Can you tell me what the hub torque value means?

I run all 2.4 PT hubs but not sure what this means.

I have had the 404's. Hed 90 and now have the 100's. Personally I would sell the Zipp due to higher resale value. The 100's are fast and light and more reasonablly priced, plus since Blackwell may not be around anymore, the resale values have been low. So to get the biggest bang for you buck sell the over priced Zipps.
S

I agree with keeping the 100's, in regards to resale value as mentioned previously and the Blackwells are great wheels. Good fit with tires and everyone rides Zipps.

So a question...if my hub torque value is greater than 512 (519) is my power underestimated?

In Reply To

  My understanding (if you want to call it that) is that there's a torque value that the hub uses to help calculate wattage using algorithms and other information from the hub. You can get there by going into test mode (squeezing both buttons past find and past clear then release) and hit the mode button until the t flashes. Once there you hit the select button to toggle to the third option which is hub and the middle line is the torque value for the hub (please consult the owners manual as I am going by memory and I'm sitting here enjoying Chilean wine and aged cheese). The default value is 512 and Saris says that +/-10 of the value is "within specs". I'm pretty sure that if your middle number is below the 512 default then the hub is over-estimating your wattage. I have two and they are at 516 and 517 and when I rode another set of wheels with a value of 506 my numbers on those wheels were noticeably higher (which I liked heh heh).

Accuracy of wattage can be calculated by comparing your previous performances (knowing your CdA) and all other variables (wind speed, slope, air density etc.) and using the models (e.g. on analyticcycling.com) to calculate required wattage to meet the performances you have achieved. I plan on doing this if I could get off the dang computer!!!

In Reply To

 
Ummm....no. The torque value you are describing is the "zero offset" value, or fixed bias. Basically, it's the torque reading the head needs to subtract from the torque output to get the "true" torque being input. If it's different that 512, it makes absolutely no difference on the power readings IF YOU'VE PROPERLY SET THE ZERO OFFSET for your combo of head unit and hub.

When first pairing a hub with a head unit, it is ALWAYS good to manually "zero" this offset (see your manual), since if the offset transmitted by the hub is too far outside a certain range of the number stored in the head unit, it can't automatically zero. Oh yeah...ALWAYS set the head unit to "auto-zero" (again, see manual) since this zeros head unit every time you coast. That way, any drift of the offset during the ride due to temp changes, etc. will be removed.

Now, if you properly did the zeroing when switching wheelsets and you felt that one was reading higher or lower, then the "slope" of the torque curve for one may be off. This can be checked with the "torque" display by hanging known weights from the crankarm and comparing the torque reading with the torque calculated at the hub. This is sometimes know as "stomp" testing (since a quick and dirty, but not very accurate, way to do this is using body weight). See the following for a description:

http://midweekclub.ca/powerFAQ.htm#Q23

So...in short, when zeroed properly, the zero torque numbers you described above have absolutely NO affect on the hub's accuracy.

Just my 2 centavos ;-)

I have two powertaps sl 2.4 a 2008 model and a 2007 model.
2007 model gives 548
2008 model gives 526

What does the 548 number mean? It's complete out of the range you gave (512 plusminus 10)

Man I just join this community and I already am probably leading people astray!!!

Glad I put the caveat that I talked to Saris about this once.

I am referring to the 2.4 hubset and was told that that the stomp test was not applicable because of the test mode in the new CPU. According to the manual it says on page 37 that:

The bottom display is the offset torque value directly sent by the hub torque with no correction applied. the hub sends a value of approximately 512 as the zero point. The middle line display is the correction offset value currently stored in the computer. This is the value that is changed when you do a manual zeroing of the torque. the top display is the corrected actual torque (the bottom value minus the middle value).

Not sure that this helps. I was relaying what Saris told me to explain why the Aeolus wheels were giving me "nicer" numbers. I originally called them to ask whether the 16 spokes in the rear wheel impacted the accuracy of the hub because of less spokes impacting the accuracy of the strain gauge (to which there was a NO repsonse). The then took me to the value mentioned above and it was 506 and then it was offered that was a possible source of amplification because I told her yes I did do a zero-torque before using the wheels because the rider who's wheels I used could not recall the last time he performed the zero-torque.