Pack Dynamics v4 Speed Tests: 4-Rider Drafting with Road Bikes

April 17, 2023

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UPDATE: the test results below are now outdated, due to Zwift modifying their Pack Dynamics.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 in 2021, Zwift rolled out Pack Dynamics 3, and we ran a second test to see what, if anything, had changed regarding TTT dynamics. (We found speeds hadn’t changed, it was just harder to hold an efficient single-file formation due to a lack of sticky draft.)

In August 2022, Zwift+WTRL announced enhanced TTT features, including the ability for TT frames to draft in TTT events. So we ran our tests using TTT frames.

Recently, Zwift announced the rollout of Pack Dynamics 4.0 game-wide. How would this impact TTT races? We tested TTT formations with TT frames and PD4, publishing the results in this post.

But we weren’t quite done! Desiring to test PD4 with road bikes and to confirm the TT frame test results, we ran similar tests using road frames. These results are summarized below.

Test Goals

We set out to answer two questions with these tests:

Is road bike power savings in the draft with Pack Dynamics v4 different than the savings with Pack Dyamics v3?
Do our findings with PD4 tests using road bikes back up our findings with TT frames and PD4?

Test Parameters and Methodology

All test riders were set to 183cm height, 75kg weight, and rode Zwift Carbon road bike frames with Zwift 32mm carbon wheels.

Tests were done in an isolated event on Watopia’s Tempus Fugit route because it’s the flattest on Zwift and has a timed section (Fuego Flats Reverse, 7.1km long) which could be used to measure the speeds of each test formation precisely.

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 disorganized group of riders, where some would rotate to the front then drop back after being in the wind for a few seconds. There was churn, but not as much as we saw with previous versions of pack dynamics.

All riders @ 300W
Segment time 10:04.5
Speed: 42.08 kph

Notably, the segment time with Pack Dynamics v3 was 10:13.4! So even though there’s less churning happening in PD4, somehow this pack moved faster. (This is the same unexpected result we saw with our PD4 TT frame test.)

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 @ 212W, Rider 3 @ 196W, Rider 4 @ 191W
Segment time: 10:36.7
Speed: 39.9 kph

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 29.3%, 34.6%, and 36.3% – significantly higher power savings than we saw in our PD3 tests. As expected, the further back you are, the bigger the draft effect.
In a TTT situation with all riders taking equal pulls on the front at these wattages, each rider would average 224.8W. (With PD3, the average wattage for the same group speed was 246W.)
Test 2’s segment time was over 32 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.
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 @ 252W, Rider 3 @ 236W, Rider 4 @ 217W
Segment time: 10:02.2
Speed: 42.2 kph

Notes:

Riders received power savings of 28%, 32.6%, and 38% (2nd, 3rd, and 4th rider respectively). Almost identical to the power savings seen in the 300W single file test.
In a TTT situation with all riders taking equal pulls on the front at these wattages, each rider would average 263.8W (compared to 290W with PD3).
This is the crux of why TTT formation is so important: even with Zwift’s “speed churning” from test 1, the four riders in this test beat test 1’s time by riding efficiently in single file formation at a much lower average wattage.

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 @ 290W, Rider 3 @ 261W, Rider 4 @ 255W
Segment time: 9:32.1
Speed: 44.4 kph

Notes:

Riders received power savings of 27.5%, 34.8%, and 36.2% (2nd, 3rd, and 4th rider respectively). Very similar to the power savings seen in the 300W and 350W single file tests.
In a TTT situation with all riders taking equal pulls on the front at these wattages, each rider would average 301.5W (compared to 326W with PD3).

Test 5: Hybrid

Lastly, we tested 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 @ 273W steady
Segment time: 9:32.1
Speed: 44.4 kph

Notes:

The wattage required to sit behind the front rider is much lower in PD4. PD3 required 313W, while PD4 only required 273W!
In a TTT situation with all riders taking equal pulls on the front, each rider would average 304.8W (compared to 335W with PD3). That means the hybrid format results in nearly the same average wattage as the 400W single file test, while being a much easier formation to hold!

Conclusions

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

Is road bike power savings in the draft with Pack Dynamics v4 different than the savings with Pack Dyamics v3?

Absolutely! Here’s a table showing approximate power savings with PD3 and PD4 based on your position in a TTT group of 4 riders:

Position	PD3	PD4
2	17%	28%
3	25%	33%
4	30%	37%

Why is there so much more savings with PD4? Because PD4 uses double-draft, so the draft effect is stronger than PD3.

Do our findings with PD4 tests using road bikes back up our findings with TT frames and PD4?

Yes. The power savings table above very closely matches what we found for TT frames with PD4.

Additionally, we saw the same anomaly with the “churn” (test 1) being faster in PD4 than it was in PD3, both with road bikes and TT bikes.

While PD4’s auto-braking is supposed to help stop riders from accidentally moving up and into the wind, our churning pack of 300W riders held steady power, so auto-braking never kicked in. That meant they got a bit of a double-draft slingshot from behind the front rider, then shot into the wind momentarily before slowing and being replaced by the next slingshotting rider. We believe this accounts for the faster speed of the churn group in PD4.

What It All Means

For road racers on Zwift (vs TT), Zwift’s new pack dynamics should result in races that more closely reflect IRL racing.

Double-draft is enabled for all races (since it’s built into PD4), so the draft is delivering more power savings than before.

Additionally, with autobraking and other changes accidental churn is decreased on the front of packs, keeping pack speeds a bit lower and forcing riders to be more intentional if they want to take a turn on the front.

These changes should lead to more breakaway possibilities, as smart breakaway-loving riders can get more rest in the draft, and work together to stay away from the slightly slowed pack once a breakaway is established.