Where's the other side?

Stuck in R&D?

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/kj

http://kjmcawesome.tumblr.com/

It's only the left crank (kinda like Stages). 4iiii has an option for both sides (two units) for slightly less than twice the price.

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ECMGN Therapy Silicon Valley:
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Ha, cute. Drive side units will arrive from production in May. I might order one.

How would using a rotor Q-ring change the power readings? I've been looking at getting a power meter for a while now...

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My take on the Q-ring/power deal is that a hub-based solution (Powertap) is probably your best option for accuracy. However, so long as it's repeatable that's all you really need for training purposes so long as you have some means of correlating to your current power data.

It actually exists!

(sorta kinda pink)

Systems that use accelerometers to keep track of crank speed may not be negatively affected by non round rings. Just depends on the firmware. But they need not be counting single revolutions and assuming speed was constant during each revolution, like magnet based setups do.

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
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Perhaps that explains P2M claims on round vs q ring power accuracy.
So does that make Quarq which relies on accelerometer vs magnet more accruate for Qrings?

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A false humanity is used to impose its opposite, by people whose cruelty is equalled only by their arrogance

When did you order, and did you buy the crank arm from 4iiii as well?

I'm not the original poster but I just received my confirmation email that they will be shipping me the box to send my crank in so they can install the 4iii. I ordered October 10th.

Not sure what the turn around time is from this point. Hopefully less than two weeks.

I thought the 4iiii was supposed to be self-install?

Gov't regulations on shipping glue was sort of the problem that prevented self installation.

It is possible to buy a crank arm through them and have it factory installed and calibrated for no extra cost. Might be a great deal if they deliver.
http://4iiii.com/crankprogram/

If you preordered through clevertraining the cost for a ultegra crank arm with PM installed was $395. Still waiting for mine, though..

I think the delivery schedule that 4iiii put up today gives a pretty good idea of what is happening next. I recall they said they're also sending out personal emails shortly, with an exact shedule for each one.
http://4iiii.com/friday-files/
Mine is on a new crank, from 4iiii.

I noticed that Wakeham/4iiii claim an accuracy of <1%, while Stages and Power2Max claim 2%, Powertap 1.5%. Any merits to these claims or is this just advertising? "More accurate than most" also implies there are better ones.

"His dual sensor design uses both primary and secondary sensors to reduce error to well below 1% for torque measures making Precision more accurate than most power meters currently available."

The original plan has been to be transparent about our claims. Nobody tells of their methodology, so I'm just going to spill the beans as much as I can on ours. This has been said over and over at trade shows.

A bending bridge on the inside face of the arm isn't +/- 2% but if you want to it means claiming three assumptions at least.

1) The Cleat interface NEVER changes EVER (not true, sprinting tends to change... Garmin's PCO offers you information on this on Vector)
2) Someone cannot twist their ankle causing a torque along the axis of the crank arm (Again, use Vector and look at Pedal center offset, you'll see that's not true)
3) The calibration occurs exactly where a user puts their feet. (Speed play has a shop kit to specifically change the width!)

So, since all of these are false, if you test against them you can literally get +/- 10% error over a 40 mm distance. I've ripped apart commercial products and tapped into their strain gage with professional instrumentation equipment. 10% is kind for the scientific side of things. The real world usage will be less but you'll get more error in sprinting or out of seat hill climbs.

What's the risk? Well, if you are training with power, changing your seat, pedal, shoes, insoles, cleats, cleat types.... they can all cause a slight change causing this pedal offset to change or causing you to create ankle torque in different positions. This means you could lose 10% off your FTP with a single change of these interface parts, or you could gain. The reality is you didn't, the reality is your meter couldn't compensate.

Below are the results of a Cannondale SL crankset with our product on it that I calibrated for one of the managers bike that has an SRM on it. You can see the bike on my twitter feed. (Yes you can put a hollowgram on a specialized transition, don't know why people say you can't)

Offset (mm) Torque (N-M) Bend Only Bend + Secondary B (err) B+S (err)
0 0 1.599E-02 1.779E-02 N/A N/A
0 0 1.372E-02 1.684E-02 N/A N/A
0 0 1.655E-02 1.904E-02 N/A N/A
16 2.357E+01 2.180E+01 2.357E+01 -7.5% 0.0%
16 2.357E+01 2.179E+01 2.356E+01 -7.6% -0.1%
36 2.357E+01 2.349E+01 2.357E+01 -0.3% 0.0%
36 2.357E+01 2.350E+01 2.359E+01 -0.3% 0.1%
56 2.357E+01 2.515E+01 2.354E+01 6.7% -0.1%
56 2.357E+01 2.520E+01 2.358E+01 6.9% 0.1%
0 0 -3.890E-02 -4.481E-02 N/A N/A
0 0 -7.215E-03 -8.784E-03 N/A N/A
0 0 -1.415E-04 -7.544E-05 N/A N/A

It's a little gross to read, but when we turn off our secondary gage we get effectively what is on the market by two other companies. One of them offers a high speed torque reading method to smart phone. Hang a weight at different offsets and see what you get, or better yet, put your pedal in backwards and see what you get. Our torque sensing is 0.1% on this particular unit. Some are better (I've seen 0.08% error over the calibration). The worse factory calibrated unit when tested with other weights (random sample points) gave about 0.3% error, but that might be me being conservative.

So what else? Rotational algorithim. We're using our 32768hz crystal for timing, but our accel runs slower. It runs through an algorithim (designed by the inventor and father of all footpods) and as a result gives us 0.4% maximum error. That's a three sigma outlier (three standard deviations away, for I believe if my stats knowledge doesn't fail me, 99.9% of the time). We're looking at doing a method that will improve that via the firmware update, but right now we're happy with that.

The last puzzle piece, and it saddens me we didn't have to implement this calibration out of the gate was the active thermal compensation. We have the physical hardware and with user installation the plan the idea was that the device would learn it's thermal calibration over time via user zeroing. We wanted to bring it in house and pre-cal them, but there wasn't time. It's still a project for factory install, but the algorithim will still come in via OTA. However, everyone else got to claim 2% beforehand without active, so that's where we are. That'll be fixed in a few months though.

Okay, So how does everyone else measure their error.... no seriously I'm asking, because NOBODY is talking. Ours is a summation of errors (torque and rotational) + buffer room for crystal drift or stupid things. Yes, I said we have buffer on that claim.

One says "scientifically proven", several just seem to have went with "sure stain gages, 2%... ya whatever", and I read once that one spider based unit in the wattage google group was the maximum error from calibrating a spider in 4 positions.

Their is only one crank arm unit on the market that straight up doesn't need correction. We aren't compatible with them -- so you can figure out who that is.

Thank you, Keith. Not that I'm in a position to validate these, but your readiness to go into this level of detail publicly certainly convinced me. I assume other manufacturers will not join the discussion, so I look forward to that drive side pod.

Having now educated myself a bit about temp compensation, from this thread
http://forum.slowtwitch.com/gforum.cgi?post=5328833
should we, until active compensation is implemented in Precision, do a manual calibration, using the head unit, just before each ride? And if there are major temp changes during the ride, also calibrate mid-ride?

Does the dual sensor design refer to left and right pedal sensors or something else? If you need both L/R sensors to be more accurate than the others then I assume it might be better to wait for the similarly priced ring-based powertap since its from a proven company

It's a crank based PM so it refers to L/R crank sensors. There is a picture in the 1st post. You can choose to buy one side or both sides. What PM manufacturer to choose? To each his own, this is what I decided to get.

What's the risk? Well, if you are training with power, changing your seat, pedal, shoes, insoles, cleats, cleat types.... they can all cause a slight change causing this pedal offset to change or causing you to create ankle torque in different positions. This means you could lose 10% off your FTP with a single change of these interface parts, or you could gain. The reality is you didn't, the reality is your meter couldn't compensate.

It sounds like you are describing pedal based PMs. I can see that they have a lot of error sources. But what about yours compared to say an SRM or a Powertap hub? Measuring the strain on the crank arm is going to have greater error sources than those.

Keith:

Thanks for being upfront. That's great.

I've always assumed that because others don't explain how they get their error claim it really means "don't even think about asking for warranty service unless it's off by more than this."

Few points to clarify.

To Acco: Yes, it's best to zero before the ride. Even with ones that do active compensation I still zero them. Personally I'm not a huge fan of Autozeroing methods. For some things they work very well, and some can actually add measurable error. So large temp swings would be good to zero again.

RVArunner: The two sensors are on the same arm under the pod. So it's not that there is one sensor for a left and right... for dual side there would be 4 strain bridges made up of 4 gages each, so a single pod has 8 gages (2 sensors) and a left + right has 16 gages (4 sensors). Hopefully that's clearer. We have very different math to come up with torque from the two strain bridges. With Precision you see two zero offsets per pod when you zero and there at two calibration factors minimum.

To Rruff: I checked the calibration on a "Version 7 spider based meter" yesterday. My zero's were 530 +/-1 and my readings from the drive side on big and small ring were 1228 and 1227, my readings on the non-drive side was 1217 and 1207. Their procedure says to average them... however that means a slope difference of (1207-530)/(20kg*9.81*0.1725) = 20.003 versus (1228-530)/(20*9.81*0.1725) = 20.623. The error between the two is (difference)/(average) [YES, Average!, this is where 90% of all papers on error go wrong for PM's, you can't assume one is right) gives (20.623-20.003)/((20.623+20.003)/2) = 3% error between calibrations. That specific unit I believe claims 1.5%. I repeated and it produced the same problem. So my calibration produced +/- 1.5% error using similar methodology I used in developing precision (which gave +/- 0.1%).