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.

Here’s a post with more hard numbers of Zwift’s draft savings, and another post with an example of this savings from a real race.

*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.)

Tools like Bike Calculator can give you good numbers here, and Zwift Insider’s speed tests can show you exactly how this bears out in Zwift over various courses, rider weights and wattages.

**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.

**The takeaway**

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.

Glad you are addressing this. I think that your math at the end in your simplified example isn’t quite right. In your example, you say that the lighter rider needs to do 75% of the puller’s 360 watts. That is not quite right. The lighter rider needs to put out 75% of the watts they would need to go the same speed as the heavier rider’s speed at 360 watts. Even on the flats that is typically going to be less for the lighter (and importantly usually shorter rider). Let’s say the heavier rider solo would do 40kph at 360 watts and the lighter rider would do 40kph at 320 watts for example. The lighter rider in the draft needs to do 75% of 320, not 75% of 360.

Joseph–yes, you are correct that the equation isn’t quite this simple. I left CdA out of it and assumed no gravity component. (That’s why I said it was a “simplified” example.)

That said, according to http://bikecalculator.com/, and as I mentioned above, on a flat it’s almost all about watts. A 90kg rider will go 39.04kph at 360w, while a 60kg rider will go 39.72. Or to put it another way–a 60kg rider would need to put out 343w to go the same speed as a 360w 90kg rider. So right between your 320 estimate and my 360. ðŸ™‚

I’m going to update the post right now so these numbers are more accurate. Thanks for the suggestion!

Thanks for taking the time to try to update things based on my response. I’m suspicious that your example numbers still may be a bit high from my experience as a lighter guy sitting in the pack behind faster guys in the A races. I wonder if the pack draft effect at race speeds is even more than a 25% benefit, which would obviously impact these estimates. Still, the overall point you are making is valid even if the numbers aren’t perfect.

Just another note: Isn’t the real equation on a flat Watts/CDa?

That being the case, I think Zwift only calculates speed based upon watts per kilo. Correct? Do we enter our height into our profile on Zwift? I can’t remember, but I think not.

Yes, they could be. The 25% number comes from my ZwiftBlog drafting tests, but I was only able to test drafting behind one or two riders. It wouldn’t surprise me at all if the group draft effect was stronger, just like the real world. But I don’t know what those numbers are!

What about going down hill? On a number of occasions I have watched riders hold or increase a small gap [<5 sec] on a 10 percent down grade when I have been putting out two to three more wkg 5.0 wkg vs 2.5 wkg. They have not been on TT bikes.

A heavier rider will be faster than a lighter rider with the same W/Kg for ALL simple power/weight calculations on the entire range of gradients (with the only exception being a hypothetical climb up a vertical cliff face where they would travel at exactly the same speed). The difference is just massively exacerbated on flats or downhills. Realistically their speeds converge well before we have to consider the hypothetical cliff face…W/Kg is not quite what many people seem to think it is.

Ahh, very useful blog.. I am new to Zwift, I chose the TT bike as that is what I had on my trainer.. I did a group ride and I was putting out much higher watts than all around me.. I was getting very sad (put off my Zwift experience).. I am going to change bikes (in Zwift) and try another group ride, see if my experience is any better.

Apparently I need to do some spin downs.

And this is why it is difficult for most women to race against men even when they have comparable w/kg. We typically get flogged on the flat and flogged again on the downhills. Uphills are the only place where normalizing w/Kg helps. Still fun to race with the guys, but have to have lower expectations!

Meanwhile on Zwift: it’s 2018 and Zwift racing category system is still based on watts per kilo, even though vast majority of courses are dead flat.

I’m one of the lucky few that experience the one-three punch, the third one being supremely bad latency to their (only?) server. Not only am i fighting to keep up with the bunch, i also constantly get teleported at the back even if i push 500 watts. It makes for a killer workout, but man, it will drive you insane after a while. If virtu go can provide a lag free experience i’m jumping ship for sure!.

WHY is there no drafting benefit using the TT bike??

Because the whole point of a TT is to not be drafting on others. That way you can test yourself easily without draft effects coming into play.

I would agree with that statement but only some of the time. There are times you just want to do a group ride with your friends and draft. Please do retort with TT riders should not do group rides b/c its too dangerous, TT bikes unstable, etc..

I’m confused. A Zwift TT bike is too dangerous?

My apologies Eric, I mean that a TT bike is frequently criticized for being dangerous in a group ride (in the real world) because it’s less stable and the brakes are not in immediate reach if in the aero position.