Getting Crank-y about Accuracy Testing

By Andrew Buckrell, 4iiii Director of Innovation

Let’s face it: when you buy a powermeter, you buy it because you want to know your exact power. You don’t want to know “I might have set a new FTP today”, or “I approximately had my best workout”. Accuracy is everything, but what does accuracy actually mean?

What is Power?

First off, let’s start with the definition of power. Power is the mechanical energy generated or dissipated by a system. In general, power is calculated as a force multiplied by a velocity (F x V). Similarly for rotating devices, this is equated to torque (𝛕) multiplied by angular velocity (𝛚) or (𝛕 x 𝛚). A rider turning their power meter will generate a torque on the pedals (turning force) and will rotate at some angular velocity (say, 80rpm). By calculating both of these values, we can now calculate the power generated at the crank by the rider.

Put simpler: the faster you turn your pedals or the harder you push, the more power you generate.

But we don’t turn the crank evenly! No matter how smooth you are, there are pushes, and jabs, and most importantly, every rider is different and no machine can replicate this. So, companies that test with a machine that spins the cranks at a constant torque aren’t testing for accuracy in the way a real cyclist actually pedals.


Instantaneous torque data from 3 pedal strokes on a 4iiii Fliiiight Smart Trainer. Note that the rider goes from a minimum of 4Nm to nearly 18Nm – that’s a 4x swing in torque every pedal stroke!

How to Measure Power

In a perfect world, everything is 100% efficient, and it would be very easy to measure power output. Unfortunately, thermodynamics is a harsh mistress, and we lose a bit of power with material flex (hysteresis losses), chain and drivetrain (bearing and chain friction), tire losses (rolling resistance), etc. Anything that generates heat or noise is a form of lost power.

This is important because it becomes very difficult to establish an accurate testing protocol to make sure that what we (or anyone) claims is actually accurate. Accuracy is really just a measure of how close we are measuring when compared to a real value, but how do we even measure that “real value”? You have to go much further than just putting a few different powermeters on a bike and comparing results. All that would accomplish is that the different options are precise and not accurate, since there is no real baseline measurements.

Historically, measuring accuracy has been done with a dynamometer (no, this has nothing to do with Jurassic Park) which accurately measures torque and speed. These devices have been used for years in the automotive industry, but are generally not well suited for the torque input variations that even the smoothest rider provides. Besides, even the automotive industry has a history of “cheating” and manipulating results in their favour.

How to Measure Cycling Power Accurately

Instead, we went to Prof. Dr. Rodger Kram, professor of Integrative Physiology and manager of the Locomotion Lab, University of Colorado, for a ground-up, physics based measurement approach. Dr. Kram and his team have developed a simple, but very accurate method of measuring the actual power getting to the ground with a given rider. By running a treadmill at a very accurate speed and inclination, and by measuring and eliminating variables such as rolling resistance and drivetrain efficiency, he is able to measure the power output with unprecedented accuracy. Another major advantage of this laboratory test protocol is that it takes out a lot of the variability for test conditions, like wind, temperature, and even allows us to test against a variety of different rider pedaling styles. He is even able to pre-cool or pre-heat the cranks to test the extremes of temperature. And let’s face it — would you want to be volun-told to maintain 350W in 50°C desert heat, or would you prefer to do it under controlled lab conditions?


An example of testing rolling resistance during the power measurement protocol

The whitepaper produced during this test, including accuracy results is available here: Boulder Whitepaper.

The Results

The testing was extremely encouraging. Our PRECISION Pro Powermeters were able to achieve less than 1.58% error, while the Podiiiium Powermeters were better than 1% error. These weren’t some “perfect” cranks that were chosen because of repeatability either — they were actually purchased by Dr. Kram’s lab from our website — our fulfilment team didn’t know they were going out for testing — which is the only way to guarantee that selection bias has been eliminated.

What does this mean to me?

So, at this stage, you might be saying, “I really appreciate the physics lesson, and now want to devote my spare time towards the pursuit of science (or maybe that’s just what I’m saying), but why does it really matter to me?”

The answer to that is that it’s a real world demonstration of accuracy. By testing at an independent university lab, we are confident that the results are accurate, unbiased and based on how real cyclists pedal, not machines. It’s also further proof that our 3D strain-gauge technology can capture the unique pedalling dynamics of real cyclists, instead of just a machine that spins the cranks at a constant torque all of the time.

You will be very hard pressed to find any other manufacturer publishing such transparent accuracy data, but for the sake of everyone we encourage them to step up for this benchmark testing at Dr. Kram’s University lab as well. In the meantime, we think you should buy the most accurate powermeter on the market!

