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Does anyone know the difference between hub based power measurement and crank based power measurement? When I think about the 2, I always think the hub based system would be better as it measures the power going into the rear wheel. The crank type would be the power the rider puts into the bike but not necessarily what is going into the rear wheel. My thought is... if I were to choose an inefficient set of gears I could be cranking as hard as possible but not efficiently delivering power to the rear wheel. In this scenario I would guess that the two systems would read different power levels.

Has anyone seen anything on this or had experience with it? Am I off base in my view of the two systems?

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triviral wrote:
Has anyone seen anything on this or had experience with it?
Yes.
Quote:
Am I off base in my view of the two systems?
No.
Most of the extreme differences noted are due to calibration issues. It depends on what metric you want to base your training off of: wattage output of legs or wattage transfer to the road. With all variables held constant, they are both equal training metrics, and entirely relative.
carlwithac wrote:
Most of the extreme differences noted are due to calibration issues. It depends on what metric you want to base your training off of: wattage output of legs or wattage transfer to the road. With all variables held constant, they are both equal training metrics, and entirely relative.
Well, that may be true from a training perspective but since I do drag testing I'm pretty happy to have a PT (if I were doing drive train loss estimates I'd need a crank-based PM).
What is the difference between torque and power?
Power = torque x distance (x sinus to an angle if the distance and the torque is not in the same direction). For crank-based powermeters you can effectively think of it as power = torque x cadence. For hub-based powermeters the equivalent is power = torque x wheel rpm.
MTM wrote:
Power = torque x distance (x sinus to an angle if the distance and the torque is not in the same direction). For crank-based powermeters you can effectively think of it as power = torque x cadence. For hub-based powermeters the equivalent is power = torque x wheel rpm.

Thank you. Let me explain what I am getting at. I know that torque equals r cross f (as a vector product). You apply the torque at the pedal. It seems to me that a crank based power meter would be more accurate because that is where you are applying the torque. How does a hub based power meter measure torque since you are not apply the torque at the hub?
triviral wrote:
Does anyone know the difference between hub based power measurement and crank based power measurement? When I think about the 2, I always think the hub based system would be better as it measures the power going into the rear wheel. The crank type would be the power the rider puts into the bike but not necessarily what is going into the rear wheel. My thought is... if I were to choose an inefficient set of gears I could be cranking as hard as possible but not efficiently delivering power to the rear wheel. In this scenario I would guess that the two systems would read different power levels.

Has anyone seen anything on this or had experience with it? Am I off base in my view of the two systems?

You are not applying force at the hub, the chain is. There are strain gauges within the hub, just as there are crank assembly of crank based meters.
QRNub wrote:
You are not applying force at the hub, the chain is. There are strain gauges within the hub, just as there are crank assembly of crank based meters.

Is there any power loss through the chain? Is a crank based power meter more accurate because that is where you are applying the force?
ToKnowMore wrote:
QRNub wrote:
You are not applying force at the hub, the chain is. There are strain gauges within the hub, just as there are crank assembly of crank based meters.

Is there any power loss through the chain? Is a crank based power meter more accurate because that is where you are applying the force?

Yes, there is power loss in the drivetrain...but, both types of power meters (correctly calibrated and maintained) are equally accurate. One is just "upstream" of the drivetrain losses, while the other is "downstream".

Personally, I prefer crank-based for monitoring the "engine" (i.e. training purposes) and hub based for monitoring the "load" (i.e. field testing), since both choices reduce the variables for each purpose.

http://bikeblather.blogspot.com/
Tom A. wrote:
ToKnowMore wrote:
QRNub wrote:
You are not applying force at the hub, the chain is. There are strain gauges within the hub, just as there are crank assembly of crank based meters.

Is there any power loss through the chain? Is a crank based power meter more accurate because that is where you are applying the force?

Yes, there is power loss in the drivetrain...but, both types of power meters (correctly calibrated and maintained) are equally accurate. One is just "upstream" of the drivetrain losses, while the other is "downstream".

Personally, I prefer crank-based for monitoring the "engine" (i.e. training purposes) and hub based for monitoring the "load" (i.e. field testing), since both choices reduce the variables for each purpose.

Thank you, Tom. That makes sense. Have you ever ridden with both at the same time and compared the results?
RChung wrote:
carlwithac wrote:
Most of the extreme differences noted are due to calibration issues. It depends on what metric you want to base your training off of: wattage output of legs or wattage transfer to the road. With all variables held constant, they are both equal training metrics, and entirely relative.

