Cadence/Speed Sensor

[Hugh Manatee]

i have a G armin 800 complete with the above add on. The Cadence bit of it I totally get. Where does the speed bit come from? Am I missing something? I am hoping it might be something to do with turbo/rollers indoors?

Or is it just a name?

 

[Racing roadkill]

i have a G armin 800 complete with the above add on. The Cadence bit of it I totally get. Where does the speed bit come from? Am I missing something? I am hoping it might be something to do with turbo/rollers indoors?

Or is it just a name?

The speed sensor is an inertial sensor that fits on the hub, it looks a bit like a get me home light, but is bit bigger. If you’re using it on a Turbo it will have to be on the rear wheel, unless it’s a direct drive turbo, where there is no back wheel.

 

[DCLane]

@Hugh Manatee - yes, it works for rollers, etc. Basically it interacts with the cadence sensor to identify speed. My 14yo uses it for training to link with Zwift or just when he's using the bike.

 

[Racing roadkill]

@Hugh Manatee - yes, it works for rollers, etc. Basically it interacts with the cadence sensor to identify speed. My 14yo uses it for training to link with Zwift or just when he's using the bike.

Yes, but the speed and cadence sensors are separate sensors. I’m presuming his set up isn’t giving speed, because he’s got the sensor on the front wheel, which doesn’t move on a single roller type turbo ( but does on double rollers) or he hasn’t fitted the speed sensor at all.

 

[DCLane]

Yes, but the speed and cadence sensors are separate sensors. I’m presuming his set up isn’t giving speed, because he’s got the sensor on the front wheel, which doesn’t move on a single roller type turbo ( but does on double rollers) or he hasn’t fitted the speed sensor at all.

No - the speed sensor goes on the rear wheel hub.

 

[Racing roadkill]

No - the speed sensor goes on the rear wheel hub.

That’s right, on a roller Turbo. I put my speed sensor on the front wheel in real life. It’s easier to just swap front wheels if I need to swap it to another bike. It saves having to get scraped knuckles / be a contortionist trying to extract the sensor from between spokes / with sharp sprockets in the vicinity. If you only have one bike set up for speed and Cadence sensing, and you use it for road and roller turbo, the rear wheel is the only sensible place to put it. If you ( as I do ) never use a turbo, and have several bikes used on the road, it’s easier to have the sensor on the front wheel, and swap the front wheels around as and when necessary. It saves having to buy lots of separate sensors / faffing about.

 

[Ajax Bay]

i have a G armin 800 complete with the above add on. The Cadence bit of it I totally get. Where does the speed bit come from? Am I missing something? I am hoping it might be something to do with turbo/rollers indoors?

Having fitted the speed sensor (as @DCLane says) on the rear wheel so one can use the bike on a 'normal' turbo, when you ride on the road the 800 determines your speed using the GPS signal(s). It also autocalibrates the speed sensor data and determines what the 'real' circumference of your wheel/tyre combo is. You can switch the 800 to 'use speed sensor' ie not GPS. On the road the speed and other integrated data (ie distance, average etc) may not be as accurate as using the GPS. But when you ride on your turbo (or rollers), the 800 has an accurate figure for the wheel (saves you calculating it or doing a roll out test) and you'll get a good (ie relatively accurate) 'speed' etc readout as you drip sweat on the 800 indoors.
Noting the 'easier to swap front wheels' argument (above note @rr never uses a roller/turbo), I suspect rather few riders fit their Garmin speed sensor to the front wheel. Personally, I have a speed sensor attached to the hub of my turbo wheel. I don't have a speed sensor attached on any of my road bikes - what's the point? I get my speed from GPS.

 

[Racing roadkill]

think the point is that it's more accurate/reliable than pure GPS-based speed because it doesn't rely on the vagaries of the GPS signal. So you won't get fluctuating speed readings, say, under heavy tree cover or if you go through a tunnel. At least that's what I've read. I don't own one.

This is very true.

I think they self-calibrate (or some of them do), so I guess they assume the GPS signal is good when calibrating.

Mine does self calibrate. It’s the inertial type, so doesn’t use GPS.

 

[T.M.H.N.E.T]

It doesn't require calibrating every ride IIRC, if it does, I've never seen it nor allowed it to be updated

 

[Racing roadkill]

Surely it has to use the GPS as a reference when it's self-calibrating? Otherwise it won't have a clue what the wheel diameter is. Without calibration all it can tell you is that the wheel is rotating as X rpm. Unless you manually enter the wheel diameter it can''t give you speed.

I always assumed that they calibrated themselves by thinking "hmmm, the wheel is rotating at X rpm, and the current speed is Y km/h, therefore the wheel diameter must be (insert a bit of arithmetic and pi here) ... Z mm" Then, having calibrated itself it can say "The wheel is rotating at X' rpm. I know the wheel diameter is Z mm, therefore the speed must be ... insert arithmetic ... Y' km/h"

That is right, I just let it do it’s thing. You can manually enter the wheel diameter, and how far 1 rotation is, on my Garmin 1030, or hit the calibrate button and ride a bit, then it sorts itself out

 

[Hugh Manatee]

Thanks everyone. I might just stick the old cat eye wired thing back on the bars. It picks up off the back wheel. I only have the cadence gizmo that fits to the chainstay.

My thread was moved. The irony is, I have the fitness forum on ignore!

 

[Ajax Bay]

it's more accurate/reliable than pure GPS-based speed because it doesn't rely on the vagaries of the GPS signal. So you won't get fluctuating speed readings, say, under heavy tree cover or if you go through a tunnel.

This is very true.

This is only true for moment to moment speed read-outs during the minute percentages of time where a rider is riding through tunnels or under heavy tree cover. The rest of the time it is less accurate. And the average speed and distance ridden corrects itself as soon as a decent GPS signal is received (quickly - sometimes one may see a spike thus caused in the readout when reviewing the ride once downloaded (eg onto Garmin Connect)). Pretty sure you have to check the "Don't use GPS" button and rely on the speed (read revolutions of wheel) sensor by itself. Once you do this the speed sensor stops auto-calibrating. On the plus side the rider can be lazy and just fix the bike to the turbo and ride, without having to change the speed default to "Don't use GPS".
There was a thread on here recently where a rider couldn't understand how on multiple rides his mate, who had started, ridden the same routes and finished with him was recording faster average speeds. Reason was, like @rr, his mate had entered a larger than life (wheel) circumference into the Garmin and turned the GPS off.

Mine does self calibrate. It’s the inertial type, so doesn’t use GPS.
I just let it do it’s thing. You can manually enter the wheel diameter, and how far 1 rotation is, on my Garmin 1030, or hit the calibrate button and ride a bit, then it sorts itself out

It can't "self-calibrate" unless the Garmin is using GPS. And if you "ride a bit [and] it sorts itself out" what's sorting it out is the GPS auto-calibrating it.

It doesn't require calibrating every ride IIRC, if it does, I've never seen it nor allowed it to be updated

As I've tried to explain, the auto-calibration is not necessary for providing a speed (etc) read out while riding on the road and you don't 'see' it recalibrating. But if you jump on the turbo, the sensor allows an accurate 'speed' (etc) readout. If you fitted a larger tyre (with the effect of a greater circumference) it would auto-change, but you wouldn't see it.