We’ve caught wind of a way to gain an advantage in races related to user weight to lower the recommended category in the category enforcement system. This problem cropped up after we changed the frequency and timing at which the user metrics are calculated and was initially overlooked by our team.
Given that, we have made some changes with the goal of preventing this problem. Instead of using the most recent set weight from any Zwifter, we have started using the weight for calculations in the as explained below.
These changes are live as of this posting, so this post details what the current behavior is.
The weight used for category enforcement calculations in watts/kg (using zFTP and zMAP) will be determined by the average weight of all weights that were in effect at the moment that certain key peak power values of the power curve were set (within the last 60 days).
This means that as Zwifters change their weight, it influences all of their power metrics we record as opposed to just a single set of power metrics for a given activity.
The key durations of time that we examine are:
As an example, Zwifter X set their peak power records for various times as follows:
3 min. : 400w at 70kg on 23rd of October
5 min. : 380w at 72kg on 13th of November
12 min. : 340w at 73kg on 14th of November
20 min. : 320 at 73kg on 14th of November
The weight that will be used on the present day (e.g: 10th of December) will be 72kg (average of the 4 weights)
So, assuming zFTP=310w and zMAP=370w, this will equate to 4.30w/kg and 5.14w/kg respectively used for this Zwifter’s recommended category.
With the changes mentioned above, it didn’t make sense to keep using the VO2max value for the category determination for two reasons:
Because VO2max is a metric that already includes weight normalization, it uses the weight currently set in the user account. It is also calculated at the same time the other power metrics (zMAP, zFTP) are. Continuing use of VO2max with these changes would cause inconsistency with the weight average that will be used for the category enforcement moving forward.
The use of zMAP - a proxy of the user’s maximal aerobic capacity - allows us to discontinue using VO2max in the category determination.
3. Categories Table
Here is how the new category limits should look given the above information:
|Open Races||zMAP (watts/kg)||zFTP watts/kg and watts|
|Category A||≥5.4W/kg||≥4.2W/kg and ≥250W|
|Category B||≥4.2W/kg||≥3.36W/kg and ≥200W|
|Category C||≥3.3W/kg||≥2.625W/kg and ≥150W|
|Category D||<3.3W/kg||<2.625W/kg and <150W|
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