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Old 2019-08-13, 01:57 AM   #1
BruceC
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Cycle Computer Settings

I'm installing a cycle computer on the 36" so the circumference of the wheel is required. This is only accurate when the tyre is loaded by the rider's weight, but rather hard to measure while sitting on the unicycle. I usually take a guess at the reduced diameter when loaded and adjust later if speed and distance are out by too much compared to GPS. My guess for the loaded diameter is about 450 mm making the circumference 2827 mm.

What do others use for their 36 wheel circumference (using standard Nightrider tyre) ?
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Old 2019-08-13, 02:18 AM   #2
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Quote:
Originally Posted by BruceC View Post
I'm installing a cycle computer on the 36" so the circumference of the wheel is required. This is only accurate when the tyre is loaded by the rider's weight, but rather hard to measure while sitting on the unicycle. I usually take a guess at the reduced diameter when loaded and adjust later if speed and distance are out by too much compared to GPS. My guess for the loaded diameter is about 450 mm making the circumference 2827 mm.

What do others use for their 36 wheel circumference (using standard Nightrider tyre) ?
Put a chalk mark on the tyre and pass through a revolution while sitting in the saddle. Then measure between the marks on the floor.

I use 2785mm.
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Old 2019-08-13, 03:48 AM   #3
BruceC
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Originally Posted by lightbulbjim View Post
I use 2785mm.
Thanks! Asking the forum was easier than using chalk !

Next part is installing. Had to go wireless which can be convenient but I find they often don't have the range to go from the wheel to the T-bar. This is made worse by the larger gap in between spokes and fork near the top of wheel. This means the sensor has to mounted nearer the axle, even further away.

Perhaps I can do some custom brackets to hold the sensor near top of wheel. Something to pass the time on a cold and windy winter night.
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Old 2019-08-13, 03:28 PM   #4
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Quote:
Originally Posted by lightbulbjim View Post
Put a chalk mark on the tyre and pass through a revolution while sitting in the saddle. Then measure between the marks on the floor.
I do that too, except I do it in the underground carpark and I ride 10 revolutions in a straight line. Then I measure the distance I covered (and divide by 10 obviously).
My bike computer couldn't be programmed for a 36" wheel so I ended up getting a second magnet trigger and placing it opposite the first one and dividing the circumference by 2.
I'm away from the 36" at the moment so can't give you my measurement, but lightbulbjim's is probably the best you'll get without having to measure yours.

Last edited by pierrox; 2019-08-13 at 03:28 PM. Reason: Typos
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Old 2019-08-14, 01:55 AM   #5
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I do that too, except I do it in the underground carpark and I ride 10 revolutions in a straight line. Then I measure the distance I covered (and divide by 10 obviously).
I think that sounds a more accurate measment, may have to give a try if 2785mm is not right for my uni.


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My bike computer couldn't be programmed for a 36" wheel so I ended up getting a second magnet trigger and placing it opposite the first one and dividing the circumference by 2.
Clever. I was thinking of this to get better accuracy as it's a few metres every time the sensor is tripped. I can do a couple of KM just trying to doing free mount
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Old 2019-08-16, 06:32 AM   #6
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The wheel size does vary quite a bit with the amount of wear, rim width, tyre pressure and weight of the rider. So it is always best to measure it under the conditions you ride.
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Old 2019-08-16, 08:18 AM   #7
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On my 2 wheeler, I have a Garmin computer with an ANT+ hub mounted speed sensor. It's self-calibrating with the GPS. Presumably there's an accelerometer in the sensor that detects when the wheel rotates. I have no way of knowing whether the speeds I see are from GPS, the sensor, or both. This would avoid the issue with mounting a magnet.

If the calibration can cope with a 36" wheel, this would be a simple but expensive solution. If not, you would be out a lot of money unless you could borrow one.

The other thought I had was, calibrate against a ride measured with GPS, using the Strava smartphone app or similar. Supposedly GPS uncertainty is about 2 metres or so -- a ride of > 2 km should counteract that.
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Old 2019-08-16, 08:22 AM   #8
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Quote:
Originally Posted by Richard C View Post
On my 2 wheeler, I have a Garmin computer with an ANT+ hub mounted speed sensor. It's self-calibrating with the GPS. Presumably there's an accelerometer in the sensor that detects when the wheel rotates. I have no way of knowing whether the speeds I see are from GPS, the sensor, or both. This would avoid the issue with mounting a magnet.

If the calibration can cope with a 36" wheel, this would be a simple but expensive solution. If not, you would be out a lot of money unless you could borrow one.

