Originally Posted by njkayaker (Post 22091631)

For a curvy path, the GPS (measuring once per second) will measure a distance shorter than a wheel sensor (measuring about 3 times per second) using the correct circumference.

All you have done with this effort is make the wheel sensor less accurate.

Originally Posted by wkc (Post 22091656)

NJK, you could be right with your two comments. But thinking about it, at least my eventual outcome is some sort of consistency between my GPS using MMR and my BC, no??? :p

Originally Posted by wkc (Post 22091678)

When you say “curvy path”, yes, I agree if there were lots of relatively “small” curves. But as I was measuring rides between 2km and 30+km with straights and curves, I would think that the impact of “curves” over those distances would be somewhat negligible when compared to the circumference of my tyre.

I think you are absolutely correct if I was riding “short” and “curvy” distances every time. But that is not the case.

I think I will start my third list next week and see what the outcome is. Maybe I could compare my third list measurements/distances with measurements on Google Maps or some other map to check if gentle* curves make any difference over “larger” distances.
* gentle road curves vs concentric or tight circles and curves.

Originally Posted by wkc (Post 22091645)

Spelger, you correctly say “if the bike computer has the correct information”. IF!!!
I have always been a little suspicious about using “standard” conversion tables for translating wheel “size” to wheel circumference. (eg 700 x 23C = 25-622 = 2096)

My general knowledge tells me that standard wheel size will only equal distance travelled for any given tyre pressure or even tyre wear. A flatter, less inflated 700x23C will have a smaller circumference compared to a well/highly inflated 700x23C. Tyre wear and even manufacturing tolerances (exact amount of rubber all the way around a tyre) would affect circumference in a similar, albeit to a smaller extent.

Therefore, using 2096 in a BC assumes that your actual wheel on your bike rolls 2096mm for every revolution. Thus, using standard conversion table circumferences is assuming your BC is using the “correct” information. To me, that is not necessarily a good assumption and necessitated some sort of recording and comparison.

Originally Posted by blacknbluebikes (Post 22091772)

Well, let's chime in with this angle...
The units being measured (miles, km, feet, whatever) are actually pretty arbitrary. Cubits? why not. The higher order question is "what are you doing with it and what precision is required?"
So this thread gets into precision, which I do find interesting. But IRL, I find that most situations rarely benefit from more than 4 significant digits, usually 3 will do and 2 is often good enough for decision making.
If I had ridden 36 miles, does 36.4 matter?
If I did a season of 3000 miles, would 30 matter? would 3?
I can find you both cases in finance, where the way we need to work sometimes says "need it to the penny," other times we round to the nearest million.
So, what I'm asking this thread is : "what level of precision is _useful_?" and when does a lack of precision become troublesome?

Originally Posted by wkc (Post 22091368)

...large discrepancies...circumference in my bike computer...tyre pressure...calibration....amended...distances recorded...my calculation...spreadsheet...absolutely negligible.

Originally Posted by BCDrums (Post 22091994)

wkc,

I admire your rigorous approach to tracking down the errors. But I am lazier (by a lot!) than you are.

I have a Cateye Astrale wired bike computer that is C&V-worthy, and I track my rides on a phone using Ride/w/GPS. When they did not agree, I lowered/raised the circumference number in the Cateye until they did agree within a tenth! Took a few rides, but not many, to get them synced.

Which one was correct to begin with? I don't know, but I think the GPS. (There are no cul-de-sacs on my routes.) Why must they agree? Because I like them to agree! :lol:

So now, they're either both right, or both wrong, but they agree.

All best to you in Oz.

Originally Posted by chaadster (Post 22091898)

You’re absolutely right on, although another high-order question which could be asked is which info is most important: distance traveled or distance and location?

Only the GPS can record where one has traveled, and as such is a much richer data source than a $20 cyclometer. It’s hard for me to imagine the point of this “accuracy exercise” in 2021; GPS data can be analyzed more easily and in a more consequential manner than the simple data from the cyclometer.

Originally Posted by kahn (Post 22092309)

Just did a ride which was

Phone apps using gps:
GAIA: 15.25
Mission Control: 15.44

Specialized Display Unit: 15.4

I guess I could join the "lose sleep over it" group or just learn to live with the differences. Elevation tracks differently, too, but gps elevation stats are always a bit hit/miss. Mission Control is always higher.

Originally Posted by blacknbluebikes (Post 22091772)

So, what I'm asking this thread is : "what level of precision is _useful_?" and when does a lack of precision become troublesome?

Originally Posted by wkc (Post 22092663)

I promise you that I did not edit this pic. That was my max speed at the very start of this ride. :giver:

Originally Posted by rsbob (Post 22092574)

My GPS on a downhill today gave me a top speed of 58.2 MPH where it was more likely around 45. ‘Technology is a wonderous thing.... when it works.

Originally Posted by wkc (Post 22092257)

Sure, the phone can do and record more but I don’t like having it strapped to my cockpit. So, now my inconspicuous BC on my stem at least shows similar speed and distance data to that on my phone in my back pocket. :D

Originally Posted by kahn (Post 22092309)

Just did a ride which was

Phone apps using gps:
GAIA: 15.25
Mission Control: 15.44

Specialized Display Unit: 15.4

I guess I could join the "lose sleep over it" group or just learn to live with the differences. Elevation tracks differently, too, but gps elevation stats are always a bit hit/miss. Mission Control is always higher.

Originally Posted by njkayaker (Post 22092779)

That's one reason people use speed sensors.

Originally Posted by rsbob (Post 22094120)

Ayup. My other bikes have them. Strava consistently under reports my average speed by 0.5 MPH with no way to correct which is a bit frustrating after a good ride.

Originally Posted by 1979schwinn (Post 22094330)

Most GPS (phone and watch), update (talk to satellite), once every 5-6 seconds.
+- 0.00'

Originally Posted by prj71 (Post 22092885)

My god...I disappear for a week and you guys are still babbling about GPS accuracy.

https://www.singletracks.com/mtb-gea...cyclocomputer/

Originally Posted by 1979schwinn (Post 22094330)

Most GPS (phone and watch), update (talk to satellite), once every 5-6 seconds.

Originally Posted by 1979schwinn (Post 22094330)

More accuracy would require a dedicated GPS device.

Originally Posted by 1979schwinn (Post 22094330)

Another note on longer runs, you are handed off to different satellites every 20 minutes.

Originally Posted by 1979schwinn (Post 22094330)

I work with surveying grade GPS and we have to set up and wait the 20 minutes to reconfirm our accuracy, +- 0.00'

Originally Posted by njkayaker (Post 22094520)

And a week later, you are still whining about it. :rolleyes:

Originally Posted by prj71 (Post 22094747)

I haven't whined once in this thread.