Bicycles and Aerodynamics - http://www.sheldonbrown.com/rinard/aero/aerodynamics.htm
| 자전거 /Tech 2010. 3. 5. 05:35Bicycles and Aerodynamics
by Rainer Pivit
published in Radfahren 2/1990, pp. 40 - 44
Translated by Damon Rinard from the original German language article at:
http://www.lustaufzukunft.de/pivit/aero/aerodynamik.html .
(Numbers in parentheses refer to the pertinent bibliography)
Other articles by Rainer Pivit published in "Radfahren" magazine:
The aerodynamic gains in bicycle racing are of great significance. While aerodynamic technology was first shown on the track in the '84 Los Angeles olympics, aerodynamic improvements in road racing had a piercing effect in the last stage of the '89 Tour de France. LeMond adhered uniquely to better aerodynamics than Fignon. Can the everyday rider also benefit from better aerodynamics?
The idea that at higher speeds the aerodynamic drag of the bicycle consumes nearly all the rider's effort is very old. As early as 1895 disk wheels were offered for bicycles to reduce aerodynamic drag. There were even wheels with 4 aerodynamically shaped spokes for the front wheel (23). This type of wheel has now become generally accepted with extreme Triathlon bicycles - made today from composite materials instead of sheet metal as they were before the turn of the century.
Restrictive Regulations
Also the idea to achieve a more favorable frontal area and thus a smaller aerodynamic drag by assuming a different seating position had already emerged before the turn of the century. Mochet's recumbent set several new hour world records. Starting from 1913 records were broken with aerodynamically faired racing cycles (5, 6). However, the governing body of bicycle racing, the Union Cycliste International (UCI), did not view these as regular records and tried to prevent any possible technical advantages to individual racers by changing the regulations. Racing should serve as a comparison of athetes, not a comparison of technology. Because of that the most important incentive to aerodynamic improvements to the bicycle was omitted going forward.
The revival of the topic of bicycle aerodynamics is due to a professor who had no more desire to explain to his students over and over again why water boils and why the perpetual motion machine is not a good idea. In 1973 Chester R. Kyle planned a project to improve the bicycle. The first measurements were made because of a bet about whether tubular tires or clincher tires were better. It turned out that for the cyclist the actual physical enemy is air resistance.
This led to quick improvements to the conventional bicycle (frame and wheel side covers made from thin sheets) and an aerodynamic shell for a normal racing cycle, which reduced air resistance by 80%.
In 1975 Kyle and Jack Lambie organized the first race for streamlined "human powered vehicles" (HPVs). Out of the the 14 participants 4 went faster than any bicycle before - if one allows riding in the lee of the wind. One year later the International Human Powered Vehicle Association (IHPVA) was formed, in order to organize independently of the limiting regulations of the UCI races, and to support the technical development of HPVs (16).
Kyle's Olympic Project
Starting in 1982 Kyle and others developed the technical configuration for the US Olympic Cycling Team for the '84 olympics in Los Angeles. Some aerodynamic components already existed beforehand, e.g. the aero helmets of the Czechoslovakian team. But now for the first time the complete system of the bicycle and rider was aerodynamically optimized. UCI regulations specify a conventional seating position and also forbid any aerodynamic accessories. Not forbidden, however, is the aerodynamic arrangement of functionally necessary components.
This means for example that covering a spoked wheel with plastic sheet is forbidden, since this has no basic function - it serves only aerodynamics. It is different however, if the wheel has so few spokes that it is not sufficiently stable in itself for racing applications and sufficient stiffness can be achieved only by the additional basic function of the disk (made from composite material). Later rules were added that continue to limit this principle somewhat; for instance the main frame must consist of 3 tubes which are not allowed to be arbitrarily broad. Wheels must have (at present still) at least 16 spokes or a full disk.
The bicycles developed by Kyle for the US olympic project, known as "Funny Bikes", were very successful at the olympics. The redesign was so broad that only a very few components remained unchanged, for example the tires. By the '88 olympics in Seol, however, teams from the other countries had nearly caught up, so the US cycling team members could no longer benefit so much from the technical advantages of their Funny Bikes.