The other thought I had was, calibrate against a ride measured with GPS, using the Strava smartphone app or similar. Supposedly GPS uncertainty is about 2 metres or so -- a ride of > 2 km should counteract that.
I agree with everything you say... I do the same, but I did a little research on to the accuracy of this method as a way of measuring longer race distances. For some reason it is not as accurate as you would expect from all the reports I have read and I can not get a proper explanation why either. It should be very accurate, but for some reason the garmin software is not making it accurate enough.
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Old 2019-08-16, 10:37 AM   #9
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Quote:
Originally Posted by Richard C View Post
I have a Garmin computer with an ANT+ hub mounted speed sensor. It's self-calibrating with the GPS. Presumably there's an accelerometer in the sensor that detects when the wheel rotates.
Totally off topic but I think the Garmin ones might use magnetometers instead of accelerometers, ie they detect their rotation in relation to the Earth's magnetic field.
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Old 2019-08-16, 11:49 AM   #10
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Totally off topic but I think the Garmin ones might use magnetometers instead of accelerometers, ie they detect their rotation in relation to the Earth's magnetic field.
Interesting! Accelerometer was a complete guess on my part, thinking that there must be lots of stock mobile phone sensors available.
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Old 2019-08-16, 12:10 PM   #11
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Both GPS and cycle computers have their errors, the GPS more so especially if just using a phone like I do. Strava often says my max speed was over 40 kph!

That's also why I do not use a GPS based cycle computer, I need a way to confirm the GPS. That and not having to charge it every few rides and waiting for the GPS signal to come in.

Can't imagine an accelerometer alone based computer would do the job. Before GPS, airlines used megabuck "INS" (inertial navigation system) and they needed constant calibration.

Set up of the cycle computer always takes time, a little trial and error. Starting with the 2785 mm as suggested is just that, a starting point. How to get to that initial setting was what this thread was about.

Now my computer is setup, I will test and update as required comparing to Strava distances but only for the longer and straighter runs. Today I did 33.61 km on Strava and 33.3 km on the cycle computer. I'd call that close enough, but lets see how it goes.

After nearly 4000 km on my KH29 the Duro 3.25"tyre has worn all the knobbly bits off, and it shows with the cycle computer now over reading by more than 10%. Initially it was within 1 or 2% on most rides. As mentioned by @rogeratunicycledotcom, after initial setup cycle computers do need adjustment.

Last edited by BruceC; 2019-08-16 at 12:12 PM.
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Old 2019-08-16, 12:31 PM   #12
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Quote:
Originally Posted by BruceC View Post
Both GPS and cycle computers have their errors, the GPS more so especially if just using a phone like I do. Strava often says my max speed was over 40 kph!

That's also why I do not use a GPS based cycle computer, I need a way to confirm the GPS. That and not having to charge it every few rides and waiting for the GPS signal to come in.

Can't imagine an accelerometer alone based computer would do the job. Before GPS, airlines used megabuck "INS" (inertial navigation system) and they needed constant calibration.

Set up of the cycle computer always takes time, a little trial and error. Starting with the 2785 mm as suggested is just that, a starting point. How to get to that initial setting was what this thread was about.

Now my computer is setup, I will test and update as required comparing to Strava distances but only for the longer and straighter runs. Today I did 33.61 km on Strava and 33.3 km on the cycle computer. I'd call that close enough, but lets see how it goes.

After nearly 4000 km on my KH29 the Duro 3.25"tyre has worn all the knobbly bits off, and it shows with the cycle computer now over reading by more than 10%. Initially it was within 1 or 2% on most rides. As mentioned by @rogeratunicycledotcom, after initial setup cycle computers do need adjustment.
Canít wait to do some riding with you once Iím back home!
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Old 2019-08-16, 01:12 PM   #13
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Quote:
Originally Posted by BruceC View Post
Both GPS and cycle computers have their errors, the GPS more so especially if just using a phone like I do. Strava often says my max speed was over 40 kph!

That's also why I do not use a GPS based cycle computer, I need a way to confirm the GPS. That and not having to charge it every few rides and waiting for the GPS signal to come in.
Phone is definitely worse than Garmin, just looking on the map I used to see my track wandering metres from the road. Even Garmin GPS isn't exact, which is why the speed sensor exists. I'd also expect that GPS would systematically underestimate, since it's a set of point readings. I'm just happy to take whatever number my computer gives me as the truth.

Quote:
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Can't imagine an accelerometer alone based computer would do the job.
I had envisaged something like the sensor that tells your smartphone which way is up. However, I've checked and lightbulbjim is right, the speed sensor uses a magnetometer
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Old Yesterday, 04:39 AM   #14
BruceC
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Canít wait to do some riding with you once Iím back home!
I think it's time you unicycled the M7. Oh, and it has hills!
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Old Today, 06:16 PM   #15
Mikefule
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Why not just use Map My Run, Map My Ride, Strava, or similar?
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