25 to 40 percent.  Figure 7 (below) offers a great illustration of the effects of aerodynamics and drafting:  a world class track team time trial riders can produce the following average wattages in a pace-line (traveling around 35 mph):  
First rider will produce around 607 watts (+/- 45), 
2nd rider 430 watts (+/- 39), 
3rd rider 389 watts (+/-32), 
4th rider 389 watts (+/-33).  

Notice that there is a decreasing advantage drafting in 3rd position over 2nd, but no further advantage after 3rd position.  (From this and other points within this post you can deduce that your front wheel is more important than your rear wheel concerning aerodynamics and performance, yet the rear wheel still matters!)

Figure 7:  Drafting Aerodynamics Illustrated
Ideal drafting greatly reduces a riders energy expenditure (as discussed above) and is a critical component of bicycle racing.

In order to increase your velocity while sprinting, it is extremely valuable to have a good aerodynamic form.  This means producing the smallest frontal area possible, along with a streamlined position.  So put your head low, back flat, and ideally keep your elbows in (if power can still be generated sufficiently).  Mark Cavendish and all the sprinters pictured in the photo in this link have perfect aerodynamic form while sprinting.  
 
Conclusion:

To summarize, an aerodynamic wheel is more valuable than a lighter wheel for most racing applications.  Weight plays a larger role concerning velocity during acceleration and hill climbing (especially for rotating parts such as wheels, shoes, pedals and cranks).  On flat courses, after accelerating, the weight of a wheel (etc) is almost a non-factor when compared to the performance effects of a highly aerodynamic wheel. 

A rider would be wise to ride in an aerodynamic position at all times where the speed is above 10mph even when drafting in a field (riding in the drops vs sitting up with hands on the bars).  
Pedaling hard down descents is not very productive due to the increasing effects of wind resistance.  Average speed can be increased on a hilly courses by careful disbursement of increased effort on uphills and lessening to no effort on descents vs a constant effort over that same distance.

Consider the information provided here and ride accordingly.  Your senses can not perceive the energy savings or the speed increases from good cycling form, but all of the measures devices (speedometer, watt-meter, etc.) can and will.  It can make the difference between winning and losing.  

If I can think of any more useful points I will add them to this post over time.  Any suggestions are appreciated!   


15 comments:

  1. I ride bullhorns in a very stretched out position when my hands are on the ends. I'm wondering how that would compare to hoods or aerobars, since they're sort of close to each. Typically, I end up so low that my forearms are on the bars, with my hands out as far as possible.

    ReplyDelete
  2. Aaron, I would say in-between. Bull horns are a bit uncommon and not listed on the bike velocity calculator.

    Your question reminds me of the '89 Tour De France where Fignon used bullhorns and Lemond used aerobars in the final and deciding time-trial
    Had Fignon read this post before July 23, 1989 (and was reasonable) he would have easily won the Tour de France that year by over a minute, instead of Greg Lemond winning by 8 seconds

    ReplyDelete
  3. David,
    Does cycling jersey and expensive helmets make a difference in aerodynamics, or are they purely for comfort reasons?

    ReplyDelete
  4. Yes, they make a difference, but with qualifications. If we compare an inexpensive jersey with an expensive one then no on the performance (aerodynamics), but yes on comfort and function (zippers, fit, pockets etc).

    If we compare helmets, then more expensive typically does improve both comfort and performance (lighter, better vented, aerodynamic, etc), but generally the performance differences are small.

    Aerodynamic performance really comes into play with Specialized time-trial helmets (long tail)and skinsuits. Both are fairly expensive (compared to standard helmets and jerseys), plus aero helmets aren't typically very comfortable.

    ReplyDelete
  5. Curious on your thoughts of going with a aero bike like a Scott Foil vs a typical roadbike. The data above suggest I would spend less energy with a road aero bike than I would with a standard roadbike even with a 1lb weight difference. I plan on buying a new bike soon for this season, I typically do road/crit racing. Thoughts?

    ReplyDelete
  6. JRodgers, yes I believe that an aeroframed bicycle has a slight advantage over the larger tubed lighter frames (the exception being long hills).

    A couple points: water bottles can be an issue (mounting). They can be put behind the saddle, but this is not UCI legal. In the jersey pocket works though.

    I'm pro aero frames and I think that bicycle manufacturers will start to trend (as an option) this way in the future.

    In terms of performance in racing, first focus on good drafting skills and body position on the bike. This will produce the greatest benefit (by far!), then equipment starting at wheels, helmet, and then frame.

