GPS distance measured on a 26in vs 29in bike different?

[Battery_Chick]

OK so I have a strange observation and I wonder if anyone else has experienced something similar. The group I ride with do weekday rides following the exact same route every time. I recently upgraded from a 26 inch bike to a 29 inch bike and my ride distance, measured by a Garmin Edge 820, changed from 15.13km to 14.94km. That's 190m less and is a 1.3% difference. 

Now I thought it was a new downhill section that may be shorter, but looking at the map, both rides I'm comparing used the same route! Could it be possible for a Garmin mounted on the handlebars to measure a difference in how I track through corners on these new bigger wheels and could it be that big a difference? It seems so crazy yet I can't (yet?) see another explanation?? 

So, am I crazy or what? 

[pedal menace]

In my opinion - bike has no influence whatsoever on the GPS computer.  You can ride a scooter, 26 inch, 29 inch or your car - it should measure the same. However every ride can differ slightly for the same course - sometimes you cut corners, take it wide, ride slightly zig-zag etc - all of these can influence distance. But the device used for forward motion has no affect on the computer

[Neill Du Toit]

GPS devices or phones will never be 100% accurate. Or at the least the cycling ones we use. 

[RobertWhitehead]

Funny fact: I had a friend who did the Baviaans both weekends, same route, same distance but major difference in elevation - also Garmin. 

[Jbr]

7 minutes ago, pedal menace said:

In my opinion - bike has no influence whatsoever on the GPS computer.  You can ride a scooter, 26 inch, 29 inch or your car - it should measure the same. However every ride can differ slightly for the same course - sometimes you cut corners, take it wide, ride slightly zig-zag etc - all of these can influence distance. But the device used for forward motion has no affect on the computer

it's not an opinion, it's a fact. The cycling computer/watch measures the movements of your gps chip according to satellites, it doesn't matter what's underneath it.

Edited December 10, 2021 by Jbr

[Martin PJ]

My Giant GPS measures less than my phone Strava app if I do a local ride with lots of corners. If I ride out and back, more or less straight, they are both similar in distance. I think the Giant GPS has lower sampling rate, so basically is cutting the corners. Difference is about 0.5 km over 30 km with a local ride.

[The Ouzo]

I ride the same route daily, always stop my garming on 27.45km, the point in the road where I hit the stop button is seldom the same. Elevation often is a little different too.

As others have mentioned, does not matter what you used to traverse the route, it the device's relation to the GPS satellites that counts.

[TheoG]

12 minutes ago, Jbr said:

it's not an opinion, it's a fact. The cycling computer/watch measures the movements of your gps chip according to satellites, it doesn't matter what's underneath it.

Yip, I couldn't put it any better than this.  The 190m difference is purely due to GPS accuracy and maybe you riding the route slightly different mainly through the corners, but the largest percentage would be due to GPS accuracy.

Edit:  My Polar V650 seems to be more accurate than my watch (Vantage V) and even on that, the distance always vary slightly for the exact same route.

Edited December 10, 2021 by TheoG

[Battery_Chick]

Yeah I also can't see how it would change for such small changea in position. Another spanner in the works. I averaged 5 rides on the 26er and 3 rides on the 29er and I still have a 1.3% difference! At this stage I'm going to put it down to a difference in sampling or some difference in the route that I'm not consciously aware of. 

So strange! 

[Battery_Chick]

1 hour ago, RobertWhitehead said:

Funny fact: I had a friend who did the Baviaans both weekends, same route, same distance but major difference in elevation - also Garmin. 

Now this may be due to barometric differences between the rides. I believe the devices we use often use barometric pressure to gauge elevation and changes in elevation.

[Eldron]

A few hundred meters is nothing. You never start and stop your computer at exactly the same place and if you do it takes varying time to pick up satellites and start tracking.

Also - GPS tech breadcrumbs - it isn't live so when you take corners etc it may take a sample at different points or even skip a short, tight corner completely if you have your sample rate set to 15 or 20 seconds.

I'm sure the fundies on here will know a few other reasons why GPS is not perfect...

[Battery_Chick]

1 minute ago, Eldron said:

A few hundred meters is nothing. You never start and stop your computer at exactly the same place and if you do it takes varying time to pick up satellites and start tracking.

Also - GPS tech breadcrumbs - it isn't live so when you take corners etc it may take a sample at different points or even skip a short, tight corner completely if you have your sample rate set to 15 or 20 seconds.

I'm sure the fundies on here will know a few other reasons why GPS is not perfect...

Thanks for the explanation of the sampling, I think it's probably safe to say that this is the real difference, and I'm just picking up a streak of unusual data. The good news I ride the route every week so I will keep on sampling, lol! 

[Fisan]

1 minute ago, Eldron said:

A few hundred meters is nothing. You never start and stop your computer at exactly the same place and if you do it takes varying time to pick up satellites and start tracking.

Also - GPS tech breadcrumbs - it isn't live so when you take corners etc it may take a sample at different points or even skip a short, tight corner completely if you have your sample rate set to 15 or 20 seconds.

I'm sure the fundies on here will know a few other reasons why GPS is not perfect...

The upgrade from 26er to 29er obviously made OP faster, meaning fewer samples on corners, thus shorter distance.

OP should borrow a 27.5" and do the loop a couple of times. This might end the war on wheel sizes once and for all.

#yayitsfriday

[RobbyB]

The only way that there can be a difference in distance in wheel sizes is if you have a speed sensor fitted to a 26" wheel and then transfer the speed sensor to a larger wheel (29") and then don't change the wheel circumference on the receiver/GPS unit. This would be the case in this matter (if there is a speed sensor installed) as the bike would be travelling further per wheel revolution but the 26" circumference measurement would give a slower speed/distance travelled therefore resulting in a lesser distance travelled. Does the OP have a speed sensor installed??

[RobbyB]

Question: is the circumference change from 26" to 29" wheel size a 1,3% difference??

[117]

Michelle, the ruskies repositioned one of their birds and this is giving a new position to your GPS receiver, giving you 1.3% difference. It's basically the conversion between imperial back to metric. 

Just kidding, wheel sizes has nothing to do with it. It's the time interval between GPS spots which might cut a corner between the spots, hence giving a shorter distance perception. If you can set the GPS to record every second (make a spot every second I mean) and do a few rides on each bike you'll find it'll be the same distances