29 X 2.0 Polar Wheel Size Setting

[flymango]

So I've searched a few threads on a similar topic, and am quite confused. I'm trying to enter the circumference of a Specialized Captain Control 29 X 2.0 into my Polar.

 

From some online chats (Cateye) and my LBS, I get the following:

29 X 2.1 = 2288

29 X 2.3 = 2326

 

Using interpolation, that gives:

29 X 2.2 = 2307

29 X 2.0 = 2269

 

Then some websites advise to subtract between 7 - 10 for weight, tyre pressure, etc.

 

Then some other websites and Hub threads suggest measuring a full revolution. If I do the latter, I get 2295. That's not near the 2269?? Please help!!

[G-Unit]

Measure it. Make a mark on the ground and using the valve as a reference roll out 2 or 3 wheel circumferences (whatever you have space or a long enough tape measure). Divide by the number of revolutions and there you go. Obviously the more revs, the more accurate.

[gogo@]

Tyre pressure?? I'm sure you've measured carefully so why not just go with that? Lots of people note differences between gps and bike computers... so no doubt those ERTRO thingamies are often wrong??

[flymango]

Measure it. Make a mark on the ground and using the valve as a reference roll out 2 or 3 wheel circumferences (whatever you have space or a long enough tape measure). Divide by the number of revolutions and there you go. Obviously the more revs, the more accurate.

 

I did measure a revolution - 3 times to triple check. That's when I got 2295, which is way off the 2269 interpolated from the charts.

[G-Unit]

 

 

I did measure a revolution - 3 times to triple check. That's when I got 2295, which is way off the 2269 interpolated from the charts.

 

I'm no expert, but I would say what you measured is what it is. Practical over theory...

[Bianchi]

From some online chats (Cateye) and my LBS, I get the following:

29 X 2.1 = 2288

29 X 2.3 = 2326

 

Using interpolation, that gives:

29 X 2.2 = 2307

29 X 2.0 = 2269

 

You should rather go the measurement route. My [Kenda small block 8] 29 x 2.1 gives me 2275, as measured and I find it accurate when testing it with my GPS.

[Frosty]

Compare:

• 3.14 x wheel diameter (in millimeters) measured with a tape measure across the centre (hub) of the wheel.
   
  http://babs.co/tyres/tyre_09_800.jpg
  http://biketouringnews.com/wp-content/uploads/2011/03/tires-019.jpg
  

 

and

• the roll out test
  

 

It should give the same result to within a few millimeters.

[jcza]

Go with what you measured less 4mm.

[flymango]

Go with what you measured less 4mm.

Trial and error I guess. Pitty I'm doing 115km on my first ride. Hate to think I've done 115km when it's actually only 90 km!!

[NINER_boy]

Flymango

1. Pump the tyres to your preferred riding pressure
   
2. Make a mark on the ground
   
3. Start on this mark with the valve
   
4. GET ON THE BIKE
   
5. Ride / roll one (or more) revolutions with your weight on the bike
   
6. Stop with the vavle at the bottom again
   
7. Measure the distance (and divide by the nr of revolutions)
   

Whoilla

 

My measurement came out to be 2240mm, and very accurate when riding with other people with bike computers / Garmin etc.

 

Happy Hoopin

[flymango]

Flymango

1. Pump the tyres to your preferred riding pressure
   
2. Make a mark on the ground
   
3. Start on this mark with the valve
   
4. GET ON THE BIKE
   
5. Ride / roll one (or more) revolutions with your weight on the bike
   
6. Stop with the vavle at the bottom again
   
7. Measure the distance (and divide by the nr of revolutions)
   

Whoilla

 

My measurement came out to be 2240mm, and very accurate when riding with other people with bike computers / Garmin etc.

 

Happy Hoopin

Best advice. Getting on the bike and at typical pressure. Thanks. Will give feedback later on.

[flymango]

2269 now that I deflated the tyres to 2.1. Gonna try 2266 to adjust for my weight and any errors of parallax.

[Showtime]

Trial and error I guess. Pitty I'm doing 115km on my first ride. Hate to think I've done 115km when it's actually only 90 km!!

 

The difference between 2269 and 2295 is 1.1%.

 

So your 90km ride measurement will be accurate to within 1km. No need to worry