POWER CADENCE

FROM CYCLINGPOWERLAB

One of the mistakes most frequently made by amateur cyclists taking on difficult challenges in mountain environments, the Etape du Tour for example, is that of arriving at the event with insufficiently low gearing. All too many riders arrive at a course they have never ridden before relying on the assumption that a couple more teeth on their easiest sprockets and a good dose of bravery will see them through the task of riding a half day challenge that includes 2 or 3 hard alpine or pyrenean climbs. Writing from experience, having mashed our way up the 13% Col de Marie Blanque with a minimum gear of 39x25 and more importantly on a distinctly amateur power to weight ratio which afforded a cadence of 40 RPM we can say that such a lack of forethought is inadvisable. It's also completely unecessary since a power meter equipped rider has the essential tool to predict speed and cadence on any terrain.

Speed Given Power

The speed we can expect to ride uphill is mostly defined by 3 factors:

The power output we can sustain for the length of the climb. And let’s be honest here – the number we select isn’t going to be the same as our time trial power on the final climb of a difficult sportive with perhaps 50k of climbing already in the legs, the searing heat of the afternoon, possible dehydration, and altitude effects thrown in for good measure.

The gradient of the climb. When selecting gearing the important consideration is not the average gradient but the steepest gradient that persists for any more than a few metres.

Our weight. This includes the rider, the bike, the water bottles, the spare clothing and anything else we have to carry that the pros usually don’t.

Now given those 3 factors we can calculate the minimum speed we might expect to be riding in an event. All we need is a model of speed given power of the type that can be found throughout this site.

Cadence Given Speed

We also need to know how the speed that comes out of the power model translates to cadence and perhaps torque or pedal force given available gearing. Only then can we know that the drivetrain choices we propose to make include gears that will be comfortable or even optimal. 

The internet is awash with bicycle gearing calculators. Quite why the world needs so many tools to calculate gear inches, metres of development/rollout or gain ratio we aren’t exactly sure. But as far as we know there is no calculator which allows the user to start with the 3 factors above and then calculate possible cadences from expected speed. This calculator will do exactly that and is accordingly a unique tool that allows the user to make smart, science based gearing decisions in preparation for any event that incorporates a significant climbing challenge.

Torque & Pedal Force

In addition to evaluating Cadence given climbing speed and gearing (briefly, if you climb at X kph then your cadence is X / 1000(metres) / 60(minutes) / metres rollout) this calculator can also return Torque in Newton Metres or pedal forces given the initial power input and the cadence output. We calculate pedal force in terms of “Average Effective Pedal Force (AEPF)" where average applies through one revolution of the crank. 

AEPF is a measure of pedal force which has become popular through its adoption in the “Quadrant Analysis” promoted by the Peaks Coaching Group but we include it because we feel it can offer a more intuitive meaning in terms of “How badly will I be mashing a big gear?” than more abstract measures of pedal force such as torque. AEPF is measured in Newtons (the Newton being a standardised unit of force) but it could also be interpreted in kilos of weight (aproximately 10% of the measure in Newtons) being forced onto the pedal.

Minimum versus Optimal Cadence

The primary goal of this calculator is to assist with the selection of apropriate minimum gearing in a climbing situation but we would encourage riders to think also in terms of what may represent optimal cadence because another benefit of riding with a power meter is the ability to test for optimal cadence. The most common way to do this is to complete a field test, such as a 20 minute effort, at a self selected cadence and then to repeat it at higher and lower cadences. In this way a potentially more optimal cadence in terms of the ratio of power to heart rate response can be identified. The process of identifying optimal climbing specific cadence should be executed in a climbing environment.

Inputs

• Sustainable Power Output (Watts). What do you expect can be maintained on the climb in question. Be honest. Factor in fatigue, heat, dehydration, altitude and any other potential issues.

• CdA. Specifies a riders aerodynamic drag profile. Use the estimator to specify a value - this is far less important in a climbing model such as this.

• Weight (Rider + Bike, Kilos). An accurate measure of weight is essential to model a rider on a climb. Include the bike and any otjher items you may he carrying such as water bottles.

• Chainring Teeth. Select one of the available values. This is an important input for gearing calculation.

• Tyre Radius (metres). Select one of the available formats. This is an important input for gearing calculation.

• Crank Length (mm). Select a crank length. This is relevant to the calculation of Torque and AEPF.

• Show Output. Choose the table of data you wish to see.

Outputs

• Speed (Kph). The model calculated speed of the cyclist on the specified gradient.

• Cadence. The cadence implied by the speed and gearing (as a combination of chainring, tye radius and sprocket choices.)

• Torque (Nm). The torque (in Newton metres) implied by the power and cadence.

• AEPF (N). The Average Effective Pedal Force (in Newtons) implied by the power, cadence and crank length.

• AEPF (Kg). The AEPF divided by 9.81ms (the gravitational constant) such that AEPF can be considered in Kilograms or "weight on the pedal".