PRECISION and Accuracy

By Andrew Buckrell, 4iiii Director of Innovation

We often hear discussions about precision and accuracy, with the words being used interchangeably. Although I’ll admit, many things about how people perceive engineers drives me batty, this one possibly tops the list. Precision and accuracy are not interchangeable, and although they are both goals in measurement, they are far from the same thing.

My favourite visual description is related to targets: if you have several attempts to get a bullseye, there are many different results that could be achieved.

If you are accurate but not precise, your measured average would still match your real world average, but you wouldn’t necessarily have confidence in a given sample or measurement being the right measurement. This is like putting 200W to the pedals, but one measurement showing 210W and the next showing 190W. Even though it is highly accurate, the measurements lack precision. Your average measurement will still read as the same, but how you get there could be slightly different.

Let’s consider the other possibility: what if you have something that is very precise, but not accurate at all. Taking the same ride of 200W input at the pedals, what if your measurements came back as reading 140W? Knowing that it felt like a lot more power, what if you exactly repeated the ride and it came back at the same 140W? This is a perfect example of a highly precise powermeter, but also inaccurate. I don’t think anyone would take consolation in the fact that their measurement was precise, but still 60W lower than what they actually achieved — I know that I certainly wouldn’t be happy about it!


Possible reaction from high precision, but low accuracy, Huffy Toss

The best of both worlds is to have precision and accuracy. This means, if you measure 190W at the powermeter, it actually means you’ve ridden 190W, and you can be confident in that being the actual power. Many people will argue that precision is more important than accuracy (since it will help you gauge your actual physiological input during a ride), but why not strive to have both?

Simple answer, you can! We have gone to great lengths to show that 4iiii Powermeters provide unprecedented accuracy, reliability and durability. When even the Canadian Olympic mountain bike team trusts us, you know it’s good enough for you! Check out our online shop to make sure that you get your raceday best.

“Why does this matter? I’m only a regular athlete”, you might ask. Let’s take a page out of my own not-very-illustrious racing career as an example. In November 2019, in the long-long ago days before COVID-19 changed the racing landscape, I was competing in IRONMAN Cozumel. Despite having a phenomenal swim (for me), I could tell it was hot that day. Much hotter than my training days leading up to the race. I could tell that something was a bit off. In maintaining my target race power, my heart rate was not going down to where it should have been. The accurate data I know and trust from my 4iiii Viiiiva Heart Rate Monitor and 4iiii PRECISION Powermeter were telling me that my body wasn’t reacting well to the heat. Knowing, and more importantly, trusting this information told me that I really needed to be cautious and dial it back a bit. In my infinite wisdom, and with a solid case of “race-brain” (trademark pending on that one), I decided not to listen to the data, or my body, and kept pushing on at my target power. Although I maintained my target power for the first lap, my HR kept climbing from my target of 145 to about 165!! At the 130km mark, I pulled out with a borderline case of heat stroke, and spent the next 30 minutes sitting in the shade without being able to dip my HR below 120bpm. Lesson learned — listen to the signs, and trust in accurate data. Have a solid plan, a solid backup, and trust your numbers.


A rare photo of me actually racing at Cozumel 2019 — note the “retro-cool” STAC tri suit

Although my next race(s) have all been cancelled so far, I’m very much looking forward to getting back out racing (fingers crossed for IM Arizona, if not 2021 Victoria 70.3 then 2021 IM Canada), and this time I’ll be “wise” enough to not make poor race day decisions, and trust my accurate and precise data from my 4iiii PRECISION Powermeter and Viiiiva Heart Rate Monitor.

A Summer Without Racing – Creating the Ultimate Fondo

By Mac Potter, 4iiii Sales Key Account Manager – Rest of World

It was 4:50 am and I woke up to what sounded like a car alarm going off outside of my house. In a tired daze I came to realize that it’s just my phone alarm. I wiped the sleep from my eyes and I got out of bed, it was time to ride. Before setting off I slammed a cold brew coffee (that had been in my fridge since last summer’s Sea Otter Canada) and three stale pancakes made the day before which I drenched in maple syrup, magically bringing them back to life. After a quick breakfast,  kit on, and power meter calibrated, I headed out the door for what was to be an epic day on the road.

