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Home » Lose Weight Quickly » Power-to-weight ratio: check out our expert guide to this important fitness measure – road.cc

If youve ever read any books about professional cycling, youll probably have read about power-to-weight ratio: riders at the peak of their abilities killing themselves on a benchmark climb to ascertain the magic figure. And if youve spent any time sweating indoors with any of the current crop of training apps you may have come across it there too. So whats it all about? And as a non-professional rider, how important is it? Read on to find out and you can watch us demonstrate what it means in the real world in our video below too.

Its not a clever name or anything: its literally just the ratio of your power to your bodyweight. Its normally expressed in watts per kilogram (W/kg). At this point someone will normally point out in a nasal voice that the kilogram is a unit of mass, not of weight, but unless youre doing some of your training on Earth and some on the moon, thats not likely to be an issue, since gravity is a constant, near as dammit. Those people need to take a long, hard look at themselves. But I digress.

Your weight is, well, your weight. And the figure you use for your power output in watts is generally your functional threshold power, of FTP. Thats the power you can nominally put out for a whole hour. Most of the indoor training apps currently available will have some kind of FTP test for you to complete so you can assign yourself a number; its generally not much fun finding out, but its not complicated.

Well, youll need a pair of bathroom scales, but you likely already have them. And then youll need a bike thing that measures power. That might be a power meter on your bikeor it might be an indoor trainer that can give you a repeatable number. Then you just get the one number and divide it by the other one. Easy!

Well, higher is better.Im a pretty big rider at 92kg, and my FTP is around 315W, giving me a power-to-weight ratio of about 3.4W/kg. Thats fairly average.Im a good enough rider to make it off the bottom rung of the racing ladder into the third cats, but Im not exactly pulling up any trees there. Its accepted Tour de France lore that to be in with a shout of winning the race you need to have a power-to-weight ratio approaching 6.5W/kg. So a pro rider who weighs 65kg would need to be capable of putting out over 420W for an hour. Yikes!

Obviously none of us are going to win a Grand Tour, but that doesnt mean that your power-to-weight ratio is a meaningless metric. In short: yes, it matters. It matters most when youre going uphill, and the steeper a climb, the more its about your power-to-weight, which is why its so important in a Grand Tour: thats where the race is usually decided. And it matters most there because youre moving slowly.

If youre riding a bike then youre working all the time against certain forces that will slow you down. Theres mechanical friction: the rolling resistance of the tyres on the road, the bearings in the hubs and pedals, friction losses between cleat and pedal, that sort of thing. Then theres wind resistance, as you push the air out of the way as you move through it. And if youre riding uphill, youre also working against gravity.

Theres a clever tool herewhere you can input numbers for all the variables* and get an idea of how many watts would be required to ride at a given speed. By working up a couple of scenarios you can see how these different forces come into play.

First, lets assume Im riding along on the flat, on a still day, at a reasonable speed, say 35km/h. For a 92kg rider on the hoods, the calculation suggests that about 278W of input is required to keep going at that speed. Were assuming youre on your own here, as drafting another rider makes an enormous difference; we wont go into that here!

Of that 278W, by far the biggest component is air resistance at 225W four fifths of the total. Thats because cyclists arent particularly aerodynamic, and air resistance increases in a cubic relationship with velocity: if you go twice as fast, theres eight times as much air resistance. Thats why time triallists spend so much time obsessing over their bikes and riding position. It really matters to be aero when youre going quick.

Secondly, lets pretend Im grinding up a 12% hill at 8km/h. Thats also going to require me to put out 278W of power, but a lot has happened to the mix. Air resistance is almost negligible at less than 3W, but the work Im doing against gravity to climb the hill is 260W, 93% of the total. When Im on a steep hill, moving slowly, its basically all about me working against gravity. On a steep climb, its pretty close to being a direct relationship between the power from my legs and my speed up the climb.

Its interesting to see how weight affects the speed of riders of different weights on the flat, and on climbs.

Take, for example, Liam and I. Liams a svelte 63kg and Im almost half as much again. His FTP is lower than mine at 298W, but because hes a lot lighter his power-to-weight is much higher at over 4.7W/kg, and he generally smokes me on the hills.

On the flat, all other things being equal, that 298W would mean that Liam was rolling along at around 36.5km/h. For my 315W Id be able to manage nearly 37km/h, because on the flat its more to do with your absolute power than power-to weight. So even though my W/kg number is lower, I could theoretically go a bit faster. If I managed to gap him, I might be able to pull away.

However, on our 12% climb Im dead in the water: my 315W buys me about 9km/h, and Liams up at nearly 12km/h, over 30% faster than me. The climbs are where it matters, and the steeper they are, the more its about power-to-weight.

If you want to go faster, then there are two things you can do. You can put out more power, or you can reduce the amount of mass thats working against gravity, either by losing weight or spunking a huge wad of cash on a really, really light bike. Its worth pointing out here that for mortals like us, your power-to-weight using your whole system weight you and the bike is sometimes more meaningful, because you can realistically save weight on your bike too. For professional riders, everyones on a UCI-limit 6.8kg bike (or near enough), so its not really a variable. If youre a hillclimber, and you can shed a couple of kilos by sawing bits off and drilling holes in things**, that can make a significant difference to your time up a climb.

For an average rider, though, most of the easy gains are with you, not the bike. You can make an effort to lose weight, and you can embark on a training programmeto increase the power you can put out. In reality those two things often go hand in hand: as you spend more time training, your weight often decreases. When I was originally training to race my weight dropped by around 10kg, and my FTP increased from about 280W to over 300W. Those kind of gains just arent available by swapping your wheels for a set of nicer ones. Not everyone has 10kg to shed, but theres usually something that could go

A final thought: your numbers can take you so far, and in some disciplines time trialling, individual track pursuits theyre very important, but a lot of the time its one factor in among many others. Pacing knowing how hard you can push for how long is vital in a range of situations.

Take a sportive;if youre fit enough to get round in three hours but you go off much too hard from the start and blow up spectacularly halfway, then it might take you four, or you might end up being sick in a hedge and never make it to the finish, not that Id know what that was like.

If youre in a crit race then it doesnt matter if your power-to-weight numbers are the highest of the whole field if youre in 20th place coming out of the final bend for the sprint: youre not going to cross the line first. Racecraft and tactics are a huge part of doing well in that discipline. Then theres nutrition, and your technique, and conditioning, and peaking at the right time... The list goes on. Power-to-weight is a useful thing to know, but its not the be-all and end-all of riding your bike.

*The figures I used were:

Frontal area A: 0.4 and coefficient of drag Cd: 1 (from https://www.cyclingpowerlab.com/CyclingAerodynamics.aspx)

Coefficient of rolling resistance Crr 0.005 and air density Rho 1.22601 (both default figures)

**This is not recommended

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Power-to-weight ratio: check out our expert guide to this important fitness measure - road.cc

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Home » Lose Weight Quickly » Power-to-weight ratio: check out our expert guide to this important fitness measure – road.cc