Well, that may be true from a training perspective but since I do drag testing I'm pretty happy to have a PT (if I were doing drive train loss estimates I'd need a crank-based PM).
Well yes. The best type is the one one that suits the individual's needs.
ToKnowMore wrote:
Tom A. wrote:
ToKnowMore wrote:
QRNub wrote:
You are not applying force at the hub, the chain is. There are strain gauges within the hub, just as there are crank assembly of crank based meters.

Is there any power loss through the chain? Is a crank based power meter more accurate because that is where you are applying the force?

Yes, there is power loss in the drivetrain...but, both types of power meters (correctly calibrated and maintained) are equally accurate. One is just "upstream" of the drivetrain losses, while the other is "downstream".

Personally, I prefer crank-based for monitoring the "engine" (i.e. training purposes) and hub based for monitoring the "load" (i.e. field testing), since both choices reduce the variables for each purpose.

Thank you, Tom. That makes sense. Have you ever ridden with both at the same time and compared the results?

Yes...and it's partly how I discovered I was losing over 10W in my drivetrain due to some "misadjustments"! Of course, prior to this I made sure that the torque slope calibration of the 2 was within 0.5%.

I've also had more than just the 2 PMs mounted to my bike at one time... ;-)

And, in the past, I've ridden with more than just the 2 :-)

http://bikeblather.blogspot.com/
Last edited by: Tom A.: Feb 25, 11 7:43
From a techy perspective I find a lot of this fascinating. I got to use a power meter (rear wheel) for the first time while I was away in France last September and it was quite a learning experience. At the newbie level it made me think about efficient use of my stored body energy on hills etc.

One question If you are running a bike with 3 power meters and the associated recording/display heads, how many watts are being used to carry all that extra weight? :o)
WappidWojj wrote:
From a techy perspective I find a lot of this fascinating. I got to use a power meter (rear wheel) for the first time while I was away in France last September and it was quite a learning experience. At the newbie level it made me think about efficient use of my stored body energy on hills etc.

One question If you are running a bike with 3 power meters and the associated recording/display heads, how many watts are being used to carry all that extra weight? :o)

Why only 3? ;-)

http://bikeblather.blogspot.com/
That is all well and good to delve into and understand, but in practical terms for your training and performance, it is a red herring.

Through constant training and testing you will be able to determine and utilize whatever the powermeter reads to optimize your training and performance. Through attention to the metrics, you will easily determine what combinations of gearing, power output, etc. produce your best efforts at a given distance or effort.

What matters more than anything is that your particular powermeter is able to produce repeatable numbers day-in/day-out. 250 watts today should be 250 watts tomorrow. It doesn't even matter if your powermeter's 250 watts is the same as someone else's 260 watts. Its the relationship of your numbers to you that is important and that allows you, and your coach if you have one, to plan or alter training to produce increased performance.

SRM, Quarq, and PT are all robust, proven systems which are used successfully, and in the same manner, by cyclists around the world to incorporate power into their training. Each type have their practical +/- points. PT is the most obvious as it forces a choice on how you'll approach training and racing wheels to maintain access to power information. You may already have a great set of race wheels that you don't have \$ to replace with a PT-hub version. If so, maybe that makes you choose one of the other two. Regardless....you will get where you're trying to go with any of the systems.

Last, I would remind you to review the scientific definitions of "accuracy" and "precision". In practical usage of a powermeter as I discussed above, precision trumps accuracy. Said another way, you might have a VERY accurate powermeter that suffers from a relative lack of precision. That powermeter will be less useful to you in day-to-day usage than one with great precision but relatively poor accuracy. Luckily for you, the three main contenders are all very similar these days.

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Last edited by: TriBriGuy: Feb 25, 11 8:17
Tom, could you elaborate more on what you found re. drivetrain losses? How were you losing 10w and how small were you able to get the drivetrain losses?
Help those of us without 2 power meters take steps to get our drivetrain losses minimized.

Thanks.
TriBriGuy wrote:
What matters more than anything is that your particular powermeter is able to produce repeatable numbers day-in/day-out. 250 watts today should be 250 watts tomorrow. It doesn't even matter if your powermeter's 250 watts is the same as someone else's 260 watts. Its the relationship of your numbers to you that is important and that allows you, and your coach if you have one, to plan or alter training to produce increased performance.
That's true only so long as you don't have to worry about an external reality. It's like saying you don't need to worry about the accuracy of your car's speedometer as long as it's repeatable -- try that argument on the judge when the police say you were speeding. The most important thing about a power meter is the quality of the data it produces. Everything else is a feature. Being able to check the accuracy and precision of your power meter is key. One might even say a "9 key."
ToKnowMore wrote:
Have you ever ridden with both at the same time and compared the results?

