Andrew Hamilton explains the concept of critical power, and explores recent research on using it for monitoring race fitness and event pacing MORE
Cycling aerodynamics: don’t let drag ruin your cycling performance
Whether you’re riding against the clock in a time trial, or competing in a long-distance stage race or triathlon, you need to understand aerodynamic drag in order to beat it. Cycling coach Joe Beer explains all and presents some fascinating findings on cycling aerodynamics, including his own recently collected data.
This drag is the product of the turbulent wake that a cyclist creates as he or she attempts to displace air in order to move forward. At 20mph (32kmh) a rider will displace over 1,000lb of air per minute, and the wake it causes requires a lot of work to overcome(2). Aerodynamic drag is to a speeding cyclist what any excess weight is to a hill climber – the enemy.
The drag acting on a rider is caused to a small degree by the friction of air over the surface of the moving object (man and bike). More importantly for cyclists is pressure drag, caused by a low-pressure area behind objects that are not streamlined and so creating air separation. Drag increases as the square of velocity; however, the power required increases as a cube of the velocity. For example, recent research data (3) show that to travel at 21mph requires around 190 watts; however, it requires another 110 watts just to go 4mph faster (3).
Table 1 shows the power required to ride at various speeds over 40km (24.8 miles). You can see that more power equals more speed. At 19mph you need just 7.2 watts for each mph of speed; however, this figure rises to a staggering 13.1 watts at 30mph. If you want to ride fast, you need a lot of power! For a full explanation of bicycle aerodynamics see Wilson, DG (2004) Bicycling Science (3rd Ed) MIT Press, p173-205.
If we accept that we live in an atmosphere that impedes our ability to ride ever faster, then knowledge about how to steal speed by cheating drag is a worthwhile goal. You cannot turn off drag; unlike gravity, which only comes into play if you persist in riding uphill, drag is always there to a greater or lesser degree.
As the bike is responsible for around 30% of aerodynamic drag and the rider well over 60% it is clear that the human body is the object where most focus should be targeted (4). There are plenty of aerodynamic frames, components and wheels that can and do make a difference to speed for a given power. However, the right rider position can give significantly more speed still.
For most riders, therefore, the first thing to do is to hone the riding position to one that allows power to be produced with minimal drag. Professional riders spend hours in wind tunnels at great expense to ensure that every watt created produces the greatest speed possible. Chris Boardman, former hour record holder and still ‘absolute’ hour record holder, made a career out of a super low position and trick components like bars built into the forks and sleek aero helmets.
The keys to a good aerodynamic position
Time trial bike
- A flat torso, possibly with a seat slightly forward to allow the knees to come up without hitting the abdomen/lower rib cage;
- Aero bars allowing forearms 15-20cm apart, in a horizontal or slightly downward angle, upper arm at 50-80 degrees;
- Knees coming close to the crossbar of the frame and just behind or slightly inside the elbow/triceps area of the upper arm;
- A head position allowing forward vision with any gaps behind the head filled with an aerodynamic helmet;
- A tight skinsuit, ideally with full-length arms and shoes covered by a tight overshoe.
- Drop bars allowing a low tuck for descending, solo rides into headwinds and fast group riding;
- Ability to tuck low, with horizontal pedals on fast descents for minimal drag yet maximum speed without pedalling;
- Optional mini clip-on aero bars (Spinachi or similar) to allow narrow arm position (check for legality in your chosen sport).
Changing the rider position from a full tuck, to the hoods, to riding upright shows progressive increases in comfort, but sadly rises in drag too. The fastest positions are rarely ever going to be comfortable. However, drag-beating tuck or aerodynamic crouch should never compromise power. Your speed is a product of power versus drag; less of the former is not helpful.
As part of the faster time trial (TT) or road position, narrower arms courtesy of aero bars or clip-ons represent possibly the most significant technological advance since their inception in the late 1980s. Reichenbach et al tested elite cyclists in various positions on standard bars and aero bars and they made a stark finding: although the aero bars reduced cycling efficiency by around 9 watts, the aerodynamic savings of 100 watts more than made up for it(5). This is confirmed in practice – aero bars do make riders faster when used correctly. Other data suggest that, in field tests, oxygen consumption was reduced by aero bars compared with standard drop bars (6).
