Vector 3 Pedals vs PowerTap Hub

I have a Powertap hub from around 2013 that i have been using on & off since then, just got left side Vector3 pedals. Have correct crank length setup, calibrate before each ride.

The pedals give me much higher readings. A 16k TT course I do pretty regularly i was able to do 301 watts where typically i am in the 270s

Then i was listening to an old ask a cycling coach podcast and someone made an off the cuff remark about a powertap hub being 10% low.

Can anyone explain why there is a difference? Is one less accurate? Is it where the power is measured?
I have always thought the powertab hub was great, super consistent and trouble free. Problem is its a 10 speed hub and i got a new bike…

I can’t speak to your issue of PowerTap hubs running low, but I think there are two other potential things that would explain the difference

First is you’re measuring total power at the hob and only left side power on the pedals. Thus if you have a slight leg imbalance that would explain some of the difference. This is not likely as you’d need to have a pretty severe imbalance, but it is worth mentioning since you’re switching from total power to left only

Second is drive chain losses. If you’re running a worn, poorly lubricated chain and haven’t thought about bottom bracket maintenance since spring then something approaching 10% difference is definitely possible.

Fortunately there is a relatively easy way to test these things! Unfortunately it requires a third power meter. Get access to another power meter and put that on your bike to compare. You could either throw your pedals on a buddy’s bike with a power meter and have them do a ride and compare the power readings from the two or throw another power meter on your bike to compare all three at once.

If you don’t have access to another power meter, or if the one you test against matches neither of the others, you could try replacing and optimizing your drive chain to see how close the two get to each other

All of this said - this isn’t much of an issue except in your brain. Pick which one you’re going to trust and use that moving forward. If you’ve got an 11 speed bike and can’t use the powertap anymore then who cares? Make sure the pedals are measuring consistently, reassess your FTP with them and move forward with your new values

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Thanks. Actually, my drivetrain is probably 10,000+kms easy. Though i do lube regularly and clean the chain and cassette ocassionally.
Anyway, yes i am done with the powertap anyway, just feel funny that my FTP just jumped like that!
But hey, its my new number and will train according to it.

The thing that sucks I guess is this website has all these career stats and records and now its all kind of useless to me…

Sadly, it’s just another sign of the problems in the power measuring industry. As much as we like to think it’s all the “same”, there is more variation than the manufacturers are willing to admit. Presumably they all think that what they make is “right” and the others are “wrong”. But with so many players and different products, the reality is in question in my eyes.

If it helps you for some reason, your FTP didn’t change, just the instrument you used to measure it! :grimacing:

I went from a PowerTap hub to Vector3 pedals and I certainly didn’t experience a 25W change in measurement. Granted, that was between seasons and I hadn’t done a lot of riding when I made the switch, but I’d usually enter seasons around 230W FTP with the PowerTap, and I started out with the Vectors at 232W last season, so not as much of a difference. I reached a 255W FTP on the PowerTap as an all-time high, and now I’m 272W with the Vectors. My performance is significantly better now than it was before, so I think the 10% difference cited is significantly overstating things, and maybe also assuming you are running a stretched out, rusty chain on an old cassette or something… A properly maintained drivetrain won’t experience 10% losses to friction/inefficiency.

Exactly. Consistency is what matters. You use the same device on the same bike and track your performance like-vs.-like.

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I have a powertap hub and powertap P1 on my bike and I notice the same thing. My difference can be anywhere between 2-10% based on how clean/lubed my chain is. I’ve actually seen it start around 2% and by the end of a 10hr ride be closer to 10%. I’ve been riding with this setup for 3yrs. I do not believe it is an issue with either power meter. IMO both are working accurately and consistently. It’s just a difference in where it is measuring the power at.

I would chose 1 power meter to follow and not worry about it. I would use the pedals, since drivetrain losses can vary. The measurement at the pedal really is the best as it has less things that can effect it.

Along side the ease of moving the pedals from bike to bike, this was why I went with pedal-based power meters in the first place.

Contrariwise, if you’re doing DIY aero testing, the measurement at the hub is more informative about the power that is actually being used to overcome air resistance. That’s a circumstance where you may be less concerned about the physiological cost, and more about the actual output.

Agreed and great point. But for setting zones and ftp, the consistency of the pedals can’t be beat. I had a hub only for 2yrs and there were days when I couldn’t hold the same power as normal, even though i felt good. Come to find out, it all depended on how clean/lubed my chain was.

I also have Vector 3 pedals and I may be warrantying my 2nd pair. There have been issues with battery fretting and massive power spikes, but frankly if you have had consistent readings on your rides then it’s probably going to be more accurate than a hub-based power meter.

I wouldn’t say hub based is less accurate. It’s just measured in a different location. My powertap hub is just as accurate as my p1’s. But for the scenario of setting zones and training, the pedals are better. There are other scenarios where the hub might be better.

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Have you considered running the vector and the hub simultaneously? I’m getting the vectors here pretty soon, I have the PowerTap hub now, and plan to run one on my computer and one on my watch, should be interesting!

Yes, it was frustrating. I couldn’t figure a really good way to do this, i put trainer road on my phone and hooked it to the pedals while my garmin was still paired to the powertap. The results showed the big differential in real time as i was looking at my phone in my hand.
I had to start some tr workout, then put the phone in my pocket hoping i could compare the files later but with trainerroad, if i stopped pedaling at all even for 1 or 2 seconds, it would stop recording so in the end it was very hard to compare the files. I tried to grab a 10min section where i was pedaling constantly and again, the 10% or so difference was there. After that i just said F it, move 100% to pedals and forget all that other stuff

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If you race or do any rides where you care even a little bit about speed, you should look into why it is close to 10%. Because it is actually the power the hub reports that is directly creating the speed. Is it the lube you are using? Maybe dirty or unlubed chain? Maybe a crappy or worn chain? Could also be flex in frame causing power loss? The setup you’re running is very beneficial. That’s why I still run both pedal and hub power meters and from time to time will record both. With my bike in top condition, I see as low as 2% power loss, which is close to the accuracy of the power meters listed in their specs. Like I stated above, when my chain lube wears off (squirt wax) I see up to 10% loss.