Triathletes Improve Handlebars and Wheels
Some of the ideas that developed within the framework of the US olympic program were later offered commercially, so today normal bicycle racers can also benefit from better aerodynamics. The new ideas were adopted particularly intensely in Triathlon. Since technical regulations are not as rigid here as in conventional bicycle racing, there were some technical improvements in this area which improved the chances of victory.
The most remarkable innovation was the triathlon handle bar (Scott aero bar), which leads to a more favorable aerodynamic position and reduces fatigue on long distances. Another recent development are wheels made from composite materials with 3 to 5 broad, aerodynamically shaped spokes (trispokes). In a cross wind, bicycles with such front wheels are easier to control compared to a full disk front wheel.
Thus three trends in bicycle aerodynamics can be summarized: first of all the racing bikes according to the restrictive regulations of the UCI, secondly the triathlon machines with large tolerance of technical advantages, but with conventional rider position and without aerodynamic fairings, and thirdly the HPVs without any technical limitation.
Developments Usually Oriented Toward Racing
For the everyday rider the HPV trend is surely most interesting - why should the street rider follow road traffic laws and also the regulations of sport federations? The everyday rider would like to travel as comfortably and quickly as possible from point A to B, so to him technical advantages are very welcome. A comparison of the athletic performance of the everyday rider in traffic with a given technical regulation makes no real sense now. Therefore it is to be much regretted that the industry (and also the press) still orients itself to a large extent toward future developments in racing.
If one does not consider the regulations which limit the application of technology, efficient structures are possible. The speed record for bicycles over a distance of 200 m with flying start is a good 105 km/h (May 1986 at 2400 m altitude). The world hour record is at present 73 km/h (September 1989). Both records are held by the vehicle "Gold Rush", built by Gardner Martin, with Fred Markham ("Fast Freddy") as rider. Gold Rush has very good aerodynamics: an effective frontal area of 0.046 m2 is indicated - a twelfth of a conventional racing bicycle; the vehicle weighs only 14.5 kg (19). Vehicles such as the Gold Rush are not suited to everyday life. Besides, other vehicles quite suited to everyday life have lower drag than racing machines which meet the regulations of the sport federations.
Aero Shopping List for the Normal Racing Cycle Rider
How can the normal racing cycle rider improve his aerodynamics? The aerodynamic drag of a conventional racing cycle without the rider is a third of the bicycle and rider together (12). Thus it is already clear that it is unreasonable to ride an aerodynamically optimized racing cycle with aerodynamically unfavorable clothes (e.g. normal everyday life clothing).
The aero shopping list, ranked by Kyle (13), shows possibilities of reducing aerodynamic drag. The costs are rough estimates; the proportional reduction of aero drag are relative to a conventional racing cycle and a rider with the usual racing clothing (racing shorts, jersey, cotton socks, gloves with knit backs) and without a helmet; the time gained is relative to a 40 km time trial at approximately 37 km/h - elapsed time 1 h 5 min. At higher speeds the time gained becomes smaller because of the shorter riding time.
|
Aero Shopping List |
Approx. Cost |
Aerodynamic Advantage |
Time saved over 40 km |
Costs per % of Aerodynamic Advantage |
|
Part |
DM |
% |
seconds |
DM/% |
| Remove water bottle and cage |
0 |
2,8 |
26 |
0 |
| Tape over shoe laces |
0 |
0,8 |
7 |
0 |
| Pump under top tube instead of in front of seat tube |
0 |
0,8 |
7 |
0 |
| Shave the legs |
0 |
0,6 |
5 |
0 |
| Remove the small chain ring |
0 |
0,3 |
3 |
0 |
| Fill the front tire gap at the rim |
2 |
0,2 |
2 |
10 |
| Benotto Aero bottle with cage |
30 |
1,6 |
15 |
19 |
| Smooth nylon socks |
8 |
0,4 |
4 |
20 |
| Pearl lzumi lycra shoe covers |
32 |
1,4 |
13 |
23 |
| Aero helmet. ANSI approved. Bell Stratos. |
140 |
5,2 |
47 |
27 |
| Aero front wheel: Araya aero rim, 28 Hoshi bladed spokes, Dura Ace hub, Avocet 190 g tire |
180 |
4,8 |
44 |
38 |
| One-piece skin suit, short sleeves and legs |
180 |
3,2 |
29 |
56 |
| Aero rear wheel: as above, but 32 spokes |
90 |
2,0 |
18 |
45 |
| Aero brakes and levers, Dia Compe AGC 300 |
