Weird Powertap Torque Readings

[kssz2]

Powertap conversion question - after converting an old standard powertap to a fixie/track hub with the surly fixxer and custom axle, I find all the displays on the head unit seem to work well including the 'watts'

So after a while I get off the bike, zero the torque, turn back to 'watts' and ride away - but no watts are now reading - nothing on the watts at all. I get off and check the torque which I've zero'd and its hovering around 000 (+/- 1or 2), get back on and still nothing on the watts, no matter what speed/effort I'm doing

Yet if I do a reset on the head unit - the watts will now display, but with no pressure/movement on the pedals the torque hovers around 75-80. If I leave it the watts display, if I zero it, no watts will display

Would this have something to do with the conversion, I have two head units and they both do the same thing. The watts that do display when the torque is hovering around 70-80 seem to have a reading consistent to my actual power output

Can anyone assist
thanks
KC

 

[BikingBrian]

I think - and I'm not completely sure - that it's not really possible to simply convert the hub like you did. IOW, that Surly kit works for regular road hubs, but maybe not for a PT.

If you go to Wheelbuilder.com | Precision Wheels for Serious Athletes they now convert PT hubs to fixed....it's possible that there's more to it than just simply removing the freehub, in order to have the torque read properly. A clue on the website is that it says the modification is "permanent", unlike a Surly fixer or other solution.

 

[kssz2]

its possible with the older PT hubs before they updated the CPU in them, the conversion I have done is quite common amongst trackies and fixxies. Wheelbuilder actually sell the Surly Fixxer with their own custom axle which is where I bought the conversion kit from

Its not possible with the newer hubs to self mod with the surly fixxer as the newer CPU just won't work with this conversion.

" A clue on the website is that it says the modification is "permanent", unlike a Surly fixer or other solution." = this refers to the conversion Wheelbuilder can/will do on the newer PT hubs which can also be converted, but only by them, by doing so it makes the new hubs a permanent mod

If my torque is reading 70-80 after reseting the head set what find of gross error would this normally produce, say if your road wheel was reading 70-80Nm and you left it and went for a ride what kind of error in Wattage would you expect, its just odd as the power readings I'm getting with the torque sitting around this figure seem to be close, if not what if should be for the efforts I'm puttng in

thanks for the feedback - keeps me thinkin
KC

 

[kssz2]

MadeinChina < snip >. REPORTED SPAM

 

[daveryanwyoming]

I run a converted PT pro hub with the Surly Fixxer kit with no troubles. Have you disabled the auto torque-zero mode in the CPU setup? If you don't you'll have trouble as the zeroing algorithm can't handle the negative torque produced by fixed gear bikes. That also means you have to manually zero the torque frequently like between each event at the track.

Other than that the only things I can think of are assembly problems that result in unreliable torque readings. If auto-zero isn't the problem I'd probably carefully reinstall the Fixxer kit and make sure everything is well aligned and properly torqued.

Good luck,
-Dave

 

[BikingBrian]

Dave, that's interesting, I had no idea that it was possible to convert a PT hub like that....so basically, you're saying it's the same as converting a regular road hub, just gotta make sure to turn off the auto-zero....gotta try this, as I also do track a few times a year and was looking for a way to get power readings. Thanks for the info!

 

[kssz2]

Hey Dave, thanks - I may try the uninstall/reassembly if I cannot sort the problem. The head units I have are the old black casing, quite old I believe as the serial numbers 134** and 167** but they are not the first generation, I can still download the data. They don't have a setting for disabling the auto torque-zero so maybe this is the problem, I can manually zero the torque by pushing/holding the 'select' button down while in 'watts' display taking me to the torque display, and zero from there. This is the only option I have. The only available options in 'settings' on the old black head unit is 1(mi/km) 2(wheel size) 3(1/2 for data recording), nothing else so maybe I need to try a Pro head unit first?

I'm on the learning curve with this hub and I don't have anyone close by to pick at there knowledge, so if I was to buy a head unit for this older hub/CPU which one to look for, would any of the newer 'wired' head units be compatible, is there a STD PRO and a 2.4 PRO both 'wired' units so only the STD PRO would be compatible?

 

[daveryanwyoming]

....so basically, you're saying it's the same as converting a regular road hub, just gotta make sure to turn off the auto-zero....gotta try this, as I also do track a few times a year and was looking for a way to get power readings. Thanks for the info!

You can't convert any of the PT SL or newer hubs using the Surly Fixxer kit, but the older pro hubs with the yellow or black caps held on with the torx screws can be converted. I had one of the yellow covered pro hubs, sold it a while back when I was living far from a track and picked up one of the black covered hubs from a friend and converted it last summer. Both worked fine. There are instructions floating around online on how to do the conversion, the only issues are getting the spacers right for your frame and using the right axle. Wheelbuilder sells a custom axle that's threaded appropriately for a Surly Fixxer when used on a PT Pro hub, it's threaded differently to account for the hub design.

Here's some info from Fixed Gear Fever: FixedGearFever Forums-viewtopic-powertap track conversion - initial observations

And yeah, if your CPU always attempts to autozero the torque you will likely have problems.

-Dave

 

[kssz2]

Okay, I tried something else (after dismantling and replacing the surly fixxer), I did a basic stomp test to see what if any results I'd get.

Put the PT head unit in torque mode (as above post -its hovering around 70-80) put pedals at 9-3, body weight on the crank at 3 and nothing, no change still hovering at 70-80 whether weights on or off. change back to watts display, do the same thing and get '000'

Yet if I free spin the pedals, power reading goes up (torque does not)

Whats going on - hub stuffed? or is it something to do with the torque autozero

 

[daveryanwyoming]

O...Whats going on - hub stuffed? or is it something to do with the torque autozero

Yeah, it sure sounds like the torque tube is broken and the strain gauge is broken. If you've zeroed the torque the display should read zero with no load and should show your torque in pound-inches when you load it with body weight. Your PM shows some power when you spin the cranks, but that's just the non-zero offset (70-80) that you've observed multiplied by the angular velocity of the wheel. If you were able to successfully zero the torque you'd measure zero power when spinning the unloaded cranks.

It definitely sounds like a torque tube failure or something in the associated electronics, bummer since those older PT hubs (the one's that can be converted to fixed gear use with a Surly Fixxer kit) are no longer supported by Saris so repair isn't an option.

-Dave

 

[Mike Lawson]

It sounds like your problem may in the hub, but I've tried using the original black CPU with my Surley equipped hub and don't get "sensible" numbers. Works fine with the yellow CPU's though.

Mike