"zero out" torque on PT SL - even when zero already??

[grv]

Hi all -

First post here. Looks like a good resource! I did not see the answer to my specific question though, so here goes.

I have a PT SL which I have had for a few months, and it has worked quite well. Early on I became aware of the need to zero the torque on this thing before every ride, which I do. Generally, it reads a low but positive torque value while coasting or at a standstill until I zero it.

Yesterday, it read zero to begin with, so I did not zero it out. I rode. I felt like I had put in a good effort, but the PT said otherwise. It was a dog of a ride power-wise, but my time was comparable to past rides where my avg. power was up to par.

Today, I rode on my I-magic, which never quite agrees with the PT, but I have enough experience with the two to know what a "normal" variance is between them. The torque on the PT was again zero, so I did not zero it out. Again the PT said I was having a bad day, although the IM did not. I stopped mid ride, zeroed the PT even though it read zero torque, and continued.

MUCH BETTER!

So my question: If the PT read zero torque before and after zeroing it out, what's the difference here. It obviously did something...

Also, as long as the hub is awake, can I zero at a standstill, or do I have to be coasting?

Sorry for the long post, and thanks for any help!!

 

[PSUcycling]

Hi all -

First post here. Looks like a good resource! I did not see the answer to my specific question though, so here goes.

I have a PT SL which I have had for a few months, and it has worked quite well. Early on I became aware of the need to zero the torque on this thing before every ride, which I do. Generally, it reads a low but positive torque value while coasting or at a standstill until I zero it.

Yesterday, it read zero to begin with, so I did not zero it out. I rode. I felt like I had put in a good effort, but the PT said otherwise. It was a dog of a ride power-wise, but my time was comparable to past rides where my avg. power was up to par.

Today, I rode on my I-magic, which never quite agrees with the PT, but I have enough experience with the two to know what a "normal" variance is between them. The torque on the PT was again zero, so I did not zero it out. Again the PT said I was having a bad day, although the IM did not. I stopped mid ride, zeroed the PT even though it read zero torque, and continued.

MUCH BETTER!

So my question: If the PT read zero torque before and after zeroing it out, what's the difference here. It obviously did something...

Also, as long as the hub is awake, can I zero at a standstill, or do I have to be coasting?

Sorry for the long post, and thanks for any help!!

I believe that you will get a more accurate number if you zero while standing still. Coasting will add a small amount of neg. torque through the hub and will slightly over-inflate your numbers.

 

[Billsworld]

Thats not what the manual says to do. Do you have some reasoning behind your statement? Not arguing

 

[PSUcycling]

Thats not what the manual says to do. Do you have some reasoning behind your statement? Not arguing

mostly because when I switched to my D/A Pro, my numbers all tended to be a bit lower, which leads me to believe that the PT was reading a little high, I just wonder if that would be a good way to counteract this. If some one had both systems they could test both ways on the PT?

It just seems to me that coasting is going to put a few neg. watts through the hub, so that when you are at 0W it would be reading ~5W. Although I don't know this to be the case.

 

[Billsworld]

I will try to zero the pt at a stop tonight. I dont think it works though. I usually zero Tq by hand spinning the wheel and then zero Tq. When Tq is off I have noticed spikes to 2000 and also readings on the low side. Between Analyticalcycling an lots of work with the PM , you just know when something is off.

 

[acoggan]

when I switched to my D/A Pro, my numbers all tended to be a bit lower, which leads me to believe that the PT was reading a little high

It's more likely that your SRM is incorrect.

 

[grv]

Thanks for the replies guys.

Any thoughts as to why power readings when torque just happens to be zero would be different (lower) than when the torque is reset to zero?

Haven't tried to verify, but I wonder if the PT computer displays negative torque as a zero for some reason (maybe it cannot show a negative number?). That would explain the low wattage readings, I suppose.

Thanks again.

Steve

 

[zaskar]

I may be totally wrong but, i thought if it wasn't zeroed it would read something other then 0 when coasting. is that correct or not?

 

[PSUcycling]

It's more likely that your SRM is incorrect.

That means that my SRM might be reading a bit low.

however, a buddy and I did an uphill TT together. We weigh within a kg of each other. I went 10:13, he went 9:45. After looking at our files, are Average Power was exactly the same. He is done the calibration test, by hanging the weight from the pedal. He feels that his calibration is very close. From that I thought maybe my SRM was reading a little high....

Who knows, i just need to do the test I guess...

 

[PSUcycling]

Thanks for the replies guys.

Any thoughts as to why power readings when torque just happens to be zero would be different (lower) than when the torque is reset to zero?

Haven't tried to verify, but I wonder if the PT computer displays negative torque as a zero for some reason (maybe it cannot show a negative number?). That would explain the low wattage readings, I suppose.

Thanks again.

Steve

i believe that is true.

 

[HammerHead]

How old is your PT? I rarely have to zero mine, maybe once every few months. The only time I had to continually zero the torque, I ended up having a bad torque tube.

 

[HammerHead]

How old is your PT?

