# Anyone ever double up the computer wheel sensor magnets to increase speed resolution?



## @[email protected] (Aug 25, 2017)

You know, halve the tire circumference in the computer setting and double up the magnets to get a more accurate and faster reading on wheel speed? 

I think this might come in handy for slower climbs.

Anyone ever done this?


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## Harold (Dec 23, 2003)

I think I heard about people doing this a long time ago when more people used basic cyclocomputers. My question is why does it matter? I don't know about anyone else, but I'm not looking down at my computer to check my speed WHILE I'M RIDING.


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## @[email protected] (Aug 25, 2017)

Harold said:


> I think I heard about people doing this a long time ago when more people used basic cyclocomputers. My question is why does it matter? I don't know about anyone else, but I'm not looking down at my computer to check my speed WHILE I'M RIDING.


I check my speed while I'm driving my car too. You should try it. It's good to know speed .


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## Harold (Dec 23, 2003)

@[email protected] said:


> I check my speed while I'm driving my car too. You should try it. It's good to know speed .


Pretty useless while riding. If you have trail speed limits, I am sorry, but there is still a better way. Better computers allow you to set audible warnings for all kinds of things, so you can have the computer beep when you exceed the speed limit. And no need to look at it when precise handling is essential.

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## Procter (Feb 3, 2012)

That wouldn't necessarily make it more accurate. 

If the computer knew there were two sensors and took the average of the readings to try to get a more accurate number, it might work. But the computer isn't doing that. 

Without that, I think the uncertainty on the two readings would be just be additive.


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## @[email protected] (Aug 25, 2017)

Procter said:


> That wouldn't necessarily make it more accurate.
> 
> If the computer knew there were two sensors and took the average of the readings to try to get a more accurate number, it might work. But the computer isn't doing that.
> 
> Without that, I think the uncertainty on the two readings would be just be additive.


It would increase the resolution because the computer is taking a reading every 1/2 turn instead of every turn. It definitely would do that. Since climbing is very slow, like 3-6mph, it would register a truer reading if you accelerate or decelerate while climbing.


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## Harold (Dec 23, 2003)

@[email protected] said:


> It would increase the resolution because the computer is taking a reading every 1/2 turn instead of every turn. It definitely would do that. Since climbing is very slow, like 3-6mph, it would register a truer reading if you accelerate or decelerate while climbing.


The only reason that might be relevant would be if you are recording that data for analysis later (though frankly, I can't even imagine a scenario where a higher resolution for speed measurements is going to do anything for you). It's not remotely useful real-time.

Why not buy a GPS that records at 10Hz? That's going to be a much higher resolution than doubling wheel magnets, especially when climbing is in question. But the real benefit of 10Hz GPS recorders on a bicycle is not from increased frequency of speed measurements. The real benefit is from more frequent position recording, which more accurately captures twists and turns in a trail at riding speed. You'll find that most 10Hz GPS devices are marketed towards the motorsports crowd, where higher speeds result in large gaps between recorded positions (especially with 1Hz recording) and screwed up recorded distances. The reason for the increased recording frequency definitely isn't in order to more quickly capture changes in speed. That does happen, but it's not really useful for much of anything.


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## Procter (Feb 3, 2012)

@[email protected] said:


> It would increase the resolution because the computer is taking a reading every 1/2 turn instead of every turn. It definitely would do that. Since climbing is very slow, like 3-6mph, it would register a truer reading if you accelerate or decelerate while climbing.


I can kinda see it but let's think this through.

There are two ways the measurement could be calculated:

A) Revolutions per unit of time * circumference = distance per unit time, which can then be converted to MPH w/ simple units conversions.

In this case, yes, 'resolution' is important, for example if your time window is 3s, and you got 2.9 sensor hits in 3s, then the computer would assume 2 revolutions and the speed would be inaccurate (to the low side).

B) Circumference / (Time between current sensor hit and the prior sensor hit), then unit conversion to MPH

This seems like a better way to calculate it, because it avoids the '2.9' problem in method A.

In this case, extra sensors would not make it more accurate, in fact, they might make it less accurate because there are uncertainties in each measurement of time (the magnet might not trigger the sensor with the exact same precision each time, there might be a +/-1% or something). One longer revolution with a single time interval measured between the two sensor hits, might be a better sample than a shorter sample every 1/2 revolution, because time uncertainty is a larger in proportion to the unit time.

For example, let's say your wheel is rotating 1x per second and the uncertainty in the time measurement is +/- .05 sec.

With a single revolution that's MPH = Circumference / 1s +/- 5% (then convert), with two sensor measurements per revolution, your calculation is MPH = Circumference (halved this time) / .5s +/- 10%.

See what I mean? The error is a larger percentage of the time measurement here.

