Power - but speed and rpm sensor

Hi. I have assiomo power pedals (dual). I’m using wahoo bolt v1 head unit. But i have a separate magnetic speed and crank rpm sensor (blue and ant). Basically , i want power meter to only report power and my rpm/speed to report RPM and speed. I don’t want Assiomo to report rpm. How do i set this up correctly? Thank you. Doing sprints on track and want accurate speed and rpm.

Have you found the Assioma’s to not report accurate rpm? I use them and have never had an issue. I’m not sure how it could accurately record power without recording accurate cadence.


Yes. But i’m talking specifically with Track cycling efforts like standing starts. So, road stuff is very accurate.

Usually when connecting the sensors you can connect the different elements separately.

So for power you would use the assiomas, for cadence and speed connect those sensors separately. I’ve not used a Wahoo head unit myself but this is the way to do it on Garmin, they are probably very similar/the same.

  • That is not a safe assumption. The process is different since the Wahoo essentially auto detects the device category via the device ID, and makes the association to that device. Garmin seems to rely on you defining the category first and then pair the device.

To the OP, I don’t know if you can deselect cadence data from the Power Meter device type. It’s been ages since I added a sensor and don’t remember the precise options that may be hiding other than the discrete ones in their pairing guide.

You may be best to contact Wahoo directly if you haven’t tried, since the will know for sure one way or the other.

Thank you Malcolm. I thought that connecting the Assiomo pedals automatically gives you power and cadence. And let’s say I’ve connected my separate rpm/speed sensor too. What will report to the wahoo head unit?

Thanks Chad . I will now contact Wahoo but originally wrote my post to see if others have been in my situation and what they have done to solve the isdue.

Sure, no harm in asking. And from my experience setting up 8 different PM’s on my Bolt (directly & via app), I don’t remember a way to isolate or select the specific cadence device if 2 are paired (PM & dedicated sensor). I never tried to figure out which device the Bolt showed since it didn’t matter. My guess is that not many people have tried what you are aiming to do.

To a degree, I am not sure there will be enough difference to matter. If you have a typical cadence sensor with the magnet tripper, it will only kick off that one time per rev. So there will be a delay from a start related to your leading pedal and the location of the cadence setup.

And if you care that much about the power data, the reality is that any flaw in it’s own cadence sensor will directly impact the power data. Perhaps the sample rate or max cadence are limiting, but that will have an influence on the power data even if you capture the cadence data in your head unit from the dedicated cadence sensor.

Excellent points. Sampling rate is sampling rate even with the separate sensor, right?

Speed and rpm may be most valuable to what I need.

Very helpful. Thank you.

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I did say ‘probably’ and was just trying to help :slight_smile:

The PM rpm is going to be a lot more accurate than the sensor. The sensor needs multiple passes by the sensor to change values (one data point per spin). The PM same rate is going to be much higher than that. They both report to the Wahoo at about the same rate, both values are likely smoothed/averaged.
Are you trying to measure wheel slip? That’s the only reason you’d want to use the same sensor. The speed cadence sensor just doesn’t have the fidelity to measure that because of the single reading per rotation. You’d need a 3-axis sensor for that to measure variances in input vs acceleration or wheel hop.

Another option here might be to add magnets to double the reading. 1) you’d be sure what sensor the Wahoo is picking up 2) you’d double the fidelity of the reading. This would easy enough for the speed, but the cadence you’d need to get creative.

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Thanks for the remarks. No, I heard that a separate RPM sensor would be more accurate at capturing Rpm at low rpm (say from a standing start) but as you pointed out, not possible. Most track riders use SRM because of the quicker rate. Let me ask you this though, would a separate speed sensor be more accurate than GpS configured speed. In other words, what should I use to get very precise speed over say 10 to 40 second efforts?

Assuming you get a proper circumference entered into the speed sensor setup, it will almost certainly be more accurate and reliable. GPS is great, but has known limitations that can lead to errors in data.

Related to accuracy, I recommend airing up the tires to proper pressure, sitting on the bike with full gear and doing a single tire revolution forward. Start and stop with value stem at the bottom and measure the actual linear distance. The common charts are a fine start for most people, but individual variation will impact the linear distance for each situation.

Once you do that, the speed will be very accurate from the speed sensor.

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Yep, definitely will do a roll out and put in accurate circumference. Thanks.

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You’d need a direct measurement speed sensor (like a geared wheel or roller reading the wheel) or maybe Wahoo’s inertia(maybe email Wahoo to get the sample rate / computer update rate of the SC and S sensors) based sensor to get a more accurate reading. Otherwise, your cadence reading would be the better ‘speed’ reading - this is how your car gets a reading for example. The once/revolution speed sensor, you’re getting a data point about every 6ft (2meters) or 7 readings per second at 50kph.

There’s no such thing as precise because your EXACT speed doesn’t really matter as much as how fast you eat up the course. If you can go .25mph faster, but you’re going L-R-L-R or taking a less efficient line, you’re not going faster. Additionally, I presume your rear tire slips and squats under efforts, do any crank/hub/wheel measurement, is just an estimate. This is why tracks have specific measurement systems. Your best number may even be just using the GPS because it is exactly marrying the GPS Position with the speed on these 1-2 lap efforts.

You might want to get two Wahoos, hook one up to the magnet based speed/cadence sensor and the other hooked up to the PM and a Wahoo speed sensor. Do a lap and then have a look at the .fit file. Graph the data in Excel and see which seems more correct. Maybe try putting 4 or 8 magnets on the wheel and then adjust the “rollout” distance. That should get you the best number.

Thanks for helping.

Q: what are the differences with regard to measurement when using an accelerometer type speed sensor versus magnetic? Is the magnetic ideal in my situation or not?

With the note that I have not actually done this hack, here is one option I heard may work to increase effective sample rate for magnetic tripped wheel speed sensors.

  • Install 2 or more magnets on the wheel spokes, evenly spaced from each other around the wheel (180* opposite sides of the wheel for the example of 2 magnets).
  • When you enter your wheel & tire circumference into the head unit, use the real linear distance divided by the number of magnets (like 2 in this example would mean Circumference / 2).

What you get is two trips per wheel rev and effectively double the sample rate. If this works, the limit would be the number of spokes on the sensor side if you wanted to get crazy high sampling. I am not sure how fast the accelerometer based version actually sample by comparison.

Do you really need cadence to come from the speed/cadence sensor?

Assume that you are still getting the speed measurment and if you are on a single speed track bike the cadence will just linearly follow the speed depending on your gearing?