Speed Tests: 4-Rider Drafting (TTT) with Pack Dynamics 3.0

November 2, 2021

LAST UPDATED August 24, 2023

UPDATE: the test results below are now outdated, due to Zwift modifying their Pack Dynamics. See the latest version of these tests for accurate data.

Our original TTT speed test post from 2020 gave team time trial riders some very welcome guidance about how to most efficiently ride their races. Then last week, Zwift began rolling out “Pack Dynamics 3.0” (PD3) – an updated version of the code which controls how riders’ avatars interact with each other on Zwift’s virtual roads. This includes difficult-to-get-perfect issues like avoiding rider collisions, having realistic spacing between riders in a pack, and deciding how “sticky” the draft is.

PD3 is currently live throughout Watopia, London, and Makuri Islands worlds. So we figured it was time to revisit 2020’s TTT test on Tempus Fugit to determine if and how PD3 may change the game for TTT racers.

Test Goals

These follow-up tests set out to answer three questions:

Has the benefit of drafting changed with PD3?
Is it still possible to hold a single-file traditional TTT formation in PD3, where the stickiness of the draft has been reduced?
Has anything changed which would affect the speed of a “churning” group?

Test Parameters and Methodology

All of the test riders were set to 183cm height, 75kg weight, and rode Zwift Carbon bikes with 32mm Zwift wheels.

Tests were done in Meetup-Only View on Watopia’s Tempus Fugit route because it’s the flattest on Zwift, and it has a timed section (Fuego Flats Reverse, 4.4 miles long) which could be used to precisely measure the speeds of each test formation.

All of the tests were done with four riders.

Tests and Results

Test 1: the Churn

For our first test, we put all riders at the same 300W power setting. This resulted in a churning group of riders where one rider would surge from being in the draft near the back to being in the wind at the front, then dropping back to do it all over again. This “washing machine effect” is what you see at the front of many Zwift races. Our question was – has PD3 resulted in a change in the speed of a churning pack?

All riders @ 300W
Segment time 10:13.4
Speed: 41.46 kph (25.76 mph)

Notes:

Segment time was 1.4s faster than our original test from 2020. So just slightly faster (it works out to 120 meters further over an hour).

Test 2: Single File @300W

The second test had the lead rider holding 300W, with the other three riders in single file behind, holding the minimum wattage possible to stay in formation. This is what you would see in an outdoor team time trial:

Rider 1 @ 300W, Rider 2 @248W, Rider 3 @ 225W, Rider 4 @ 212W
Segment time: 10:36.98
Speed: 39.9 kph (24.79 mph)

Notes:

The “minimum wattages” stated for riders 2-4 on this test and other tests below should be considered approximations, as it is impossible to figure out the precise wattage required to hold formation due to Zwift’s dynamic physics engine and very small undulations in terrain, even on Fuego Flats.
Riders received power savings of 17%, 25%, and 29%. As expected, the further back you are, the bigger the draft effect. But what we did not expect was that the draft savings has decreased somewhat in PD3. Our 2020 test showed a saving so of 23%, 32%, and 34% for the same three riders.
In a TTT situation with all riders taking equal pulls on the front at these wattages, each rider would average 246W. This is an increase from our 2020 test which showed an average of 234W.
Test 2’s segment time was 23.6 seconds slower than Test 1’s, despite riders holding no higher than 300W in both tests. This may seem odd at first, but it’s a result of the “churn”, riders are speeding up while in the draft, then shooting ahead into the wind, only to be slowed and have another rider shoot past them. This little speed boost accounts for a significant time difference, as we see here!
Our 2020 test was 11.7 seconds slower, but with a lower overall average power. It’s hard to determine what exactly has changed between the pack dynamics of 2020 and PD3 to account for this time difference, but Zwift has said that the draft effect hasn’t changed. We do know that the stickiness of the draft has certainly changed, with PD3 being much less sticky. Perhaps the sticky draft actually let us stay on a wheel with a bit less power?
It’s worth noting here that we did a solo rider test at 300W steady, because we were curious if there was any advantage to the front rider if there were riders behind. There is not. Our solo rider turned in the same time as this 4-rider group.