200 |
2,0 |
18 |
100 |
| Gloves with Lycra backs |
24 |
0,2 |
2 |
120 |
| Disk wheel in front |
1000 |
7,2 |
66 |
140 |
| Clipless pedals |
240 |
1,0 |
9 |
240 |
| Disk wheel in back |
1000 |
3,6 |
33 |
280 |
| Cinelli aero bar |
80 |
0,2 |
2 |
400 |
| Edco Competition Aero crank set |
250 |
0,6 |
5 |
420 |
| Shimano Sante aero Schaltung |
320 |
0,4 |
4 |
800 |
An economical re-evaluation of the aerodynamics of a bike and rider with two aero spoke wheels, aero brakes, aero bottle, ANSI approved aero helmet, one-piece skinsuit, aero shoe covers, gloves with Lycra backs, silicone filling the gap between tires and rims, shaved legs and pump under the top tube costs about 1100 DM. In a 40-km time trial the rider equipped in such a way is 3 min 6 seconds faster than his conventionally equipped colleague with the same power, because his aero drag is less by around 21%. The speed of the aero cyclist is 4.8% higher than his conventional colleague (the last Tour de France was won with a lead of only 0.0025%).
Anyone who wants to invest somewhat more cash in his chances of victory (900 DM) and installs an aero crank set, an aerodynamically acceptable Schaltung, an aero bar and clipless pedals, can undercut his conventional colleague's aero drag value by around 23% and thus gain a lead of 3 min 30 seconds with the same performance.
Clothing and Helmet
In (15) Kyle points out that compared to the usual combination (long sleeved wool road jersey, Lycra racing shorts) aero drag can be reduced by 7.5% with a one-piece long sleeved skin suit made from Lycra; the same suit in rubberized coating gives an advantage of 8.4%. Aero helmets, as they are used for racing, which do not however meet the ANSI safety requirements, reduce the aero drag by approximately 2% compared to a bald head or a rubber cap over the hair. The Bell Stratos, an ANSI approved helmet, increases the aero drag by approximately 1.3 % over a bald head. Short hair worsens it around 4.6%, long hair around 8.6%. The leather hairnet helmets which can often still be found with racers - although completely insufficient according to ANSI - increases the aero drag by 6.3%. The wide-spread ANSI approved helmet Bell V1 Pro gains around 9.8% compared to a bald head. So far no measurements have been published concerning the influence of beards.
Disk Wheels are Front Runners
New wheels offer the largest aerodynamic advantage with the bicycle. Disk wheels are the front runners. Problemetic, however, are the high price and the severe impairment of the steering that comes from using a disk wheel in a cross-wind. Besides, an aero steel spoked wheel's air resistance clearly can be lowered already. If possible, few spokes in radial arrangement should be used.
Bladed spokes have 85% of the air resistance of normal round spokes (13). Narrow 18 mm tires likewise reduce air resistance. The rear wheel runs in an area where the air flow is already influenced by other components. Thus the effect of aerodynamically better material at the rear wheel is not as pronounced as at the front wheel. The use of a disk in the rear wheel is not worthwhile with a limited budget; a rear wheel with aero rim and bladed spokes also does the trick.
A particular example, which comes from measurements made by LeHanneur (10), is a bicycle with conventional wheels that has an effective frontal area cwA of 0.05 m2. Roval racing wheels, however, (developed in 1977: deep rim, bladed spokes with hidden nipples, 24 spokes for each wheel) have a value of approximately cwA = 0.03 m2.
From measurements by Kyle (17), a good, spinning disk wheel (AeroSport flat disk 26"; Kyle is involved in the company AeroSport) has an aero drag 35% of an appropriate conventional 27" wheel (normal rim, 36 round spokes). However he determined another disk had a value of 54% of the conventional wheel. There are thus noteworthy differences between the individual disks. In addition, a 24" wheel with aero rim and 18 aero spokes had only 40% of the aero drag of the conventional wheel.
Comparative measurements between normal wheels, the combination with disk in the back and spoke wheel in front, as well as disk both in front and in back were executed by the editors of "Bike Tech" on a time trial bike (with a measuring procedure whose accuracy is not yet known) (22). Replacing a conventional rear wheel with a Campagnolo Ghibili disk resulted in a reduction of aero drag by around 2.8%; replacing the 26" front wheel with an identical disk reduced the aero drag by 7.1% compared to the conventional configuration.
