We bang on about it all the time – and we’re right to. How much your bike weighs is crucial, argues James Spender
‘A rider, said Anquetil, is made up of two parts, a person and a bike… and in climbing the thing is to make the bike as light as possible… So at the start of every climb, Anquetil moved his water bottle from its holder to his back pocket.’
I’ve always loved this tale from Tim Krabbé’s The Rider. I have no idea if it’s true, though I can quite imagine this happening in an era when riders stuck pins in their legs to stop cramp and only raced on tubulars that had been ‘seasoned’ – stored in a dark room for several months in the belief this hardened the treads, thereby making for faster, more puncture-resistant tyres. But I digress. The point is, Maître Jacques was onto something. How much your bike weighs is absolutely crucial.
Let’s consider Anquetil’s situation: climbing. Now, I did maths at A-Level, which was a long time ago, so please cut me some slack, but in mechanics I know that ‘work = force x distance’. Force is defined as mass (in kg) multiplied by gravity (9.81m/s²), so the equation for the energy needed to climb a mountain is ‘work = mass x gravity x height’.
Imagine scenario A: Jacques (70kg + 10kg bike) is climbing Mont Ventoux (vertical gain 1,601m), that means his work done is 80 x 9.81 x 1,601 = 1,256,465 joules. But we are cyclists, so we don’t know what joules means. But we do know a watt is the equivalent power of a joule per second (J/s), so if Anquetil is to make his ascent in a respectable 70 minutes (4,200 seconds), his power output must average 1,256,465/4,200 = 299W.
Now let’s imagine scenario B, in which we add a kilo to Anquetil’s bike and ask him to make the same time. Suddenly his required power shoots up by a whopping… three watts. Ahem. Not as much as I thought. Still, it’s a useful takeaway: the relationship between weight and watts is directly proportional, so if you increase weight by X%, you increase the necessary wattage by X%.
The spin
Let’s consider the energy cost of weight in terms of food, because fast riders are fuelled riders. A kilocalorie (how we tend to measure our food, erroneously known as a calorie) is roughly 4.2kJ, so in scenario A, Anquetil needs to eat around 300kcal to crest Ventoux, or about 200g of pheasant and a glass of champers. Scenario B Jacques needs a whole 3.74kcal more, that’s… a twelfth of a ginger nut. I can see I’m not convincing you. But let’s move Anquetil�
