Interbike 2012: StageONE Add-On Crank Based Power Meters Explained

2013 Stages add-on crankset power meter for training on a budget

Stages Cycling’s StageONE crank based power meter was announced just prior to Interbike as a more affordable way to use power for your training. With typical crankset-based power meters topping $2,000, Stages’ $699-$950 offerings certainly look tempting. We met up with them at Interbike to get a little tech breakdown – here’s how it works:

They start with a new crank and surface prep the crank arm. Just looking at it, one naturally wonders why they can’t just sell the unit and let anyone stick it on their own cranks. The main reason is that the location and orientation is critical to proper performance.

The other reason is that they do a good bit of surface prep to ensure the unit won’t come off your crankarm. Once it’s cleaned and ready, the strain gauges are attached directly to the crank arm, which are then wired to the circuit board. All that’s covered by a custom plastic cover that fits the profile of the specific crank it’s going on.

The circuit board has a lot going on. There’s an accelerometer that provides info about rotational velocity correlated with the strain measurements to determine both cadence and power. That means you no longer need a cadence magnet strapped to your crankset. It’s also what puts it to sleep or wakes it up, and they say it wakes and knows what’s going on within one second.

Another cool feature: It communicates power data via Bluetooth Smart (4.0) and ANT+, meaning it’ll send power data to your iPhone 4S or newer (and any other phone that’s using the latest Bluetooth standard) without it needing a 3rd party dongle.

It’s calibrated for temperature by using an active temperature compensation that measures both outside temp and the temp of the electronics and continually zeroing out the calibration to keep it spot on. To set it up initially, you only need drop the left crank to 6 o’clock and use your cycling computer to zero reset it. For absolute accuracy, you should do that before each ride, but they said it’s not super critical since the unit is constantly self-calibrating for temperature anyway.

The StageONE only measures the left leg’s power, but it does it for the full 360° and then doubles the number. They’ve tested that against a left/right system and shown that power data averages out virtually identically between.

With different cranks getting the same basic unit, how does it know what you’re doing? They calibrate the strain gauges to each different crank arm by putting a known load on the arm to get both zero and a fixed number, then it knows the load range. For example, they check the strain on the crank with no load and with XX number of pounds. They wouldn’t disclose what that number was, but it gives them a low and high number. With that info, they have a strain curve (or line) and can figure out any amount of force (aka “power) within that range.

The only limitation seems to be that you’ll need to be running an alloy crankset from Shimano, SRAM or Cannondale…or willing to mix and match an alloy non-drive crank arm with your carbon setup. They’re not mounting it to carbon cranks.

Comments

WG - 09/22/12 - 4:27pm

Got to say it looks impressive, esp. at this price. Actually it’s a price that could make me consider purchasing this kind of equipment (but honestly speaking, at this point I’m more inclined towards buying Brim Brothers’ Zone if only it makes it to the market).

I’ll definitely be following the reviews and see how it works in real life.

AAR - 09/22/12 - 5:34pm

Are they not installing their product carbon fiber cranks for now or will they never offer their product on carbon fiber cranks?

Ck - 09/22/12 - 5:50pm

As someone really interested in training with power data, but really only being able to afford a used PowerTap at best, this is very intriguing. Sure, it’s not going to be as good as a crank-based one, but i’d rather get 90% of the performance at up to 1/3rd of the price. I can skip things like left/right separate measurements. Not being available for non-alloy cranks is certainly a downside, but I don’t think this product is aimed at that market level. Sign me up for a Rival crank in the near future.

Whatever - 09/22/12 - 6:43pm

You dolts are missing the point. It doesn’t have to be accurate at measuring POWER, it simply has to be almost perfectly repeatable in ITS measuring from day-to-day and temperature-to-temperature.

The actual number doesn’t matter, so long as it is consistent. We’ll see.

Max - 09/22/12 - 8:25pm

Active temperature compensation… oh guys using a “Wheatstone bridge” isn’t that magic high tech feature. It’s technology from 1843. In the real industry world all guys would laugh, if you mention this as a feature :D
http://en.wikipedia.org/wiki/Wheatstone_bridge

Never the less, it’s very simple, effective ans accurate solution. Should work fine for many people. Measuring of deformation with this method has a typical pitch of 1-2% is you are using the 0,18 € sensors (some better stuff, good electronics and you can measure the deformation with 0,X% Pitch)

@Whatever: The accuracy is important. If you are going to an professional performance analysis or training on an ergometer you should be able to compare your results. It would be pretty senseless to have three different results ;)

Joey B - 09/22/12 - 9:02pm

@Whatever – You are spot on. I talked to these folks at Interbike and I walked away very impressed, enough so that I will purchase one. It would be near perfect if it could used on carbon cranks but as they explained it to me, they can’t get quite get the mounting process right (yet). It was one of the top things I saw at Interbike.