Aerodynamic Frames
Finally we come to the frame. Investigations by Kyle (12) show aero drag is reduced by around 5% for a manned track bike with aero frame, like the ones developed for the 84 Olympics, compared to a conventional track bike. It is particularly interesting that with a light side wind at about 10� the track bike with aero frame cuts aero drag around 12%, and with a 20� side wind has around 11% better drag than the conventional track bike.
In (17) Kyle presents investigations of commercial frames. Compared with a Gios steel road frame, an aluminum Cannondale frame with rider brought a reduction in aero drag of around 1.6%; a Trek aluminum frame was appropriately even with the Gios, a Kestrel 4000 composite frame brought a reduction of 4.7% and a very complex aero bike by Gleb - this time with 32 aero spokes instead of the 36 round spokes with the other bicycles - obtained an advantage of 7%. The track machines for the 4000 m individual pursuit riders of the US team in the 84 olympics showed an aero drag reduction of about 16% compared to the Gios road bike.
Tour de France in the Wind Tunnel
In wind tunnel studies Steve Hed (18) tried a reproduction of Fignon and LeMond on the last stage of the '89 Tour de France. LeMond rode with a plunging handlebar (bull horn bars) with Scott clip on aero bar, whereby he could assume the same very favorable aerodynamic position as with a normal Scott handle bar. Additionally he wore a Giro aero helmet.
Fignon however rode without a helmet - with a pony tail - and only with the plunging handle bar. Hed's measurements show a 22% advantage in aero drag for LeMond compared to Fignon. If Fignon had ridden with his team's aero helmet, then the difference would still have amounted to 17%. The difference between Fignon and LeMond was not really quite so large however, since Fignon used a front disk wheel, and LeMond only used one with 32 spokes. Hed did not use the different wheels in the wind tunnel. In each case measurements show clear advantages for the Scott aero bar, particularly with a position where the elbows are brought close together.
Now, after all the racing cyclist stuff, where is the bicycle as a means of transport? Aerodynamics are nowadays primarily a topic for the racing cyclist. Here each aerodynamic advantage - no matter how small - must be bought for the victory, so long as the additional weight is not counted as excessive. In addition, with the everyday rider different criteria must be consulted for evaluation. For example the racer pays attention to certain clothing conventions. Racing shorts are even fashionable, but the one-piece Lycra skin suit of the racer is (still?) not.
HPV Development Could Use the Everyday Rider
Sheilds raise aero drag by approximately 5% (11), but nevertheless I would not like to omit it. Am I to ride on vacation with a stripped down bike because the panniers and water bottle would increase the aero drag by approximately 12% or 2%? No, no, that won't do. The aero developments of racing bring almost nothing for the everyday rider.
Nevertheless, there is a certain hope that perhaps wheels made from plastic with a few aerodynamic spokes can be established within the normal bicycle arena on a long-term basis. Apart from the better aerodynamics - comparable with disk wheels - this would have the positive side effect that the problem of broken (steel) spokes would disappear with wheels that are almost always badly built. Factory built wheels are technologically on the lowest level, and often they do not get a grasp on their production quality, were replaced by other operations with high tech plastic technology.
In relation to the extensive investigations into the racing cycle there are relatively few measurements of ordinary bicycles. In (8) the influence of (weather-related) clothing was determined. A rider with summer sport clothing (running shorts and sleeveless t-shirt - fresh from the gym) served as reference on a Dutch style upright bike.
In contrast to this, aero drag increases by 19% for a long-sleeved shirt and long pants. Adding a closed wind jacket it was 24% more than with sport clothing. For winter a German Federal Armed Forces Parka and gloves measured 40% higher aero drag. With a rain cape and rain trousers the cyclist became a parachute: 69% worse aero drag than the summer sport clothes. Hopefully it does not rain very often!
Nevertheless the fact that aerodynamics and rain protection can get along together shows some developments with HPVs. The every day rider can benefit from the efforts in HPV development (hopefully) on a long-term basis; however, to a large extent racing cycle development is irrelevant for the everyday rider.
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