# calibrating your Garmin for a 29'r?



## raceboy (Jun 24, 2008)

Garmin does not have 29'r wheel size calculations in their manual. I came up with 2285 for a 29x2,0, is this correct? I told them 29'rs are here to stay and they needed to update their manual.:thumbsup::rockon:


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## ATBScott (Jun 4, 2006)

??? GPS shouldn't need to have your tire size in - it calculates speed and distance from the satellite feeds. Unless you are setting it up with a rear-wheel magnet for use on a trainer - there is no need for a wheel setting that I can think of!


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## Squash (Jul 20, 2003)

ATBScott said:


> ??? GPS shouldn't need to have your tire size in - it calculates speed and distance from the satellite feeds. Unless you are setting it up with a rear-wheel magnet for use on a trainer - there is no need for a wheel setting that I can think of!


While the tire size isn't needed for speed and distance, there are other calculations that the Garmin does where it is required for accurate results. I don't recall of the top of my head what they all are, but the tire size is needed in the bike profile.

To the OP, that sounds about right. How did you measure? A roll out or did you physically measure the circumference of the tire. Either method works about the same. Like I said, sounds about right.

Just remember if you change tires at any time to do it again. Even if it's the same size tire but a different brand or model you'll likely end up with a different figure. Probably won't be a huge difference, but often it's enough to make a difference in the accuracy of the information you get from your unit.

Good Dirt


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## hike-run-bike (Dec 31, 2011)

Cadence and power output. 

Sent from my A500 using Tapatalk


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## raceboy (Jun 24, 2008)

I talked to the Garmin CS, one of the CS guy rides a 29'r and figured it out on his own bike. They have a table of wheel size and circumference in their manual.


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## WR304 (Jul 9, 2004)

You need to make sure that your wheel circumference is set correctly in the Garmin. A mountain bike tyre's circumference is reduced significantly when you sit on the bike. This is more pronounced for a rear tyre (the rear wheel circumference is used for speed if you have a Garmin GSC-10 or Powertap hub) so you can't rely on wheel size tables.

The compressed tyre's circumference will vary depending upon tyre pressure and rider weight. A heavier rider will compress the same tyre more than a lighter rider, changing the circumference. Putting a blob of grease on the tyre, and then measuring the actual distance between the marks left on the road as you ride your own bike is the most accurate method.

Eg: Garmin Tyre Circumference Tables
https://support.garmin.com/support/...caseId={8c7b7b10-0424-11de-76c5-000000000000}

26x2.1" Specialized Ground Control rear tyre measured circumference with 149lb rider:

- Garmin Tyre Table for 26x2.1" tyre: 2068mm
- Measured Uncompressed tyre circumference: 2110mm 
- Measured rear tyre compressed circumference whilst riding @60 psi 2060mm 
- Measured rear tyre compressed circumference whilst riding @20psi 2020mm (This is my actual tyre circumference for offroad use. If you used the Garmin tyre table figure it would overestimate your speed and distance.)

Different brands of tyre also tend to vary in width. My suggestion would be to make a spreadsheet of measured tyre circumferences so that whenever you swap to a different tyre you can enter the correct tyre circumference into the Garmin, keeping your ride files consistent.


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## drmayer (Apr 19, 2007)

Use the auto size setting in the bike profile. This will compare the GPS speed to the sensor speed and calibrate the wheel size automatically. I use this method and have not had any problems.


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## shiggy (Dec 19, 1998)

raceboy said:


> Garmin does not have 29'r wheel size calculations in their manual. I came up with 2285 for a 29x2,0, is this correct? I told them 29'rs are here to stay and they needed to update their manual.:thumbsup::rockon:


I just turn it on. It is a GPS. Does not need a wheel calibration. If you are using the speed/cadence sensor set it on auto calibration.


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## ATBScott (Jun 4, 2006)

Again, the rear wheel magnet/sensor are only useful if you are using the bike on a trainer, and probably aren't using a knobby off-road tire on that little aluminum roller. There are a ton of "700 x ##" road sizes out there that might be listed in the guide in a size that would be used on a trainer. Cadence, IMO is not that useful for Off-road riding, but on a trainer or road ride for fitness is nice to ensure your revs. If you ARE using it on the trainer, the roll-out should be measured with you on the bike to account for the small amount of tire deflection with the tire compressed.


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## boseta (Jul 26, 2008)

just measure it:thumbsup:


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## shiggy (Dec 19, 1998)

ATBScott said:


> Again, the rear wheel magnet/sensor are only useful if you are using the bike on a trainer, and probably aren't using a knobby off-road tire on that little aluminum roller. There are a ton of "700 x ##" road sizes out there that might be listed in the guide in a size that would be used on a trainer. Cadence, IMO is not that useful for Off-road riding, but on a trainer or road ride for fitness is nice to ensure your revs. If you ARE using it on the trainer, the roll-out should be measured with you on the bike to account for the small amount of tire deflection with the tire compressed.