A few months - got it. It helps to completely read the post you're respondint to...

 

[Billsworld]

Thanks for the replies guys.

Any thoughts as to why power readings when torque just happens to be zero would be different (lower) than when the torque is reset to zero?

Haven't tried to verify, but I wonder if the PT computer displays negative torque as a zero for some reason (maybe it cannot show a negative number?). That would explain the low wattage readings, I suppose.

Thanks again.

Steve

Thats my excuse for low readings too. Looks like I need a better story. Kidding aside, I have wondered the same thing.

 

[cPritch67]

Hi all -

First post here. Looks like a good resource! I did not see the answer to my specific question though, so here goes.

I have a PT SL which I have had for a few months, and it has worked quite well. Early on I became aware of the need to zero the torque on this thing before every ride, which I do. Generally, it reads a low but positive torque value while coasting or at a standstill until I zero it.

Yesterday, it read zero to begin with, so I did not zero it out. I rode. I felt like I had put in a good effort, but the PT said otherwise. It was a dog of a ride power-wise, but my time was comparable to past rides where my avg. power was up to par.

Today, I rode on my I-magic, which never quite agrees with the PT, but I have enough experience with the two to know what a "normal" variance is between them. The torque on the PT was again zero, so I did not zero it out. Again the PT said I was having a bad day, although the IM did not. I stopped mid ride, zeroed the PT even though it read zero torque, and continued.

MUCH BETTER!

So my question: If the PT read zero torque before and after zeroing it out, what's the difference here. It obviously did something...

Also, as long as the hub is awake, can I zero at a standstill, or do I have to be coasting?

Sorry for the long post, and thanks for any help!!

Have you tried contacting Saris and asking? I have a PT SL as well, and I can tell you firsthand that I wish the instruction manual was a little more user friendly and provided a little more information.

FWIW, I have the PT 3000, indoor trainer with power and that computer will actually post a negative number. I have never shown a negative reading on the PT SL computer yet.

 

[grv]

Have you tried contacting Saris and asking?

What a great idea! Honestly I had heard some horror stories about Saris support, and figured I wouldn't even start there. I just got off the phone with them.

My suspicion was correct - the PT can actually be sending a negative torque value when coasting, but the computer cannot display the negative number, so it shows a "0". Moral of the story, zero your torque, even when it reads a "0" before you do do it. That "0" may really mean -20 or something...

The only time I had to continually zero the torque, I ended up having a bad torque tube.

The tech also said it was not uncommon to need to zero torque at the beginning of every ride. Not sure I buy that yet, as it sounds like some of you don't need to - do you check torque every time, or only when power seems out of whack?

The tech walked me through the process of checking calibration, which I will do when I get home later. Absent a problem with that, or the need to zero torque more than the one time at the beginning of a ride (so far just zeroing at the beginning is OK for me), he was not worried about what I described.

I guess we'll see...

Thanks!

 

[Woofer]

That means that my SRM might be reading a bit low.

however, a buddy and I did an uphill TT together. We weigh within a kg of each other. I went 10:13, he went 9:45. After looking at our files, are Average Power was exactly the same. He is done the calibration test, by hanging the weight from the pedal. He feels that his calibration is very close. From that I thought maybe my SRM was reading a little high....

Who knows, i just need to do the test I guess...

A correctly calibrated SRM should always read a little high to a checked PowerTap on the same bike.

I've done a local hill dozens of times and I can get variations in time of 30 seconds out of 23 minutes total with the same average and normalized power so even doing the comparison with your friend is going to not be as accurate as doing your own calibration. Just do it!

 

[PSUcycling]

A correctly calibrated SRM should always read a little high to a checked PowerTap on the same bike.

I've done a local hill dozens of times and I can get variations in time of 30 seconds out of 23 minutes total with the same average and normalized power so even doing the comparison with your friend is going to not be as accurate as doing your own calibration. Just do it!

I agree, but we were both on D/A Pro SRMs.....your right though, the pacing strategies could have been different.

 

[Billsworld]

Have you tried contacting Saris and asking? I have a PT SL as well, and I can tell you firsthand that I wish the instruction manual was a little more user friendly and provided a little more information.

FWIW, I have the PT 3000, indoor trainer with power and that computer will actually post a negative number. I have never shown a negative reading on the PT SL computer yet.

Does power seem consistant? Have you done any hard intervals on the 3000?

 

[PSUcycling]

A correctly calibrated SRM should always read a little high to a checked PowerTap on the same bike.

I've done a local hill dozens of times and I can get variations in time of 30 seconds out of 23 minutes total with the same average and normalized power so even doing the comparison with your friend is going to not be as accurate as doing your own calibration. Just do it!

i know its been posted before, does anyone have that link on how to do it yourself?

 

[Woofer]

i know its been posted before, does anyone have that link on how to do it yourself?

I had trouble with finding it on their website myself.

Squint helped me out.

http://www.cyclingforums.com/showthread.php?t=325397



Downloaded the whole web page and saved it on a memory stick just in case...