In the first section of this review (go to bicyc.com.au), I had a look at the iBike Newton+, the Quarq Riken and the Garmin Vector – three quite different power meters with the common goal of improving training. The reviews continue; here are my experiences with the SRM, the PowerTap and PowerCal, and the Stages. Again it is a collection of four quite distinct devices that have the same goal of improved training.
These cranks are the baseline by which all other power meters are judged. They were among the first to market and still are nearly 25 years later! SRM (Schoberer Rad Messtechnik) is almost like Hoover or Walkman in defining a product class by its name. Thanks to their long history and deep penetration into the ranks of pro cyclists, they offer a crank solution for virtually every kind of cyclist (road, MTB, track, and BMX); at a cost. The SRM system is clearly more expensive than any of the others on this test. SRM definitely wins the nice packaging contest amongst all power meters. The packaging, manual and accessories are all excellent.
Previously SRM purchased Dura-Ace cranks from Shimano, sliced off the spider and attached their instrumented solid spider to the bare arm. Now Shimano makes for SRM an SRM-specific DA9000 11-speed right arm that fits without any initial modification. As my bike is 10-speed, I received an older model DA7900 crank that is more 10-speed friendly (which displays minor evidence of where the spider was cut off).
Installation for me was a bit more involved than it might be for some – my bike uses SRAM cranks and the Shimano axle doesn’t work with the SRAM bearings. So job one was to remove my own cranks and left bottom bracket bearing (my bike uses press-fit bearings without cups) before pressing in a Shimano-sized left bearing and installing the SRM cranks.
As I wrote in the prior article about the Quarq meter, the SRM requires a magnet to trip the cadence sensor portion of the power meter. No cadence equals no power, so this is critical. Rather than abandon the bike and put the SRM on my other bike (which has a SRAM external bottom bracket and therefore would require a new bottom bracket which I didn’t have) I persevered with my race bike and found a way to temporarily mount a magnet in the required location. One of the guys I coach has lost the magnet off of his bike at least twice, with power instantly dropping out. This is proof that it pays to spend some time securing that magnet.
A System Rather than a Meter
SRM also makes their own head unit; the Power Control (now in its seventh generation). I remember when the Power Control required wires running down the frame to the cranks in order to obtain the power data – those days are long gone! With the seventh generation
the entire system is ANT+ compliant, meaning you could pair a Power Control with any other power meter in this review and my Garmin worked just fine with the SRM meter.
Advantages to using the SRM Power Control to record your data include at least two notable differences: lots of memory suitable to a pro who rides hours every day and four hertz data capture (that is four times the industry standard). One clear disadvantage is the fact it lacks
GPS, meaning no maps and no Strava bragging rights after your big ride.
The battery in the SRM is not userreplaceable (it has to go to New Zealand for service) – that’s the bad news. The good news is, it lasts 3000 hours in the latest 11-speed version (and half that in the reviewed 10-speed version). For a club rider who rides a lot, that is about
four years of use. Even a busy professional on 10-speed cranks would only require a battery once a year.
RRP: $4,990 ex GST (NZD)
Distributed by www.srm.com.au
One issue with just about any power meter is installation. Most of the meters I have discussed put the instrumentation in the crank somewhere. Cranks are not easily swapped between bikes, and with different lengths for different riders they are not easily swapped between cyclists. The PowerTap was released back in the ’90s on the premise of easy interchange between bikes. Based on a rear hub, the vast majority of road bikes will take installation of a PowerTap in, literally, seconds (assuming tyre and cassette are already installed).
Over the years the PowerTap system has been updated to keep abreast of new technology (such as ANT+ compatibility) and to make the hub less expensive and more robust (the first generation PowerTap I used 15 years ago was much more expensive and also more delicate than the current model). The easy installation of a wheel is the benefit of using the hub as the measuring platform. The inability to use ‘race wheels’ and measure power at the same time, unless your race wheel has a second PowerTap hub in it, is the drawback to a hub-based system. And a racer loses his power data if he gets a flat and takes a neutral spare.
After mounting a tyre on the rim and moving the cassette from the bike’s current hub, the wheel fits straight onto the bike. A quick check of brake pads and shifting are always wise when swapping wheels, to prevent trouble later on. Spinning the rear wheel wakes the PowerTap from its ‘sleep’ state (and will also wake the speed/cadence sensor, assuming you have one of those too), ready for the first pairing with the head unit. My Garmin found the hub on the second try and offers to zero the meter every time they are reconnected.