Chris - 09/23/12 - 12:38am

@Whatever – What was interesting is that spoke with Matt at Stages a few days ago in Vegas, and he had me convinced that the Power data was actually going to be withing the 2% realm of data readings from other devices (Quarq, SRM, PT). I was sold. Until I visited the SRM booth. I rode on their torque analysis demo which showed my left leg was between 4-6 Nm lower than my obviously dominant right leg, consistently for about 10 minutes (granted I was in flat pedals and it wasn’t ‘my’ bike). This was seated and standing at 70 rpm and at 95 rpm. My left left was only contributing 40% of the force to the total power of one revolution. The Stages power meter will only measure my weaker leg and multiply by two. My data won’t be comparable to anyone, other than myself. This has me less likely to invest in a device with truly unknown capabilities for long term ‘accurate’ measurement. The guy at SRM made a solid case for why measuring power accurately is critical for the purposes of training with a power based training program. It seems that the Stages device is dangerously close to the same measurement that was from Ergomo years before – though from a different method, the same left side only.

WG - 09/23/12 - 3:45am

What’s the point of comparing your watt output to that of other cyclists? The time and place to compare your performance is a bike race, not statistics.

Jacob - 09/23/12 - 7:37am

I like this a lot. It is another tool for training purposes. It will never be as good as systems costing $2,500+ but it isn’t claiming to be. What @whatever says is very true. All you need is consistency. That way you have something to compare it to when you go out again.@Chris, It’s not unusual to have one leg stronger than the other but a system like SRM won’t help you strengthen your left leg. Most people have a dominant leg (and they know it) but it not a big deal. If you have $2,500+ to spend then don’t buy this system but if you don”t have unlimited funds but still want to be able to take your training to another level then this system is a good option. I myself ride with a SRM system and it is absolutely amazing and I love the extra information that I get compared to a normal cycle computer but having talked to my coach today about this system he would be happy with the information that this system would give him.

djconnel - 09/23/12 - 12:01pm

Left leg only is non-starter. If I change my fit a bit and my L-R balance changes from 49% to 51%, for example, all of a sudden that’s a 4% power change, which would be really good for a year of training. They should come out with a 2-sided system, but until then, are not anywhere close to comparable to the spider-based units. On the accelerometer-based cadence, they are obviously tracking the gravity vector as the crank moves in a circle. This works properly only if there’s no accelerations. It will be interesting to see how it’s affected by rapid acceleration. Power = torque multiplied by cadence, so the unit is only as accurate as its cadence extraction. Finally, Rotor power has strain gauges drilled through the entire crank arm. I wonder how accuracy of torque is compromised by measuring the crank arm only in a limited region. Time will tell…

gatouille - 09/23/12 - 2:00pm

Must be careful with gap between crank sensor box and chainstay !!!
I’m sure we can have a clash on some bikes…

Yevgen - 09/25/12 - 2:22am

With the way “thing” measures the power, considering MTB usage, fe harsh landings staying on pedals and you could take your power graph to Pro-Tour managers )))

Frankie - 09/26/12 - 1:41pm

@djconnel, Wouldn’t changes in fit would typically require you to re-test your thresholds anyway?? If you retest, you wouldn’t mistakenly think you had all of a sudden increased your ftp by 5% or something.

Comparing absolute numbers from one PM to another is really only for the eWang crowd comparing “size” at the coffee shop. Even multiple PowerTaps can give different results. The absolute number doesn’t really matter. What matters is consistency. If your power meter says you’ve got an ftp of 300w at 30 minutes, and is reproducible, you can build your training regimen around that number. It doesn’t matter if another power meter would say your FTP is really 310w or 290w or something. You still would build training around a consistent number.

I’m dubious of the +/-2% accuracy Stages is claiming but it would be awesome if that is indeed the case. The problem is that this level of accuracy is based on the critical assumption that both left/right legs generate equal power. For most people, that’s probably not true, though the difference is likely small. So given their assumption of left/right leg equality, it is possible to achieve the level of accuracy they’re claiming. But in the real world, most people will have at least small differences in power between their legs. With that small difference added to the +/-2% accuracy range, there could be an absolute accuracy range greater than that 2%, so comparing numbers from the Stages power meter to any other power meter would be problematic at best. But again, they’re going for consistency, not absolute accuracy.

MJL - 10/08/12 - 4:34am

Agree with others about problems using left measurement only and x 2. No need for two pods as this could be easily overcome by designing firmware update which enabled user specific L/R balance to be used. So if my known L/R balance was 47/53 I could enter this and a factor of 2.1276 could be used to multiply left crank arm reading to derive total power. Of course this is depednent on knowing what your L/R split is…..but it is a very quick and inexpensive work around.

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