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Bike Computer Accuracy

Old 01-16-21, 10:24 PM
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bikehoco
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Bike Computer Accuracy

For rides, I use a bike computer (magnet in the spokes) and Ride With GPS on an iPhone. For distance, both provide similar results (20 miles vs. 19.9 miles). But for speed, the difference is bothersome (14 mph vs. 13.5 mps). Is this typical?

I ride along the road (with some trails) in the suburbs..
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Old 01-16-21, 10:28 PM
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Bike Computer set with Roll OUT Measurements is most accurate..
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Old 01-16-21, 11:38 PM
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Originally Posted by bikehoco View Post
For rides, I use a bike computer (magnet in the spokes) and Ride With GPS on an iPhone. For distance, both provide similar results (20 miles vs. 19.9 miles). But for speed, the difference is bothersome (14 mph vs. 13.5 mps). Is this typical?

I ride along the road (with some trails) in the suburbs..
I have two GPS devices that donít agree on distance - my iPhone running Strava consistently reads 0.6% higher than my Garmin head unit. Probably as good as it gets for consumer devices
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Old 01-17-21, 12:25 AM
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Go with whichever is higher.

Last edited by aplcr0331; 04-29-21 at 03:37 PM.
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Old 01-17-21, 08:53 AM
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The speed and distance on the computer are only as accurate as the value used for the tire circumference. Standard values listed for each tire size are close, but I measure the length of 3 revolutions, several times, then convert my inches measurement to millimeters. Having your weight on the bike will further improve accuracy. The tire will squish and the effective radius will be smaller.
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Old 01-17-21, 09:03 AM
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Old 01-17-21, 09:55 AM
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My simple Cateye with a magnetic sensor matches usually within .10 of a mile with the ridewithgps route planner app on 50+ mile rides.
I stopped using Garmin because it grossly inflated elevation gains. It is more accurate to just use the ridewithgps mapping function to figure out elevation gain.
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Old 01-17-21, 10:32 AM
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Originally Posted by DaveSSS View Post
The speed and distance on the computer are only as accurate as the value used for the tire circumference. Standard values listed for each tire size are close, but I measure the length of 3 revolutions, several times, then convert my inches measurement to millimeters. Having your weight on the bike will further improve accuracy. The tire will squish and the effective radius will be smaller.
NOPE.
A Roll Out is Necessary as it relates to Tire PSA and Weight of the Rider.
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Old 01-17-21, 10:43 AM
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The correction for distance Strava uses if you're on pretty well known roads and routes is pretty accurate. However, you "instantaneous" speed shown on the GPS is most accurate if using a wheel speed sensor with accurate info inserted in for that sensor (rollout, tire size, etc...).

One thing I've seen is that if you're a runner, I have the slowest runs by far on curvy greenway/MUP runs where the curves are so small that the GPS mesh coords are much larger than the curves. Thus, you lose a solid amount of distance that it can't really correct for.

I could assume MTB folks also suffer from this and rollout may work better for distance the curvier your route is.

My most accurate distance/times for running pace are on arrow straight streets and roads with really steady curves to them when they do turn.

Either way, I train on rides to time spent in zones combined with overall TSS. I don't really bother looking at the speed or distance as it is irrelevant sometimes. I only went 18mi yesterday in an hour on a 30/30 vo2 workout but went uphill almost 1900ft. I was chasing X sets of 30/30 and an overall TSS score for the workout. I hit my sets, then free-rode at Z2 and tempo till I hit my TSS score. Then went home.

It doesn't really bother me ever unless doing my super nerd time trial equipment testing. Then I care about the details a bit more.
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Old 01-17-21, 11:16 AM
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Originally Posted by bikehoco View Post
But for speed, the difference is bothersome (14 mph vs. 13.5 mps).
Just wondering if this a look at actual current speed sometime mid-ride. Or, if itís the average speed you see after the ride has finished? Just asking because I think the two would likely be different while looking at them mid-ride due to the frequency of how often the speed is determined.

Dan
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Old 01-17-21, 04:42 PM
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In my experience, GPS devices are inaccurate as to speed at that moment. There’s always a few seconds of lag in the display. Is why I use speed sensors.
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Old 01-17-21, 07:42 PM
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I think the OP is saying that speed is pretty accurate between both.. So if the problem is the Avg Speed/Hr, then the problem lays in the difference in how the 2 devices are measuring time (moving, stopped, overall, etc etc.. lots of possible differences).
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Old 01-17-21, 08:30 PM
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Originally Posted by Sy Reene View Post
I think the OP is saying that speed is pretty accurate between both.. So if the problem is the Avg Speed/Hr, then the problem lays in the difference in how the 2 devices are measuring time (moving, stopped, overall, etc etc.. lots of possible differences).
^^ This... I wrote a database app many moons ago that I stored all my ride info in and I noticed that my avg time and speed were always off slightly. Came down to rounding from the computer and my db. I carried out digits 3 places for some off reason and the computer app only carried it out to 1. I changed my coding to only show a single digit east of the decimal and boom.....match.
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Old 01-17-21, 11:26 PM
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Auto pause/resume with most apps and *some* bike computers will also influence readings for speed, distance and time. Some auto-pause/resume settings are adjustable, some aren't.

I've run a dedicated bike computer and two phones with apps (Wahoo Fitness on one, Strava on the other) and they all differ slightly. Not enough to worry about. I was never close enough to a KOM for a 0.5 mph or 1 minute difference to matter over over my favorite routes. It might affect a top 10 position, but as younger, stronger and faster riders use those same routes I'll never be anywhere near a top 10, let alone a KOM. So my best times/speeds will be jockeying for middle of the pack honors, along with almost every other MAMIL.

