Pack Dynamics v4.1 Speed Tests: 4-Rider Drafting with TT Bikes

August 24, 2023

LAST UPDATED August 23, 2023

After months of testing, this week Zwift rolled out their latest version of Pack Dynamics (v4.1) to all worlds. We’ve already summarized how PD4.1 differs from PD4 in this post, but now it’s time to measure those changes and learn how they affect the everyday Zwift experience!

Below you’ll find the results of our standard 4-rider drafting tests using PD4.1 and TT frames. (We also published test findings for road frames, which line up nicely with our TT frame results below.) Let’s dig in!

History of our 4-Rider Speed Tests

Our original TTT speed test post from 2020 gave team time trial riders some very welcome guidance about how to ride their races most efficiently. Then in 2021, Zwift rolled out Pack Dynamics 3, and we ran a second test to see what, if anything, had changed. (We found speeds hadn’t changed, it was just harder to hold an efficient single-file formation due the 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. Then several months later in April 2023, Zwift announced the rollout of Pack Dynamics 4 game-wide. So we ran our standard tests with PD4.

This week, Zwift rolled out Pack Dynamics 4.1 gamewide. We tested it with road frames first, but this post is all about TT frames. Let’s go!

Test Goals

We set out to answer three questions with these tests:

Is power savings in the draft with Pack Dynamics v4.1 different than the savings with Pack Dynamics v4?
Does Pack Dynamics v4.1 alter pack speeds, and if so, how?
Are there any other observable differences between PD4 and PD4.1?

Test Parameters and Methodology

All test riders were set to 183cm height, 75kg weight, and rode Zwift TT frames with Zipp 808 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 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 rather disorganized group of riders, but crucially, there was much less churn than with Pack Dynamics 4. The riders would often be 3-4 abreast across the road, with no rider taking the “pull” and no riders benefiting from a significant draft.

All riders @ 300W
Segment time 10:11
Speed: 41.83 kph

Segment time with Pack Dynamics v3 was 9:53, which dropped to 9:40.5 with Pack Dynamics v4. Pack Dynamics v4.1 makes a huge difference with this particular formation of riders, dropping the average speed by almost 2 kph!

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 @ 234W, Rider 3 @ 214W, Rider 4 @ 198W
Segment time: 10:11
Speed: 41.83 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 22%, 28.7%, and 34% – lower power savings than our PD4 tests, but still significantly higher than PD3.
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 237W. (With PD4 the average was 233W, while PD3 was 247W.)
Crucially, test 2’s segment time perfectly matched test 1’s. That means the front rider set the speed of the pack in both tests, and there was no churn/slingshot effect unrealistically driving up the pack speed.
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 @400W

This test was similar to Test 2, we just bumped the front rider up to 400W, then increased the power of the riders behind accordingly.

Rider 1 @ 400W, Rider 2 @ 309W, Rider 3 @ 277W, Rider 4 @ 268W
Segment time: 9:10.2
Speed: 46.47 kph

Notes:

Riders received power savings of 22.8%, 30.8%, and 33% (2nd, 3rd, and 4th rider, respectively). Very similar to the power savings seen in the 300W single file test.
The additional draft benefit (only 2.2% more) for the 4th rider is less than we’ve seen with previous pack dynamics.
In a TTT situation with all riders taking equal pulls on the front at these wattages, each rider would average 314W. (With PD4 the average was 307W, while PD3 was 325W.)

Test 4: Hybrid

Lastly, we tested a formation that many Zwift TTT teams have used, 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 @ 308W steady
Segment time: 9:10.5
Speed: 46.39 kph

Notes:

If there wasn’t at least 1 rider holding 308W or more in the pack of 3 behind the front rider, the pack of 3 would get dropped. (This number, not coincidentally, matches the wattage of the 2nd rider in the single file line of test 3.)
The pack of 3 did not benefit from the increased speed of the “churn” like they did with PD4. PD4 allowed three riders holding 274W to churn and sit on the front rider’s wheel, but here with PD4.1, if we required all 3 riders to hold the same power, it had to be 308W minimum to hold the wheel.
In a TTT situation with all riders taking equal pulls on the front, each rider would average 331W. (With PD4 the average was 306W, while PD3 was 339W).
The hybrid format was a solid option with PD4, but it is now terribly inefficient compared to single file formation in PD4.1.

