PT Auto-zero vs. manual zero torque

Thom_y

Perhaps this is a stupid question, but the manual for the PT SL 2.4 is not that informative. If I set up my CPU so that it is on auto-zero function, is it still recommended that I do a manual zero torque on each ride ?

Thanks.

Alex Simmons

I would suggest a manual zero before each ride and a short period of coasting early in the ride to allow the auto zero function to have a chance to check for any movement in torque zero.

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Steve_B

I would. Here's why:

One of my hubs is permanently skewed so that it's nominally at around 32 ft-lbs without pedalling. When I turn on the computer while using that hub, the auto-zero seems to be able to account for it and tells me zero torque without pedalling. However, if I now use that computer with a different hub (the computer having been auto-zeroed with the skewed hub), the auto zero doesn't seem to want to work and I'll be stuck with 32 unless I zero it myself. So I've gotten in the habit of manually zeroing every ride. But that's just my unusual situation.

I have seen some occasional instances where the auto-zero did not work on the non-skewed hubs for whatever reason (sometimes it's due to temperature changes or due to pressure on the hub from my trainer) and I caught it in time because I manually zero at the start of each ride.

Repairing a file that had torque offset is a pain in the backside so I try to avoid that pain. Doing a manual zero takes about 5 seconds, just unclip from your pedals, zero, clip back in again. Done. Repairing a file takes much longer.

Thom_y

Thanks, But...

Do you have to take your feet off the pedals, if you just rest on pedals while coasting would that be sufficient to zero. Or alternatively, can you zero while stationary and not on the bike before your ride ?

lawrence

jws

Yes to the OP. The auto-zero works well, but it won't auto-zero when torque is less than 7, I think. That's why Steve B finds that it stays at 32.

If you want to watch the auto-zero function at work, do this:

Go to test mode by going into setup, then advancing to "t" with the SELECT button. Then hit MODE.

Keep clicking MODE to go to test 6.

The bottom line of display shows the raw (uncorrected) torque value sent by the hub.
The middle line shows the offset value stored in the cpu which is subtracted from the bottom raw value.
The top line shows that difference which is recorded and displayed as actual torque and used to calculate power.

As you coast, you will probably see the raw value change by 1 or so periodically, and you'll see the offset value quickly adjust to the same. If the raw value changes again before you pedal, the offset will change immediately with the raw value.

Alex Simmons

The auto zero only works when the torque offset is less than about +/- 8ft-lbs. Outside of that range and you'll need to manually zero. Hence why you find the need to manually zero when switching CPU to a different hub.

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DesFlurane

I keep my feet on, if I'm aboard.

I have actually started just lifting the back wheel in the air and zeroing then, having spun the wheel.

Spunout

Perhaps it is more relevant to zero when coasting, riding the bicycle. The bearing drag and weight on the rear axle should be accounted for when setting 'zero'. Hence different observations when on the stand vs. coasting on the road.