Specialized Power Cranks Power Meter In-Depth Review


Today Specialized has finally announced their new power meter crankset, aptly called Power Cranks.  This crankset has been in the works for quite some time, both in rumors as well as on pro team riders bikes (as I showed at the Tour Down Under).  The unit is offered not just on Specialized bikes, but also as a product you can buy like any other power meter. Both in a dual-sided version as well as a single-sided variant. And both in Carbon as well as non-carbon options.

Now Specialized wasn’t entirely alone in this venture, in fact as the inside of the battery compartment alludes to – Specialized enlisted 4iiii for much of the underlying power meter technology.  But it’s actually not a simple re-branded 4iiii Precision power meter. In fact there are numerous internal and external differences that are unique to the offering from Specialized.  But fear not, we’ll get into all those details down below.

Note that as usual I’ll be sending back the two bikes and one standalone power meter that Specialized sent over for me to put through their paces. That’s just the way I roll. With that, let’s dive right into it.


There are basically two ways you can get the Specialized Power Cranks.  The first (and most expensive) option is to simply buy a new bike with it equipped.  No installation required. The second (and far more reasonable) option is to buy the boxed variant as an aftermarket accessory.  Note, there’s no specific requirement for a Specialized bike, as I’ll show you in a bit.

Starting with that first option, you can go for broke with something like the new S-Works Tarmac Disc, which was also announced today.  This particular bike is spec’d out with all sorts of bling including Shimano Di2 and disc brakes, and some sort of rolling thingies that cost more than my entire family’s return plane tickets to Australia ($11,000 to be precise).  Speaking of which, if you want to increase the price of this scenario, request that the review unit be shipped to Australia…preferably the least close coast to California.


And, for those bike geeks out there, here’s the full setup for how this bike is configured, minus the pedals which I added – another power meter, the Favero Assioma dual-sided units.


While undoubtedly many of you will want me to ‘review’ this bike, that’s not really my thing (see James Huang’s review of the bike aspect). I’m a simplistic person when it comes to bikes, as long as it feels fast despite however many cookies I eat, I’m happy.  Also, it should stop when I use the brakes.  This bike achieved both.  Nonetheless, here’s a gallery of cookie-friendly goodness gallery I shot:

Now, back to that less expensive option – just buying the power meter itself. In this case, you get a box almost identical in size/shape to that of the Garmin Vector 3 box. It’s even black. But of course, there’s no similarities in what you’ll find inside.


Once you crack it open you’ll have some brightly colored paper stuffs looking up at you. That’s the manuals and safety junk.  Then below that you’ll find the crankset.  Note that it lacks any chainrings – so you’ll need to figure that out yourself (don’t worry, it’s easy).


Here’s a closer look at the two crank arms. Each has their own power sensor on it – hence the ‘dual’ part.  Whereas Specialized also makes a left-only variant that’s the single option.

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Then there’s a small box of parts, that box contains a plastic bag which actually has more bags of parts in it. It’s like one of those Russian stacking dolls. Here’s all those parts, which are largely spacers and bolts to attach the chainrings to your crankset.

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And then there’s a battery cap removal tool.  But more on that a bit later.


In my case, my secondary bike was already tied up with another power meter review (Shimano R9100P), so I couldn’t quite swap everything out too easily since the bottom bracket types were different.  Thus I largely just used the bike that was shipped to me…since that was prettier.

(Fun tidbit: This is actually the second bike equipped with a Specialized power meter that I complexingly had them ship to me elsewhere in the world.  The first was last June, to France.  You didn’t realize it at the time, but that ‘rental bike’ as per my Strava listing was a bit more special than that. You just couldn’t tell a special…ized, power meter was on there.)

