The ANT+ Bike Speed/Cadence Sensor: Everything you ever wanted to know

After the popular ‘everything you ever wanted to know’ post I did back this winter on the footpod, I decided it was time to continue the series with the Speed/Cadence sensor.  Thankfully, the speed/cadence sensor is actually a bit less mysterious than the footpod, primarily because it’s a very mathematically simple device (unlike the footpod which calculates distance based on a number of calibrated factors).

So let’s start out – what exactly is it?

Well, there’s actually three different ANT+ sensor types related to this area, which align to the three different official ANT+ sensor device profiles for speed & cadence sensors.  You have:

1) Speed Only Sensor: This unit uses a wheel sensor and magnet that’s mounted on the frame with a magnet on the rear wheel to measure speed and distance.  Because the wheel circumference is known (manually entered, or calibrated via GPS), each time the magnet passes the sensor a simple mathematical calculation can be made to determine overall speed and distance.

2) Cadence Only Sensor: This unit uses a cadence sensor on your frame and a magnet on your crank to measure cadence.  The crank is the arm that the pedal connects to (and in turn, your cleat then shoe then foot). Each time your crank arm passes the sensor a single revolution is recorded.

3) Speed/Cadence Combo Sensor: This is by far the most popular type. This unit type sends both speed and cadence information wirelessly to the head unit, and does so usually using a single device (either one piece physically or two small pieces connected via wire)  mounted near the rear wheel.  It’s simply a combination of the first two unit types I noted.

Each of these different devices uses a different ANT+ ‘profile’ type, which means that just because a device supports one profile type, doesn’t mean it’ll support the other types.  The combo sensor type was the oldest, and is the most widely supported.  I know of no ANT+ devices that fail to support it.  Meanwhile, the speed-only and cadence-only sensor types are newer and far less supported.  For example, the Garmin FR305 doesn’t support these newer ANT+ profiles.

What do I mean by ANT+ profiles/device types?  Well, every ANT+ device has a profile, think of it like a ‘classification’.  There are ANT+ Heart Rate Straps (HRM Device Profile), Power Meters (PWR Meter device profile) and others.  There are even new profiles coming for data streams like Skin Surface Temperature and Electric Bikes (LEV).  Each of these is simply a standard identifying a device, as without standards ANT+ as a ecosystem wouldn’t work too well.

Speed/Cadence Sensors:

For most folks, the most common Speed/Cadence sensor is the $35 Garmin GSC-10.  Without question it’s the Microsoft Office of the ANT+ world – almost everyone has it.  This simply has three pieces.  The first is the spoke magnet (seen left), then the cadence magnet (seen right), and finally the electronics pod mounted to your frame that has two magnet sensors capturing both speed (upper moving arm off unit) and cadence (lower right side of unit).

But there are others that have created Speed/Cadence combo sensors as well, most with slightly differing designs.  Some of these designs are meant to solve certain bike frame configurations where the GSC-10 doesn’t work well by using a small wire to bridge a separate pieces for the speed and cadence data.  This gives more flexible installation options Ultimately the unit combines the data together though so it shows up at an ANT+ combo sensor.  For example, below is the Wahoo Fitness combo sensor:

I suppose the above picture probably deserves some explanation (I generally feel any photos with cutting devices probably do).  The Wahoo sensor is in the middle about to get snipped, as I was redoing the cabling to play around the idea of extending the short cable in the middle to accommodate a recumbent bike.  I never did get a bike to test with, though conceptually it’s pretty simple and worked just fine on my desk (albeit in a very non-weatherproofed sorta way):

Speed Only and Cadence Only Sensors:

Next we have those units that are either speed sensors or cadence sensors…but not both.  These sensors are dedicating to delivering either speed data, or cadence data.  They can’t do both, and can only measure what they are designed for (meaning, you can’t use a speed sensor to measure cadence).  The most common reason people by these sensors over combo sensors is if their bike has a specific configuration that the combo sensor won’t fit.  In particular, these are more popular with unique bikes like recumbent and tandem bikes.

Here for example, is a speed-only sensor from Bontrager:

Bontrager also makes a speed sensor that mounts directly into the fork as well, similar to the frame mounted sensors noted below in the next section.

Bontrager also makes a cadence-only sensor, though, these are generally harder to find simply because the demand is so low for them.

