Calibrating Bike Computer with Wheel Size.

Worztron

Hi.

I'd like some advice on calibrating my old bike computer with the wheel size of my mountain bike. Attached is an image of the guide that came with the bike computer. On my tyre, it says "26x2.00" - indicating I presume the wheel height and thickness.

Am I correct in choosing 2114 in the bike computer? I don't recall how I came up with the blue writing - it was a long time ago.

What do 47-305, 47-406, etc. in the left columns represent? And what does the section that reads "26 x 1.75 x 2" mean?

I'd appreciate any help. Cheers guys.

cdaly_

The 47 is the nominal width of the inflated tyre while the 305/406 is the rim to rim diameter of the wheel without tyre. A wheel to take a 26" tyre is 559mm in diameter while a wheel to take a 700c tyre is 622mm.

Your choice of 2114mm would appear correct but the definitive way to measure this is:

1. Find a willing helper
2. Inflate tyres to correct value
3. Sit on bike and position the wheel with the valve stem at its lowest point
4. Have your helper mark this position on the ground.
5. Still sitting on the bike, roll forward for one full revolution of the wheel (about 2 metres)
6. Position the wheel with the valve stem at its lowest point
7. Have your helper mark this position on the ground.
8. Measure the distance between the marks in mm
9. This is your calibration value

ninoon

Well explained above. Alternatively mark the tyre with thin line of tippex and roll out on black tarmac, then measure

Worztron

cdaly_ wrote: »

The 47 is the nominal width of the inflated tyre while the 305/406 is the rim to rim diameter of the wheel without tyre. A wheel to take a 26" tyre is 559mm in diameter while a wheel to take a 700c tyre is 622mm.

Your choice of 2114mm would appear correct but the definitive way to measure this is:

1. Find a willing helper
2. Inflate tyres to correct value
3. Sit on bike and position the wheel with the valve stem at its lowest point
4. Have your helper mark this position on the ground.
5. Still sitting on the bike, roll forward for one full revolution of the wheel (about 2 metres)
6. Position the wheel with the valve stem at its lowest point
7. Have your helper mark this position on the ground.
8. Measure the distance between the marks in mm
9. This is your calibration value

Hi cdaly_.

My tyres were hard enough so I presume they were the correct pressure. I got about 80.5 inchs which is 2044.7 mm. Maybe I should have done it at least 3 times to be sure. Does it have to be an exact straight line?

cdaly_

An exact straight line would be best but you might get away with a bit of wobble. You were sitting on the bike when you did the rolling? Your body weight will affect the measurement so it will only be correct if you were on the bike.

Worztron

cdaly_ wrote: »

An exact straight line would be best but you might get away with a bit of wobble. You were sitting on the bike when you did the rolling? Your body weight will affect the measurement so it will only be correct if you were on the bike.

Yes, I was on the bike. I did the measurements alone with the help of a coin and biro. My result differs a lot from 2114mm. So would those chart measurements be approximates at best for all/most bikes?

Unknown Soldier

Does anyone run strava as an app on iphone etc and use a bike computer too? Are they closely matched for distance/avg speed?

I used to run two strava accounts for testing. One iPhone and one Garmin 500.
iPhone strava for May = 1031. Garmin = 1015

Since I added a Garmin cadence yoke last week, I have the ability to choose wheel size (Don't know if it was there before)
I took it off auto which was 2096 and having Conti 4000s @ 25 on the rear and 23 on the front, did that rolling wheel thing and so far using 2130 has been the closet at matching the Garmin to iPhone.


Anyone else use a bike computer and "Strava App"? If so are there discrepancies? If you have 25 on the bike what size did you put in the Garmin?

Just looked at the OP's pic and it's 2146

Worztron

I stayed with the initial setting of 2114mm from the manual and tested it this morn. Then used Google Maps to calculate the distance - it's pretty much spot on.

Worztron

On seconds thoughts, Google Maps did not pick the exact same route that I chose. Also, I went through a park on the way home.

These guys used slightly different methods:

• https://www.youtube.com/watch?v=5Ff6R4Io8Qs
• https://www.youtube.com/watch?v=ITJiguy-T3U

Are the manual settings a nonsense and should people only rely on manually measuring the wheel circumference?

G1032

Unknown Soldier wrote: »

Does anyone run strava as an app on iphone etc and use a bike computer too? Are they closely matched for distance/avg speed?