General Use Overview:


Now before we go too much further, it’s important (like, really important) to point out one interesting tidbit: Specialized didn’t make this unit entirely by themselves.  Nope, instead they enlisted the help of 4iiii, who has been making power meters for a couple of years now (respectable ones at that).  But that doesn’t mean the Specialized variant is just a simple rebrand of the 4iiii Precision.  There are actually some notable differences, mainly externally focused – but also some key important internal difference with respect to strain gauge design.  The first piece you’ll notice is the battery cap situation, which is quite different than the 4iiii Precision (either Pro or Podium), which has a much smaller pod than the Specialized variant.


Why you ask?

Well Specialized wanted to improve the waterproofing aspects a bit more and bring it to a higher waterproofing standard than the existing 4iiii Precision units.  So they went with a system that requires removal of two screws (using a hex wrench) to access the CR2032 coin cell batteries (which are set to last 125-150 hours each).  The main driver for this increased waterproofing standard was to allow teams to more comfortably use pressure washers with the power meters (because let’s be honest, they already do on the two team’s that use/used 4iiii’s Precision units over the past 2-3 seasons):


Now, there’s one downside to this new battery compartment: It’s a pain in the ass to change.  Sure, it’s silly simple if you have the included tool – but it’s a long-affair if you don’t have the super long and tiny hex wrench to get that close to the downtown in a rather awkward spot.  It’s not something you want to forget on a road trip:


Sure, it’s possible to do without the tool (been there, done that), but it’ll take you forever with a bike tool since you can only complete half a rotation with each twist.  On the flip side, at least it’s well waterproofed.  I used the first test bike with this design through plenty of rain and crappy weather in Paris last year, and it’s still alive.

The second notable change to the 4iiii design is internal – and totally not visible. They re-arranged the strain gauge design/pattern (which they declined to get into the details of, as expected) to get slightly better accuracy on carbon crank arms.  As any power meter company will tell you – getting repeatable and accurate results on carbon crank arms is incredibly difficult, in large part because every carbon crank arm off the assembly line is slightly different (not just model to mode, but each individual unit coming off the line).  They also tend to age over time slightly different as well.


Now, Specialized appears to have gone a bit further along in the accuracy claims backing department than most power meter companies.  They went to the University of Colorado Boulder’s Locomotion Lab to have them put together independent verification of the power meter accuracy claims.  Of course, obviously, Specialized is funding some of that occurring, though after listening/watching/discussion via web conference the non-Specialized scientists involved in the testing, I’m reasonably confident we aren’t seeing any funny business there.

What might be funny is how out of the box the design is for validating the power meter.  In large part because it doesn’t depend on using another power meter to validate.  Instead, it’s math based with an incredible amount of energy spent on covering the bases for details like measuring sweat loss.  In a nutshell, the university built a custom treadmill that allows them to measure rolling resistance, drivetrain loss, even spoke resistance by using known weights and the resistance pulled on the weights.

About 5-10 minutes into this presentation after they had presented the man in charge (Dr. Rodger Kram) – his Harvard backed credentials and peer reviewed papers and the general overview – my alarm bells were starting going off.  In large part because the vast majority of very smart academics usually really screw up power meter testing. Mostly because they don’t understand the nuances of the technology itself, or where things could go wrong on test setup.  So, I casually started to wade into these nuances, which included (paraphrased below):

DCR Question: How do you account for the rolling resistance of the treadmill relative to the drivetrain loss of crank arm measurement?
Dr. Kram’s Answer: We invert the power equation using a known drivetrain loss value, in this case – 1.02%.

DCR: Most power meters today tend to have issues with temp shifting scenarios, did you test for that?
Dr: Glad you asked, first, we got a crapton of ball bearings, and then we heated them to 50*C. We validated the temperature with an infrared thermometer. Then we moved them to a larger cooler where we submerged the crankset into the cooler to have it reach the 50*C (122*F) temp.  Then we quickly attached to the bike. We found initially the power was 4.1% low, but within 10 minute it was within 1.7%.

DCR: Ok, a cooler is kinda impressive. But what about cold?
Dr.: We did the inverse, this time putting them in a freezer validated at 0*C, after temp stabilization we installed crankset in under 2 minutes and then found power was also within 2% within 10 minutes. There were no manual zero offsets.

