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?
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.
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).
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.
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?
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.
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.
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?
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.
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
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)
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)
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.
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.
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.”
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.