Test 3: Single File @350W

This test is similar to Test 2, except we bumped up the front rider’s wattage to 350W to make sure the group would be faster than the churning pack in Test 1.

Rider 1 @ 350W, Rider 2 @297W, Rider 3 @ 270W, Rider 4 @ 241W
Segment time: 10:02.73
Speed: 42.15 kph (26.19 mph)

Notes:

1.5 seconds slower than the 2020 test
Riders received power savings of 15%, 23%, and 31% (2nd, 3rd, and 4th rider respectively). Our test from 2020 showed a savings of 23%, 30%, and 33%.
In a TTT situation with all riders taking equal pulls on the front at these wattages, each rider would average 290W. This is, not surprisingly, a bit higher than 2020’s result of 275W. This is crucial to understand: that even with Zwift’s “speed churning” from test 1, the four riders in this test significantly beat test 1’s time by riding efficiently in single file formation at a lower average wattage.
That said, the difference between Test 1’s time and Test 3’s time is a bit smaller with PD3, even though the average power has increased by 15W for this test. So there’s definitely more power required to reach the same speeds in a single file TTT format with PD3.

Test 4: Single File @400W

This test was very similar to Tests 2 and 3, we just bumped the front rider up to 400W.

Rider 1 @ 400W, Rider 2 @323W, Rider 3 @ 297W, Rider 4 @ 283W
Segment time: 9:33.93
Speed: 44.28 kph (27.51 mph)

Notes:

2 seconds slower than the 2020 test
Riders received power savings of 19%, 26%, and 29% (2nd, 3rd, and 4th rider respectively). Again, less savings than 2020, where we saw a savings of 23.5%, 31%, and 35%.
In a TTT situation with all riders taking equal pulls on the front at these wattages, each rider would average 326W. So just like Test 3, the average power is 15W higher than our 2020 test.

Test 5: Hybrid

For our final test we wanted to look at a strategy that many TTT teams use, wherein there is one designated rider in front, and the riders behind simply churn in the front rider’s draft. This reduces the hassle of trying to maintain single-file positioning, while receiving some of the benefits. But how does it impact efficiency?

Rider 1 @ 400W, Riders 2, 3, and 4 at @313W steady
Segment time: 9:33.52
Speed: 44.28 kph (27.51 mph)

Notes:

2 seconds slower than the 2020 test
In a TTT situation with all riders taking equal pulls on the front, each rider would average 335W. So not as efficient as single-file riding (average wattage is 9W higher than the single file test). But a much easier formation to hold!

Conclusions

Let’s answer the three questions we stated at the top of the page:

Has the benefit of drafting changed with PD3?

While Zwift says the draft “shadow” put out behind riders hasn’t changed with PD3, something has made it less efficient to ride in a single-file TTT formation with PD3. Our guess is that the reduced stickiness of the draft makes it a bit more challenging to hold a wheel in PD3.

Is it still possible to hold a single-file traditional TTT formation in PD3, where the stickiness of the draft has been reduced?

It is possible. But it’s harder to do. PD3 feels like there’s almost no stickiness at all, so holding the wheel of another rider (whose power is also fluctuating, like yours) is even more challenging now than it was previously. TTT racers: consider yourselves warned.

Has anything changed which would affect the speed of a “churning” group?

With our test 1 time being only 1.4 seconds faster than the same 2020 test, clearly nothing has changed that significantly impacts the speed of a churning group.

Single File or Hybrid Formation?

While single file is still the most efficient TTT formation on Zwift (and IRL), it’s made much more difficult by the lack of draft stickiness in PD3.

We predict that more TTT teams will begin using a “looser” hybrid formation (1-2 lead riders with everyone else churning in the group behind) rather than single file in upcoming events. This is the logical result of the increased difficulty of maintaining a single file formation and the reduced advantages of doing so.