Either way, time uncertainty is a problem with both single and double sensors, so my guess is that the system would average the reading over the past 3 or 4 revolutions at any given time, which, would just wash out the effect of your double sensors really.

If I'm wrong, I think any accuracy gain you might get (perhaps +/- 1-2% max) is nullified by the uncertainty in the circumference of the tire anyways. Despite measuring the circumference on paper or with the tape-on-the-ground method, there will be real-time uncertainties in the actual circumference, like how far the tire deforms under your weight, your PSI on that day, how high or low the knobs ride due to the hardness of the surface, etc.

Those may seem small but when you're dealing with, say, a 26" wheel, 2.7" - 3.2" of tire height (varying) = ~ 90.5 - 92 inches of circumference, that's +/- 1-2% error in your MPH anyways.


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## @[email protected] (Aug 25, 2017)

Procter said:


> I can kinda see it but let's think this through.
> 
> There are two ways the measurement could be calculated:
> 
> ...


I don't think it would introduce any more error at all. All you'd be doing is taking the 2334mm circumference input into the computer and replacing it with 1167mm. Having two sensors exactly 180* apart would mean that you'd get a more accurate speed meaurement as the speed is a function of the distance travelled (in this case the circumference of the tire that is input manually) and the time it takes for each revolution of the magnet. It actually takes a 29" tire a little while to make a complete revolution when going uphill at 3mph. It takes so long in fact, that any accelerations or deccelerations seem to impact the output reading greatly. I can try my hardest to maintain a steady cadence, but the actual readout will vary by 6 or even more tenths of a mph continuously. I think I might be able to smooth that out by doubling the magnets. It just annoys me that no matter where I am, flat, incline, or decline, I can't get a consistent reading. It jumps around too much, and I think the reason is that the circumference is too big to be taking only one reading per revolution.

Anyway, now that it's in my mind, I don't think I'll be able to let it go until I try it. Luckily, I have two magnets right now. One for each of my wheelsets. I'll just take one off of my slick tire set and see what it does.


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## @[email protected] (Aug 25, 2017)

Okay, so, that's blown up.

The way my rims are laced, I can't get two spokes on the same side of the rim exactly 180* apart, so, it won't work on my rim. Oh well.


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## Harold (Dec 23, 2003)

@[email protected] said:


> Okay, so, that's blown up.
> 
> The way my rims are laced, I can't get two spokes on the same side of the rim exactly 180* apart, so, it won't work on my rim. Oh well.


Not surprised.

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## bbbr (Nov 6, 2005)

why stop at 2? One magnet per spoke, divide wheel circumference by the # of spokes and get supreme accuracy. Or just accept the fact that there is a normal level of error in all systems and roll with it.


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## Procter (Feb 3, 2012)

Right, it's trying to extrapolate a measurement on the order of ~100 inches / sec into one of 64k inches / 3600 seconds. That's just inherently noisy.


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## @[email protected] (Aug 25, 2017)

bbbr said:


> why stop at 2? One magnet per spoke, divide wheel circumference by the # of spokes and get supreme accuracy. Or just accept the fact that there is a normal level of error in all systems and roll with it.


Because the lowest entry that can be made for the circumference is 1000mm.


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## Cuyuna (May 14, 2017)

@[email protected] said:


> It would increase the resolution because the computer is taking a reading every 1/2 turn instead of every turn. It definitely would do that. Since climbing is very slow, like 3-6mph, it would register a truer reading if you accelerate or decelerate while climbing.


How would knowing your speed with higher resolution affect your riding while you're riding? I don't get why knowing your speed while climbing would be important, let alone knowing your speed with high resolution.

I just recently, finally, put a computer on my mountain bike, but that was mainly for distance and for heart rate. Of all the measurements that that computer is capable of, speed is the least useful for the all-mountain-type riding that I do on that bike. I'm sure that your two-magnet proposal is both cheap and feasible...just pointless.


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## @[email protected] (Aug 25, 2017)

Cuyuna said:


> How would knowing your speed with higher resolution affect your riding while you're riding? I don't get why knowing your speed while climbing would be important, let alone knowing your speed with high resolution.
> 
> I just recently, finally, put a computer on my mountain bike, but that was mainly for distance and for heart rate. Of all the measurements that that computer is capable of, speed is the least useful for the all-mountain-type riding that I do on that bike. I'm sure that your two-magnet proposal is both cheap and feasible...just pointless.


It's just another data point. I'd love to have a power meter too. I have a heart rate monitor. More data for my engineering mind.


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## Cuyuna (May 14, 2017)

@[email protected] said:


> It's just another data point. I'd love to have a power meter too. I have a heart rate monitor. More data for my engineering mind.


OK. I get that. It's a hobby, after all. All kinds of aspects of it that we can indulge. I use three different heart rate sensors feeding four different devices/apps.


