Sunday 12 March 2023

The benefits of unlocking Zwift's rear disc wheel

Today I got to Level 35 in Zwift, which that means I was able to get the Zipp Super 9 rear disc wheel.

It looks better on any Time Trial (TT) bike, of course (if you care about such things for your Zwift avatar!).  However, I wanted to know how much faster it is.  This post explains my calculation to determine just that.  The quick answer, if you don't want to read the full post, is 2.5 Watts.

I've been using Zwift since September 2015, which was not long after the launch of Zwift.  I'm not a massive fan of Zwift, I must admit, but I quite like it and I use it for a few months every year to help take some of the monotony out of winter indoor riding.  It's taken me that long time, about eight years, to get enough XP to get up to level 35 (approximately 5,700 Zwift miles).

Fastest Zwift Wheels

When talking about the fastest wheels in Zwift, it is worth clarifying that this applies only to wheels on TT bikes.  Why?  Because once you unlock the Zwift Concept Z1 road bike (aka the 'Tron bike'), then all other road bikes are slower, with a few caveats, regardless of the wheelset you put on them. See this Zwift Insider post for more information about that.

TT bikes are different though, and will always be faster than a road bike, including the Tron bike, when riding solo or in a no-draft TT event.

Zwift Insider's analysis

The guys at Zwift Insider do an excellent job of analysing bike and equipment choices within Zwift, amongst other things.  Their post here explains the fastest TT wheels, using their testing protocol, whereby they recorded the time required to ride the Tempus Fugit route (17.4km / 10.7miles) twice at 300W.

As shown in the table to the left, the faster wheelsets provide larger time savings.  However the fastest wheelset can only be unlocked at the higher levels within Zwift.

For some years, since Level 13, I've been using the Zipp 808 wheels on the TT bike, which are the fastest wheels for people at lower Zwift levels.  Now having the Zipp Super 9 disc wheel on the back, saves me 9 seconds on that 2 x Tempus Fugit route, versus two 808s, according to the Zwift Insider table above.  Given that the entire route takes "approximately 50 minutes" when ridden at 300W, then 9 seconds doesn't sound like very much!  In fact, the time/speed improvement is only 0.3% (9 seconds divided by 3000 seconds).

What I wanted to calculate is how much that time saving equates to, in terms of power savings and CdA reduction.

Important: As with all power saving values, it's crucial to keep in mind the associated speed, which in this case was 41.52 kph (25.79 mph), because any power savings due to aerodynamic changes are proportional to speed cubed.  Beware anybody that quotes power savings without also giving the associated speed!

CdA and Watt savings

I used my performance modelling spreadsheet (described here) to model the Zwift Insider test, where they completed two laps of the Tempus Fugit route at 300W, for a 75kg rider, in about 50 minutes, as shown below:

In addition to the parameters above, I had to make a few other assumptions to calculate the CdA, and these are simply guesses, because I don't know what assumptions Zwift makes.  I'm confident, however, that the power and CdA increments won't be particularly sensitive to this choice of conditions:

  • Air pressure = 1012.5 bar (i.e. sea level pressure using the International Standard Atmosphere, ISA, conditions).
  • Air temperature = 15 degrees C (i.e. sea level ISA conditions).
  • CRR = 0.004 (which ZwiftInsider says here is the CRR that Zwift assumes for the road).
  • Bike weight = 9kg
  • Drivetrain losses = 2.5%
Using these assumptions, I calculated the baseline CdA to be 0.2710 m^2.  Then, by increasing the speed by 0.3%, giving a 9 second time saving, I calculated:

Either: 0.0025 m^2 CdA improvement (0.2710 -> 0.2685, or a 0.92% CdA reduction) at the same 300W power to achieve that 9 second saving.

Or: 2.5 Watt power saving at 41.52 kph (0.83% power saving) for the same 50 minute time, with that reduced CdA of 0.2685.

I think these numbers, especially the power saving, can be understood more intuitively by the average person.  A 2.5 Watt power saving is not large, but is not negligible either.

Are the numbers valid also for me?

Finally, I also wanted to check whether these values are valid for me, because I'm a slightly smaller and less powerful rider than was assumed by Zwift Insider.

Firstly, I calculated my CdA to be 0.2415, for my Zwift time trial bike using Golden Cheetah.  This is relatively easy to do (compared with real life), because in Zwift there is no braking, no wind and no other vehicles to mess things up!

Something to note is that my Zwift CdA is about 10% lower than the CdA for the Zwift Insider analysis, and this will be because I'm probably shorter (at 5 foot 10 inches) and slightly lighter (at 73 kg) than for the Zwift Insider study, and we know that Zwift adjust the CdA based on weight and height.

Next, I applied this CdA to my spreadsheet analysis.  Interestingly, for a 50 minute time to complete the 2 x Tempus Fugit distance, the power requirement was 270.6W, which is approximately what I could hold for 50 minutes, so I didn't adjust it any further.  Then, I reduced the CdA by the same 0.0025 m^2 value, and calculated the power saving in the same way as before:

The Zipp Super 9 rear disc wheel give the following benefits, both at 41.52kph:

                     Zwift Insider (75kg, 300W, 41.52kph)           Me (73kg, 270.6W, 41.52kph)
CdA              0.2710 -> 0.2685 (-0.0025 or -0.92%)      0.2415 -> 0.2390, (-0.0025 or -1.04%)
Power saving           2.5W saving (-0.83%)                               2.4W saving (-0.89%)

In summary, the Zipp Super 9 disc wheel will save me about two and a half watts at typical flat TT speeds.  This benefit is fairly small, but certainly not negligible.