After downloading the data the cadence sensor call tell you where the hike-a-bike sections were.


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## slocaus (Jul 21, 2005)

Search the Garmin forums and you will find that auto calibration works best, since it will compensate for changes in tire pressure and rider weight (how much clothing you have on, or how loaded your pack with gear.


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## zarr (Feb 14, 2008)

tagged for future ref.


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## Nürburg Nomad (Jan 23, 2012)

Perfect timing on this question! I was just going to ask the same thing for setting up the Edge 800 on my Rumblefish! Thanks.



shiggy said:


> After downloading the data the cadence sensor call tell you where the hike-a-bike sections were.


Agreed... and at least for myself, on long singletrack it's nice to have a cadence sensor firing to be sure you're keeping the RPMs up at a glance which is much easier on the knees of course.


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## shiggy (Dec 19, 1998)

Nürburg Nomad said:


> Perfect timing on this question! I was just going to ask the same thing for setting up the Edge 800 on my Rumblefish! Thanks.
> 
> Agreed... and at least for myself, on long singletrack it's nice to have a cadence sensor firing to be sure you're keeping the RPMs up at a glance which is much easier on the knees of course.


You misunderstood what I said.


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## gored996 (May 15, 2016)

shiggy said:


> I just turn it on. It is a GPS. Does not need a wheel calibration. If you are using the speed/cadence sensor set it on auto calibration.


Auh yes you do need to calibrate. If you ride in woods like I do, the trees will lessen the satelite signals and give you reading short of actual distance. By have the tire calibration and speed sensors setup, with the GPS, it will result the best accurate distance. Road bikes sure you can get away with GPS signals only, but MTBs in woods will not be accurate without this calibration.


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## BmanInTheD (Sep 19, 2014)

This thread is over 6 years old, but, yes, if you ride in the woods you can be robbed of up to 15% of the mileage you rode if you use GPS.


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## hey_poolboy (Jul 16, 2012)

Do a rollout on each wheelset. Inflate tire to desired pressure. Place valve stem at bottom of wheel. Make a mark or place a stick by the stem on the ground. Sit on bike and roll forward two revolutions of the wheel until valve is down again. Measure (in mm) from mark or stick to the new location. Divide by 2 and you have your extremely accurate circumference.

Sent from my XT1650 using Tapatalk


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## Harold (Dec 23, 2003)

gored996 said:


> Auh yes you do need to calibrate. If you ride in woods like I do, the trees will lessen the satelite signals and give you reading short of actual distance. By have the tire calibration and speed sensors setup, with the GPS, it will result the best accurate distance. Road bikes sure you can get away with GPS signals only, but MTBs in woods will not be accurate without this calibration.





BmanInTheD said:


> This thread is over 6 years old, but, yes, if you ride in the woods you can be robbed of up to 15% of the mileage you rode if you use GPS.


Has little to do with "the woods".

Has a WHOLE LOT MORE to do with the way GPS-only distance calculations inherently work. Recording single points and "connecting the dots". The reason a wheel sensor is more accurate than GPS-only distance measurement has to do with the wheel sensor measuring the actual path of travel (the full arc of a curve) rather than connecting the dots (and shortcutting the curve). The sinuosity of a trail (curviness/twistiness) is another big factor. If you ride straighter trails, there are fewer curves to shortcut. More (and tighter) curves mean more distance is lost through GPS-only measurements. Speed is another factor. The faster you go, the more spaced out the recorded points are, which results in corners being shorted by larger amounts. Forest cover is a MUCH smaller factor than all of these others.

To emphasize - you cannot calibrate the GPS itself. Rather, you calibrate wheel sensors (if you have them). Garmin's auto calibration works pretty well, but you can manually override that if you prefer.


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## Lone Rager (Dec 13, 2013)

Auto calibrate works well if done over a relatively non-tortuous track, otherwise it'd have the same problem as GPS alone has on twisty tracks and report too short of a distance. Of course the wheel sensor simply reports a wheel revolution. What's calibrated is the wheel circumference which gets multiplied by wheel rotations to determine distance. The times I've done it, it seemed like Garmin took something like 1/2 mile...or maybe 1km to do it. I've done the wheel roll out method too in the past. I figure either is more than good enough for my purposes.


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## thasingletrackmastah (Nov 15, 2005)

Garmin will only auto-calibrate on a straight, with good satellite reception.
It'll do this at the start of any ride.
Turn your gps on a couple of minutes before taking off, to make sure you have a good satelitte fix.


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