DCR: Any testing of cross-chaining?
Dr.: Yes, we did full cross-chaining sweeps from biggest cog to smallest cog, and while I’m at it we also did full cadence tests from 50RPM to 114RPM. The power didn’t vary more than 2% across the entire cassette.

DCR: What about vibrations, that’s really the second area that power meter struggle with?
Dr.: We couldn’t test that on this treadmill design, but we have some specific ideas on how to test that down the road with designing a new treadmill.

This sort of discussion went back and forth for about an hour as they walked through the testing methodology including aspects like how many units they tested (6 unique units), as well as the differences between the various prototype iterations (earlier prototypes saw a worst case scenario 2.2% during standing sprints).  Or that they also tested pedal spacers

The part that I appreciated in talking to Dr. Kram was simply that he was refreshingly honest.  When there was a gap in the methodology (like vibration testing), he noted it.  He also made a reasonably good point in that he doesn’t actually get paid anymore as an emeritus professor, instead, the funding from Specialized goes to the lab itself.  He noted that “I’d be pretty damn stupid to stake my career and reputation to be a shill for one company”, and instead wanted it really to be “ more about the method”, and said they were definitely looking at ways to document the method in more detail allowing other entities (universities or otherwise) to duplicate it.

In any case, I’ll dive into more of the accuracy pieces from my perspective down below.  For now, we’ll leave that massive diversion about differing strain gauge design and get back to some basics.

When we look at the power meter from a pairing standpoint, it transmits both ANT+ & Bluetooth Smart, just like virtually every other power meter on the market today.  So when searching from a head unit you’ll find it listed both as an ANT+ unit and a Bluetooth Smart unit. As a general rule of thumb for power meters, you’ll want to use ANT+ instead of Bluetooth Smart, since usually compatibility works better.


It transmits both power and cadence, as well as additional metrics in certain cases.  Here’s the total listing:

ANT+ Power (total)
ANT+ Power Balance (left/right)
ANT+ Cadence
Bluetooth Smart Power
Bluetooth Smart Power Balance
Bluetooth Smart Cadence

In addition, by tomorrow they’ll be adding in ANT+ Pedal Smoothness & ANT+ Torque Effectiveness via firmware update.  Since these aren’t enabled yet on my unit, I can’t speak to their accuracy yet. But given that 4iiii has had these metrics for a while and they were fine there, this is really more just of a firmware enablement of a well understood feature than anything else.

Speaking of head unit goodness, you’ll trigger a calibration just like you would other power meters by going into the ‘Calibration’ comment on your head unit.


Like the 4iiii units, the Specialized units reads back identical status codes during calibration, which display the two sides (left + right) as a single four digit number

10 – Good, ready to ride
20 – Unstable data (bike not stationary)
30 – Low battery (less than 10%)
50 – Calibration error – contact support
99 – Power meter side not found
0 – Power meter not found

So basically: 1010 as seen above is ‘Good’ on both crank arms.  The positive side to this is that it’s far easier for most people to understand.  The downside is that it can make tracking calibration/drift variances harder since you’re not given a specific zero offset.

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In addition, you can also use the newly published Specialized Power Cranks mobile app, which will connect to your Power Cranks via Bluetooth Smart and allow you to check various settings and status information.

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For example I can pull open the status of either side of the crank set. And if you had bought a single-sided unit, there’s the option to upgrade to a dual sided unit.

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You can also perform a zero offset there as well.

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For the most part the app has aspect that mirror the 4iiii app, and indeed you can even use the 4iiii app too if you so desire – though honestly the Specialized app looks prettier.  Beyond what I’ve outlined above, the setup and configuration is all pretty straight forward.

Power Meter Accuracy Results:


I’ve long said that if your power meter isn’t accurate, then there’s no point in spending money on one.  Strava can give you estimated power that’s ‘close enough’ for free, so if you’re gonna spend money on something it shouldn’t be a random number generator.  Yet there are certain scenarios/products where a power meter may be less accurate than others, or perhaps it’s got known edge cases that don’t work.  Neither product type is bad – but you just need to know what those use/edge cases are and whether it fits your budget or requirements.

