Today I had to collect the car I dropped off on Monday, having had a new brake servo fitted. The ride is a fairly straight line to the north. Do you find when travelling north, and this may only apply to the northern hemisphere, that you seem to do more uphills than downhills? If you were to travel north on the M6 motorway in England there’s no way that you could imagine you’d not gone significantly up hill. We’re not climbing up the globe so it must be psychological. My trip today actually has a net loss of around 180 feet which may be nothing in a car but by bike may be significant. I’ll do a bit of maths to see what the effect might be.

If me and the bike weigh 100kg, I’ve dropped by 55 meters then the energy I’ve gained is given by MGH, mass times acceleration due to gravity (9.81 m/s2) times the drop so I gain 53,955 joules of energy. A meaningless figure in some ways but if we divide this by the time taken in seconds which was 75 minutes times 60 or 4,500 seconds then that gives me 11.99 watts! In fact that is in comparison to riding on the level. Since my journey on Monday climbed by 180 feet it required an average of 24 watts of extra effort.

Fortunately the cycling app. Strava estimated my average power output in watts for each trip so how close did it get to “the truth”? I took a different route with only 416 feet of ascent on the return compared to 693 feet on the outward so this is getting a bit complex.However, since “what goes up must come down” we could ignore this difference because, although climbing is hard, coming down is easy. I rode 0.38 miles further on today’s ride but it took me slightly longer at 75 minutes rather than 70 minutes. That’s 7.4% longer but only 2.4% further. On Monday I used an estimated 209 watts whilst today I used only 174 watts. This was a difference I could feel. I was making distinctly less effort today as a practice for my big ride in April. I wanted to know what it felt like to finish a trip with plenty left in the tank, energy I could then use to do around 3 times the distance.

I’m going to assume that the average speeds will allow me to estimate the difference in effort. I won’t go into details but my estimate is that today’s 12.7 mph was 7.4% easier than Monday’s 13.2mph. Applying this to the power figures I’d expect today’s run to reduce the 209 watts to 7.4% less or 15.47 watts less giving 193.53 watts. Taking away the 24 watts for the net downhill today gives 169.53 watts. Strava gave 174 watts. A difference between measured and calculated of only 4.5 watts. I did a little over 2 miles off road today compared to 0.8 miles on Monday which might account for the 4.5 watt discrepancy anyway. I like numbers but appologise if you hate them. Still, if you do you possibly stopped reading a while ago!

I’ve got to conclude that Strava’s estimates were really rather good, in this instance. Only a power meter could better this, though as I said in my last post, Strava seems to estimate a much greater energy use on the road than off road. After 2 mainly road rides I hope to get seriously muddy next time.

Strava’s estimated power figures range from “fairly accurate” to “rubbish” and don’t take into account a multitude of factors such as wind, road surface, change in rider aerodynamics, etc. Still, they can provide interesting numbers to ponder over. 🙂

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I think you’re definitely right about Strava power figures. In this case a lot of the variables were very similar so the estimates were good.

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