Wheel Circumference Physics
08-16-24 | 08:51 AM
#51
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From: Far beyond the pale horizon.
08-16-24 | 08:56 AM
#52
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From: Far beyond the pale horizon.
Yes, it's a mess (and a day late).
The "effective" radius is the radius for horizontal travel with a full wheel rotation. It's the radius "you want". Exactly what the value of that radius is is complicated (and it varies).
These distances vary depending on a bunch of things. The image isn't saying PC=CA. It's just showing the spatial relationships of these things (not their sizes).
08-16-24 | 09:22 AM
#53
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GPS is far more capable of the accuracy the OP is interested in, over the distances that the OP is interested in, than any distance measurement based on the circumference of a tire.
08-16-24 | 09:30 AM
#54
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https://www.dcrainmaker.com/2019/04/...th-review.html
Of course, the main reason you’d use this sensor is to display speed and distance on your bike computer, most likely off-road. For the most part, GPS is more than accurate and stable enough these days on-road for most cyclists. Whereas off-road mountain-biking in dense forest or with switchbacks, that’s where you’d probably want a speed sensor to give you better distance accuracy and better pace stability.
Last edited by njkayaker; 08-16-24 at 09:36 AM.
08-16-24 | 09:38 AM
#55
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On edit: oops, it wasn't the OP that stated the above requirements.
The wheel sensor also works better for instantaneous speed.
Last edited by tomato coupe; 08-16-24 at 10:01 AM.
08-16-24 | 09:40 AM
#56
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08-16-24 | 09:50 AM
#57
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That was my initial post, injecting myself in the midst of physics and geometric debates, on the very specific question of why one might need more accuracy than a cyclocomputer can offer. This is a multithreaded and largely pointless conversation, as these things tend to become.
08-16-24 | 10:01 AM
#58
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They aren't "speed" sensors anyway. They are "rotation" sensors. The speed and distance have to be provided by the head unit (there are a few that do this calculation in the sensor but that's a recent thing).
Last edited by njkayaker; 08-16-24 at 10:08 AM.
08-16-24 | 10:05 AM
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No. Speed sensors that are meant to be used with GPS computers offer better speed measurements at short times/distance where GPS doesn't work as well. At longer distances, GPS computers rely on GPS positions to determine distance more accurately.
08-16-24 | 10:44 AM
#60
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The early GPS units might not have been that good at measuring distance. (They still aren't great at measuring short-term speed.)
The distinctive thing GPS provided was the location of where you were riding (and recording data). GPS didn't improve much on the results (distance and speed) of using rotation detectors and were more expensive.
GPS has improved such that it might now be fine or better for distance generally. There are still situations where GPS doesn't work well at all.
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https://support.garmin.com/en-US/?fa...623Z535geTx2e9
The Garmin Speed/Cadence Sensor (GSC10), Bike Speed Sensor (BSS), and Speed Sensor 2 override the speed and distance data from the GPS satellites when used during an outdoor timed activity.
The GSC10 uses a magnet on the wheel and the bike profile's wheel size to calculate distance and speed with improved accuracy, while the BSS and Speed Sensor 2 use a magnetometer and orientation to accomplish the same thing.
These sensors are useful when traveling through tunnels, areas with heavy tree cover, and on rides with substantial changes in elevation where GPS is limited. When the device is recording distance with GPS, it is recording the distance from one point to the next, and may not be factoring in the elevation ascent or descent. The sensors give a more accurate reading by recording distance based on wheel rotation.
The GSC10 uses a magnet on the wheel and the bike profile's wheel size to calculate distance and speed with improved accuracy, while the BSS and Speed Sensor 2 use a magnetometer and orientation to accomplish the same thing.
These sensors are useful when traveling through tunnels, areas with heavy tree cover, and on rides with substantial changes in elevation where GPS is limited. When the device is recording distance with GPS, it is recording the distance from one point to the next, and may not be factoring in the elevation ascent or descent. The sensors give a more accurate reading by recording distance based on wheel rotation.
Last edited by njkayaker; 08-16-24 at 11:52 AM.
08-16-24 | 02:51 PM
#61
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The most accurate way to calibrate a rotation sensor is to use GPS to determine tire circumference. As a consequence, distance measurements based on the sensor will not be as accurate as those based on direct GPS measurements, if the distance is greater than that used for calibrating the sensor.
