Non-zero watts when coasting...

[Terry Ferguson]

PT Pro has been working fine until this morning when I get on and it won't zero. If I coast or just spin the back wheel, it will read a low wattage - about 15-40 depending on the speed the wheel is turning (faster the speed, higher the watts). The first thing I did (I normally do it anyway) is to try to zero the torque - makes no difference. It does go to zero when the wheel stops. Any idea where to start my diagnostics? - TF

 

[Terry Ferguson]

Here is the power distribution curve showing the 'coasting peak'. Any idea what I would do to clean up this data to something realistic? - TF

EDIT: Better picture.

 

[Terry Ferguson]

Also, looking at the raw data - even as the 'coasting power' drops with the speed as I coast to a stop, the torque remains a constant 1.2 N-m. How can the torque not be proportional to the power? - TF

 

[sugaken]

Also, looking at the raw data - even as the 'coasting power' drops with the speed as I coast to a stop, the torque remains a constant 1.2 N-m. How can the torque not be proportional to the power? - TF

I'm afraid I don't have a clue why the torque remains constant non-zero value, but given that, it makes sense that you get power proportional to speed since power = torque x angular velocity; if the torque remains constant, power becomes proportional to angular velocity of the wheel which is, of course, proportional to speed of the bike.

Did you zero torque as per the manual (i.e. hold down select button 2sec to enter torque mode, then hold down select again until "0" is shown)? If you did that and still get non-zero torque reading when not pedaling, I think you should contact Saris.

Ken

 

[fabiosav]

I'm afraid I don't have a clue why the torque remains constant non-zero value, but given that, it makes sense that you get power proportional to speed since power = torque x angular velocity; if the torque remains constant, power becomes proportional to angular velocity of the wheel which is, of course, proportional to speed of the bike.

Did you zero torque as per the manual (i.e. hold down select button 2sec to enter torque mode, then hold down select again until "0" is shown)? If you did that and still get non-zero torque reading when not pedaling, I think you should contact Saris.

Ken

I would call Saris, 100% sure. I had a "bent torque tube" and they changed it. A woman named Maggie on t eh help desk is great, if you get her. There is a test they do, so have the bike around, and if possible on a trainer, on in a way you can spin the wheel. Takes two seconds, if you get the right tech...

 

[Woofer]

PT Pro has been working fine until this morning when I get on and it won't zero. If I coast or just spin the back wheel, it will read a low wattage - about 15-40 depending on the speed the wheel is turning (faster the speed, higher the watts). The first thing I did (I normally do it anyway) is to try to zero the torque - makes no difference. It does go to zero when the wheel stops. Any idea where to start my diagnostics? - TF

If you have a PowerTap Pro CPU ,look for the post by grv where he explains how to check the offset of your wheel.
http://www.cyclingforums.com/t-345071-15-2.html

 

[Terry Ferguson]

PT Pro has been working fine until this morning when I get on and it won't zero. If I coast or just spin the back wheel, it will read a low wattage - about 15-40 depending on the speed the wheel is turning (faster the speed, higher the watts). The first thing I did (I normally do it anyway) is to try to zero the torque - makes no difference. It does go to zero when the wheel stops. Any idea where to start my diagnostics? - TF

It appears that it is adding the speed in Km/hr to the watts??? I took a few places where I was coasting to a stop and pulled the speed and watts out. A linear regression was 1.115x - 0.032. What the hey? - TF

 

[Terry Ferguson]

And curiouser yet...I have to Pro hubs and two head units. One set (hub & cpu) works great. The other hub OR the other cpu does the 'add the speed to the watts' thing. ?????

Saris has not answered my email. Guess I will have to call. I HATE THAT!

TF

 

[acoggan]

It appears that it is adding the speed in Km/hr to the watts??? I took a few places where I was coasting to a stop and pulled the speed and watts out. A linear regression was 1.115x - 0.032. What the hey? - TF

With the PowerTap, power is calculated as the torque times the angular velocity of the hub, i.e., the rate at which the hub rotates. Speed is also calculated from the rate at which the hub rotates. Thus, if the reed switch isn't working properly, both power and speed will be off, but torque will still be correct, whereas if the torque "tube" isn't working properly, torque and power will be incorrect, but speed will be registered correctly.

 

[frenchyge]

It appears that it is adding the speed in Km/hr to the watts??? I took a few places where I was coasting to a stop and pulled the speed and watts out. A linear regression was 1.115x - 0.032. What the hey? - TF

I doubt that it's adding the speed, but rather that a very low, constant torque is being sensed while coasting. Since power is torque x speed, that very low constant torque produces a power reading proportional to speed, as your regression shows.