Can someone explain this

I’m trying to wrap my head around how for the same ride with slightly different average powers (234 vs 235) the time for the 235 W is 12 seconds slower. Same weight, same bike, no draft or power ups. This is for the Alpe du zwift so lots of changes in the gradients. If you attacked the climbs vs flats but averaged the same power would the times be different? I’m sure it’s true but I just can’t wrap my head around it. Could someone explain it?


What was your avg wkg for both races?

I’ve wondered myself which number is more impactful wkg or watts. Since Zwift does all its categories by wkg I assume that this number has a greater impact on race times.

I’m interested to hear what the experts say as well.

Air resistance.

Increased power going uphill saves more time than increased power on flats.

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For the best efficiency, you should work the hardest when you are going the slowest. Average power doesn’t tell the whole story.


Avg w/kg was 3.217 vs 3.231 (234 vs 235 Average W respectively).

So if you are putting out more power in the flatter sections you are going faster and increasing your wind resistance. So a greater percentage of your power is lost to wind resistance when you power through the flatter sections? Thank you

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So, if I read this correctly you had a higher wkg and a higher avg watts but a slower time? You expended more energy and went slower? All due to air resistance?


A couple of the (many) sources:


This is a major principle in cycling pacing: wind resistance is exponential . In other words, the faster you go, the more resistance you’ll get. If we want to have the fastest time, we must plan for higher power output on climbs where speeds will be lower and wind resistance is less, creating a higher speed gain.


On a hilly course, it is imperative that you increase your power in conjunction with increases with gradient. With shallower gradients it’s not as important. A 5% power increase, at 200W, yielded a 16.36 second gain on a gradient of 1% over 2.5km. However at 6% that 5% rise gained the rider nearly a minute and a half (78.42 seconds). As the gradient increases so does the rider’s need to up the power from baseline.

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Steve, this is really cool stuff to know and thank you for sharing the references. As I have done most of my racing IRL I understand how air resistance works but totally clueless as how Zwift incorporates it. Do you have a reference on how Zwift increases or decreases wind resistance?
Conceptually I know Zwift must incorporate it in order for drafting to work as well as the aero and super draft power-up but power-ups aside is air resistance solely a function of speed?
So if I am going up an incline of 6% my speed is down so my air resistance is down but if I increase my power and speed on the same grade will air resistance increase proportionally?

The average power was 1 singular watt different?

Sounds like basically a rounding error, and like others have said where you put bigger efforts in counts

This article explains it well I think


Thank you guys!! That really helps a lot!! Great references and explanations. Thank you.

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