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## Procter (Feb 3, 2012)

@ride, assuming the measurement mode is B above (circumference / time between this sensor hit and the last one), here's an analogy:

Go out to your garage, and measure the width of your two car garage door, with a 1 foot ruler. You'll notice, each time you place the ruler, there's some uncertainty in where you place it relative to your last placement. Measure all the way across, 3 times, and take the average of the readings. 

Now, do it again, with a single tape measure. You'll notice you measurements with the ruler were not consistent with either themselves or the tape measure method. 

It is much the same with two time measurements per revolution, each starts from the last, so the total uncertainty is the sum of the uncertainty of each measurement. 

Make sense? 

If the used measurement method A above, then more resolution is better. But we just don't know, we'd have to look at the programmers algorithm.


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## ghettocruiser (Jun 21, 2008)

From a prior thread:



ghettocruiser said:


> Note that with a wheel magnet (or cadence magnet) sensor, the magnet has to pass the sensor twice to give data.
> 
> So if you take off like a batoutofhell, the lag should be less because the head unit is waiting less time for the second ping compared to a gentle start.
> 
> ...


You are right that it will not increase distance measurement accuracy in a meaningful way, and it will not have any discernible effect on the speedometer display once the bike is up to speed.


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## Procter (Feb 3, 2012)

That's great, it's great to see the experimentation here. But given there are two possible algorithms for determining realtime speed, we cannot know which GPS has which algorithm, even within a brand. Findings on a particular computer or GPS don't necessarily translate to others. Only the programmer knows. 

A) Revolutions per unit of time * circumference = distance per unit time, which can then be converted to MPH w/ simple units conversions.
B) Circumference / (Time between current sensor hit and the prior sensor hit), then unit conversion to MPH


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## @[email protected] (Aug 25, 2017)

ghettocruiser said:


> From a prior thread:
> 
> You are right that it will not increase distance measurement accuracy in a meaningful way, and it will not have any discernible effect on the speedometer display once the bike is up to speed.


^interesting post.


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## Guest (Oct 17, 2017)

@[email protected] said:


> Okay, so, that's blown up.
> 
> The way my rims are laced, I can't get two spokes on the same side of the rim exactly 180* apart, so, it won't work on my rim. Oh well.


 Spacing doesn't matter. So long as your computer can differentiate between the two inputs and you calculate the tire size correctly it will still average correctly. I'm still stuck on the "Why". My elcheapo computer records speeds down to 3mph. If I go slower than that I'm either doing a track stand or falling over.


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## bachman1961 (Oct 9, 2013)

Forster said:


> I'm still stuck on the "Why". My elcheapo computer records speeds down to 3mph. If I go slower than that I'm either doing a track stand or falling over.


lol

I think of slow going and noticed my "speed" riding up High Dr. a few times. At 2.5 mph, I'm amazed that wee bit of torque keeps me upright.

For me, that's a steep sumbitch.


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## ghettocruiser (Jun 21, 2008)

Forster said:


> Spacing doesn't matter. So long as your computer can differentiate between the two inputs and you calculate the tire size correctly it will still average correctly.


Distance would remain correct. Displayed speed will jump up and down rather widely; I know because I got the wrong spokes on the first attempt.

Fuzzy math aside, the double-magnet did mostly solve the problem of the gopro video showing the bike taking off, but the overlaid speedometer display briefly lagging at zero, and the reverse effect when stopping. That was the only purpose of my zero-cost undertaking.


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## NordieBoy (Sep 26, 2004)

I find speed very useful on the single speeds.
After going down anything, I want to know when my speed is approaching 22kph so I don't resume pedaling too soon.

On a geared bike, I rate cadence higher, but on the single speed, speed over cadence.


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## Sidewalk (May 18, 2015)

I'm a pretty data driven person. Speed is the last thing I am interested in when riding the MTB.

On my GPS I have feet gained (so I'll know what I have left to climb for pacing purposes), elapsed time, mileage (for pacing), HR, HR average (for pacing). The only time I care about speed is on my roadie, when I am on a bike path. Speed and cadence aren't useful at all (no cadence sensor). If I had power, I would only use normalized power.


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## Guest (Oct 17, 2017)

Sidewalk said:


> I'm a pretty data driven person. Speed is the last thing I am interested in when riding the MTB.
> 
> On my GPS I have feet gained (so I'll know what I have left to climb for pacing purposes), elapsed time, mileage (for pacing), HR, HR average (for pacing). The only time I care about speed is on my roadie, when I am on a bike path. Speed and cadence aren't useful at all (no cadence sensor). If I had power, I would only use normalized power.


 Right there with you on HR. After my back surgery I used one for every ride and every run for the next three years. I've sort of learned to gauge my exertion level based on perceived effort, but I don't really track that data anymore. If I want to know exactly how fit I am I ride with other people, when they leave me behind I know I'm not fit enough yet.


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