While it’s definitely not unique to the PowerTap, it is worth mentioning that during a long ride where the ambient temperature could change by tens of degrees, one must re-zero the meter to preserve accurate measurements. Imagine starting your ride at 7am when it is 12 degrees out and completing that ride at 11am when it is approaching 40 degrees (this happened to me during the testing period). That big increase in temperature has the potential to raise the measured power. Similarly if you like twilight racing and the temperature drops by 15 degrees or more, your power values will decline with the temperature.
I don’t know what went wrong – a bad zero perhaps – but one ride came back with an average power of two watts and a max of four watts. Fearing it had broken, I re-paired the PowerTap with the head unit and subsequent rides were just fine.
Included in the PowerTap box is the tool required to remove the non-drive side hub cover. Inside this space sits the battery that powers the meter (as well as providing access to the hub bearings). When the battery runs low, it is a quick job to open the hub and replace it with one available from a pharmacy.
This latest version of the PowerTap is 11-speed compatible (for SRAM or Shimano) and includes the spacer required to fit an 8/9/10-speed cassette as well, so all the bases are covered. The wheel was well built (round, true and tight) with a standard box-section rim suitable for training and club racing. Cosmetically the latest PowerTap looks a bit different from prior models (now dark grey and orange) and blends in easily with the look of modern road bikes.
Distributed by Monza Imports
Anyone who has used a heart rate monitor to characterise their athletic endeavour knows that some days your heart responds quickly while other days it doesn’t, no matter what you are doing. When riding at a steady speed you will settle into a steady heart rate – but exactly what rate it is depends on the speed, the rider and the day. In short, heart rate is extremely variable in all aspects.
Why wouldn’t it be? Your legs are doing the pedalling and your heart is supplying the blood to them. It is a bit like monitoring the activity of the fuel pump in your car to determine how fast you are travelling. Yes they are related, but the relationship is not exactly direct.
Most power meters measure force. Force in the pedals, the crank spider or the freehub body. Force per unit time is power. Thus there is a very direct pathway from measurement to output. To ride at 20kph requires a certain number of Watts; to go 40kph requires eight times as many Watts. The required wattage doesn’t change (much) from day to day and the relationship of eight times as much power for twice as fast is fixed.
A Better Heart Rate
The PowerCal fills in a gap between merely measuring heart rate and actually measuring power. This is because the PowerCal is an ingenious device that monitors heart rate variability instead of just heart rate. The change in the heart rate with time and every beat-to-beat interval is analysed inside the chest strap.
On one ANT+ frequency it sends out a heart rate signal, but it pretends to be a power meter on a second frequency and sends out a power signal.
The power it reports is not necessarily any more accurate than a good guess but it is surprisingly consistent. A multi-lap race using the PowerCal reveals that the reported power levels are internally consistent; that is two 30-minute laps report virtually identical power averages, while a 28-minute lap is notably higher and a 32-minute lap is clearly lower. If you stop pedalling, the ‘power’ value soon drops down close to zero even while your heart rate is still sky high. How did they do this? As creators of the PowerTap force measuring hub, they had countless hours of ride files containing heart rate and power data. In collaboration with a university research team, they developed an algorithm that accounts for the average power output from the average rider depending on what their heart was doing.
Not Quite Power
The error margins are larger than the difference between any individual’s unfit and fit self, meaning that you will not see changes in your power values as your fitness changes. The variability from moment to moment is even greater than the variability seen with power meters that measure force, the plot from the PowerCal is all over the place with higher spikes and lower low values than any other ‘power’ meter. At a steady rate however, climbing a long mountain pass or a time trial effort over 10km, the average power recorded via PowerCal and via another power meter can be extremely close to each other. Uncannily so, in fact.
The fatigue monitoring that coaches love to use power files for, works quite well too. Monitoring the level of training and recovery works better with PowerCal than with just heart rate alone.
While the PowerCal is sold as a power meter, and it connects to your computer as if it were one, it still isn’t an actual power meter. Currently there isn’t a name for this type of device, however it is very useful and well worth its small asking price. PowerCal makes a particularly smart choice if you need to replace your current heart rate strap anyway – in which case the cost to add the extra data is close to zero (that is, the PowerCal is not much, if any, more expensive than other ANT+ heart rate straps).
