Bicycle Power Meters: Garmin Vector 2 compared with Powertap

Bicycle Power Meters: Garmin Vector 2 compared with Powertap

For the past 18 months I’ve been using a power meter on my bicycle as one of several tools to help me improve my cycling.  For me, using a power meter enables a much more accurate view of my cycling fitness. It’s kind of like watching your biometrics in high definition.  

There are four general types of power meters:  Crank-based (Quarq & Stages), hub-based (Powertap), pedal-based (Garmin Vector), and passive (Newton). I started out with the Powertap because a friend ‘sold’ it to me cheap.  The Powertap hub-based power meter works fantastic.

It’s very reliable and the battery lasts forever. Unfortunately, it has one drawback.  Because the power meters is built into the rear hub of my training wheels, when I switch to my race wheels, the hub-based power meter does not transfer.  That means I’m training with a power meter and racing based on perceived excretion and/or heart rate.  That’s like navigating with a GPS in training and then having to use a map/compass during a race.  That’s actually not that big of a problem for short races such as a Sprint or Olympic distance triathlons because you just focus on going really fast.  However, it is a big problem for long distances, such as a 112 mile bike leg of an Ironman race, where consistency and pacing is paramount. Heart rate monitors work well but they are lagging indicators of effort so using both a HR monitor and power meter in a long-course race is optimal.

I recently picked up the new Garmin Vector 2 power meter at Destination Kona.  The Vector 2 is located on the pedal which is ideal for me because:

1. I can switch between training wheels and race wheels while keeping my power meter
2. I can easily switch the pedals from my roadbike to my triathlon bike.  The length of my road bike cranks is 172.5mm while my triathlon cranks are 170.0mm long which is a common trend for triathletes as the shorter cranks open-up my hip-flexors which allow me to get more aero and run better off the bike. 

 Initially I was skeptical of the Garmin Vector.  I had heard some reports that the Vector is unreliable.  Based on reviews, it appeared that most of the challenges with Vector were a result of not torqueing the pedals to the 25lbs spec. So, I decided to test the Vector myself.  I carefully installed the Vector 2 which took all of about 15 minutes the first time. I can now transfer my pedals between bikes in about 3 minutes.  Simple!

Once the Vector 2 pedals were installed, I kept my Powertap on my bike to do a series of comparison tests.   I synched each power meter with one Garmin 910xt (thanks to my brother for loaning me his 910XT).  Then, I went for a couple of really long bike rides which included some up-tempo interval sets.  At each interval I hit the ‘lap’ button on both of the Garmin 910xt’s which gave me average watts for the last segment.  Interestingly, during the ‘up-tempo’ intervals, the difference between the Vector 2 and Powertap averages was one (1) watt.  Equally interesting is that during the easy segments of my ride I noted the average differences in watts between the Powertap and the Vector were about five watts with the Vector 2 reporting higher watts.  I’m not sure why but here’s my speculation:

1. There may be some degradation in watts as power is transferred from the pedal to the rear hub, perhaps as much as five watts.  However, that’s inconsistent with my ‘up-tempo’ interval results.  
2. The Vector appears to be more sensitive to coasting.  For example, if I coast for a moment, the Vector almost immediately reports 0 watts while the Powertap gradually descends to 0 over several seconds (maybe 7 seconds?).  If true, that means the Vector 2 could perhaps be slightly more accurate.

 In short, I’m satisfied that both Vector 2 and Powertap power meters are accurate.  In fact, both are surprisingly accurate.  Even a five watt difference on easy segments is nothing to worry about.  I’m very pleased with how easily I can transfer the Vector 2 from bike to bike.  If I wasn’t worried about race wheels, I would have just kept the Powertap.  Both offer greater insight into cycling biometrics which enables more efficient training and racing.

Side note, CycleOps, the maker of Powertap, now sells not only hub-based power meters but also a pedal and chain-ring system.  Love the innovation!