Conclusions

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

Is power savings in the draft with Pack Dynamics v4.1 different than the savings with Pack Dyamics v4?

Yes, but the difference is smaller on TT frames than road bike frames. Here’s a table showing approximate power savingswith TT frames on PD3, PD4, and PD4.1 based on your position in a TTT group of 4 riders:

Position	PD3	PD4	PD4.1
2	17%	24%	22%
3	25%	30%	30%
4	30%	35%	33%

Does Pack Dynamics v4.1 alter pack speeds, and if so, how?

Our first test seeks to emulate the front of a hard-charging race, and that test returned a significantly slower result (2kph) with PD4.1. So yes, we can confidently say that PD4.1 slows overall pack speeds because it reduces the churn/slingshot effect that drove pack speeds up previously. We predict that TTT teams will find it very challenging to break their past course records now that PD4.1 is in play.

Are there any other observable differences between PD4 and PD4.1?

Yes. The four tests above showed marked differences from PD4 in two areas not already mentioned: the “shape” of the draft, and the potential for wasted watts.

Draft “Shape” Notes

Zwift altered the “shape” of the draft cone (the draft “shadow” behind each rider) with PD4.1. These changes could be clearly seen during our tests, in two ways:

The cone’s “fall off” was faster. That is, riders lose the draft benefit sooner as they drift back and off the wheel of another rider. This may explain why the draft benefit for the 4th rider in our tests above was only slightly better than the 3rd rider’s.
The cone is narrower. That means in a small group or single-file situation, left-right positioning is more crucial than ever. Steering may be beneficial here, but in a TTT without steering, teammates behind the front rider may need to make room so their friends can “slot in” and not waste watts sitting on the edge of the draft cone off to one side.

Wasted Watts Notes

What you don’t see above is something interesting we observed while wrangling our bots into single-file pacelines: PD4.1’s modified overtaking algorithm has, in some sense, created a new “sticky draft”.

In a single-file scenario like test 2 above, it’s now possible to have one rider (say, the 4th in the line) holding 30 more watts than the 3rd rider, without overtaking the 3rd rider. This wouldn’t have happened with PD4, but it definitely happens with PD4.1. It’s not the same as the old sticky draft, but the result is similar, if not more pronounced. In fact, when we were running our hybrid formation test (test 4 above) we often found our riders strung out single file, even though the back 3 were all holding 300W steady!

We assume this is due to whatever algorithms Zwift uses to reduce forward/backward movement in the pack. Especially in a single-file scenario, it now takes much more intentional ramping up of power to overtake the rider ahead, even if that rider isn’t the front rider in your group.

What does this mean for racers? It means PD4.1 makes it much easier to waste power in the pack. Similar to how the old sticky draft might have stuck you on someone’s wheel whether you’re doing 250W or 270W, you may now find yourself able to maintain a pack position whether you’re doing 300W or 340W!

So when you’re sitting in the pack, try easing off your power to see how it affects your position. Learning how to ride most efficiently with PD4.1 may take some trial-and-error practice, but you’ll probably find you can save watts by riding smarter.

What It Means for TTT Racing

With Pack Dynamics 4, the Single File and Hybrid formations were nearly identical in terms of efficiency. But this is not the case with PD4.1, where Hybrid is now much less efficient.

Along with everything noted above, TTT riders may notice two more things about PD4.1:

Pack speeds will be lower (PD4.1 added ~30 seconds to our ~10 minute test segment at 300W);
Rotating to the front or even moving forward/backward in your paceline will require more purposeful action as opposed to accidental movement caused by the churn/slingshot effect.