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Cadence Sensor Question

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Old 01-15-18, 10:19 AM
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rzsionak
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Cadence Sensor Question

Simple question, but haven't found a clear answer in search.

I'm looking at getting a cadence sensor to use while on my bike trainer, but wanted to know if a speed sensor is also needed to calculate speed and distance. I'm using the Sufferfest for training sessions, and they already ask your wheel size. I would think from physics class that wheel size plus cadence could determine speed and distance, but wanted to check on other's experience.
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Old 01-15-18, 10:32 AM
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TimothyH
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Originally Posted by rzsionak View Post
I would think from physics class that wheel size plus cadence could determine speed and distance, but wanted to check on other's experience.
Only on a fixed gear bike with a known gear ratio. On a geared bike the ratio of speed to cadence changes depending on the gear ratio in use.

I don't know much about trainers and will leave the original question to those who do.


-Tim-
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Old 01-15-18, 10:34 AM
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I have no need personally I dont use a bike computer, but on bike tours, to count miles ..

, but typically, a magnet is attached to the L crank arm. and sensor on the chainstay functions
just like a wheel magnet passing by a fork sensor , in a trainer-stand that needs to be on the rear wheel
2 sensors tick '1' as the magnet passes.
you have input wheel size, so the bike computer adds up circumference roll out for distance,
and pulse frequency is combined for speed.
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Old 01-15-18, 10:36 AM
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rzsionak
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Originally Posted by TimothyH View Post
Only on a fixed gear bike with a known gear ratio. On a geared bike the ratio of speed to cadence changes depending on the gear ratio in use.

I don't know much about trainers and will leave the original question to those who do.


-Tim-
On the trainer, I'm riding in ERG mode, which basically fixes the gear ratio the entire riding time. That's where I'm confused a bit as to what I need to get.
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Old 01-15-18, 11:48 AM
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I think ERG mode holds your power steady, eg if your pedaling cadence slows, the resistance increases, and vice versa. That shouldn't prevent you from changing gears, which means cadence and wheel size are not enough information to calculate distance.
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Old 03-10-18, 06:24 PM
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cadence sensor & rear wheel pickup

Originally Posted by fietsbob View Post
typically, a magnet is attached to the L crank arm. and sensor on the chainstay functions
just like a wheel magnet passing by a fork sensor , in a trainer-stand that needs to be on the rear wheel
2 sensors tick '1' as the magnet passes. you have input wheel size, so the bike computer adds up circumference roll out for distance,
and pulse frequency is combined for speed.
This is exactly how it works. Speed is calculated from wheel size, and speed of back wheel, which is
picked up by the rear wheel sensor. I got a Cateye micro cyclometer a few decades ago and it still works great.
The pedal magnet wore out but an inexpensive earth magnet replaced it. The cyclometer shows instant but not average
cadence, which I would like to have, although it is easy to calculate afterward. The Strada Cadence model appears to be similar.

Last edited by jlvs2run; 03-11-18 at 04:55 PM.
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Old 03-10-18, 07:19 PM
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A few years ago I used the speed sensor by putting the magnet on the crank instead of on a spoke to give me the cadence. I do not remember the numbers now, but I calculated what number to enter into my computer for wheel circumference to fool the computer to give me a speed read out that was exactly one tenth of my actual cadence. For example, I had my computer set so that 7.2 km/hour on the computer was really 72 cadence. Or maybe it was in MPH instead of km/hour that I used, I do not remember the numbers now and the computer battery died so the computer forgot them too.
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Old 03-11-18, 06:58 AM
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I don't know any software, trainer or cycling computer that uses your pedaling cadence to calculate speed and distance. Although as one poster mentioned cadence can measure speed on a single speed, but again I don't know of any software etc that actually applies that.


Depending on your trainer, it may already be transmitting some kind of speed information -- based on the revolutions of the flywheel, for example.


So the only reason to get a cadence sensor is to allow your software to display your cadence, which I suppose can be important since The Sufferfest also has cadence targets in addition to power.
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Old 03-11-18, 04:02 PM
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Fietsbob is living in the past. Sensors these days have no magnets and don't count the clicks. Just look at your iPhone or whatever. A cadence sensor and a speed sensor are really the same thing now, and the computer does the math internally to give you cadence or speed, depending on what you call it. A trainer in ERG mode can potentially tell you speed, since the flywheel is turning at a rate related to the wheel. For a given outer tire size and ratio of flywheel size to that, you can determine speed. That said, does speed matter? Speed indoors is unrelated to real speed outdoors. Power OTOH does matter, and that you already get. Power in watts is absolute, it doesn't matter if you are indoors or outdoors. Cadence matters as a riding style. You have slow grinding efforts uphill vs. fast cadence on sprints, and a whole mix in-between. HR doesn't matter so much during the ride until you get past the point where you are still living. In that case you may want to dial it back a little. I find HR mostly useful after the fact so I can see where my best efforts were and how I was able to hold them.

So all that said, why not just buy them as a pair? Wahoo sells a pair of magnet-less sensors for like $70 or less. There are similar cheap Chinese ones for like $20. Garmin must have them too. The best thing is if you have more than 1 bike, you can move the sensors easily.
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Old 03-11-18, 04:08 PM
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Im delaying microwaving my brain holding a transmitting phone against my head too.. by not owning one..
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Old 03-11-18, 09:20 PM
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getting a cellphone was not one of my best ideas. People think they can call you whenever they want
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Old 03-12-18, 08:47 AM
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Originally Posted by unterhausen View Post
getting a cellphone was not one of my best ideas. People think they can call you whenever they want
They can call you. That doesn't mean that you should answer them.
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Old 03-12-18, 06:08 PM
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Originally Posted by unterhausen View Post
getting a cellphone was not one of my best ideas. People think they can call you whenever they want
The very very few people that know my cell number know that I only bought it for travel, thus it is almost always turned off. I go for months without making a call or checking messages. The $10 USD/month plan I have is no longer offered to new customers, but I am still on that plan.
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Old 03-12-18, 06:34 PM
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Originally Posted by zacster View Post
Fietsbob is living in the past. Sensors these days have no magnets and don't count the clicks. Just look at your iPhone or whatever. A cadence sensor and a speed sensor are really the same thing now, and the computer does the math internally to give you cadence or speed, depending on what you call it.
While there are sensors that don't use magnets, the new ones still are rotation counters. Cadence and speed sensors were "the same thing" before (except they used magnets).

The computer always "did the math".

The math hasn't changed. (It's the same math.)

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Old 03-12-18, 07:35 PM
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You can have a cadence sensor on your shoe these days. Same as RPM on your hub. No magnets. Just acceleration.

BTW, it used to be worry worry worry check-in at a pay phone.
Then it was worry worry worry call on the cell phone or get called.

Now, with live tracking, the blue dot quenches all worries. (Even though nobody ever looks at the blue dot.)

-mr. bill
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