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

[Battery_Chick]

1 hour ago, Fisan said:

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

LOL, love the way you think! Unfortunately I'm not faster on the new, heavier bike yet, but I love him dearly and I will be quicker soon! 

[love2fly]

3 hours 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. 

As far as I know elevation is measured by changes in atmospheric air pressure. I know when I set up my Bryton I had to enter my elevation into user settings. We always find differences when comparing m climbed with that of organizers. Its not exact. We've found that a weather front during a stage can really mess with m climbed.

[love2fly]

55 minutes ago, gerriemtb said:

Have you been vaccinated recently? The chip that is now in your body might influence your bike's GPS #tinfoilfriday

Like Jeremy Clarkson said: " How can the government track you via the vaccine when they can't even fix potholes?"

[Battery_Chick]

1 hour ago, RobbyB said:

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??

No speed sensor, just GPS. It was a good line of thinking though 

[Battery_Chick]

1 hour ago, copperhead said:

I don't want to insult anyone but this is the best thread I have ever read. A GPS works off of satellites. So as stated you could be in a humveeeeee and it won't make a difference. Computers work off of a barometer and will almost never be accurate. Starva elevation works off of maps???? So I think it is more accurate. While all the questions and answers i think are legit please do not not hate on me for saying thanks for giving me a smile today. I mean I could even be bloody wrong. It just made me laugh though. So thanks. Okay back to riding. 

Glad to bring a smile. Maybe one day I will remember that we made some route change and that's responsible. For now I'm putting it down to a weird confluence of data that looks consistently different! 

[Battery_Chick]

1 hour ago, gerriemtb said:

Have you been vaccinated recently? The chip that is now in your body might influence your bike's GPS #tinfoilfriday

Dang it! Maybe that's it! Hahaha! 

[Battery_Chick]

57 minutes ago, Shebeen said:

you would need to get a 29er specific GPS unit to resolve this issue

The good news is this Garmin came from a 29er! So what I'm reading now must be the accurate reading 🤣

[TheoG]

3 minutes ago, Battery_Chick said:

No speed sensor, just GPS. It was a good line of thinking though 

The difference between 26" and 29" is more like 11% anyway, so definitely not it.

[Battery_Chick]

52 minutes ago, dexterdent said:

It's because you are faster on the 29er and therefore relativity comes into play

Well my smile is bigger... 😁

[Battery_Chick]

44 minutes ago, DieselnDust said:

Do you have a speed sensor on the bike and is the gps bike computer set to use it as the main sensor for measuring distance?

No speed sensor

[TheoG]

1 minute ago, Battery_Chick said:

No speed sensor

As far as I know your Garmin can take a speed sensor.  I suggest you get one for better accuracy.

Edited December 10, 2021 by TheoG

[Battery_Chick]

33 minutes ago, Frosty said:

Do you, the OP, wait for your device to find satellites before you start every ride?

I'd say yes but it's a factor to consider

[TheoG]

https://www.garmin.com/en-ZA/p/pn/010-12843-00

Speed sensor for the Garmin Edge 820.

[Battery_Chick]

3 minutes ago, TheoG said:

As far as I know your Garmin can take a speed sensor.  I suggest you get one for better accuracy.

Yeah I got one with the second hand kit, it just needs batteries. I'll look into fixing that up and using it! Then I can k ow THE TRUTH!! (or something closer to it, taking into account tyre pressure, time spent in the air etc etc) 😄

[Shebeen]

2 hours ago, copperhead said:

I don't want to insult anyone but this is the best thread I have ever read. A GPS works off of satellites. So as stated you could be in a humveeeeee and it won't make a difference. Computers work off of a barometer and will almost never be accurate. Starva elevation works off of maps???? So I think it is more accurate. While all the questions and answers i think are legit please do not not hate on me for saying thanks for giving me a smile today. I mean I could even be bloody wrong. It just made me laugh though. So thanks. Okay back to riding. 

Barometric sensor can be in a watch and a dedicated GPS unit. They are much more accurate than GPS for altitude, but need to be calibrated to the correct altitude when you start, and during the trip as the weather (and barometric pressure) changes. 

The reasons why altitude is inaccurate for GPS simplistically is because the 2D position is a latitude/longitude fix. The altitude is much harder to get accurately as it is an approximation to the position vs the model of the earth (WGS84 - not a perfect ellipsoid)

 

[TheoG]

59 minutes ago, Shebeen said:

Barometric sensor can be in a watch and a dedicated GPS unit. They are much more accurate than GPS for altitude, but need to be calibrated to the correct altitude when you start, and during the trip as the weather (and barometric pressure) changes. 

The reasons why altitude is inaccurate for GPS simplistically is because the 2D position is a latitude/longitude fix. The altitude is much harder to get accurately as it is an approximation to the position vs the model of the earth (WGS84 - not a perfect ellipsoid)

 

Amen!  Both my watch and bike comp have barometric altimeters and 95% of the time for the duration of a training/race, the change in weather is not significant enough to make the data inaccurate/invalid.

For speed and distance a sensor on the wheel is still the best.  I use the GPS just for a nice pic of the route afterwards.