If I was serious I'd try a wired bike computer calibrated to my wheel/tire diameter, etc., as described above.
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Old 01-18-21, 09:55 AM
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Originally Posted by CAT7RDR View Post
My simple Cateye with a magnetic sensor matches usually within .10 of a mile with the ridewithgps route planner app on 50+ mile rides.
I stopped using Garmin because it grossly inflated elevation gains. It is more accurate to just use the ridewithgps mapping function to figure out elevation gain.
If you ride through an area with high rise buildings, you can get some wildly funny elevation gains. I ride through downtown Pittsburgh while using Map My Ride and Road ID's app. Both sometimes think I'm riding over the rooftops! They also miss datapoints, presumably blocked signal by the buildings, so that'll throw the distance off.

On my Novara, I have a Sigma with mag sensor. It's set pretty accurately - it matches every mile marker around the river trails here and on the GAP even over long distances.
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Old 01-19-21, 08:55 AM
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When I'm feeling obsessive-compulsive over distance and/or speed, the wired (or wireless) cyclocomputer with a wheel sensor is my go-to measurement device. I've got a nice downhill ridge near home I can coast down, and compare the computer's measured distance with the mile markers. If I'm within 1%, it's fine; more than 2%, time to adjust the calibration. I've gotten it within 0.5% some months, and then the tire gets worn and/or replaced, and the next time it's up to 1% match.

A few key features of the above. First, I'm coasting downhill, so there's minimal side-to-side pedal tracking to affect the distance measured. Second, because it's a U.S. highway, I trust the mile markers to be accurate. (Don't try this in Kansas or Missouri -- I think they sell mile markers at Walmart in those states, and farmers pick out a pretty green sign to put up near their farms without regards to location or numbers) Finally, curves are pretty wide, even on the ridge.

GPS devices sometimes match the cyclocomputer pretty closely, as long as the comparison is on a straight, treeless road with no large buildings nearby. On my more typical rides (curvy, trees, hillsides blocking some satellite signals), the GPS distance is off. It's still good if I need help mapping or navigating, but I don't trust the distance the GPS measures. Ergo, I figure the instantaneous speed measurement is in the ballpark, but it's more for entertainment than precise measuring.
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Old 01-19-21, 09:46 AM
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I've had a similar ambiguity with GPS vs. my taffrail log.
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Old 01-19-21, 09:53 AM
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I use RWGPS post ride to fix the speed and elevation numbers.

Those complaining about a 0.6% difference is kind of insane in my book. That’s 15.00mph vs. 15.09mph, or for most practical purposes ‘the same’ number. The speedometer in your car is often has no better accuracy wise than this.
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Old 01-19-21, 10:15 AM
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Originally Posted by bikehoco View Post
For rides, I use a bike computer (magnet in the spokes) and Ride With GPS on an iPhone. For distance, both provide similar results (20 miles vs. 19.9 miles). But for speed, the difference is bothersome (14 mph vs. 13.5 mps). Is this typical?

I ride along the road (with some trails) in the suburbs..
I'd be irate if my magnet based bicycle computer was a half mile per hour off at 14 MPH. I'd pick up the phone and call someone's manager.
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Old 01-19-21, 04:29 PM
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Meh. My Garmin Edge 25 is close enough for me, although it's a bit off from my riding partner's computer of unknown make. I ain't settin' any speed or distance records.
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Old 01-19-21, 07:53 PM
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Originally Posted by DaveSSS View Post
The speed and distance on the computer are only as accurate as the value used for the tire circumference. Standard values listed for each tire size are close, but I measure the length of 3 revolutions, several times, then convert my inches measurement to millimeters. Having your weight on the bike will further improve accuracy. The tire will squish and the effective radius will be smaller.
unless you are using the computers GPS for those features.
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Old 01-19-21, 09:26 PM
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You can't ride an hour (or a half, or fifteen minutes) and see how far you've come, calculate your average speed, and know which one is right????
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Old 01-20-21, 10:15 PM
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Originally Posted by Paul Barnard View Post
I'd be irate if my magnet based bicycle computer was a half mile per hour off at 14 MPH. I'd pick up the phone and call someone's manager.
Bugger off smartass.
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Old 01-20-21, 11:20 PM
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If the sensor and magnet are correctly installed, the wired computer will be virtually100% accurate at counting wheel revolutions. Converting wheel revolutions to distance, measuring time, calculating speed from time & distance, then displaying speed on the screen, are matters of calibration, software, and hardware. There may be calculation inaccuracies and delays updating the display. Generally I think most cyclocomputers are pretty good, but they are not all equal.

Theoretically, if well calibrated, this should be far more precise than a GPS at both instantaneous speed and speed and distance over a long ride.
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Old 01-21-21, 07:24 AM
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The difference in measured distance between my cycle computer and Strava is pretty substantial, around 2%. Since I have a wired computer from a brand I have had only good experience with (Sigma), and I measured the tire circumference carefully when setting it up, I'm inclined to trust the computer instead of GPS. I mean, 2% of the measured 2220mm circumference (which also corresponds to the figures cited online for this tire size within a few mm) is 44mm - it's virtually impossible to be off by that much. The accuracy of speed measurements aren't that important to me, but such a difference in distance would translate to almost a whole century over my yearly mileage.
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