Even primitive time trial bikes with cowhorn-style bars from the 1990s allow a significant 7% drop in oxygen consumption at 25mph(1). Remember that at 32kmh, two-thirds of the total power is used to overcome air resistance. Even changing your bars from standard bars to the wing-shaped aero bars (eg HED aero bar) can drop your 40km time by around 40-70 seconds, depending on rider speed(7). Wind tunnel testing shows that when a rider rises out of the saddle and fails to hold a good position, all the other potential aerodynamic gains from equipment choices are negated.
After your riding position and the right equipment choices, the next most important factor in aerodynamic efficiency is how you ride. There are huge drag reductions to be had if you ride behind another rider, in pack or catch a draft from a passing car or juggernaught. Data from testing in the 1990s showed that riding 30cm behind another rider at 20mph reduces drag by around 20% – just for taking someone’s wheel (1). This is the reason why stage race contenders rarely are seen at the front until it matters; they let other riders shield them until their climbing, sprinting or attacking legs are needed.
Technique and tactics therefore are vital to add to your ‘aero armoury’. If you can stay close to another rider’s wheel, wind drag can be 44% less than he or she is experiencing. Drop off the wheel to 2m, however, and you will experience three quarters of the drag they do (2). As speed rises, the need to hold the wheel becomes ever more important; behind a rider at 30mph, you only feel the same drag as if you were riding solo at 24mph.
Bikes have become hi-tech carbon, steel, titanium and hybrid works of art with 20 to 30 gears and some as low as 6kg in weight. Although cyclists love to buy performance, you can’t buy a better engine. You have to train smart to get that right. However, all other things being equal, the right equipment can make a difference to drag and speed.
We recently investigated various scenarios using a rider pedalling at 25mph on the Wales National Velodrome in Newport using SRM cranks. These cranks measure the rider’s power output with strain gauges mounted inside the chain ring, allowing a real measure of the work done.
Our team, including industry experts from Giant and Planet-X bikes, looked at the effects of a standard vented helmet (Uvex FP1 with 24 vents) verses an aerodynamic version (Uvex FP2 – as used by the T-Mobile team in time trials) on power, drag and speed. This was a highest drag/lowest drag comparison at ~40kmh (25mph) to see just how much of an advantage an aero helmet could provide.
For every watt that the rider puts out (approx 300 to get 25mph) the aero helmet gives an extra 5.38 metres, or to put it another way, enables the rider to go 1,614metres (1.002 miles) further in an hour. A 4% gain sounds small until you realise that it equates to around 2mins to 2mins 20secs over 25 miles. The aero helmet allows the rider to go faster for the same effort, or ride at the same speed with less effort.
There are a number of other potential gains to be had by utilising low-drag components over standard. Over 40kms the following components can produce significant time savings (7):
- Aero frame vs standard frame 1-2.5mins;
- Aero forks vs round forks 0.5mins;
- Disk + tri spoke vs spoked wheels 1-1.5mins;
- Aero position vs standard position up to 6mins.
Data available at www.biketechreview.com suggests that choice of rim type, spoke count and even if the rear wheel is covered or not, can make a significant difference. For example, a box rim with 20-30 spokes is around 30 seconds slower over 40km than a deep-rimmed or composite wheel. Our data from the track testing at the Wales National Velodrome suggest that even low-spoke-count wheels (eg 18 front/20 rear) on a box rim require 18 watts more than a disk and deep-rim front wheel combination (3).
A box-section set of wheels (18 spokes front/ 20 rear) requires 312 watts for a rider to speed along at 25mph. Throw some cash at some aero wheels and the effort gets easier; our test rider cruised at 25mph using just 294 watts.
A solid carbon fibre disk wheel and a front wheel with a teardrop-shaped deep rim provide watt-saving technology for the rider who wants to race at speed. These so called ‘deep-rim’ wheels are even being ridden in road races as clued-up teams realise that power savings on the flat, fast sections can be had with no weight penalty.
Wheels create turbulence by spokes striking the air, and they also create a wake behind the normal box-shaped rim. Both cause drag, slowing the speeding cyclist. A deep front rim keeps the air in contact for longer, reducing the low pressure drag. At the back of the bike a solid disk smoothes the air flow. Interestingly, a disk can gain a ‘sail effect’ with certain side winds actually causing forward motion like a sail on a boat.
