CLEAN UP TO POWER UP

James Bracey discovers a selection of low-hanging watts

- James Bracey

The bicycle itself is a very efficient system – the design is solely focused on taking a rider’s input and translating it into forward momentum. And it’s a job it does incredibly well.

Indeed, maximum efficiency of energy transfer from the chainring to the cassette is as high as 98.6 per cent, according to a study carried out by a group of researchers at John Hopkins University.

This study was carried out in a clean laboratory environment. The real world has far more factors that can impact efficiency. The researchers – led by James B. Spicer in 1999 – concluded that once the real world gets in the way, the 98.6 figure might come down to nearer 80 per cent.

Let’s put that into a form most riders should identify with: if a rider is pedalling at a constant 100 watts of power, at its most efficient the drivetrain will convert those 100 watts of energy into 98.6 watts at the rear wheel, whereas the least efficient drivetrain only converts 80 watts – a difference of nearly 19 watts. Ride at a higher wattage and those differences can be staggering.

So when we are all too quick at considering an expensive upgrade in the hopes of saving a handful of watts over the course of a ride, should we instead be looking at how we can maximise the efficiency of the simple drivetrain?

From the research of Spicer and his team two factors have been identified as to having the most impact on drivetrain efficiency: sprocket size (chainring and cassette) and chain tension. So how can we apply this in the real world in order to maximise our own efficiency?

Sprocket size

If you want to be efficient you need to think big. It seems that when chain links have to wrap around larger cogs the less acute angles created between each link reduces friction. Less friction means greater efficiency.