 

I was not setting out on this adventure alone, and admittedly this ride was not even my idea. Due to  Covid-19, races across North America were cancelled. My co-worker Bailey, who is 4iiii’s Sales Account Manager for America, thought of this crazy idea – what if we trained to do Highwood Pass door to door in one single ride? Highwood Pass (located in Kananaskis Country, Alberta, Canada) is a cyclists paradise. Up until June 15th the road is completely closed to all traffic, allowing cyclists to have the mountain pass all to themselves. It’s a once a year opportunity to enjoy the beautiful vistas of the Rocky Mountains worry free. What makes this pass significant is that it’s also the highest paved road in Canada, topping out at 2206m. Once the snow melts in late May, you will find your Strava feed inundated with Highwood Pass rides. It has become an annual tradition with most cyclists in Southern Alberta, Canada to complete before that June 15th date. Most riders choose to do this ride as an out and back making the typical route 110km, but starting and ending in Calgary was going to turn it into a 347km mission.

 

By 6:00am the four of us, Bailey, Mike, Scott and myself, were together and  we officially started our ride. As we knew that we were in for a long day, we set one ground rule at the start of the ride. Pulls were to be kept at 250w and around the 5 minute mark. Mike commented that holding 250w was going to be hard…because it’s tough to go that easy. Mike, who is 4iiii’s Director of Engineering, has been using Trainer Road to train religiously in 2020. Since January, he’s raised his Threshold from 290w all the way up to 333w, or roughly 4.8 w/kg. Riding with Mike is like riding with a kid on a sugar high who just finished a package of Fun Dip, but the sugar high lasts the whole ride and seems to peak on any presence of an incline. Admittedly, the 250w pull rule was more for Mike then the rest of the team that day. Heading out of Calgary Mike took to the front, riding tall and relaxed on his hoods leading us up the first kicker. At 250w, I let Mike know that he passed his first test and we were off, getting us that much closer to the mountains. 

 

After about an hour and a half of riding a low flying owl swooped across the quiet road while we approached our first challenge of the day. “A good omen!” Mike commented as the owl flew off into the distance while hunting for a field mouse with the same determination as Bailey looking for an expired Clif Bar in his back jersey pocket. Bailey, who manages our North and South American sales channels, was adamant that we added a 7km gravel portion to our ride even though we were all rolling on 25c road bikes. My guess is that for 7km he wanted to relive his glory days as a pro cyclist and bring back the now distant memories of racing in the Tour of Alberta. Although Bailey has pro pedigree in his blood from 5 years of racing the North American continental circuit for Red Truck and H&R Block, he was the one that we were most worried about on this adventure. Bailey’s training for this event consisted of 1 hour mountain bike rides followed by beer and hot dogs in the parking lot, randomly selected 45 min Trainer Road workouts that looked “fun”, adjusted to what he felt his FTP was that day, and a few “repeats” on a 1km climb around the corner from his house. Although Bailey didn’t put in the training hours for this “mission” that he created, he will unapologetically remind you that he can still hold 1000w for 20 sec and close to 500w for 2 minutes…even if he can only do it once. Bailey is a soul rider now, but you can always pull the inner racer out of him with a few pokes.


Scott and Bailey fueling up before hitting the Unofficial Strade Bianchi Alberta 7km Group Ride Championship portion of our ride

As Bailey and I flew through the gravel section, Scott and Mike were closely behind but riding a bit more cautiously, clearly on less of a suicide mission then Bailey who was busy leading the charge through the loose and washboarded gravel road. We finished the sector with no flats, no crashes, and Bailey was crowned the unofficial Strade Bianchi Alberta 7km Group Ride Champion. We took a quick pit stop to chow down a few Snickers bars and turned onto the Trans Canada Highway. For those not familiar, the Trans Canada Highway connects Canada from Coast to Coast and is the main corridor for anyone travelling across the country. We decided to up the pulls to 300w to get through this 25km section quickly as even at 8 am, it was still quite busy with adventurers on their way to the mountains and semi drivers blasting their way to Vancouver, British Columbia. Having ridden from Vancouver to Calgary a few years prior I was unphased by the passing semis, but Bailey was eager to push the pace as apparently riding the Trans Canada was his worst nightmare come true.

