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

April 10, 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UPDATE: the test results below are now outdated, due to Zwift modifying their Pack Dynamics.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 in regard to 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.

Last week, Zwift announced the rollout of Pack Dynamics 4.0 game-wide. How would this impact TTT races? There was only one way to find out, so we set up a private test event, booted up 4 Zwift PCs, pulled the TT frames out of the garage, and did several test laps of Tempus Fugit. The results of these tests are summarized below.

Test Goals

We set out to answer three questions with these tests:

Is the power savings in the draft with Pack Dynamics v4 different than the savings with Pack Dyamics v3?
Is the game’s positioning of TTT riders noticeably different with PD4 compared to PD3?
Do PD4’s modified pack dynamics result in different overall speeds in small churning packs?

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, 4.4 miles long) which could be used to measure the speeds of each test formation precisely.

The ever-helpful James Bailey at Zwift HQ flipped the switch on our event so our TT bikes could draft each other.

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 9:40.5
Speed: 43.8 kph

Notably, the segment time with Pack Dynamics v3 was 9:53! So even though there’s less churning happening in PD4, somehow this pack moved faster.

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 @230W, Rider 3 @ 210W, Rider 4 @ 190W
Segment time: 10:08.9
Speed: 41.7 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 23.3%, 30%, and 36.6% – 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 232.5W. With PD3, the average wattage for the same group speed was 247W!
Test 2’s segment time was almost 29 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 @266W, Rider 3 @ 247W, Rider 4 @ 229W
Segment time: 9:36.9
Speed: 44.1 kph

Notes:

Riders received power savings of 24%, 29.5%, and 34.9% (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 273 (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 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 @299W, Rider 3 @ 273W, Rider 4 @ 256W
Segment time: 9:08.2
Speed: 46.4 kph

Notes:

Riders received power savings of 25.3%, 31.8%, and 35% (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 307 (compared to 325W 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 @274W steady
Segment time: 9:06
Speed: 46.6 kph

Notes:

The wattage required to sit behind the front rider is much lower in PD4. PD3 required 319W, while PD4 only required 274W!
In a TTT situation with all riders taking equal pulls on the front, each rider would average 306W (compared to 339W with PD3). That means the hybrid format results in (essentially) 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 the 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%	24%
3	25%	30%
4	30%	35%

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

Is the game’s positioning of TTT riders noticeably different with PD4 compared to PD3?

Yes. This doesn’t really show up in our test numbers, but what we saw is a lot less forward/backward movement of riders, and less of the “slingshot” effect of boosting your speed by coming around the front rider and heading to the front of the group.

Riders doing lower power numbers than the front rider would sometimes start to come around the front rider, but once they hit the wind (with their front wheel roughly equal to the pulling rider’s front wheel) their speed would decrease and they would drop back without ever being fully on the front.

Do PD4’s modified pack dynamics result in different overall speeds in small churning packs?

Also yes! Our first test of 4 riders holding 300W resulted in a significantly faster time than the same test with PD3. This is surprising, given PD4’s reduced churn on the front. We have two theories as to why this is happening:

One of the stated goals of PD4 is to “Make breakaways have a slightly better chance of sticking if the attackers work well together.” Perhaps Zwift is giving a speed boost to smaller packs of riders? UPDATE: Zwift HQ assures us this is not the case.
Somehow the increased “double-draft” effect is still causing small speed boosts as riders rotate to the front.

Single File or Hybrid Formation?

Clearly “The Churn” is not a good TTT formation. So what’s better – the single file formation, or the “hybrid” approach? With PD3, our 400W tests showed that single file was the most efficient approach since the average power needed was 14W lower compared to hybrid.

But with PD4, the single file and hybrid approaches delivered the same average wattage in our 400W tests! So go with either formation – the important thing is that your front rider hits their power/speed targets.

The good news is, with PD4 it’s easier than ever to hold your desired position in the pack. So whether you’re shooting for the single file or hybrid approach, your TTT team will be able to stay in formation more than ever.

Let’s Break Records!

Released in September 2022, the Cadex Tri frame trimmed 4.5s off a 50-minute flat course compared to the previous fastest frame. Now PD4 appears to give even more speed advantages to small churning groups while increasing the draft so riders can rest more between pulls.

This means Zwifters should be able to set TTT records now, more than ever. Ride on!