In both of these situations these singular-function sensors require that the ANT+ head unit supports the single-function device profiles.

Newer bike computers like the Edge 500 and Edge 800 do – but many older units do not, so it’s something to keep in mind.

Frame Integrated Speed/Cadence Sensors:

The last category of speed/cadence sensors out there is the frame integrated sensors.  Technically from an ANT+ standpoint this is merely a speed/cadence combo sensor.  And today, only one company makes them – Bontrager – as part of their Duotrap line.  And further, today only one bike frame vendor  support these – Trek.

That said, for those that have these bikes – these are awesome.  The pod simply fits right into the bike.  I installed one of these on The Girl’s back this past winter…really cool stuff.

Here’s what it looks like installed into a Trek Speed Concept:

As you can see above, the design is incredibly streamlined – and perhaps more importantly, not susceptible to getting bumped out of alignment.

Power Meter Speed/Cadence Information:

Finally, it should be mentioned that most cycling power meters will include either speed information, cadence information, or both.

The type of data gathered by these power meters will vary based on how the specific power meter measures its power data.  That in turn drives whether or not speed or cadence data is determined – and if so – whether it’s measured or calculated (estimated).

Take for example crank based power meters (like Quarq and SRM). 

These power meters use magnets to determine cadence – it simply measures each revolution of your crank (the thing your foot ultimately connects to).  Here’s an example of a Quarq Power Meter, with the magnet visible below, attached to a small metal ring near the bottom bracket (taken looking straight down onto the crank & chain rings from above):

Then you’ve got other crank power meters like Power2Max, which doesn’t actually use magnets at all – but rather uses mathematical equations to determine your cadence based on other known variables.  In this case, these calculations are usually given a specific cadence range they are accurate to.  For example, the Power2Max is specified as 30 to 180 RPM.

And finally, you’ve got hub based power meters, like the PowerTap, which can do both speed and cadence.  In the case of speed it can measure that directly since it knows revolutions based on hub rotation.  But for cadence, it actually calculates that mathematically.  This is generally pretty accurate, though like the Power2Max it has known ranges or fringe scenarios where it doesn’t work quite as well (really high cadence work).  But for typical everyday use, it’s absolutely more than sufficient.

In most head unit configurations (that’s the display unit), cadence information from a power meter will always override cadence information from a separate standalone sensor.

Why do you want cadence or speed data anyway?

Well, it depends.  Let’s start with cadence.  There’s actually been quite a bit of research on both sides of the cadence coin.  One side says that a higher cadence – such as 90-95RPM is ideal, while the other side says that a ‘self selected’ cadence is most efficient.  Many folks do high cadence drills though to be able to spin at higher RPM’s in the event it’s required – without taking as large a hit heart-rate wise simply to spin the cranks faster if required (i.e. a sprint to a finish).

Virtually all ANT+ cycling head units will display cadence information in real time on the bike, such as below (see as CAD):

During the ride the data is recorded for later access, easily viewable in charts/graphs:

Looking at the speed side of the equation, most folks tend to use speed sensors indoors on trainers.  This allows them to gather both speed and distance data when inside.  And while speed and distance while on a trainer is purely a function of your gearing combination and resistance applied by the trainer (meaning, by changing gears and resistance you can dramatically change speed without changing effort) – it can still be an interesting metric for some.

Afterwards, like cadence, you can view the speed and distance information in charts/graphs:

For mountain bikers a speed sensor can be more accurate than GPS in determining distance – as the speed sensor measures speed purely based on revolutions of the wheel and isn’t dependent on GPS correctly tracking on quick switchbacks in forests.

And finally, the same can also be said of indoor track cyclists, who are looking for data in places where GPS isn’t going to work well.

Wrap Up:

As you can see, speed and cadence sensors are both quite common – but also pretty integral to many cyclists.  The good news is that they are pretty cheap, with most ANT+ units costing about $35 – well within the budget of most folks.

Hopefully I’ve covered everything you ever wanted to know about speed and cadence sensors – but if not, feel free to drop a note below and I’ll compile it into a Q&A and update the post – similar to what I’ve done on the footpod post.

Thanks for reading!

(Note: I’m currently away on my honeymoon, but due to the magic of automated publishing, you’ll be enjoying content in the meantime.  Thanks for reading!)