As always, I set out to find that out.  In power meters today one of the biggest challenges is outdoor conditions.  Generally speaking, indoor conditions are pretty easy to handle, but I still start there nonetheless.  It allows me to dig into areas like low and high cadence, as well as just how clean numbers are at steady-state power outputs.  Whereas outdoors allows me to look into water ingest concerns, temperature and humidity variations, and the all-important road surface aspects (e.g. vibrations).  For reference, Specialized has a claimed accuracy rate of +/- 1.0%.  It also does not require any magnets for cadence, while also automatically correcting for any temperature drift. Both of these are pretty common though on most power meters these days.


In my testing, I generally use between 2-4 other power meters on the bike at once.  I find this is the best way to validate power meters in real-world conditions.  In the case of most of these tests with the Specialized I was using these other power meters concurrently:

Favero Assioma Pedals
Garmin Vector 3 Pedals
JetBlack WhisperDrive Smart Trainer
Wahoo KICKR 2017/V3 Trainer

Typically I like to also use a PowerTap G3 hub on my bikes, but in this case I don’t actually have a disc PowerTap hub wheelset yet, so I had to skip that 3rd concurrent unit (since the bike I received has disc brakes).

Now in actuality, I also used it with other power meters/trainers too, back on previous beta units on a non-disc equipped bike, these included:

Tacx NEO Trainer
Elite Direto Trainer
PowerTap G3 Hub
PowerTap P1 Pedals

However, for the purposes of the accuracy section here, I’ll just focus on the most recent production version.  Though honestly, my data from all the way back last summer was perfectly fine for me too.

In general, my use of other products is most often tied to other things I’m testing.  Also, when it comes to data collection, I use a blend of the NPE WASP data collection devices, and a fleet of Garmin head units (mostly Edge 520/820/1000/1030 units).  For the vast majority of tests on the Specialized units I used an Edge 1030 and Edge 520.


Note all of the data can be found in the links next to each review.  Also, at the end is a short table with the data used in this review.  I’ll likely add in other data not in this review as well.

First, let’s start indoors with a Zwift session from a few weeks ago on Zwift. In both cases though I’m recording the data via head units to keep the playing field straight forward for data consistency.  Here’s the DCR Analyzer link for this set if you want to dive in more deeply.


What you see above is that all three units tracked very closely, though there was a tiny bit of separation mostly from the Favero Assioma pedals.  This was actually the first ride these pedals had been put on the bike.  And while I did do a bunch of sprints/etc before starting the ride, sometimes you still see a settling period. Still, ignoring that slight offset there, things are very close.  Especially if we look into the sprint a bit later in the file.


The above is smoothed at 10-seconds just to keep things easier to view above, but you can see all three track through the sprint quite well. Even if I turn off smoothing and just look at the 1-second power for the sprint, they’re all very close. Getting any set of power meters to match 1-second power values for a sprint is virtually impossible, primarily due to the different recording and transmission rates of head units and power meters.


Also of note is that if I look at the breakouts of left and right power from both dual-sided units (Favero Assioma and Specialized Power Cranks), you’ll notice they roughly mirror each other in terms of which side I have higher balance on.


And if I look at the cadence on this ride, they’re virtually identical between the two units (the KICKR doesn’t transmit cadence):


Next, if we look at another indoor ride from this past weekend – we can see that once the Favero pedals have settled a bit, things look incredibly close together.


Those 800-900w sprints are also super tight between the units. Some slight variants as you’d expect from recording rates, but overall pretty clean:


And again, cadence is indistinguishable here:


And if we turn to the mean-max graph, that too is very close. Some slight offset differences there – as in theory I’d want the KICKR to be the lowest and the Favero to be the highest, but if we account for the claimed accuracy of the units involved, the overall range is correct.


