Zpower after the last update on KK


(Robert Lockhart) #1

I have my indoor training bike setup on a Kurt Kinetic Rock & Roll. I use Trainer Road and I match their virtual power to a Stages power meter by adjusting the amount of turns on the resistance unit. It has a trainer tyre set at 110psi and the resistance unit tightened to 4.5 turns of the screw.
I then started using Zwift. I rode 2 identical endurance/tempo workouts. One before the update on January 16 and one after. The one before felt spot on with the Zpower feeling as it should. The watts it was displaying felt like they were endurance/tempo and this tied in with my heart rate. The ride after the update seemed a lot harder for the same watts! My HR was more in the threshold zone than endurance/tempo.
Have the power curves for the KK been adjusted to make it read lower so therefore making it harder to push the higher numbers?
I know that you’re not necessarily going to have the same power curve as Trainer Road. So to run Zwift on my setup I would have to match Zwift with my Stages power meter by using less turns on the resistance unit of my KK.
This would make it difficult for me to use both Trainer Road and Zwift and switch between the 2 when I want to ride one or the other.


(Robert Lockhart) #2

Also noticed after the update that the power readings seem to fluctuate a lot. When keeping a very steady cadence I’m seeing it varying in a region of 100 watts or so.


(Clint Westhoff) #3

Quick question - If you have a Stages why are you using Zpower or Virtual Power?


(Robert Lockhart) #4

I thought that question would come up!

I use my Stages power meter on my outdoor training bike and have a dedicated bike set up on the indoor trainer. So I use virtual power that is set up correctly instead of having to swap cranks every session.


(Clint Westhoff) #5

Makes sense - essentially you calibrate your setup (tire, pressure and knob turns) so you have two accurate, but separate setups with different equipment.

I don’t remember when the last update was, but I did a Zpower to Powertap comparison on 1/18/15 and 1/22/15 and didn’t see any noticeable difference between those two dates.

Here is one: https://support.zwift.com/hc/communities/public/questions/202003645-Another-PowerTap-v-Zpower-Comparison

And here is the other:

https://support.zwift.com/hc/communities/public/questions/201444179-PowerTap-v-Zpower-Comparison


(Robert Lockhart) #6

Yes that’s right with my setup.

The update was 16/01/15 so just before your first test. Out of interest what’s your setup on the KK? (tyre, pressure & knob turns)
Off topic but I’ve been looking for a way to generate a graph that allows me to overlay the power from separate sources. How have you done it for your comparison tests?


(Robert Lockhart) #7

I suppose I should really calibrate the amount of turns on my trainer required for Zwift with my stages independently of Trainer Road. As I can’t presume that Trainer Road and Zwift use identical power curves. Then depending which software I use I adjust the tension accordingly.

It shows how much variation there is in riders that use virtual power if they don’t have a real power meter to calibrate to. Zpower readings can vary quite considerably depending on how they have their trainer set up.


(Clint Westhoff) #8

I have almost always used SportTracks with the overlay plugin for these types of comparisons. But I have also converted files to CSV and done it in excel as well.

My setup is always 100psi with 5 full knob turns. I have a mark on the knob so I can make sure it is 5 turns each time.

I’ve tested different tires (just two), different tire pressures and different clamp down pressure. It seems to me as if clamp down pressure and tire PSI are what can make a difference. If those two things stay consistent then the virtual power will stay consistent (except for temperature drift on fluid based trainers). I have a mag trainer that I have done overlay charts on and you can hardly tell there are two different lines and there is no temperature drift.


(Eric C. (Zwift HQ)) #9

The tyre should be set to 100psi for sure. Previously they were over-reporting wattage (a bummer as that’s how I got a sub-9 second green jersey sprint that I never saved).

They may be a little under right now but we’re dialing it in closer and closer. I don’t believe there is much appreciable difference for time right now (but, of course, everything works well in the office).