With “Zwift season” starting up here in the Northern Hemisphere we’re seeing a lot of new Zwifters coming on board. Many have little or no experience with riding with power–that is, measuring your wattage output and understanding how your watts effect speed in solo and group ride situations.
Fortunately, Zwift behaves very much like the real world in terms of physics, so any understanding you may have of outdoor cycling physics will translate to Zwift, and any additional understanding you may gain from Zwifting will transfer outside.
One common question people ask, especially in race situations, is “Why am I getting beat by people who are putting out lower watts per kilogram?”
There are several answers to this question, but first make sure you understand how Zwift is calculating your speed. Secondly, understand that watts per kilogram (w/kg), or power to weight ratio, is your current wattage (the power you’re putting into the pedals) divided by your body weight in kilograms.
For example: I weigh 84kg, so if I’m putting out 300 watts my w/kg is 300/84=3.57.
Now, back to the question: “Why am I getting beat by people who are putting out lower watts per kilogram?” Here are two reasons why this may be happening…
#1: You aren’t staying in the draft
Drafting in Zwift results in a power savings of approximately 25% (possibly more in large groups.) In other words, a drafting rider only needs to put out 75% of what a non-drafting rider is putting out to maintain the same speed.
Note: you cannot draft in Zwift while using the TT bike. Others may draft behind you, but you will not receive any draft benefit if you choose to use the TT frame.
#2: You’re a lighter rider on flat terrain
Watts per kilogram is a good pace metric for climbs, where gravity is the main thing slowing you down. But pure wattage is a better metric for flats, because once you get moving on a flat your speed is mostly determined by wind resistance and the power you’re putting down.
Because of this, when we take drafting out of the equation on a flat course a heavier rider will always be faster than a light rider when both are doing the same w/kg–because the heavier rider is putting out more watts. (This is true outside, too.)
The one-two punch
Where things really get tough is when #1 and #2 above both apply to you. If you’re a lighter rider who gets dropped from the pack on a flat course, you’ve got your work cut out for you to catch that group again!
Here’s a simplified example with numbers pulled from bikecalculator.com: let’s say you are a 60kg rider in a pack being pulled by a 90kg rider who is putting out 360w (4 w/kg). The pack would be travelling at 39kph. Since you are drafting, you only need to put out 75% of the 343w it would take you to travel that same speed solo. That’s 257w, which is 4.28w/kg. Don’t miss this! Just to maintain your position in the draft your w/kg is already going to be higher than the heavier rider who is pulling.
Then if you get dropped and lose that 25% power savings, so you’ll need to put out 343w just to keep pace with the group. That’s a whopping 5.72 w/kg you’ll need to put out, while the guy pulling in the pack is still just doing 4 w/kg. Couple this with the fact that others in the group can rotate through and take pulls, and you’re going to have a very hard time catching the pack in this situation.
So there you have it. Just like riding outdoors, lighter riders will be more challenged on flat courses, and you want to work to stay in the draft.