 

After covering 25km in only 40 minutes, we turned south and started heading into the Rocky Mountains. With the crisp mountain air and the sun radiating on our backs, we took our first pitstop at the Centex just across from the forever “reopening” Fortress ski resort junction, which has seen more movie sets than skiers in the past decade. We refilled our bottles with Gatorade and kept heading Southwest towards the base of the climb. We met my girlfriend Jaylene at the base of highwood pass who parked in the shoulder and turned her Tacoma into a fully stocked aid station. With a few mini cokes, fresh pizza buns, and a handful of potato chips we made our way to the gate. Sticking with our rule of 250w we paced the climb at a steady tempo. Starting at 1690m my heart rate was steady in the high 130’s. The effort felt manageable, but the altitude immediately took effect at the 2000m mark. Our mountain bike trails frequently touch the 1900m mark so for myself, edging above this point was an altitude that my body was far less accustomed to. It was obvious from looking at the data being broadcasted from my Viiiiva that the wheels were starting to fall off. The last 200m of climbing my heart rate climbed from 143 BPM to 160 BPM (max is around 176 BPM) all while steadily holding 250w. Mike even half jokingly asked Bailey, who formerly held the KOM on this climb, if we were at the top yet, unfortunately we were not.


Scott chose to still ride in the shoulder even on the closed road while Mike sets the tempo up Highwood Pass.

Cresting the top at 2,206m, we caught our breath and began the 37km 700m descent. We had a bit of a headwind on the way down so we stuck to our 250w pulls to keep the speed high on the way down. As it was the final weekend to ride Highwood car free, the descent was as busy as commuting along an urban bike path. We were consistently yo-yoing a group of cyclists who would push the little kickers while we stayed steady for the entire descent. Our groups joined, and by the end of the descent we had amassed a peloton of around 30 riders which was pushing the pace as if the south gate were the finish line in a Tour de France stage. Like playing tactics in a road race, we stayed at the back and enjoyed the free ride for the last few kilometers. 

Although the climb up Highwood Pass felt difficult due to the altitude, the toughest part of our ride was the South East stretch to Longview, which is a small town in Alberta best known for not only its Heartland style Alberta cattle ranches, but more importantly its famous beef jerky. Half way up Highwood Pass I started to feel a pain from my right hip that radiated down to the top of my foot. I knew something in my hip was tight but pushed through, hoping that whatever had a death grip on my IT band would loosen itself up. It wasn’t until we hit the cross winds to Longview that my right leg began to really bother me. It almost felt like I had a dead leg. I was running a 4iiii PRECISION Pro Powermeter for our ride and saw my balance go from my typical 52/48 earlier in the ride to 65/35 meaning that my right leg was only doing 35% of the work, about the same amount of work that Bailey does during booth tear down at whatever cycling festival we are working at that weekend. My right leg did the equivalent of  “I’ll go get the car. Left leg, can you pack up the 4iiii tent”?


A steady increase in heart rate once we hit the 2,000m mark.

As we battled the crosswinds towards Longview, Scott was keeping a steady tempo and was definitely the strongest rider on the day. Scott, who joined us for our ride has some serious long distance pedigree in his legs. Formerly a pro Ironman triathlete, the current course record holder for Sinister 7, and Black Spur Ultra, Scott has the ability to push power for hours on end. His “Just Keep Swimming” attitude of Dory from Finding Nemo also pairs with his forgetfulness of how badly we suffer every time it was his turn to lead in the rotation. At 5’8, Scott rides an aggressively set up Cervelo S5. Scott is the only person that I’ve ever ridden with who gives off what seems like a negative draft. Every time I saw my power spike to the 270’s I’d pipe up reminding him of the 250w limit. He’d politely reply with a simple “I haven’t gone above 240w!”. Since then, we draw straws as to who has to ride behind Scott on the group rides as you are guaranteed the equivalent of a double pull when riding behind him.

 

After all of us were worn down and tired of being thrown around in the cross wind, we arrived in Longview exhausted, in need of a coke and a piece of salty beef jerky. I chased down two Tylenol’s with the remainder of my lukewarm Gatorade to help with the pain in my right leg. At 230km in, we turned North and made our way along the foothills back into Calgary. When checking the weather earlier that morning I had seen a tornado watch was listed for South West Calgary and to be on alert in the afternoon. Looking West towards the Rocky Mountains in the distance, a dark cloud could be seen approaching in the distance which was accentuated by the bright green pastures and cattle grazing off in the distance. Asking Mike what kind of omen a Tornado represents, he responded “I’m pretty sure it just cancels out the owl”. With the wind now on our backs we made a quick stop in a small town Millerville to do another restock of food and Gatorade. At this point we were 260km in and we were all starting to fade. I reached into the back pocket of my salt stained jersey and pulled out a Maurten gel (which I purchased through my local bike shop’s customer financing program the evening before). With the consistency of a Jello shot and the flavor of icing sugar I sucked back the gel and felt the energy come back into my body.


Shooting back my emergency Maurten gel in Millerville while Scott is ready to Rock n’ Roll and get back onto the road. I’m pretty sure Mike is pondering whether to dip into the Redbull this stop or to hold off for our final stop in Bragg Creek.

