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Here's an odd ball (bike computers, tire circumference and distances)...

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Here's an odd ball (bike computers, tire circumference and distances)...

Old 10-22-13, 07:58 PM
  #26  
DX-MAN
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Dude...you need a hobby. Trying riding a bike or something.
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Old 10-22-13, 09:44 PM
  #27  
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From the OPs measurement of the unloaded tire I would deduct 4 or 5 mm and another 2mm to account for wiggling.
There are a couple meeasured highway zones here where I verify my calculations. I certainly don't accept more than
10 feet off.

This year I had the misfortune of buying a Bontrager speedo. They were apparently designed by car users. Only has tenths of a mile on the day trip. That's a fricking 176 yards !!!! How the hell do you verify distance with that ??? Can't even see that far down the road. Besides that it seemed to be recording low all the time. One day I was running both at once, I had it doing kms and my Sigma doing miles as usual. The avg was way different. The Bontrager is unbelievably complicated to set up.
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Old 10-23-13, 09:26 AM
  #28  
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Look at bicycle tire marks in the snow or after a rain or such, they are not a straight line. This means your distance made good does not equal distance travelled. The wiggle while riding can and does make a difference when compared to a "straight" line mileage.
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Old 10-23-13, 10:21 AM
  #29  
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Originally Posted by njkayaker View Post
You do like digging holes!

A circumference is a property of a circle. There isn't any real circle in a loaded tire.

The tire (in use) is a circle with a flat area under the hub touching the ground. The circular tire is deformed in use (so it isn't a circle any more).

What you are measuring is the radius of an imaginary circle.
This is correct. And if you have a rear wheel sensor the difference between the loaded radius apparent circumference and unloaded measured circumference is significant. I calibrate my computers by riding a known distance, survey pins in the centers of two intersections exactly one mile apart (at least I did back when I cared more).
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Old 10-23-13, 11:14 AM
  #30  
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Gentlemen and ladies,

Thanks for all the help, certainly a lot of info and knowledge to be gained.

I did a tire roll out measurement and then measured 3-4 revolutions and got a median out of those while I was in a normal riding position. The lucky number was 2125mm, smaller than Sheldon Brown suggestions (2155) and than my previous Google measurements (2132). Certainly a lot smaller than using masking tape around the tire and measuring that (2173)... I'll keep an eye on it and adjust it in the future if I deem necessary.

I feel as though allocating any more time to this issue would be a waste of time, besides I have to get back to life's previous engagements (such as doing the dishes).

All of you are more than welcome to keep the discussion going if you so decide, however I've found the answer to my original question.

Cheers and all the best
Be safe out there

D
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Old 10-23-13, 04:16 PM
  #31  
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Google may measure from a flat projected map. This means that uphills and downhills will be shorter distances than actually measured on the road. In other words, they measure one side of the triangle and you are riding the other longer one.
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Old 10-23-13, 05:54 PM
  #32  
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Originally Posted by Matariki View Post
Google may measure from a flat projected map. This means that uphills and downhills will be shorter distances than actually measured on the road. In other words, they measure one side of the triangle and you are riding the other longer one.
+1. GPS might also (largely) do the same.
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Old 10-23-13, 06:03 PM
  #33  
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Wow, I don't know why anyone would go through some of the gyrations above. Back when I had a computer I just put a mark on the pavement, started with the valve over it, and rolled in a straight line, leaning a bit on the bars, until the valve was down again. Such a method should result in less than a 1% error even with inflation variation. There's absolutely no point in measuring by mile markers, as you don't know how accurately they were set in place. The computer on the other hand computes exactly the correct distance for the rollout provided to it.
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Old 10-23-13, 07:09 PM
  #34  
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Originally Posted by cny-bikeman View Post
There's absolutely no point in measuring by mile markers, as you don't know how accurately they were set in place.
+1 I've seen rather a lot of variation between adjacent "mile markers" when attempting to calibrate my car odometers years ago. The odometer distance would vary noticeable over successive 10 mile intervals. I did once calibrate a cyclometer over a TAC certified 10K running course and it agreed within about 10' over 6.21 miles so I was happy.
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Old 10-24-13, 05:54 PM
  #35  
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Originally Posted by HillRider View Post
+1 I've seen rather a lot of variation between adjacent "mile markers" when attempting to calibrate my car odometers years ago.
This is caused by a geophysical phenomenon known as "gravity waves". These below-the-surface waves concentrate gravity in some areas by pulling it from others. The effect on the mile markers is to reduce the distance between them in gravity dense areas and spread them apart in the other areas. Because the waves are dynamic, the inter-sign distances may change day to day.

An interesting statistic: Most Club Tombay members live in areas prone to gravity waves.

Last edited by Matariki; 10-24-13 at 05:59 PM.
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