New to power meters - surprised by power readout

Former Member over 6 years ago

Hello,

Apologies if this has already been addressed here - I trawled this forum for a while but didn't quite get the answer(s) I had hoped for, so I'll try asking anyway.

??????I am unsure of the power data that I am receiving from my Vector 3S pedals. However, the only thing that I have to compare it to is the Virtual Power calculation from the rear wheel speed and my trainer's power curve.

For reference, I have a Tacx Booster magnetic brake trainer on level 6 together with Garmin Speed and Cadence sensors. My calculated FTP is 179W. When I first received my Vector 3S, the batteries were dead and I was forced to change them immediately to get the pedals to work at all (this issue I raised with my supplier). After installing new batteries I upgraded the pedal firmware to v3.50 immediately from Garmin Connect on my phone.

I have the correct crank length entered in the settings, auto-zero enabled and have conducted a calibration before riding (successfully, with a zero result). When I ride, I use my Forerunner 920XT as the "head unit" and keep the virtual power calculation in the training app I use to compare the numbers. I have heard and read that one usually experiences a "loss" in power when moving from Virtual to Actual power.

However, when I ride I see that the Vector 3S consistently reports a power output 60-70W greater than that of the Virtual Power, meaning that my Vector 3S FTP is near 250W. As a non-racing cyclist, I find this a little surprising and borderline unbelievable. I do not have the tools or opportunity to conduct a torque test to verify the pedals. Is there something else I can do to confirm correct operation of the pedals?

0 TrippyZ over 6 years ago

I recommend you take your bike to the LBS and have them check the torque.

0 Browner40 over 6 years ago

There's a few things that could be going on here:

You probably need to do a few sprint type segments to make sure you have the pedals fully seated down and then do a calibration.

You have single sided pedals and your left leg might be producing significantly more power than your right. The way the pedals report power is by doubling the reading that it gets from the left pedal, If you are out of balance your overall reading will be much higher. You could also have some other issues going on that are diminishing the speed of the rear tire (brakes dragging, lots of drive train loss) and since the only way the program is able to report power is based on wheel speed and "expected resistance" from the trainer, the virtual power number the training program is reporting could be way off.

The power pedals are reporting an actual measured force from your foot, not a guestimate based on a bunch of dubiously accurate figures. If it comes down to which one to believe between the two, I would definitely side with the pedals.

How much do you weigh? The real number you need to worry about is W/kg not just watts. A 250W FTP puts me at 2.5 W/kg which is just barely out of "Untrained" status, where a 50kg person would be at 5W/kg which would put them at almost Pro level.

0 Former Member over 6 years ago

Good answer - thanks!

I have done several sprint segments with the trainer app I'm using. Maybe not absolutely flat out but pretty close.

Certainly possible that I'm unbalanced. I had considered the rear tyre drag and will check it later (air pressure, brake tension, etc.), although I would say that the Virtual Power is very consistent, i.e. the same gear and cadence combinations produce a stable number, repeatedly. I had considered that the Virtual power is inaccurate on the low side but, as I wrote, this is not the feedback I get from other users who switch.

I would absolutely believe the accuracy of the pedals over the virtual calculation (which is the reason I bought the pedals in the first place) if I was happy that the pedals are operating correctly. That's the bit I'm having some issues with (for no real reason, actually, beyond expectations).

I weigh 75kg, so 250W gives 3,33W/kg. In the middle of a CAT4 cyclist. I'm reasonably fit, I guess but this seems a little high (he said modestly :) ).

0 Browner40 over 6 years ago

Hmmm... it does seem a bit high. My next steps would be:

Perform the torque test by hanging a known weight from the pedal as described here: https://support.garmin.com/en-US/?faq=AVZeQisZvi3oBYZUUZdPV6
If it is within the ~2% mentioned in the directions then congratulations! you are a mid-Cat 4 cyclist. If the test reports way out of spec, then call Garmin customer support and get a new pedal set.

0 Former Member over 6 years ago

welcome to garmin pedal issues, i recorded on my last ride 26343 peak power, was something 365watts per kilogram

0 Former Member over 6 years ago

An update on this issue: my LBS was unable to execute the recommended torque test to confirm the correct operation of the pedals and, frankly, I can't bothered to travel the required distance to find a store that can do this accurately. Through the LBS, I was told that Garmin had recommended that I ride outside and see if the power numbers reported seem sensible. I can ride outside (although now the temperatures have dropped below -7C I'm less inclined to do so) but, still, without any frame of reference I can't see how I will know that the numbers will be "sensible" or not.

As I usually just train indoors and, as such, am only ever comparing myself to myself, I think I will just accept that the pedals report those numbers (consistently, it must be said) and base my indoor training from those numbers.

0 t_smit over 6 years ago

As I usually just train indoors and, as such, am only ever comparing myself to myself, I think I will just accept that the pedals report those numbers (consistently, it must be said) and base my indoor training from those numbers.