Gee, it sure would be nice if someone created some sort of Rosetta Stone to cross-compare different power meters.
Last edited by: RChung: Feb 25, 11 8:23
Tom A. wrote:
ToKnowMore wrote:
Have you ever ridden with both at the same time and compared the results?

Yes...and it's partly how I discovered I was losing over 10W in my drivetrain due to some "misadjustments"! Of course, prior to this I made sure that the torque slope calibration of the 2 was within 0.5%.
If you had a hill, the same tires, the same position, measured mass and rho, and either a calm day or else the stick, you wouldn't need the PT. The load would be set by the hill (and mass and rho and wind) and you could vary chainring/cog to see differences in drive train loss.
RChung wrote:
TriBriGuy wrote:

What matters more than anything is that your particular powermeter is able to produce repeatable numbers day-in/day-out. 250 watts today should be 250 watts tomorrow. It doesn't even matter if your powermeter's 250 watts is the same as someone else's 260 watts. Its the relationship of your numbers to you that is important and that allows you, and your coach if you have one, to plan or alter training to produce increased performance.

That's true only so long as you don't have to worry about an external reality. It's like saying you don't need to worry about the accuracy of your car's speedometer as long as it's repeatable -- try that argument on the judge when the police say you were speeding. The most important thing about a power meter is the quality of the data it produces. Everything else is a feature. Being able to check the accuracy and precision of your power meter is key. One might even say a "9 key."

I'm not following your analogy to a car's speedometer....one is faulty equipment that leads to you breaking the law and is compared against an objective standard. The other is piece of bicycle equipment that only needs to be compared against itself.

As TriBriGuy said, the key is consistency and repeatability. If the PM reads 250 watts for a given input, as long as it always reads 250w for the same input, you are fine in regards to training. Note the emphasis. Whether the wattage reported is the actual wattage is almost immaterial. You are training against YOUR values only. As long as your measurements are consistent and repeatable, then you can train properly with those measurements. Everything is still a percentage of those readings.

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You are correct, of course, in absolute terms. But the last time I checked, we do not win races on watts input. And we don't win them on our watts readout vs our competitor's watts readout. We win them on the results of distance over time.

And let's also be clear that the different systems in question are very close in accuracy and precision for practical usage.

The majority of folks are not diving into the testing minutae like you and Tom where the absolute accuracy becomes important. How many people are riding around with 2+ powermeters on their bike because they want to determine drivetrain loss?

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Power13 wrote:
RChung wrote:
TriBriGuy wrote:

What matters more than anything is that your particular powermeter is able to produce repeatable numbers day-in/day-out. 250 watts today should be 250 watts tomorrow. It doesn't even matter if your powermeter's 250 watts is the same as someone else's 260 watts. Its the relationship of your numbers to you that is important and that allows you, and your coach if you have one, to plan or alter training to produce increased performance.

That's true only so long as you don't have to worry about an external reality. It's like saying you don't need to worry about the accuracy of your car's speedometer as long as it's repeatable -- try that argument on the judge when the police say you were speeding. The most important thing about a power meter is the quality of the data it produces. Everything else is a feature. Being able to check the accuracy and precision of your power meter is key. One might even say a "9 key."

I'm not following your analogy to a car's speedometer....one is faulty equipment that leads to you breaking the law and is compared against an objective standard. The other is piece of bicycle equipment that only needs to be compared against itself.

As TriBriGuy said, the key is consistency and repeatability. If the PM reads 250 watts for a given input, as long as it always reads 250w for the same input, you are fine in regards to training. Note the emphasis. Whether the wattage reported is the actual wattage is almost immaterial. You are training against YOUR values only. As long as your measurements are consistent and repeatable, then you can train properly with those measurements. Everything is still a percentage of those readings.

1. How do you know it's consistent?

2. What happens when you get a second PM?

3. Part of training to go fast is knowing what your drag parameters are. If you don't have accuracy how will you measure them?

People who are new to PMs often make the "precision is more important than accuracy" claim. That's myopic.
TriBriGuy wrote:
And let's also be clear that the different systems in question are very close in accuracy and precision for practical usage.
I'd say drag measurement is a pretty practical usage.

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