Zwift’s last major update included this note:

Various wheels and tires now have different rolling resistance based on the surface they are rolling on. IE, a mountain bike or gravel tire may be the best on the Jungle circuit, while a road/TT bike is fastest on the tarmac. More to come here in a future update.

This weekend’s update to version 1.041097 seems to have rolling resistance working the way Zwift intends, so we’ve started running tests to see how different wheels perform on the course with updated rolling resistance values.

First: About Crr

The change in Zwift’s latest update means the game is now setting a value for the Coefficient of Rolling Resistance (Crr) based on the wheels you are using in game and the type of surface you are currently riding.

Crr is simply a number which quantifies how much friction your tires are creating against the road surface. A fast-rolling tire has a lower Crr, while a slower tire has a higher Crr.

Our educated guess is that, before this update, Crr was a constant on Zwift, not changing based on road surface or wheel type. But now, every time the road surface changes, Zwift checks the Crr for your particular wheel for that particular surface, and uses that value to help determine your in-game speed. Zwift also sends that value to your smart trainer, where it affects the resistance you are feeling in standard Sim mode.

Here’s a snippet from the ANT+ spec showing how rolling resistance is calculated, then used to determine the resistance your smart trainer gives you:

(section 8.10.2)… the standard calculation for rolling resistance.
Rolling Resistance [N] = (Bicycle Mass + Cyclist Mass) x Coefficient of Rolling Resistance x 9.8

Calculating Total Resistance (Simulation) The total resistance applied by the fitness equipment is a sum of the wind resistance, rolling resistance and the gravitational resistance as shown in Equation 8-15:
Total resistance [N] = Gravitational Resistance + Rolling Resistance + Wind Resistance

Speed Comparison: Road Bike

We used Strava’s Effort Comparison tool to compare the speeds of an identical bike setup before and after Zwift’s Crr updates.

The frame used is the Specialized S-Works Venge, with Zipp 858/Super 9 Wheelset. One of the fastest setups in-game. Click here to access the Strava Effort Comparison, or simply watch the quick video below:

As you can see, our test bike definitely slows down in the dirt sections compared to past performances.

Speed Comparison: Road Bike vs Mountain Bike

Here’s a comparison of the same road bike above and Zwift’s new mountain bike, which is supposed to perform better than the road bikes on dirt.

While the mountain bike steadily loses ground to the road bike on most of the course, it keeps up with the road bike (and even gains a little time) on the dirt sections. (More on MTB performance in dirt coming soon!)

What Does It All Mean?

There are a few key takeaways here:

  • All dirt sections are slower with road tires, not just the Jungle Circuit (where it’s most noticeable because it’s a long stretch of dirt). The lava dirt around the volcano appears to roll at the same speed as before.
  • Soon, dirt performance will be a factor in wheel choice. Currently, all road tires perform the same in dirt, and only the mountain bike (with its MTB tires) performs differently. This will change when Zwift begins releasing more wheelsets, and they may even change the Crr values of some existing wheelsets.
  • Expect more dirt attacks. Racers already like to attack in the dirt, where dust clouds mean poor visibility. But the increased resistance on road tires in the dirt means power numbers will bump up even more in these sections.

Your Thoughts

What do you think of Zwift’s Crr changes? Chime in below!