At 270km into our ride, Scott’s tire slowly began to leak. It was bound to happen to one of us, but we finally had our first (but notably the only) flat of the ride. Rolling into Bragg Creek we reached our final stop before home. With a Snickers bar and Sprite in hand, I sat with Mike and Bailey on the steps outside of the Shell station at the Bragg Creek intersection where we watched the Canadian flag blow in the wrong direction. Shattered, we knew that only 50km remained in what was already an unforgettable day on the road. Clipping back in the flag changed directions and it seemed like that good omen earlier in the day came true. Sailing back into Calgary we averaged 41km/hr until we hit our final climb. Cresting to the top of Springbank the skyline of downtown Calgary glistened in the distance and at that point I knew that we had made it. We rolled back into Calgary and parted ways with Bailey (who proved that it’s possible to ride over 300kms on only 6 hours a week of training), while Mike, Scott and I took the bike path back to our homes where we started our journey at 5:30am that same morning.


Complete map and elevation profile of our ride. That last little hill at 330km was harder than it looks…

An hour later, after showering and refueling with a whole pizza, we met at the local Brewery to share a pint and poutine to celebrate the epic journey that we all took on that day. Although struggling to stay awake I opened my phone to Google maps in search of our next adventure and I think I found one at 428km long. I turn to the boys, “Calgary to Jasper anyone”?

Distance: 347 km
Elevation: 3,608m
Ride Time: 10h56m
Avg Power/NP: 170/195
Avg Heart rate: 124 BPM
Avg Speed: 31.7 km/hr
Calories: 6,777
TSS: 385
Tornados: 0

Straining for Power

By Andrew Buckrell, 4iiii Director of Innovation

There’s no question: we all know that pushing harder on the pedals makes you go faster.  But, what are the effects of how you push?  It shouldn’t be a surprise that everyone has a slightly different physiology, which results in a different pedal stroke, which then changes the forces on the crank.  To further complicate things, the way you push on the pedals differs between an easy recovery pace and a full-on, no-holds-barred sprint to the line.  You don’t want to sacrifice the accuracy of those numbers, especially when it could be the difference between breaking that big power number you’ve been chasing after! The real question should be: how does a 4iiii powermeter measure those forces accurately for every rider and every pedal stroke?

When calculating power, we’re relying on one very important feature.  Strain.  Strain is a measure of how much physical deformation is present in a component, which is often directly proportional to the force applied, even if it’s far too small to actually see.  These are measured with tiny gauges that stretch with the material and tell us exactly how it’s moving, but each gauge only tells us one direction of movement.  This is how your bathroom scale tells you how much you weigh (even if you disagree with its assessment), but it’s also how the scales that measure the weight of fully loaded trucks on the highway work.  Without delving too deep into the science, there are a few different types of forces that we’re actually interested in measuring.  The most common and most significant force is the bending strain. Every crank based powermeter on the market measures bending.

We didn’t stop there though.  In order to get the highest accuracy, we kept measuring, since knowledge is power! (and power is what we’re interested in, actually!).  If you imagine holding the brakes on your bike and standing on the pedal, you’re not only going to cause a bending of the crank, but also a twisting motion.  This is exactly the force that a driveshaft in a car has to sustain.  It’s not the primary loading of the crank, but it’s much more than nothing.  

Now, this is where the competition stops, but we kept going!  In order to become the most accurate powermeter on the market, we went even further, and patented our triaxial strain gauge technology to resolve the other major force acting on the crank, axial loading.  This is more or less stretching the crank to make it longer or shorter.

Why did we go to this length to measure all of these forces when others don’t? The easy answer is that we’re engineering geeks and appreciate capturing all of the available detail, but in reality, the real answer is that we’re cyclists too, and all we want is the most accurate measurement possible.  Using these three components gives us the best possible picture of what’s actually happening in the crank, and exactly how much power you’re making.  And don’t think that it’s just the elite cyclists that benefit from this, regular riders will benefit from this measurement as well!

In reality, a simulation of the crank shows the forces and strains in action:

Not to worry though, unless you’re maybe Peter Sagan, your crank doesn’t deform this much (we amplified the movement by about 200x so that you can see it), but it gives you an idea of just how complex the results can be.  Because we took the extra time to go to these extremes for measuring more forces than anyone else, you can be sure that your next sprint AND your next recovery ride will both be measured with the highest level of accuracy in the industry, with less than 1% error, and that yes, you really can be confident in your brag about that latest #WattBomb.