Aerodynamics On A Budget
Jeff Jones, editor of bikeradar.com, tells you how to shave valuable seconds off your bike splits without it costing the earth in this exclusive excerpt from the latest issue of Triathlon Plus (in shops now).
If you want to go faster on your bike, spend some time getting aero. By that I mean adjusting your position, clothing and equipment to reduce your drag so that you get the most out of the power that you’re putting in.
If you’re not convinced about going aero then consider the current world hour records: 49.7km on a ‘standard’ road bicycle, 56.375km on a time trial bike in the extreme ‘Superman position’, and – wait for it – 90.598km on a fully faired recumbent bicycle. That’s getting on for double the speed of a standard road bike, and it’s all down to aerodynamics, not extra power.
Now for some physics: the power you need to overcome aerodynamic drag on a bike is related to your drag coefficient (determined by shape and surface), frontal area, air density and speed. You want to lower your drag coefficient and frontal area while making sure you can still ride your bike. You’ll have to use your judgement on this as there’s no point in having an extreme position if you can’t pedal. But changes in position take some time to adapt to, so give them a chance.
The fastest and most accurate way of knowing how aero you are is to get yourself to a wind tunnel and get your drag measured in different positions. This costs at least £500 and goes up from there. A company called Drag2Zero offers this in two places in the UK.
If you don’t want to shell out the cash for tunnel time, do some field testing. This can be as simple as roll down tests (find a hill with an uphill at the end, start from a standstill, do not pedal and see how far up the other side you get using different positions/equipment). Or, if you have a power meter then the Aerolab function in the (free) Golden Cheetah software is an accurate way of comparing different positions by riding laps of a circuit.
Finally, there’s the most basic measure: if you’re faster in all your bike legs than you’ve ever been before, you’re probably doing something right! If you don’t want to measure it then don’t worry, the four steps below will help reduce your drag, no matter what.
Anything that is loose or flappy will catch the wind and slow you down big time. Air is a fluid, and obeys the same physical laws as water. You wouldn’t dream of going swimming with all your clothes on because they would drag. So ditch the T-shirt and rain jacket in favour of a tri-suit (£50 and up) or a close-fitting top (£20 and up) and be prepared to see your bike times drop.
If you ride a drop or flat handlebar bike, then you should seriously consider buying a set of clip-on aerobars. These start from around £30 for a basic aluminium pair. If you are considering buying some, check they will fit the diameter of your handlebars. Once you’ve fitted them and are happy with the position, do the clamps up tight enough, otherwise you’ll find your arms pointing downwards after you hit the first bump.
If you’ve got a dropbarred bike, you may want to lower your stem when you install the clipons. Otherwise your racing position will probably be too high. Aerobars work by narrowing your upper body and effectively reducing your frontal area. This lowers your drag and hey presto! You go faster. I’d say up to a minute faster in 10 miles if you can average 25mph, and if you’re slower than that, you may save even more time.
3 AERO HELMETS
Aero helmets may look funny, but they are a cost-effective way of improving your aerodynamics, with prices for a decent aero helmet starting from £100. They’re teardrop shaped with few vents, so if you’re susceptible to overheating then they may not be a good option. They work not by reducing your frontal area, but by changing the shape of your head profile and smoothing the surface so that your drag coefficient is reduced. In the drag equation this is just as important as frontal area.
In practice, I’ve found switching from a road helmet to an aero helmet saved around 20 seconds in a 10-mile time trial at 25mph. Again, the saving will be greater the slower you are. Fit is important so find one that fits snugly over the ears without being too uncomfortable. Don’t go for an extra large helmet unless you really need it. Secondly, spend some time adjusting the straps so that the tail sits flat to your back. Having the tail pointing up into the wind is not doing you any favours aerodynamically.
4 BODY POSITION
I’ve left body position until last, not because it’s the least important, but because it’s the most time consuming to sort out. You yourself account for between 70-80% of the drag in the bike plus rider equation. The bigger you are, the greater this percentage, and vice versa. Outside of radical surgery, there’s only so much you can do to reduce your size and shape. But if you have weight to lose, then you’ll not only gain speed on the hills but also on the flat by being thinner.
On the bike itself, the general rule is the flatter you make your torso, the better. Going lower – up to a point – will bring your head down so it shelters your upper torso, and reduce drag further by shrugging your shoulders and tucking your head in.
If you have adjustable tri-bars then there are a few more rule-of-thumb tweaks you can perform. Firstly, set your elbow pad width according to your size. If you’re of a smaller build (under 5ft 8ins), then go narrow to divert the air around your body.
If you’re taller than 6ft, set your pads wider so that your arms are shadowing your thighs and the air flows through your chest cavity rather than around it. If you’re in between then experiment with what’s most comfortable. Secondly, adjust the tilt of the bar extensions so that they’re flat or point slightly upward. A downward tilt can work aerodynamically (I’ve seen some riders come out of a wind tunnel with down-sloping bars) but at the expense of handling.
Hopefully I’ve convinced you of the importance of being aero on the bike and that you don’t need a big budget to do so. Clothing, aerobars, helmets and body position form the bulk of the aero equation, and by making some smart adjustments to these areas you can really save minutes. If you’re prepared to put in this time and effort, I guarantee it will pay off with faster bike splits.
on Tuesday, February 15th, 2011 at 5:30 pm under Cycling, Triathlon Training.
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Tags: Cycling, Hot, Triathlon Training