I used to run two strava accounts for testing. One iPhone and one Garmin 500.
iPhone strava for May = 1031. Garmin = 1015

Since I added a Garmin cadence yoke last week, I have the ability to choose wheel size (Don't know if it was there before)
I took it off auto which was 2096 and having Conti 4000s @ 25 on the rear and 23 on the front, did that rolling wheel thing and so far using 2130 has been the closet at matching the Garmin to iPhone.


Anyone else use a bike computer and "Strava App"? If so are there discrepancies? If you have 25 on the bike what size did you put in the Garmin?

Just looked at the OP's pic and it's 2146

I have a Garmin Edge 500. I use a manual wheel size input and it seems accurate. I was within a few hundred meters of all other cyclists recordings that I saw for the Westportif a few weeks ago. Happy with my settings.
I never thought to check against my android Strava app but I'll do that next time I'm out and let you know how it compares

G1032

Worztron wrote: »

On seconds thoughts, Google Maps did not pick the exact same route that I chose. Also, I went through a park on the way home.

These guys used slightly different methods:

• https://www.youtube.com/watch?v=5Ff6R4Io8Qs
• https://www.youtube.com/watch?v=ITJiguy-T3U

Are the manual settings a nonsense and should people only rely on manually measuring the wheel circumference?

I believe the automatic settings are nonsense.

G1032

ninoon wrote: »

Well explained above. Alternatively mark the tyre with thin line of tippex and roll out on black tarmac, then measure

Unknown Soldier wrote: »

Does anyone run strava as an app on iphone etc and use a bike computer too? Are they closely matched for distance/avg speed?

I used to run two strava accounts for testing. One iPhone and one Garmin 500.
iPhone strava for May = 1031. Garmin = 1015

Since I added a Garmin cadence yoke last week, I have the ability to choose wheel size (Don't know if it was there before)
I took it off auto which was 2096 and having Conti 4000s @ 25 on the rear and 23 on the front, did that rolling wheel thing and so far using 2130 has been the closet at matching the Garmin to iPhone.


Anyone else use a bike computer and "Strava App"? If so are there discrepancies? If you have 25 on the bike what size did you put in the Garmin?

Just looked at the OP's pic and it's 2146

Worztron wrote: »

I stayed with the initial setting of 2114mm from the manual and tested it this morn. Then used Google Maps to calculate the distance - it's pretty much spot on.

According to the chart in the OP the circumference for a 700x25 is 2146mm
So for every 5,000 revolutions of the wheel this will clock a distance 10.73km on a computer set with that circumference

I just did the Tipex method of measurement that ninoon suggested earlier. Did it 3 times on a 700x25 and came up with a circumference of 2104mm each time.
For every 5,000 wheel revolutions this will clock a distance of 10.52km

So we're getting a difference of 210m for each 5,000 revolutions which is quite a lot I think.
At a very rough estimate it'll give a difference of about 2km for every 100km cycled.

For anyone with a Garmin and a speed/cadence meter......... AFAIK the speed/cadence meter overrides the GPS as it's supposed to be more accurate (open to correction here). But it'll only be as accurate as the circumference inputted by the user. I had it set to 2126 which would have been out by approx 1km for every 100km cycled. I'll change now to 2104 and check tomorrow against the Strava Android app to see what the results are

Thanks to ninoon for suggesting the Tipex method

G1032

Just a quick update on this since Unknown Soldier got me thinking...

Went out the last evening and used both my phone and garmin to track distance.

Just cycled 22ish km

Strava app on phone recorded 22.3km cycled
Edge 500 recorded 22.25km cycled

Just a 50m difference in roughly 22km cycled which would roughly equate to just a 250m difference over 100km. Nothing in it really.

Worztron

I just did measurements again. 5 times to make more accurate. The results were (1 rotation).

Sitting on bike:
80.25"
80.25"
80"
80.5"
80.25" = 401.25 / 5 = 80.25

So that is 80.25 inches = 2,038.349 millimeters

Setting Value = 2038

---

Without sitting on bike:
81.75"
81.75"

Worztron

With the new bike computer setting of 2038, I noted the distance and then later checked it on Google Maps.

My bike computer = 1.38 km
GM = 1.4 km

Since it's hard to get exact points on GM, I will accept the small difference. Or maybe they round up/down and don't use 2 decimal points.