Heard a bit of a racket from the back wheel, stop and have a look, and notice something missing from my garmin cadence/speed sensor…

Bit of a dodgy design if you ask me. Still works as a cadence sensor though. Thinking maybe I need to plug the hole up with sealant to stop water getting in.

EDIT: Meant to look like this…

did the screw come out or the arm get hit by the spokes?

I’d say it’s designed to shear off so it doesn’t jam your back wheel if it does end up in the spokes?

What was the name of that Welsh chap way way back (1940s?) who lost an arm then went on to become a professional cyclist?

No sign of it, and I wasn’t going to look back over half a mile of road to try and track it down. Bit of a shame.

molgrips, no idea, quite impressive though if he did it. Would give him a bit of a weight advantage too!

EDIT: Is this the chap

Ialways thought it was odd because unless you have very unusualy shaped cranks/stays the cadence but ends up tilted out. And its a big unit so looks even more wrong.

Mine did that. I bought another. Then I read the instructions that said “Position the arm and tighten the screw” and immediately realised why the first one died.

You don’t need it unless on he turbo anyway

Doh! That’ll teach me to never read instructions. Is the arm meant to point upwards or be down next to the chainstay? I had mine pointed upwards.

I use it on the turbo quite a bit. Still somehow does speed and distance, I think it may be getting this from the power tap hub, seems too high/fast though.

instructions say down;

http://static.garmincdn.com/pumac/Speed_Cadencebikesensor_Instructions_Multilingual_.pdf

Bugger, major fail with my fitting. Was fine for a year or so on my other road bike, must have been lucky.

you’re not alone – do a quick search on google images…

Feels good to know that I am not alone in my incompetence.

Bit of a dodgy design if you ask me.

Workman, tools, you know the story…

If it’s pointing down and finds its way into the spokes it’ll get pushed back out again.

As you’ve found though, the PowerTap will do speed/distance, and will also do cadence if you’ve got a Pro/SL+ or SLC+, making the sensor totally pointless.

I have mine pointing up, there isnt enough clearance if it is pointing down.

I HAD to have mine pointing up – no other way as the chain stays are so close to the spokes!

How would the power tap do cadence? Wouldn’t it need to know what gear ratio you were using? Or does it have some other sensor?

Plenty of room between wheel and spoke, might have been able to save the arm if only I’d read the instructions 🙁

I have never understood the need for cadence on a bike.

If you don’t have any you’re not going anywhere 🙂

foxyrider – Member
I HAD to have mine pointing up – no other way as the chain stays are so close to the spokes!

I’ve fitted a sensor like this on several bike when I works at a bike shop (cateye though) and had the exact same problem with many of them. To keep the sensor close enough to crank and wheel the flappy bit had to be up. Shit design.

I’ve fitted a sensor like this on several bike when I works at a bike shop (cateye though) and had the exact same problem with many of them. To keep the sensor close enough to crank and wheel the flappy bit had to be up. Shit design.

Yup maybe not sh1t 😉 but not very adaptable/adjustable – perhaps they need a design that is more adjustable to fit different styles of chainstays? – I got mine for the more accurate distance measurement than the GPS not really for the cadence TBH.

cynic-al – Member

I have never understood the need for cadence on a bike.

Essential for most turbo training plans.

serves you right for bieng from the dark side

Measuring cadence really works for me on a road bike, I use it a lot for different types of training. Pretty pointless on an mtb IMO, though never had one fitted.

How would the power tap do cadence? Wouldn’t it need to know what gear ratio you were using? Or does it have some other sensor?

That much I can’t tell you, something to do with the strain sensors I guess, it knows from the ‘pulsing’ when you’re on the downstroke. I only know because I use my PowerTap on 2 bikes, and the cadence works perfectly whether the Garmin sensor’s there or not! Kinda makes the Garmin sensor pointless!

(Admittedly I have never seen one of these things but…) I thought the cadence sensor worked using a similar magnet-and-reed-switch but with a magnet on the crank instead of the wheel?

Admittedly I have never seen one of these things but…) I thought the cadence sensor worked using a similar magnet-and-reed-switch but with a magnet on the crank instead of the wheel?

It does, it’s the speed sensor that’s fallen off, the cadence sensor is in the same unit

Thought so, I was just wondering given talk of strain sensors and gear ratios whether they worked some other way…

Looks like this when all the bits are in place

Because with a PowerTap hub you don’t need any magnets or sensors, it has an inbuilt cadence sensor, which I assume is reliant on the strain gauges. It can also do crank torque, and various other things, I wouldn’t have thought cadence is that complicated!

Trust me, take your sensor off and go for a ride, the cadence will still work unless it’s one of the cheaper PTs.

it knows from the ‘pulsing’ when you’re on the downstroke.

Must resist a Finbar moment…

Sounds plausible though. Shall try whipping it off and seeing if cadence works with power hub alone. Can see how it would do speed but does seem higher than it should be based in cadence and gear.

Cadence works with just a powertap hub, no other sensor needed – it’s because no-one has a completely smooth pedal action (except for that cycling robot that rides for Leopard Trek now) so there is a pulsing in the strain being measured.