Bumping this as I’m struggling to get my head round it!

I bought a 4iiii left hand crank power meter a few months ago and it has been repeatable in terms of the same gear ratio and cadence gives the same power until recently. I’m on non resistance rollers with tyre pressures checked, power meter calibrated before riding, room temperature is stable.

I have been doing 10 min intervals in the same gear, same cadence but the average power over the 10 minutes varied by upto about 15 from one day to the next. Battery life is about 50%. Any ideas why this may be happening? Thanks

If these intervals are at z4 or above and your FTP (using 95% of your 20mins MAP) is ~250W+, I’d probably put 15W variance down to fatigue levels.

Looking at my Old Winchester cat4 times from 2018, when I was in decent shape…
19th Apr 303W (9mins20secs) https://www.strava.com/activities/1518306404/analysis/4427/4987
13th May 289W (9mins42secs) https://www.strava.com/activities/1570290147/analysis/3604/4186
2nd Jun 323W (9mins12secs) https://www.strava.com/activities/1613001845/analysis/3065/3617

Not an ideal comparison, as not consecutive days and they are part of rides where I’ve gone up other hills too, but looking at my heart rate I was giving each effort “full beans” on the day and there’s 34W variance from my 4iiii.

Could it be you use your right leg more some days? I agree same cadence same gear should be the same power but you are only measuring one side.

Fatigue doesn’t come into it. Same gear, same cadence, same resistance hence you’d expect the same power to be needed.

Could temperature be affecting the power needed to spin the rollers? More drag in the roller bearings when cold? Can you correlate with air temperature?

Could it be you use your right leg more some days?

Interesting… could be the reason.

Interesting points. I initially ignored left right inbalance as i thought that if I had it, it would be consistent from day to day. But it looks like garmin have updated their software in the last couple of weeks and it now shows torque effectiveness left ( about 75 ) and right, pedal smoothness left ( about 21 ) and right. These figures do change slightly from one activity to the next and I don’t really understand them – especially as i only have a single sided power meter.

The air temp in the room drops by about 4C over 1 hour – i have the doors open to the garden and a fan blowing, hadn’t really thought about bearings getting hotter but surely that would happen for all workouts?

Would a change of tyre type have a significant effect – same pressure and width but the one used that gave the lower power readings feels like it has softer sidewalls

Tyres will have a huge affect. Try switching to 32`s to see how much extra power it requires for same cadence/speed. Likewise just different compounds in a much more subtle way. ALso im guessing you arent anwhere near powers quoted above, i start to jump around above 240w on rollers.
Edit: when i have to do high power sets I switch to my winter tyres and heavy wheels even on rollers. Makes a huge difference.

If I’m on the turbo my power can easily fluctuate by 10/15 watts despite my cadence remaining relatively static. The difference between 79rpm and 80 rpm would account for 10 watts I reckon so unless you are keeping the exact same cadence for your interval I think you are all good. Especially given all the other variables that although small will all add up to make a difference

I’m using a stages

Are you using an actual cadence meter or the 4iii software approximation.

Tpbiker, I’m looking at the average of 10 mins and seeing this difference – not the fluctuation during the 10 min period

Trail rat, I’m using the 4iiii to give the cadence. I took my garmin cadence off when I got the 4iiii as I thought they couldn’t work together. Is one more accurate than the other?

Well I know my kickr core is not aligned with my Garmin cadence monitor.

And when I read reviews prior to buying the 4iiii it was noted that there was questionable accuracy in the cadence system.

Don’t 4iiii use an accelerometer for their cadence? I’d assumed the guys if a 4iiii would be similar to Stages – they rely heavily on accurate cadence for measuring power and use an accelerometer to do so.

OP – the most obvious culprit for me would be different tyres. Having slightly different’form’ on rollers will have an effect as well: if you’re moving around a bit more it will have a bit of impact on resistance in the rollers

Thanks all, think I will try some trials. Staying in one gear and ramping up the cadence between 90 and 100 in 2 rpm steps for 2 min intervals to see the effect of cadence through 90-100rpm. Then do fixed cadence and ramp up the gears in 2 min intervals. Then vary tyre pressure for fixed gear and cadence. And try to keep my form!