An 18 watt saving might not sound much, but it could mean finishing over two minutes ahead of a less technologically aware but equally talented rider. However, take an even worse case scenario where a rider has 32 spokes front and rear. Such a low-tech rider could be giving away three minutes over 25 miles.
The golden rule of wheel selection is to keep spoke count low (from 12 up to 20 spokes) and the rim depth in the moderate to deep size (40- 100mm). However, the deeper the rim depth at the front, the greater the difficulty you’ll experience in cross winds. You may go faster using a rim as deep as the industry-leading 101mm Planet-X in still conditions, but it would not be advised in windy conditions. Strong crosswinds, gusts caused by traffic or courses that include unexpected side winds can make aerodynamic objects with large surface areas become suddenly precarious.
Beginners or those with rusty riding skills need to think carefully about the equipment they buy. An expensive disk and deep combination may only be used a few times if you’re uneasy about the handling characteristics they impart. What works in a wind tunnel or on a velodrome may not always be applicable in the real world.
Because the rider presents the biggest cross-sectional area to the wind, what he or she wears makes a significant difference to drag, especially when you consider that surface drag over fabrics can vary immensely. Nike is one of the pioneers, with their Swift Spin suits as used by Tour de France legend Lance Armstrong. Experts in the field of speed skating have found the Nike suit to outperform others; however, these elite suits rarely reach the general market.
The best advice for mere mortal riders is to use a tight fitting top for road racing to reduce drag, and a one-piece very tight-fitting skinsuit for time trials. The latter, without pockets, makes the body a better shape than flapping hems and pockets. The fact that elite riders racing the clock insist on a skinsuit suggests that the effects are considerable, but data is scarce. Many teams have data on items of equipment but these are ‘trade secrets’ and the only people likely to tell you how good something is tends to be a manufacturer keen to sell you its wares!
The finishing straight
Aerodynamic drag is the product of the rider and bike interaction, equipment choices and the tactics of riding. In solo events, such as time trials, you cannot draft, so you need therefore to seek out drag reductions with the best equipment you can afford. In road racing or long sportif type rides (such as L’Etape, featured previously in PP224) saving energy during flatter parts by ‘sitting in the group’ is very wise. You’ll need that saved energy when the pace or gradient increases.
Reducing drag can be cheap (with a simple change of stem and a drop in the torso angle) or expensive (£1,000 for a set of carbon aero wheels). However, never lose sight of the fact that the engine is the key to the power generation; all riders can knock off around 2 minutes over 25 miles with an increase of 20 watts in power output (see table 1 on page 2). Therefore the priority should be to:
- Build a strong cycling ‘engine’ with plenty of miles, intervals and competition experience. You cannot get away from hard work if you want to set PBs or stand on the podium. A winter’s worth of training could add 20-40 watts of power to your previous season’s power output;
- Fuel your training properly. For example, in events longer than 90 minutes, taking on board carbohydrate gels and drinks makes you faster;
- Get in the right position to be comfortable but also to reduce drag. This is less vital in road racing but essential in time trials. Once your position is correct (using aero bars), you can then add other items such as aero wheels, helmet, forks, skinsuits etc. None of this equipment is cheap, but with some careful shopping around you can go ‘aero’ without being totally broke!
The very top riders can sustain between 400 and 500 watts riding against the clock or attacking on a climb. Lesser mortals cannot buy that type of engine – sadly that’s something that you’ll have to blame your parents for. However, given the right aerodynamics, training and motivation we can all ride faster, cheat the wind a bit better and move nearer to our own limit of ability.
1. J Appl Physiol 1990; 68(2):748-753
2. Burke, ER (1991) Science of Cycling Human Kinetics (0-87322-181-8)
3. Beer, JM (2006) Power Testing Observations from Velodrome Testing (unpublished data)
4. Gregor, RJ & Conconi, F (2000) Road Cycling – an IOC Medical Commission Publication. Blackwell Science (0-86542-912-X)
5. Med Sci Sports Exerc 1997; 29(6):818-23
6. Ergonomics 1994; 37(5):859-63
7. Martin, J & Cobb, J (2002) ‘Bicycle Frame, Wheels and Tyres’ in High Performance Cycling (Jeukendrup, AE) HKPress, p113-127 (0-7360-4021-8)