ProxyPower ™ explained

Title ProxyPower ™ explained
Lead Curious to understated how our Power+ software can calculate your power output without needing a dedicated Power meter? Read more to understand the logic
Image proxy_explained.jpg
Author (Boffin) Bob
Author Url @bike_boffin_bob
User 18
Id 4
Timestamp 1,411,122,256
Edited 1
Active Yes


When it comes to setting your training zones for cycling there are two choices, heart rate zones and power zones. Heart Rate measurements are accessible for a nominal outlay of about £100.00. Power meters for bikes start at around £600.00, and for the top of the range power cranks you are looking at nearer £3000.00. Power meters are pretty accurate, within plus or minus one percent as a rule. They self calibrate for the most part to compensate for temperature effects. Heart rate on the other hand, particularly HRmax, can vary by 10 bpm fluctuating with temperature and other factors, such as fatigue, illness or simply what food and drink has been consumed e.g. coffee and alcohol. So it makes sense, where possible, to train to power zones rather than heart rate zones.

One of the advantages of turbo training is that external conditions, such as wind and terrain, that cause variations in the speed that can be achieved for a nominal power value, are taken out of the equation. So it becomes possible to equate rear wheel speed with the force (power) being applied to the pedals.

There are still two sets of variables in play however and these are the characteristics of the turbo and the characteristics of the rear wheel and its “interface” to the turbo roller.

Let’s start with the characteristics of the turbo. Most turbo trainers have two forms of resistance, one is a fan (air resistance) and the other is either fluid dampening or magnetic dampening. In addition there is the inertia of the moving parts which sometimes includes a flywheel. Of the resistive forces generated by these factors some will have a small temperature dependency, for example, when starting from cold the dampening fluid will probably be more viscous than when it is warmed up. Magnetic resistance is pretty much devoid of temperature dependency. So what we find with some turbo trainers is that the relationship between the speed of the wheel and the amount of power required to maintain that speed changes in the first ten minutes or so of use. It becomes more repeatable when the temperature of the components reaches a stable equilibrium. These characteristic tend to be repeatable day to day, session to session, so we can use them with confidence when constructing a function to map “speed” to “power”.

Now for the rear wheel and its “interface” to the turbo roller. This is a much less “well behaved” variable because it is influenced by the tyre pressure and the amount of pressure put on the tyre by the turbo roller. The tyre pressure should be the same for every turbo session, but this too will be influenced by temperature. The turbo roller pressure should be set up the same for every session. Some turbos don’t have any adjustment for roller pressure, some do. One way of testing for a repeatable set up is to do a roll down test. This is explained in our “how to set up your turbo trainer” article. One way to avoid this issue all together is to use a direct drive turbo trainer where the back wheel is not used at all. The turbo trainer has an axle with a cassette on it that is fitted with a quick release, and the rear triangle of the bike fits to this as an alternative to the rear wheel. If using this type of turbo trainer for ProxyPower ™ you need to ensure that the flywheel within the turbo trainer has a built-in magnet so that a speed measurement can be taken.

So now we come to the point where we can explain how ProxyPower ™ calculates a power number from wheel speed. So for each turbo trainer there is relationship between power applied to the pedals and the speed that this will produce at the wheel. If we use a bike with a power meter on a turbo and do what is effectively a ramp test we can record the power and speed data as a set of number pairs. We collect data for power numbers between 50 watts and 600 watts, roughly speaking. Once we have collected the data we can put the numbers into a spread sheet and derive a set of coefficients for an equation that maps speed back to a power number. It’s not exact power, but given that the data were collected using a power meter it’s usually within one or two percent. We haven’t yet tested every type of turbo trainer in every resistance setting, so for those that we haven’t yet tested we have used generic values. If using a turbo trainer that we haven’t tested please set it up as described in our “How to set up your turbo” article, and use the manufacture’s lowest resistance setting, and then use the gears to increase the effort required to drive the turbo, just like you would on a road ride. (Drop us a line with your turbo make/model and we will make it a priority to test)

Before using Power+ with your turbo, you should select the turbo manufacturer and type (including setting if the turbo is adjustable) from the dropdown options in your rofile under "My Equipment". This is vital if you don’t have a Power Meter and you will be using ProxyPower ™ .

For ProxyPower ™ to be accurate it does require that the turbo be set up as near the same for every workout session as is humanly possible. Using a direct drive turbo trainer is much more reproducible than using a conventional turbo trainer that relies on the rear wheel and a roller, as the whole set of variables associated with tyre pressure and roller pressure are eliminated when using a direct drive turbo trainer. However, training with ProxyPower ™ and power zones is always going to be more real time than interval training with heart rate, as heart rate will always have lag compared to effort and recovery. If you want to verify this for yourself then try this on the turbo: do five 30s intervals with two minutes rest and record you speed and heart rate. Upload the file and look at the speed and heart rate versus time graph. The traces will both have peaks but the heart rate peak will lag the speed peak by about 20 seconds. Then do the same test using ProxyPower ™ recording power and speed. This time on the trace the peaks will be pretty much coincidental.