Pack Dynamics v4.1.1 Speed Tests: 4-Rider Drafting with Road Bikes

October 4, 2023

LAST UPDATED October 4, 2023

After digging into user feedback on Pack Dynamics 4.1, Zwift discovered, “Currently the progression inside the groups in PD 4.1 is harder than it should be. It’s like a game stuck in the hardest difficulty.” So they came up with a fix and enabled it on Monday, Oct 2.

Below are the results of our standard 4-rider drafting tests using PD4.1.1 (what we’re calling the bug-fixed version of PD4.1) and road bikes. We will follow up with a test using TT frames in a drafting TTT scenario, but we believe the same conclusions will be drawn from both sets of tests. 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.

In August 2023, Zwift released Pack Dynamics 4.1 slowing packs and reducing churn on the front, so we ran our tests with PD4.1.

This week, Zwift rolled out Pack Dynamics 4.1.1 (officially just a bug fix for PD4.1) You know what that means… time for more tests!

Test Goals

We set out to answer three questions with these tests:

Is road bike power savings in the draft with Pack Dynamics v4.1.1 different than the savings with Pack Dynamics v4.1?
Does Pack Dynamics v4.1.1 alter pack speeds, and if so, how?
Are there any other observable differences between PD4.1 and PD4.1.1?

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 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 results in a very inefficient pack of riders, but it lets us test the slingshot effect. Here’s a quick snippet of what we saw:

The pack seemed to churn a bit more in PD4.1.1 compared to PD4.1 (see that video here), while still being much more stable (less churn) than PD4.

All riders @ 300W
Segment time 10:31.9
Speed: 40.44 kph

Our test segment was completed 4.5 seconds faster in PD4.1.1 compared to PD4.1. We attribute this small time savings to having a bit of churn on the front, but it’s worth noting that the churn with PD4.1.1 is very minor compared to PD4, which turned in a time 27.4 seconds faster in this 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 @ 225W, Rider 3 @ 202W, Rider 4 @ 187W
Segment time: 10:36.3
Speed: 40.19 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 25%, 32.7%, and 37.6%. The first 3 riders had essentially the same numbers as PD4.1, but the last rider could stay on with less power, indicating that perhaps the “fall off” point of the draft has been extended in PD4.1.1.
In a TTT situation with all riders taking equal pulls on the front at these wattages, each rider would average 228.5W. (With PD4.1 the average was 232W.)
With PD4.1, test 2’s segment time perfectly matched test 1’s. This isn’t the case with PD4.1.1, because there’s a bit of a churn/slingshot effect happening.
It’s worth noting 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 @ 310W, Rider 3 @ 275W, Rider 4 @ 240W
Segment time: 9:33.5
Speed: 44.53 kph

Notes:

Riders received power savings of 22.5%, 31.3%, and 40% (2nd, 3rd, and 4th rider, respectively). Again, the first 3 riders’ numbers didn’t change much from PD4.1, but the final rider saw a significant decrease of 18W.
In a TTT situation with all riders taking equal pulls on the front at these wattages, each rider would average 306.25W. (With PD4.1 the average was 309W.)

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 @ 296W steady
Segment time: 9:33.4
Speed: 44.53 kph

Notes:

If there wasn’t at least 1 rider holding 296W or more in the pack of 3 behind the front rider, the pack of 3 would get dropped.
With PD4.1, the pack of 3 needed to hold 308W. Why the lower wattage in PD4.1.1? Because the pack of 3 was benefiting from the churn/slingshot while sitting behind the front rider. This increased their speed a bit, allowing them to hold onto the front rider while holding 12W less than they did with PD4.1.
In a TTT situation with all riders taking equal pulls on the front, each rider would average 322W. (With PD4.1 the average was 331W).
The hybrid format was a solid option with PD4, and terribly inefficient compared to single file formation in PD4.1. With PD4.1.1 its efficiency improved a bit, but based on average power it’s still not nearly as efficient as a single file formation.

Conclusions

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

Is road bike power savings in the draft with Pack Dynamics v4.1.1 different than the savings with Pack Dyamics v4.1?

Absolutely! Here’s a table showing approximate power savings with various versions of Pack Dynamics based on your position in a TTT group of 4 riders:

Position	PD3	PD4	PD4.1	PD4.1.1
2	17%	28%	24%	24%
3	25%	33%	33%	32%
4	30%	37%	34%	39%

As you can see, the big difference in draft power savings between PD4.1 and PD4.1.1 is for that 4th rider.

Does Pack Dynamics v4.1.1 alter pack speeds, and if so, how?

Yes. Pack Dynamics 4.1.1 will speed up packs slightly due to the increased churn/slingshot effect on the front. It’s worth noting that this slight pack speed increase is nothing compared to the major pack speed decrease that PD4.1 delivered! Overall, packs are still significantly slower today than with PD4 and earlier.

Are there any other observable differences between PD4.1.1 and PD4.1?

Yes. PD4.1 made it rather difficult to move forward in a pack, allowing riders to “waste watts” by pushing harder than necessary to hold a pack position (see video). That’s not the case with PD4.1.1. Now, increasing your power results in moving forward in the pack as it should. Additionally, it feels as though rider inertia has increased, because once you start moving forward, you tend to keep doing so!

Also, as noted above, it appears that the draft “fall off” point has been lengthened, so the draft “shadow” extends even further behind each rider. This is the only way we can explain the 4th rider’s ability to hold on with less power in our two single-file tests above.

What It Means for Road Racing

For road racers on Zwift (vs TT), we believe Zwift’s Pack Dynamics 4.1.1 will greatly reduce complaints from riders who said it was too hard to sit in. (Those riders were mostly complaining because they were wasting watts, but in the end, the truth is that it was harder for them.)

Pack speeds are still lower compared to PD4 and earlier, though. This means breakaways stand a chance of surviving, which means riders will be attempting more breakaways! The end result is races that feel more punchy/dynamic… harder.

This isn’t a Zwift physics issue, though: it’s an evolution of the Zwift racing mindset.