The only issue I had with the PowerCal during use was that it was slow to provide a steady heart rate reading most of the time. Sometimes a short training session was nearly over by the time correct heart rate readings were achieved, while other days it worked just fine.
Distributed by Monza Imports
Where SRM are the established players in the power game, Stages are the new kids on the block. Stages Cycling purchases left side (the non-drive side) crank arms from the various manufacturers and adds the power meter to the inside of the arm – it is that simple. Installation is thus merely the removal of the left pedal and arm, permitting substitution with the new left arm. A typical multi-bike cyclist would have several bikes with identical cranks on them and in this circumstance one Stages meter would easily swap across all bikes. That is the exact reason I purchased my review meter.
One Sided Metering
The Stages meter only measures the power output of the left leg. This is a problem for some riders – I have seen cyclists with a 50% imbalance in leg power.
Yet it was a reasonable decision by Stages because virtually all of the cyclists I have worked with have a maximum 2-4% imbalance, which is negligible. To fool the head unit into reading correct power data it simply doubles the left leg data for the right leg.
In case you are concerned that this one-leg limitation is a real impediment, know that the power-meter addicted Sky pro cycling team is using Stages on their bikes for the 2014 season. That is a convincing endorsement.
Since the metering unit contains accelerometers, it can calculate cadence without any external sensor or magnet – the power meter broadcasts both power and rpm on ANT+ frequencies to any ANT+ head unit. They also found the space to fit the latest Bluetooth 4 equipment in it as well. This offers two options; to update the firmware in your power meter via your smart phone and to use fitness Stages apps on your phone to monitor your riding. In general, ANT+ is the preferred option because any number of devices can ‘pair’ in this protocol (a unit on your handlebars, a unit on your wrist, a unit in the car following behind you and so on) while Bluetooth is a closed pairing – two devices hook up and no other devices may listen in. That and the fact that my phone battery only lasts a couple of hours on a full charge when running Bluetooth and GPS where my Garmin lasts about 10 times as long.
Stages contains a temperature sensor so it auto corrects for changes in temperature throughout the ride. SRM is the only other meter that does this – the others have to be calibrated to the current temperature to be accurate. In fact there is no calibration procedure for this meter; it does everything itself except for the zero offset. The installation of the crank on the axle induces a stress in the crank arm, setting zero offset corrects for this allowing measurements to be accurate. You only need to set zero offset once each time the crank is removed and reinstalled.
All Stages models are based on aluminium crank arms. This is no problem for Shimano users, but it means that SRAM users have to put a Rival left arm on their Force or Red carbon cranks (it fits however). The device is around 1cm thick, mounted on the inside of the crank.
This means that you require about 1cm of space between the back-side of your crank and your frame in order to use a Stages meter. The only bikes I know of that have a real problem with this have the rear brake located on the chainstays rather than up high on the seat stays.
Installation, connection with my Garmin and connection with my iPhone were all easy. The Stages iPhone app allows me to update the firmware, check the battery status and see live cadence and power figures. I first used it during a race, where it was 100% transparent: I
will never notice the few grams of weight it adds to the left arm, the left arm itself is identical to the one I removed, the cadence and power reporting were continuous (no mysterious drop-outs or any other glitches) – just too easy.
Distributed by FE Sports
A Powerful Argument
A power meter is not sexy like a pair of deep carbon rims; in fact most cyclists won’t notice that you have one. They add weight to your bike rather than reduce it. They don’t make the bike feel better, go faster or in any way provide immediate gratification for their installation.
Quite the opposite – a power meter just about guarantees that your training sessions will be harder, longer and more pain inducing than ever before.
Despite all that, there is no other item that I would recommend over a power meter to the cyclist that wants to improve their competitive performance. It is no coincidence that the most successful cyclists I have coached have had a power meter on their bike. These were people who were motivated enough by the prospect of success to trouble with a power meter and then put in the training effort required to get the results.
The bottom line is that if you want to maximise your speed on the bike, you should be considering a power meter. They don’t tell you what to do with them, so you’ll also have to factor in learning to use power or hire a coach who knows. I suggest after six months of training with power you will achieve new heights on your bike.