Zwift’s “difficulty setting”

Here’s a very interesting article with, for me, some initially counter-intuitive outcomes – until I thought about it more.

For context, I’m using the Smart Tacx Flow as an alternative to my non-Smart i-Genius trainer (a contradiction in terms…).. The Flow goes up to 6% incline and 750W. No motor for downhill, but it DOES reduce the flat riding resistance which can be felt to about 3% downhill, and then it stays the same. No motor means if I stop pedalling it eventually stops whether up or downhill. For my money it’s great value. I reckon it’s swings and roundabouts on 6% max and no motor compared to I-Genius. I work harder on big downhills but less hard on steeper hills. The Zwift “difficulty setting” offers a way to make the 6% hills and above feel tougher…

The article at http://zwiftblog.com/using-the-trainer-difficulty-setting-in-zwift/ describes resetting the resistance levels when using Zwift to increase or reduce the multiple of training effort for any given slope, WITH NO CHANGE TO THE WATTS REQUIRED. How can that be, one might ask? Surely, if you increase the resistance multiple for any hill, thereby increasing pedal pressure (the intention of the change (in my case) to circumvent the Flow’s 6% max slope simulation) then isn’t the power (pedal force X speed = work rate) increased, or do they reckon because you are likely to reduce cadence (for a given gear) as a result, the speed diminishes in proportion with the increase in pedal pressure?

Having a PhD in Theoretical Physics means I’m a little ashamed of myself even to have a momentarily different intuition initially! But people of my age who did Applied Maths at school ought to remember the old Horsepower formula HP = Pv/550 (in appropriate units). Power is proportional to the multiple of force P and velocity v. It’s why some sports car manufacturers used to be so fond of highlighting their cars’ horsepower, because by making their engines rev at very high revs (enhancing engine wear no end) they could advertise very high horse-power. Very UNusable power, given how high you had to rev in all the low gears to get performance. The American way was to make engines bigger and bigger (427 cubic inch (7 litre) V8 preferably) so that TORQUE (turning force) was very high and the engines were just ticking over at quite high speeds, generating power at much more usable revs. Even smaller diesels (whose fuel characteristic is different from petrol) produce high torque at low revs too, compared to lighter, faster revving petrol engines.

As for cycling, and using the Zwift “difficulty setting” to change the perceived resistance, I guess, as the article says, just like the automobile analogy, it’s really a cadence control. You push harder on the pedals, say, but at lower revs, so the power is the same. Watch those quads grow!

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