Next let’s shift some gears to an outdoor ride. This first ride from last weekend, a pretty straight forward loop with mostly smooth pavement, however I added a small cobblestone road section on the return to mix things up a bit.  In this case due to the aforementioned lack of a disc variant of a PowerTap hub in my arsenal we only have two viable power meters to use at once. I probably could look at adding in something like a PowerBeat here, but that’d require them to certify on this specific carbon arrangement, which obviously they haven’t done yet since it’s not out yet. While I never like to have only two power meters for testing purposes, I’ve done boatloads of 3-4 concurrent power meter rides with the Assioma’s and find them very solid.  So using it as a reference here is a good starting point.

In any case, here’s the DCR Analyzer data set, and the overview plot below:


As you can see, virtually identical. Except for a few sections when I seem to stop pedaling and the Specialized and Assioma units differ in their opinions of what happened. I’ve zoomed into about the 36-38 minute marker for the below snippet


Specifically above I’m looking at the difference between when the Specialized shows ‘0’ and the Assioma shows a higher but mostly non-powered value value (such as 47w).  Three obvious things could potentially cause this difference:

A) The Specialized power meter is produces a dropout
B) The Edge 520 head unit connected to the Specialized power meter has a connectivity issue
C) The Assioma pedals don’t catch me stoppage of pedaling for a brief moment

So which one is it? Well, it’s honestly hard to tell without a third power meter to provide some other reference point.  But, we can look at cadence instead to get an idea of what might be going on. Here’s the cadence plot for that same section:


What do you notice?

Well, my cadence stops every little bit – and usually on both units. A scenario for this would be if you’re riding in a group or next to a car and having to stop pedaling, then immediately laying down the power again. That’s exactly what you’d see above.

In that case you can see above that there’s four times the Specialized catches my stopped pedaling, but only 3 times the Assioma catches me.  Why does cadence matter? Because for power meters, cadence is used as part of the calculation to produce watts.

So in the above case, I’m relatively confident that the 1-2 second gap of me not pedaling isn’t fully being caught by the Assioma pedals, but is being caught by Specialized.

Looking at the total mean-max graph for this ride, you’ll see things overall are quite close. Not precisely the same – but pretty darn similar.


And then finally let’s look at another outside ride, this one from just an hour ago in fact. A loop that I tried to find some uglier pavement on. The bike path I selected for the first portion includes plenty of cracks, bumps and generally crapiness.  I also slotted in a section of cobbles again.  Here’s the DCR Analyzer link for this set.


As you can see in the first portion, lots of brief surges and pausing of pedaling. I did this on purpose to see if I could recreate some of what I saw in the set above.


And indeed, I was able to confirm exactly my suspicions from above – the Assioma pedals seem to be waiting just an extra second or two before registering paused pedaling.  Whereas the Specialized catches it.  But it really is just one second’s worth. Check out the below, whereby you can see that on the first highlight in yellow I paused pedaling for 2-3 seconds and it catches it, but the second highlighted dot in yellow I only paused pedaling for one second and the Assioma doesn’t quite catch the drop 100%.  None of which has to do with the Specialized accuracy, other than to clarify why those dips exist.


On the sprints, I saw a bit more difference, with the Specialized getting a higher value than the Favero by almost 100w, 796w to 700w (over a 5-second smoothed max average).  But in this case, if you scroll all the way back up you’ll notice that the Favero pedals compared to the indoor KICKR setup seemed to only just barely undershoot sprints.


Yet other times they were practically identical, like this one here at 723w to 718w.


Ultimately though, this ends up more about nitpicking mostly minor differences that you’d see when recording between any two power meters with any two head units.  From an overall accuracy standpoint either indoors or outdoors, whether on final production or nearly a year’s worth of beta devices – it’s remained remarkably solid for me.