    It all adds up!

    ReplyDelete
  7. Dave,
    What are your thoughts that for most of us simply losing 5lbs of body weight could be the best way to improve our aerodynamics, thus speed.

    Thank you,
    Gene

    ReplyDelete
    Replies
    1. Gene, I could imagine that some weight loss of the body could help aerodynamics by producing a smaller object to produce drag. However, 5 pounds of weight loss tends to be somewhat spread over a body and probably wouldn't result in significantly smaller shape (especially on a larger person). It's worth mentioning that a lot of world class time trialists are relatively big and heavy especially when compared to world class hill climbers.

      The upside of losing 5 pounds is that a cyclist will accelerate faster(starts and turn arounds in time trials will improve, etc.), and they will go up hills better and may tolerate higher temperatures better. 5 pounds for a professional cyclist is a pretty big deal. It's generally better to be physically lighter, but there's a paradox where too skinny for too long often results in susceptibility to illness and fatigue. Upper single digit percentage body fat seems to be ideal, but once a person gets into the lower single digit body fat content they are riding the razor and are potentially risking their health (from what I've read and have been told).

      Delete
  8. The most important consideration to think of when it comes to cycling is ensuring your well being and safety. Most cyclists would agree that finding clothing and gear that also looks good is important too.

    jeksports.com

    ReplyDelete
  9. David, assuming we have set standard wheels and riding on the hoods (no TT position), what speed will a aerodynamic frame come into play, I was given 47km/h, I have always believed most benefits come from a good set aero dynamic wheels, followed by a aero position which is balanced power output meaning that you might have very aero but compromised because of less breathing capacity, then aero helmet and skin. PS using the right type aero wheels for cross wind course as they may lose there effect and really slow you down

    ReplyDelete
  10. Hi Luis, largely aerodynamics are determined by frontal surface area resistance against the resistance from atmospheric gases and drag resistance along sides and rear of an object as it moves through these gases. By far the best way to improve your cycling aerodynamics is to simply reduce the frontal surface area by putting your arms forward on aerobars and dropping your torso parallel to the deck (ground). Helmet and wheels will vary a bit on which is superior for aerodynamic performance depending on the course (hilly or flat, wind direction and which particular product design is being used).

    Take the guess work out of the equation and consider an iBike NEWTON+. This device displays instant, continuous aerodynamic drag coefficient feedback on your screen even while pedaling! When you ride with both your iBike NEWTON+ and a direct-force power meter sensor, your iBike NEWTON+ provides information that enables you to find your optimal combination of aerodynamics and power.

    Basically the iBike Newton+ uses Issac Newtons 3rd law that states that applied and opposing forces must be EXACTLY equal. Therefore, using known data and proven equations the software in the iBike Newton+ can give you instant readings on not only coefficient drag, but also other helpful data such as watt power, speed, ground slope, wind speed and angle, altitude, and temperature.

    ReplyDelete
  11. I've read in several places that deep aerodynamics wheels can create some problems in lighter weight riders because of crosswinds. What do you know about this? I am 5'9'' and weight 145 lbs.

    ReplyDelete
    Replies
    1. Hello Christian. Yes, crosswinds can become a issue. Ideally, a rider has multiple wheel sets and can use a shallower rim or substitute a disk for a bladed wheel depending on the wind and terrain conditions.

      If only one wheelset can be afforded, I would recommend racing on something around 60mm or less.

      Delete
  12. Dave, I have two wheelsets. One is the Mavic Ksyrium Elite with bladed spokes, flat rim and the other is the Fulcrum Racing 7 with round spokes but a more aero rim. Which is better for a flat lander aero biker?
    Tom

    ReplyDelete
    Replies
    1. Tom, that's a good question. I can't say for sure. My guess is that they are close in aerodynamics.

      Besides a wind tunnel, or using an ibike Newton plus to measure the drag coefficient of each wheel set , a cheap method is to measure the time/speed/distance difference between the two different wheel sets by doing a coast test down a long hill.

      What you do is find a nice grade that you can coast down without using brakes or pedaling. The wind conditions must be the same, your starting speed must be the exact same, your riding position must be the exact same and you must travel the same line. Whichever test has the highest max speed, or shortest time over the same distance is the most aerodynamic wheel set (or greatest distance traveled over the same time).

      To be more sure, repeat the tests a few times; the closer the results match previous trials, the more confident you can be in your test.

      If you do a coast test, please let me know your results.

      Delete

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