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Cadence / Speed Sensors as a Proxy for Power

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Cadence / Speed Sensors as a Proxy for Power

Old 06-24-20, 06:56 AM
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Yonah
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Cadence / Speed Sensors as a Proxy for Power

I am a relatively recently back to cycling, and I have been steadily increasing in mileage and speed, but I wanted to get a more efficient method of measuring my power output. Then I saw how much power meters cost - especially the ones that assume that I already have a high-end gearset that I can ship to them to install the meter on.

I also realize that Cadence and speed sensors are a lot cheaper, but I wonder - do cadence sensors provide a good proxy for power? I don't need an exact measure, I just want something more accurate than I am currently getting from Strava's estimate. I don't have a cycling computer, but my Polar watch is compatible with most BLE cadence and speed sensors.
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Old 06-24-20, 08:36 AM
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Nothing is going to substitute adequately for measuring power than a power meter. If you are going to base your training or workouts on power, then you need to get a power meter. Otherwise, just go by one of the various HR training plans and do not try to make any sense of the Calorie function of your devices with respect to Watts of output. IMO the Calorie number on them is for ESTIMATING what you may have burned for dietary purposes. Since it is given for the entire ride, it is difficult to attribute any segment of your ride a Calorie used number. With a power meter, you can get an reasonably accurate number for any given segment of your ride.

I don't use a power meter, but I believe there are less pitfalls from misuse of a PM and misunderstanding of the data, When you use a power meter, it gives you the number you want to improve, so there is less ambiguity of what you need to do to improve it.

However, essentially all you have to do if you want to get better is just ride more. And put some short segments of riding at max effort in your rides.
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Old 06-24-20, 11:19 AM
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Originally Posted by Yonah View Post
... I wonder - do cadence sensors provide a good proxy for power? I don't need an exact measure, I just want something more accurate than I am currently getting from Strava's estimate.
No, they don't. You can be doing 100 w or 400 w at 90 rpm. One is easy enough to do for the rest of your life, the other is above your threshold. Power is cadence times pedal force, so only having the answer to one of those is no proxy at all.

Speed doesn't help much - as you can see from Strava power. Without a sensor for wind and other things, there are a lot of different reasons your power vs speed will change.

Depending what you need the data for, you may or may not be able to come up with something good enough.

The most economical way to measure power is with a used PowerTap hub. Second most affordable is a left only crank arm from 4iiii or Stages, but this isn't preferable because it only measures from one leg and you still don't actually know what you're putting out. It may or may not be close enough for your purposes to assume both legs are always equal, in reality they're not though.
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Old 06-24-20, 01:54 PM
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+1 that speed and cadence are not proxies for power. Heart rate is a direct measure of how hard your body is working, so that's a much better proxy, and plenty of info for most people to inform a training plan. If you're not sure whether or not you need power to train, then most likely you don't.

In a separate comment - I can't imagine wanting to increase mileage and efficiency without cadence and an always-on bike computer. I've internalized how my body feels at certain HR, cadence etc, but I've recently been through this with my GF who's a novice cyclist, and equipping her with cadence & HR on an old bike computer has helped her immensely. This is way more valuable than power IMHO.

Since you mention Strava be aware you'll need their paid subscription to do anything with power data anyway, that might well be a poison pill (it is for me).
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Old 06-24-20, 03:04 PM
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Thanks to everyone who responded, maybe I should've asked this question differently:

- I have HR from my watch/HRM, I know that gives me a way to measure relative efforts
- I know that power gives me a whole different set of metrics, but that getting a power meter would like cost me a few hundred $$
- Given that my watch has GPS, I don't think a speed sensor gives me much of anything.

So I guess my real question is, what does cadence give me? As someone else pointed out, I can be doing 90rpm at 100w or 400w. I could also climb the same hill 3 different times, climb at the same pace, but use different gearing for each and the RPMs would be way different too.

But cadence sensors are relatively cheap - sub $100. So what do I truly get from them?
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Old 06-24-20, 04:15 PM
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I also don't have a power meter, because they are expensive. However, it is not terribly difficult to calculate power based on a few parameters is you make some basic assumptions:
No Wind
Constant body position of quantified, (CdA)
Constant riding surface of quantified type (Coefficient of rolling resistance, generally lumped with drive train efficiency)

From here you can calculate stuff. This is basically what Strava does, but the raw speed/gps resolution seems poor and no smoothing is applied. Strava calculations looks to be quite good compared to my calculations if you use something like a 5-10 second average. It is quite good going up hills, but terrible going downhill or in the wind.