As an approximate ballpark guide, you can use your own weight to validate that your pedal is reporting decent results (at least to resolve the 179-vs-250 question):
At 75 kg, your weight is 735 N. Thus, if you rest *all* your weight on the left pedal when it's horizontal and forward (while locking the rear wheel with the brake) you should see 735*0.1725 = 126.8 N.
If you do a zero calibration on your Forerunner 920, the display should show the applied torque afterwards.
Before you start, validate that your 920 has the correct crank length set.

Ride the bike on the trainer at 60 to 90 RPM for a few minutes to ensure the spindle installation angle has been set.
Use your Forerunner 920 to do a zero calibration.
With the left crank arm horizontal and forwards, balance yourself (carefully!) on the left pedal while locking the rear brake.
You should see a torque value (depending on how much weight you can transfer to the pedal) in a range corresponding to 90 to 100% of your body weight.

Do not do this if your trainer is at all prone to tipping if you happen to lean way out to one side.

0 Former Member over 6 years ago

As an approximate ballpark guide, you can use your own weight to validate that your pedal is reporting decent results (at least to resolve the 179-vs-250 question):
At 75 kg, your weight is 735 N. Thus, if you rest *all* your weight on the left pedal when it's horizontal and forward (while locking the rear wheel with the brake) you should see 735*0.1725 = 126.8 N.
If you do a zero calibration on your Forerunner 920, the display should show the applied torque afterwards.
Before you start, validate that your 920 has the correct crank length set.
Ride the bike on the trainer at 60 to 90 RPM for a few minutes to ensure the spindle installation angle has been set.
Use your Forerunner 920 to do a zero calibration.
With the left crank arm horizontal and forwards, balance yourself (carefully!) on the left pedal while locking the rear brake.
You should see a torque value (depending on how much weight you can transfer to the pedal) in a range corresponding to 90 to 100% of your body weight.
Do not do this if your trainer is at all prone to tipping if you happen to lean way out to one side.

I'm afraid this only resulted in a calibration failure 4.

0 Former Member over 6 years ago

As an approximate ballpark guide, you can use your own weight to validate that your pedal is reporting decent results (at least to resolve the 179-vs-250 question):
At 75 kg, your weight is 735 N. Thus, if you rest *all* your weight on the left pedal when it's horizontal and forward (while locking the rear wheel with the brake) you should see 735*0.1725 = 126.8 N.
If you do a zero calibration on your Forerunner 920, the display should show the applied torque afterwards.
Before you start, validate that your 920 has the correct crank length set.
Ride the bike on the trainer at 60 to 90 RPM for a few minutes to ensure the spindle installation angle has been set.
Use your Forerunner 920 to do a zero calibration.
With the left crank arm horizontal and forwards, balance yourself (carefully!) on the left pedal while locking the rear brake.
You should see a torque value (depending on how much weight you can transfer to the pedal) in a range corresponding to 90 to 100% of your body weight.
Do not do this if your trainer is at all prone to tipping if you happen to lean way out to one side.

i have a question for T_Smit.When i have bought my garmin vector 3 about 40-50 days ago,the first rides i have been using the pedals with Rotor 3D24 crank(and p2max Ngeco ).all good power difference was (2-5 watts )so almost perfect.Then i have switched the pedals to Dura Ace 9100 cranks-same size /(i felt from the first time that they can t be torqued that hard as on Rotor cranks).Anyway power was more or less in line with what i expected(maybe a little lower).But then after 1 month or so,I switched to indoor training my power seem to be lower like 10-14 watts.i have changed batteries ,and then make a static torque test with a expected weight of 17.37 nm.My values were 16,72 left and 16,47 right(average as i did 3 for each pedal).So i changed the scale factor a little to 1.034 or so for both .My question is :are the pedals affected about the cranks model (i mean about how they sit on the crank) and if it is normal to drift a little after first 1000km and be necessary this scale modification.i always have the same crank length ,doing autozero 2-3 times indoor,before and in the ride.no spikes or other problema just this low reading after 1000 km.now the readings are ok no problem ,but i needed this correction which in first month wasn t necessary.i also use one spacer on the right(due to chain)and none on the left.can this affect something?

0 t_smit over 6 years ago

It may take a couple of rides with some hard efforts before the zero offset stabilizes.

Dura Ace cranks are definitely strong enough to allow you to torque the spindles to 34-40 N-m (as Shimano recommends). That is enough that the pedals will work properly, unless there is some debris between the spindle and the crank face. The pedals are not otherwise affected by what cranks they are mounted on. It also does not matter if you use a spacer on one, and not on the other, unless it affects your personal fit on the bike.

It is normal to not be able to achieve the same power indoors, as outside. This usually has to do with not being able to get enough cooling air.