Category Archives: Aerodynamics

Top 5 lessons from the Symposium on Cycling Optimisation

1.  Aerodynamic drag versus Gravity drag

You’ve probably heard that the power required to increase your speed goes up disproportionately with the increase in speed.  Here is how the power required to push you and your bike through the air is calculated.

  • Power = ½ρV² x CdA
    • ρ = air density
    • V = Velocity
    • Cd = Coefficient of friction
    • A = Frontal area

Energy model for ITTKnowledge is power, and Sebastian Weber, Sports Scientist at Cannondale-Garmin, explained how they model aerodynamic drag and gravity drag on a parcours to determine their equipment and race strategy. For example, based on the amount of climbing to be done and the expected wind conditions, they decide whether to use their lightest wheels or those that are most aerodynamic.

In fact, they run the model for each rider on each parcours, as their riders’ power/weight and power/CdA differ greatly.  They use this modelling to determine which riders they will use in a one day race and the role each rider will play each day on a multi-day stage race.

For non-professionals, we can select the races or sportives we ride based on analysing our own power/weight and power/CdA.  We can also figure out pretty quickly that pushing hard on a downhill to go from 30 mph to 31 mph is a VERY poor way to use our energy.  We will be MUCH better served by using our energy to go from 10 mph to 11 mph when climbing!

2.  Don’t waste money on an aero road bike  

(We can gain you 1 – 3 mph without a wind tunnel)

The number 1 thing we can do to make you faster on the vast majority of courses is reduce your profile and resistance in the wind.  If you look at the variables we can control in the equation above, there isn’t much we can do about air density, and we want to go as fast as our power will take us, so the things we can control are our frontal area and the drag created by our clothing.

cervelo-S5-review-front-rear-streamline-view02What percentage of the frontal area on the rider to the right is the rider, and what percentage is the bike? (over 80% of aerodynamic drag is from the rider, and, of the remaining 20%, half the drag is from the wheels) Now, how much money should you be spending on buying a bike with more aerodynamic tubes versus improving your own aerodynamic position on the bike?

Dr Andy Fronchini, who consults with British Cycling and many professional teams on aerodynamics, put together a list of the top 100 things you can do to go faster through the air.  Number 1 is improve your position on the bike, and number 100 is buy an aero road bike.  In other words, aerodynamic tubes, the positioning of your brakes, and other bike tweaks are the most marginal of marginal gains.  Of the aero losses due to your bike, 50% comes from the wheels, so if you’re going to spend money on faster equipment, spend it on the wheels.

Tommy's aero position comparisonThe bottom line is you should spend your money on improving your aerodynamic position.  If you do time trials, then TT bikes are optimised to get you as aero as possible.  How you are set up on your TT bike, however has to be optimised for the type of time trial you are riding.  You must be able to sustain your position for the duration of your race to benefit, so you can be much more aggressive in your position for a 10 mile TT (25 minutes) than you can for an Ironman triathlon (6 hours).

On road bikes, use your drops and get your head down.  We use an iPad app that allows us to accurately measure the area uncovered in the photo.  In top image it was 0.163 square meters, and in the bottom image it was 0.129 square meters.  This is only the rider’s head, arms and torso, so, conservatively, we improved the total area including his legs and the bike by 10%.

In this case, we are working to improve his speed in the sprint at the end of a race, so he is already travelling over 50 kph (30 mph).  With every thing else fixed, a 10% decrease in frontal area means he needs 10% less power to reach the same speed.  So, using the formula above, keeping the same power will increase his speed by 2.8 kph (1.7 mph).  That can easily be the difference between winning and coming fifth.

For you, we can improve your frontal profile and quantify the benefit it will deliver.  At Sportive speeds, you’ll gain up to 5 kph (3 mph).  We have added this service to our business, and at £75 it’s the most effective money you will ever spend on going faster.  So, rather than buy that aero bike you were thinking about, get a proper aero fitting and invest in some fitness testing and structured training!

3.  Periodising Nutrition can train fat burning

Your aerobic system is what powers over 90% of your cycling effort, and it burns a mix of fats and carbohydrates. However, your fat burning system will atrophy if you continuously fuel the aerobic system with carbs.

Feedzone-BooksWe have all been influenced by the sports nutrition companies into believing we must fuel our cycling with products we buy from them.  It turns out that simply isn’t true, and by continuously feeding our bodies carbohydrates, we can actually harm our long term development as athletes.

Team Sky and Cannondale-Garmin have both publicly acknowledged they periodise their nutrition strategy to optimise fat burning.  In the off-season, they use fasted and bonking rides to force the riders’ bodies to burn fat.  This stops once racing begins,  but even then they prepare “real food” and supplement that with gels and powders, rather than the other way around.  The books pictured are co-authored by a chef who works for professional teams, and they are filled with recipes for real food to fuel your riding.

For you, think about what and when you eat on the bike.  Your body stores enough glycogen in your muscles and liver for around 90 minutes of sub-threshold effort, so you don’t need any food for rides under 2 or 3 hours if you’ve had a meal before you ride. Occasionally, you can ride beyond your glycogen stores (bonking) and force your body to burn fat exclusively.  It’s not the most pleasant experience, but it will improve your fat utilisation for subsequent rides and your racing/sportive season.

4.  How differences in Women’s anatomy affect their bike fit

male and femail pelvisesYes, we all know there are differences between women’s and men’s anatomies, but it the bits you can’t see that make the difference between women’s and men’s bike fitting.  The two primary factors are the differing shapes of their respective pelvises and women having greater joint mobility than men.

While I knew women’s sit bones were further apart than men’s, I wasn’t aware that women’s sit bones are far less pronounced, and their pubic arch comes lower.  As a result, women carry more weight on their pubic bone than men, and, in general, women cannot not get as aerodynamic as men.  To help address this, saddle choice is vital for women, and the nose of the saddle must offer more pressure relief.

women's bar widthWhen it comes to joint mobility,  correct handlebar width is critical with women. Bars that are too wide require more work from the muscles and tendons to hold the rider’s upper body in position.  The measurement to the end of the shoulder blade should be matched by the centre line to centre line measurement of the handlebars.  With the arms reaching straight out from shoulders down to the handlebars, bone structure takes far more of the load, and that will help eliminate the upper back and neck pain many women cyclists experience.

5.  Shorter Cranks is the answer.  What was the question?

Power versus crank lengthAccording to Dr Mark Timmerman, who consults with Trek Bicycles on medical issues associated with cycling, shorter cranks help alleviate or prevent a variety of knee, leg and hip problems.  I’ve quoted him in the heading for this section, “Shorter cranks is the answer. What was the question?”

But don’t longer cranks help you generate more power? Tradition is a difficult habit to break, and traditionally cyclist have used cranks ranging from 170 – 175 mm in length.  A seminal study by Dr Jim Martin at the University of Utah,, concluded that shorter cranks actually offer a benefit in power production.

So, hip flexion and knee angle at the top of the pedal stroke, which are vital for your health and performance, are both are improved with shorter cranks without compromising power. In addition, shorter cranks allow you to get lower over the front of your bike by reducing the amount by which you close your hip angle, so you can be more aerodynamic.

Improved power, better aerodynamics and less likelihood of injury all result from shorter cranks.  I’m riding 165 mm cranks now, and hopefully you’ll soon be using shorter cranks soon as well.