If you look into this, you will likely find that your cadence falls within a small range, but how quickly you fatigue at a given cadence/power can vary greatly. It's a good tool for training and building fitness.

With a cadence sensor you can experiment cycling at a fixed level of effort at different cadences and comparing speed/hr and how long you can go. I found a while back that pushing up my cadence from being a masher to being a spinner, I could ride 16 mph at a similar perceived level of effort as i did before. At that point I did not have a cadence sensor, I just experimented until I found a happy place. Eventually I got a cadence sensor and that happy place is around 90 rpm.

Interestingly you can calculate your most common gearings as well, which you could do with more fancy electronics.

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Old 06-24-20, 05:36 PM
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Well with cadence you get... well... ah.... er.... your cadence <grin>

You don't have to have it, but it does give you another number to look at and compare your data from several of the same segments of multiple rides. IE, a hill you might always try to climb at a high effort. You might see trends that show you doing better with a higher speed at a particular cadence.

I rode for a couple months recently without cadence. I missed it and put it on my new bike too. I'd been using it for more than ten years with my other bikes and knowing what I'm doing at any one moment seems to help make sense of the other data.

I'm sure there are those that will tell you that all you have to do is count your pedal strokes and do some math in your head. But cadence isn't a constant for a ride. You'll be shifting gears and constantly changing cadence. How much math do you want to do on a ride? <grin>
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Old 06-24-20, 06:34 PM
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Originally Posted by Yonah View Post
But cadence sensors are relatively cheap - sub $100. So what do I truly get from them?
It's actually a little controversial if they're important or not, and how much.

As a general rule, higher cadence tends to be easier on your knees long term, and to use your cardiovascular system more and your leg muscles less, which helps you avoid fatigue once you reach a certain level of fitness. Many people do the opposite, so a cadence sensor can help you adjust if that's the case.

You can get one on Amazon in the $30 range.
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Old 06-24-20, 08:09 PM
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Watts without PM

I was using Strava for Watts, LINK to post
If you're running a HRM, Strava will take a guess. It was pretty close for me.

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Old 06-25-20, 07:07 AM
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Cadence can be really useful to know. Teaching yourself to spin is one of the quickest ways to more power. You do have to train for it though
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Old 06-25-20, 10:04 AM
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I don't think that monitoring cadence is that important if you are already comfortable spinning at relatively high cadences. If you're getting accustomed to higher cadences, a cadence meter will keep you honest and give good feedback.

From what I understand, it's easier to generate more power at a relatively high cadence, and your body is more energy-efficient at a relatively low cadence, but it's not a big range. If you don't need to really optimize to eke out that last few percent, there's no need—unless you like to nerd out on data, which I fully support (but then you'll really need to get a power meter too).
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Old 06-26-20, 07:40 AM
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I think enough of you (+ the cheap $20 sensors on Amazon) that it's worth trying. I will give it a shot and report back
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Old 06-26-20, 10:20 AM
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What do you plan to do with the data?
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Old 06-26-20, 10:33 AM
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I bought one of the cheap amazon heart rate straps and couldn't get it to work very long with bluetooth. Never tried it with Ant+

Now that I'm using a contact lube with my wahoo tickr, it has been working pretty well. I also rarely cleaned it, now I'm cleaning it every ride
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Old 06-26-20, 10:48 AM
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1) Track my cadence for a week, not changing anything
2) Alter my riding style over the same route so I can test to see how I respond to changing cadences
3) Use that data to help me improve
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Old 06-26-20, 03:44 PM
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Great discussion. You seem to be on the right track.
Personally I would rather buy stuff than ride, but there are some great proxies to use to train and help you see if you need other stuff.
The Subjective Exercise Experience Scale (SEES) is amazingly good, and Strava , Training Peaks and Zwift have calculated proxies that are a good and inexpensive start.
all inexpensive to try
2 $15 Magene sensors for speed and cadence with a smart phone, maybe with a heart rate strap are plenty. The $250 Power pod is worth a look, but I think its biggest value is putting you off the idea altogether.
And I have found a cheap electronic metronome useful, since I can't count and multiply effectively while riding.
Power meters have their place, but for a beginner, a plan and a smart phone will give a solid start.
Many of the sensors seem best to help you in finding a way to make a good game out of it , to help you stay with things and keep pushing.

Last edited by bikebikebike; 06-26-20 at 03:50 PM.
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Old 06-26-20, 04:14 PM
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Of all the information my Garmin collects, I think cadence is one of the least useful.
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