08-16-24 | 03:08 PM
#62
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The rotation sensors are not inaccurate (as long as a good roll out number is obtained).
=============================
Given the variability of GPS tracks, it's not clear that distance from a well-calibrated rotation sensor would necessarily be different in accuracy.
https://media.dcrainmaker.com/images...6/image-49.png
Last edited by njkayaker; 08-16-24 at 03:27 PM.
08-16-24 | 03:35 PM
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Yes, Garmin says that in their one sentence blurb. But, when you dig into it, it's a lot more complicated. You have to understand the nature of sensor calibration errors, the various contributions to GPS error, and how all those errors accumulate over multiple measurements.
You're missing the point. The most accurate way to get is good number for the roll out is with GPS. That's why the sensor cannot be better than GPS at distances greater than the distance used to determine the rollout.
Originally Posted by njkayaker
The rotation sensors are not inaccurate (as long as a good roll out number is obtained).
08-16-24 | 07:08 PM
#64
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A good exercise might be to put a sensor on the front wheel and compare front to rear. Fronts generally travel further I believe from looking at wheel tracks.
08-17-24 | 06:38 AM
#65
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Yes, Garmin says that in their one sentence blurb. But, when you dig into it, it's a lot more complicated. You have to understand the nature of sensor calibration errors, the various contributions to GPS error, and how all those errors accumulate over multiple measurements.
08-17-24 | 06:59 AM
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I'm not sure if the Garmins let you use two rotation sensors. (You could use two head units.)
Last edited by njkayaker; 08-17-24 at 07:05 AM.
08-17-24 | 07:48 AM
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I think the proposed explanation is that the deformation of the tire under weight is consistent for the entire revolution, thus the reduced radius is consistent for the entire revolution and therefore the effective circumference loaded is less than an unloaded circumference.
With tires on wheels, when tires aren't loaded the effective circumference is greater (corrected thanks to Choddo for calling me out on my error) than when the tire is loaded. It's not the distance traveled so much as the lever arm of the wheel's effective radius when loaded that affects calculations when that variable's plugged in.
Last edited by spclark; 08-17-24 at 12:51 PM.
08-17-24 | 08:33 AM
#68
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Wheel rollout measurements are only valid for the tire pressure, tire wear, road surface, plus weight of rider and equipment used at time of measurement.
08-17-24 | 09:13 AM
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It's a short, simplified answer in an FAQ. It doesn't get into the details of how these devices work.
It is the most accurate way to calibrate tire circumference.
Elevation error is already very small AND it can be corrected using readings from a barometric altimeter. Errors from cutting corners can be an issue mountain biking, but they're pretty insignificant when road cycling.
This subject has been discussed at length in several other threads. Anyone interested should just search for those threads.
You don't know whether there's really any difference between using that and other methods.
Again, this is wrong. The GPS can have drop outs and doesn't account for elevation. And it might cut corners too. And Garmin says otherwise.
This subject has been discussed at length in several other threads. Anyone interested should just search for those threads.
08-17-24 | 10:15 AM
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To my mind it's kinda like calculating ERD when building wheels. Hard to measure directly w/o some sophisticated tools but the concept is fundamental to getting spoke length right.
With tires on wheels, when tires aren't loaded the effective circumference is less than when the tire is loaded. It's not the distance traveled so much as the lever arm of the wheel's effective radius when loaded that affects calculations with that variable plugged in.
With tires on wheels, when tires aren't loaded the effective circumference is less than when the tire is loaded. It's not the distance traveled so much as the lever arm of the wheel's effective radius when loaded that affects calculations with that variable plugged in.
08-17-24 | 11:06 AM
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Rotation sensors using a separate magnet can have issues with the magnet not being picked up (alignment issues). Likely more of an issue with mountain biking. The newer hub rotation sensors should eliminate that issue.
No one does this.
Last edited by njkayaker; 08-17-24 at 12:49 PM.
08-17-24 | 12:49 PM
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08-17-24 | 03:40 PM
#73
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Yes, if you’re a top fuel dragster. At 200 watts or so, I doubt it’s significant, maybe not even measurable, within an available margin of error
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