As I said before, a large part of that can be attributed to the fact that this isn’t a brand new power meter on the market by a company that’s never produced a power meter. Rather, they leaned heavily on 4iiii for this venture, which has years of power meter experience with two WorldTour Pro teams riding their gear.  Well, actually, now they have one – since the other one is riding the Specialized version instead.

(Note: All of the graphs made here are via the DCR Analyzer, which allows me to compare any types of data sets, such as power meters, heart rate sensors, GPS tracks, cadence, even running power. The good news is now you can use it too – details here on how to get started.)

Pricing and Availability Tidbits:

Just to very briefly run through all the options Specialized is offering, here’s the bulleted list:

A) S-Works Carbon dual add-on to a new Specialized bike purchase: $1,150 for dual carbon option (available today)
B) S-Works Carbon left-side only add-on to new bike: $750USD
C) Shimano 105 left-side only with crank/spider: $450
D) Shimano Ultegra left-side only with crank/spider: $500
E) Shimano Dura-Ace dual crankset/spider: $1,500

Coming this Fall 2018:

F) Carbon dual-sided aftermarket option (not a new bike) – what I showed in the box
G) Carbon single-sided aftermarket option (not a new bike)

The units are designed to fit Specialized road frames with PF30 or OSBB bottom bracket shells, and are BB30 compatible.  Also, they’re compatible with either Shimano or SRAM drivetrains (I’ve used one bike on eTAP and another on Di2).  Total weight on all of them is 30g additional.

From a pricing standpoint, for a dual-sided S-Works carbon crankset inclusive of cranks, that’s actually pretty competitive. It’s cheaper than the Shimano Dura-Ace power meter, and $150 cheaper than Stage’s new Stages LR unit.  For the Dual-sided Dura-Ace 9100 that their pro teams are using, that’s a bit more expensive than Stage’s exact same variant which is $1,299.

Power Meter Recommendations:

With so many power meters on the market, your choices have expanded greatly in the last few years.  So great in fact that I’ve written up an entire post dedicated to power meter selection: The Annual Power Meters Guide.

I refresh that annual guide each fall, and in this case that was November – which is inclusive of all the power meter players on the market.

The above-noted guide covers every model of power meter on the market (and upcoming) and gives you recommendations for whether a given unit is appropriate for you.  There is no ‘best’ power meter.  There’s simply the most appropriate power meter for your situation.  If you have only one type of bike I’d recommend one power meter versus another.  Or if you have different needs for swapping bikes I’d recommend one unit versus another.  Or if you have a specific budget or crankset compatibility, it’d influence the answers.

I also recently published a pricing update a few weeks ago, covering where pricing stands for power meters in general in 2018, and what my predictions are going forward over the remainder of the year.



As I said at the beginning, it was no surprise that Specialized was coming out with a power meter – especially given they have their pro teams and athletes riding it out in the wild.  Further, partnering with 4iiii to deliver the underlying technology is a smart move. The power meter road is littered with companies that have tried to make accurate power meters and failed, so going with a ‘known good’ partner is one of the fastest ways to market that doesn’t burn a ton of R&D time.

Of course, it’s not simply a 4iiii Precision copy-cat. It’s legitimately different, both from the outside standpoint in areas like the tank-like battery cover design, as well as under the covers in their strain gauge layout which was specifically tuned for their carbon crankset.  The only visible mark of 4iiii you’ll find is under the battery cap, the tiny little logo.

From an accuracy and reliability standpoint it’s been very solid for me. Both the final production units but also even the beta test units I’ve tried over the last 9 months.  It’s quite frankly fire and forget, it just works. You know, like S-Works. Or some bad pun like that.

It’ll be interesting to see how well it does attached to the bike as a single purchase. Their pricing is very competitive for the full crankset on carbon, depending on how you value S-Works branded crank arms versus Shimano ones.  For the Shimano Dura-Ace offering, it’s a bit more pricey than Stages. So I suppose that becomes just a buyer or cash allocation preference. Of course if you’re buying a new bike, having them install the unit and ensure everything is good is handy.

With that – thanks for reading!

Source: dcrainmaker

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