Aug 10
4
Power to Weight Ratio
Why does it matter?
I never have really paid much attention to this concept, basically because my weight over the years has vary rarely changed. I would always be about 80kg in the off season and then 78kg going into a race.
Back in 2008 in the days when I owned a power meter and my Functional Threshold Power (FTP) was 330 watts and at the time I weighed 80kg. This gives me a power to weight ratio of 4.1watt/kg. This basically means at threshold I can produce 4.1 watts of power for every kilogram I weigh.
Fast forward to July 2010 and since I have completely stopped running and swimming, but I still ride, my weight has snuck up on me (things change once you hit 40, LOL) and I recently jumped on the scales to see 86kg!!!!! Not good I know. The thing is though I’m still fit, but I’m riding much slower at threshold efforts, why?
Well basically assuming my power to weight ratio is still the same I now need another 33 watts on the bike to keep maintaining my 4.1 watt/kg power to weight ratio. The reality is I just can’t pluck 33 watts out of thin air, so instead I find myself riding a good 3-4 kph slower at the same level of effort, ie threshold effort. This is all because of weight. The speed I notice is on a flat section of road, when there is no wind and I ride the same section at least 2-3 times PW. So over time you get a fair idea of what you can hold.
I then asked my training partner, who is my height and is around 78-80kg what his new Cannondale road bike weighs. His reply was 7kg fluro. So I return home chuck my Karbona on the scales and it reads 10kg exactly. What the hell, that’s another 3kg I’m pushing around which basically means another 3 x 4.1 = 12.3 watt/kg I need in power. So all up I need probably close to an additional 40 watts of power just to stay with my training partner, who I had no trouble staying with 6 months ago, when we were both 80kg riding clunkers! LOL.
What does this all mean?
Well for the first time in my life I now know what happens when you put on weight. It’s harder, it’s much harder, to maintain your normal speeds, you may be just as fit and have just as much power, but if you’re heavier, it WILL slow you down. That’s what has happened to me anyway.
The next logically step then is to lose weight. I took some crap off the bike and now it weighs 9kg, cool, that is an instant 4-5 watts I don’t need to worry about. In just 2 weeks I’ve dropped 2 kg and I’m now down to 84kg, and I can honestly say I already feel 1-2 kph faster. It doesn’t feel any easier riding at threshold but it is definitely FASTER. In summary being 2kgs lighter plus having my bike 1kg lighter I’m now pulling back 3 x 4.1 = 12.3 watts, that is a big saving in terms of the potential speed you can hold.
So my little experiment is going to continue, following on from the BMI discussion, I’m going to use this time, while my training is in limbo somewhat, to see how low I can go before I lose POWER. The first step I want to do is get back down to 78kg, borrow and power meter, test my FTP and then re-establish my power to weight ratio and my BMI. I’m then going to keep dropping my weight, keep testing my FTP and see how low I can go before I start to lose power. From that point onwards I’ll probably up my weigh again, but only slightly to ensure I’m in a healthier weight range.
Does anybody want to predict how light I should go? I’m 187cm tall.
Stay tuned
Fluro
P.S Rick can I borrow your Powertap every few weeks???? LOL
