Sprint-Coast/Microbursting Speed Tests: are Zwifters cheating by pedaling in spurts?

There’s been a lot of talk in the Zwift Racing community recently regarding “microbursting”, aka “sprint-coasting.” Today I want to share some test results and open up a conversation about whether this riding style is cheating… or just odd. Let’s get to it!

What Is Microbursting?

There is no precise accepted definition for microbursting, but it can be described in very basic terms as short, repeated intervals modulating between sprinting (high power) and coasting (low or no power).

WTRL recently added a microbursting prohibition to the Zwift Racing League rulebook. Section 7.1.3 says:

WTRL further defined what they consider to be “illegal” sprint coasting in this Facebook post:

Microburst Testing

Microbursting isn’t an easy thing to test. If you do it “manually” (with your legs) it’s hard to have the precision needed to prove whether or not the technique gives you an advantage. And if you automate it with a bot, there are only certain power/interval combinations you can accurately replicate due to the tools available.

There’s been some confusion in the comments on this post, so I’m adding a note to explain that my goal here wasn’t to test sticky watts, which clearly give riders an unfair advantage. Although associated with microbursting, sticky watts are not the same as microbursting. Because, while sticky watts are usually triggered via microbursts, plenty of trainers/power meters let you do microbursts without sticky watts!

My goal here is to test if microbursts themselves exploit Zwift physics and provide some sort of unrealistic speed advantage.

After attempting to test it with my legs, my conclusions were that 1) there didn’t seem to be any obvious advantage, but also 2) I couldn’t get the accuracy needed to prove this. So I devised several automated tests.

The basic idea of these tests is to compare a steady-state rider’s performance with microbursts that average out to the steady-state rider’s power. For example: if a rider holds 300W steady, will they go as fast as a rider who is alternating evenly between 150W and 450W? Both riders finish with the same average power, but their efforts look very different.

Here are the results of my tests…

Flat Solo Rider Tests

The first set of tests used a solo rider (our standard 75kg, 183cm tall bot on the Zwift Carbon bike with 32mm carbon wheels). I tested the rider on the Fuego Flats Reverse segment at steady 300W power, then at various microburst intervals. Here are the results:

• 300W Steady: 10:36.23
• 600W to 0W in 1-second intervals: 10:31.52
• 500W to 100W in 3-second intervals: 10:31.46
• 600W to 0W in 4-second intervals: 10:32.66
• 550W to 50W in 2-second intervals: 10:31.01
• 350W to 250W in 10-second intervals: 10:33.87*

Zwift’s Pack Dynamics v4 includes a CdA bonus for riders who are “attacking.” Specifically, if you are not drafting and your power is 20% higher than your last 10-second average power you get a 5% CdA reduction. This particular test was set up to attempt to exploit this feature.

Just to prove a couple of these times, and to show what it looked like, here are the bots finishing two of the tests:

These test results are interesting: clearly there is a slight speed advantage to microbursting, as all of the microbursting schemes beat the steady bot by 4-5 seconds. But is 4-5 seconds a big enough improvement, considering you’d be riding short intervals for 10 minutes to get it?

Climbing Solo Rider Tests

Many stories I’ve heard about microbursting in races seem to involve climbs. To put it another way: riders seem to utilize this technique on uphills more than flats or descents.

So I figured I should test it on a climb. How about the Alpe? I put the bots to work. Here are the results:

• 300W Steady: 49:30
• 400W to 200W in 4-second intervals: 49:30
• 500W to 100W in 4-second intervals: 49:39

As you can see, the best I could do was to get the microbursting bots to finish at the same speed as the steady-state bot.

RoboPacer Tests

My last set of tests involved riding in a pack with a RoboPacer. This test was done to observe how a microbursting rider would perform in a pack drafting off of others.

I placed a bot in the Yumi RoboPacer group. Yumi holds 240W on the flats, and if my bot was at 240W steady he sat in the group quite nicely. I tried various microbursting intervals that averaged out to 240W, including the 380W to 100W interval you see here:

None of the intervals saw my bot break away from the group, or even go to the front of the group – in fact, he struggled to even stay with the group and after a minute or two would inevitably get dropped.

Summarizing Test Results

The tests above clearly show that microbursting results in higher speeds than steady-state power on flat ground. But is the speed increase enough to conclusively say microbursting gives riders an unfair advantage?

I say no.

Modulating your power in microbursts increases the perceived difficulty of a ride, at least for most riders. The physical and mental “cost” of microbursting seems to far outweigh any speed advantage it delivers.

A Sticky Watt Hypothesis

Based on the test results above, I have a hypothesis about microbursting in Zwift racing: microbursting by itself does not give riders an unfair advantage. The unfair advantage comes when riders combine microbursts with sticky watts.

What are sticky watts? Read all about them here, but simply put, sticky watts are “free” watts riders get when they stop pedaling for short intervals using certain power meters. As shown in the chart below (click for details), if you combine a sticky power meter with sprint-coasting you can increase the average power Zwift sees by 20% above the “actual” power you’re putting out.

Someone with a lot of practice may be able to exploit sticky watts + microbursting even better than I did in the short test above. In fact, I believe there are Zwift racers doing precisely that.

Overall (Tentative) Conclusions

I’ll wrap up with four clear points:

1. Sprint-coasting is not a “natural” pedaling technique, but that by itself shouldn’t make it “illegal.” Unless it can be shown to deliver an unfair advantage, it shouldn’t be outlawed.
2. In all my testing (and there is much more than what I’ve summarized above) I have yet to find a method of microbursting that gives a significant advantage on Zwift.
3. I’ve heard enough stories of racers using microbursting to win races they shouldn’t have won that I must conclude there is a method of microbursting that gives a significant advantage. I believe that method combines microbursts with sticky watts.
4. I consider my conclusions above to be purely “provisional.” I realize my test methods and scope aren’t perfect, and there may be methods of microbursting (without sticky watts) that deliver a greater advantage than my test results show. If so, I would love to see hard evidence of such methods.

Your Thoughts

Do you agree with my conclusions? Got evidence to support or refute my provisional results? I’d love to hear your thoughts. Chime in below!