Finding An Ideal Event Position With 'The Golden Ratio' (Watts/CdA)
Right now there is something of a battle going on in the field of positioning riders on bikes. In “the blue corner” we have the aerodynamicists and the businesses with a vested interest in selling wind
tunnel time. They will tell you that aerodynamic optimisation of equipment and position is the number one priority for riders who want to ride faster time trials or triathlon bike legs on most courses.
They will point out that at race speeds small gains in terms of aerodynamic drag reduction will tend to have a greater positive effect on speed than any consequent reduction in the riders power output.
Often they are right, but not always.
In “the red corner” we have the professional bike fitters. They tend to focus on finding riders comfortable, biomechanically sound positions which in theory promote the riders ability to deliver power.
What they usually envisage when talking about power is the sustainability of power output through a particular endurance event since comfort and biomechanical efficiency
are understood to reduce fatigue – the tendency of average power output to reduce as ride time increases. Indeed we have long argued that the relative priority of comfort on the bike can only be established
having regard to the duration of the riders target events.
The aerodynamicists in the blue corner have the benefit of a scientific measure of aerodynamic efficiency – CdA – to support their argument.
Aerodynamics & CdA Primer
if you haven’t yet seen it.
They can measure the CdA of an aero position, plug it into a 'Speed Given Power' Ride Model
, and suggest how much faster the rider ought to move due to the new position, assuming always that there is no
consequent reduction in average power output. On the other hand the idea of also studying the impact on the rider’s power output in an aerodynamic testing environment, such as in a wind tunnel, has been
largely discounted. Wind tunnel time is already eye-wateringly expensive so any suggestion that the environment might be used to simultaneously study impact on a riders power output is unlikely to gain in
The bike fitters in the red corner benefit from the increasing use of power meters to support their argument.
A position choice that promotes maintenance of power output ought nowadays to
be measurable in a completely objective way on the road. For this reason we would recommend that any rider considering the investment in a professional bike fit first invest in a power meter - otherwise
how can we know if a position is truly better? Against the case of the bike fitters we have to consider that most of them will totally ignore any objective measure of aerodynamic drag pertaining to the
position they have recommended. They will speak a lot in terms of “look” and “feel” but let us remember one thing: a position that looks aerodynamic isn’t necessarily so, just as a position that feels
more comfortable or powerful comes with no such guarantees on the road. Sound bike fit is important but beware - there is a lot of snake-oil on sale out there too.
Maximising the compromise – Sustainable Watts / CdA(m^2) as an objective measure of position quality
Most riders are familiar with the concept of power to weight ratio or more specifically watts per kilo
as the standard, objective measure of a riders climbing potential. But the number that
really matters in less hilly events - time trials and triathlons for example - is watts per metre squared CdA
. This is a ratio which divides a riders sustainable power output as applicable to a
specific event duration by his aerodynamic drag metric, CdA. The resulting number therefore combines the best of what the bike fitters have to offer - sustainable power - with the best of the what the
aerodynamicists have to offer. Maximising this compromise is therefore a gold standard, objectve measure position quality which reflects both priorities - a Golden Ratio
if you like.
So what does this number look like? Earlier this year we ran some analysis on the
2011 Copenhagen Worlds Time Trial
which identifies watts/m^2 CdA estimates at the world class end of the scale, peaking at around 2000 watts / m^2. At the other end of the scale an amateur rider with a power output of 200 watts and a
poor CdA of 0.4 m^2 would register a number of 500. You can review CPM registered users data poopulation in our
The Aerodynamic Metabolic Ratio
So what is it about extreme aerodynamic positions which constrain a riders ability to sustain a certain power output? Well power is simply a manifestion, probably the best one, of everything that is going
on inside a riders body. It depends on metabolic factors such as oxygen utilisation, lactate threshold and mechanical efficiency. Review our
HR-VO2-Power Relationship Model
to explore this relationship.
Aspects of riding position which constrain power output will all constrain one or more of these variables. For example: rate and quality of breathing would constrain oxygen utilisation; restrictions on
blood flow to the legs would impact oxygen utilisation and the intensity at which lactate threshold ocurred; while suboptimal biomechanics would impact mechanical efficiency.
It is with regard for these precursors to power output that some researchers have coinned the ratio we are talking about The Aerodynamic Metabolic Ratio
. We would recommend articles by
Practical Approaches to Finding and Maximising Watts/CdA
If Watts/CdA is the the gold standard, objective measure of position quality then it must be pretty important for us all to find it and maximise it, right? But the bike fitters and a power meter alone can
only help us work on the numerator, while the aerodynamicists alone can only help us work on the divisor. Alternative, more integrated approaches are required.
The Wind Tunnel Meets the Laboratory
Imagine an ideal world, where there was a low cost, accessible wind tunnel in every city and every tunnel had an exercise physiologist on hand to measure metabolic variables such as oxygen consumption,
blood lactate concentrations and mechanical efficiency while we peddalled away at race pace in the tunnel itself. Considering what we have said above about the metabolic precursors of power output this
would surely be the ideal environment in which to expore the impact of postion choice on power output. Such an ideal world might just about be affordable by a professional super-team but consider one thing -
sooner or later we all adapt to new positions. Sure, our power output may be constrained when we take on a new, more aero position but it has a tendency to come back over time. The golden ratio then is
something dynamic and we could never keep a constant handle on it using multiple trips to the wind tunnel.
Racing & Race Simulation
An alternative way to study the ratio may be using race data or race simulation data recorded with a power meter. In this protocol the rider would experiment with different positions at race pace, we would
study his average power relative to the CdA of the position used – known or calculated using a field test protocol – and then identify the position which maximises the ratio. The risks and disadvantages with
this approach are that the rider simply has good days and bad days – he cannot be expected to deliver close to personal best race power on every trial – and so we might not truly be observing the effects of
position changes. There is also a limit to how frequently a rider could expected to ride a race pace effort, accordingly it may take a very long time to collect data through a range of positions and in this
time the riders state of training may change, introducing error to the results.
In this protocol the rider selects a fixed power output close to but below race pace – a pace that can be repeated in multiple tests even on the same day – and at that power output he rides a circuit
according to the protocol necessary for a CdA estimation field test such as outlined at
. Along with power data we also record heart rate data. Each test yields the following
- CdA. The actual CdA presented by the rider when riding under race-like conditions on the road. This is arguably a more useful CdA than a number recorded momentarily in a wind tunnel.
- Average Power & Average Heart Rate.
- Average Power / (Average Heart Rate-Resting Heart Rate). This is a proxy for metabolic stress and efficiency due to position choice expressed as watts of power produced per beat of heart rate.
- Aerobic Decoupling. We may also study any decay in the above relationship through the duration of the test run as an additional marker as metabolic stress.
Given all of the above it is then a simple matter to identify the position which suggests the highest golden ratio. All things considered this would seem an appealing and natural extension to the
discipline of aerodynamic field testing which offers our best option to maximise a riders golden ratio.
The Future of Aerodynamics?
Is it just us, or has everybody noticed the slowing down of progress in terms of aerodynamic gains coming from the aerodynamic equipment manufacturers? Diminishing returns were surely inevitable in an
area that has seen such rapid progress in recent years but what is really interesting is what will happen next. We see leading manufacturers such as Zipp
concepts such as stability, introducing the argument that aerodynamics alone is not the only consideration in component choice, rather the amount of energy a rider may have to expend (waste) in bike
handling. What they are effectively referring to is the impact of whatever component on efficiency and we can reasonably predict a significant increase in this sort of holistic approach to aerodynamics
in a similar vein to the discovery of ones golden ratio. Watch this space…