Speed Tests: 4-Rider TTT Drafting with TT Bikes

October 7, 2022

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UPDATE: the test results below are now outdated, due to Zwift modifying their Pack Dynamics.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 3.0” and we ran a second test to see what, if anything, had changed in regards to TTT dynamics. (We found speeds hadn’t really changed, it was just harder to hold an efficient single-file formation due to a lack of sticky draft.)

Then in August 2022 Zwift+WTRL announced enhanced TTT features, including the ability to use TT frames and still draft.

Zwifters have been wondering how riding a TTT with a TT frame differs from riding it with a road frame. Is it just faster, or is the draft savings different? We decided to find out.

Test Goals

This set of follow-up tests set out to answer two questions:

Is the power savings in TTT formations on a TT frame different than it is on a road bike?
Are the different formations (churning or hybrid) more or less efficient on TT frames than they are on road bikes?

Test Parameters and Methodology

All of the 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 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.

The ever-helpful James Bailey at Zwift HQ flipped the switch on our event so our TT bikes would be able to 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 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.

All riders @ 300W
Segment time 9:53
Speed: 42.9 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 @250W, Rider 3 @ 225W, Rider 4 @ 214W
Segment time: 10:09
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 17%, 25%, and 29% – the same power savings seen in our road bike TTT 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 247W.
Test 2’s segment time was 16 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 @293W, Rider 3 @ 271W, Rider 4 @ 246W
Segment time: 9:37
Speed: 44.1 kph

Notes:

Riders received power savings of 16%, 23%, and 30% (2nd, 3rd, and 4th rider respectively). Almost identical to the power savings seen in our road bike TTT tests.
In a TTT situation with all riders taking equal pulls on the front at these wattages, each rider would average 290W. This is the crux of why TTT formation is so important: 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.

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 @327W, Rider 3 @ 293W, Rider 4 @ 281W
Segment time: 9:06
Speed: 46.6 kph

Notes:

Riders received power savings of 18%, 27%, and 30% (2nd, 3rd, and 4th rider respectively). Almost identical to the power savings seen in our road bike TTT tests.
In a TTT situation with all riders taking equal pulls on the front at these wattages, each rider would average 325W.

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 @319W steady
Segment time: 9:06
Speed: 46.6 kph

Notes:

In a TTT situation with all riders taking equal pulls on the front, each rider would average 339W. So not as efficient as single-file riding (average wattage is 14W higher than the single file test). But 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 TTT formations on a TT frame different than it is on a road bike?

It is not. Given that there’s a slight margin of error here, we’re confident there is no detectable power savings (or cost) in TTT formations on road bikes vs TT bikes. You’ll just go faster on a TT frame, but your power to chase a rider holding, say, 400W in front will be the same as it was on a road bike.

Are the different formations (churning or hybrid) more or less efficient on TT frames than they are on road bikes?

We don’t think so. Again, the numbers in our TT tests were very close to those in our road tests.

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?

While the test results above confirm that single file is still the most efficient TTT formation on Zwift (as it is IRL), it’s much more difficult to hold a single file formation compared to the hybrid approach, especially on technical courses with lots of gradient changes and sharp turns.

So the “best” formation for your TTT squad will be the formation they’re able to hold. If you can pull off single file, do it! But if not, go hybrid. Either way, smash it!