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Clydesdales/Athenas (200+ lb / 91+ kg) Looking to lose that spare tire? Ideal weight 200+? Frustrated being a large cyclist in a sport geared for the ultra-light? Learn about the bikes and parts that can take the abuse of a heavier cyclist, how to keep your body going while losing the weight, and get support from others who've been successful.

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Old 07-06-11, 06:55 PM   #1
nutmegTN
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question on bike computer setup

I am trying to set up my new computer for the proper wheel size and am having some difficulty.

I have a Trek 700, which has 700x35 tires, and (I know you clydes will laugh) a 13.5" frame. (The frame size is irrelevant but it just makes me laugh to think of all you 6' plus guys and this shrimpy frame.)

The computer's instructions for the 'wheel size input' say:
Multiply the wheel diameter (d) in millimeters by 3.1416 to determine the wheel factor (c).
Then it provides a table for wheel factor (in case you don't know how to multiply)

Wheel ---------------Wheel
Diameter (d) -------- Factor (c)
26" (650A) ---------2073
26.5" (Tubular) ---------2117
26.6" (700x25c) --------- 2124
26.8" (700x28c) --------- 2136
27" (700x32c) ---------2155
28" (700b) ---------2237

(w/tire)
ATB 24x1.75 --------- 1888
ATB 26x1.4 --------- 1995
ATB 26x1.5 --------- 2030
ATB 26x1.75 --------- 2045
ATB 26x2 (650B) ---------2099
27x1 ---------2136
27x1 1/4 ---------2155


Now, my problem is that when I multiply my wheel diameter, which the LBS told me is 35mm, I get 109.956 which does not seem to be a high enough number to be correct.

I then went with the high probability that the LBS was wrong and tried using 700 instead. That yields 2199.12, which seems more likely to be the correct answer.

So, is 2199 the right answer? and why isn't "700x35" on the damn chart????

Last edited by nutmegTN; 07-06-11 at 06:58 PM. Reason: make table more readable
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Old 07-06-11, 07:09 PM   #2
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Your wheel diameter is _not_ 35mm. That's probably the tire width. If it was the diameter, your wheels would be about 1.5" tall...

Best bet - fill your tires to correct pressure, and use a tape measure (metric if you can, if not, standard and convert) to measure from the ground to the very center of the wheel axle. That's your radius. Double it, to get the diameter. Convert to millimeters (mm), and multiply by pi (3.14159). There's your wheel factor.

Even more accurate is to get a long, flexible ruler (like a tailor's tape measure) and wrap it tightly around the wheel (probably easiest to take the wheel off the bike). Convert that length to mm, and there's your wheel factor (don't multiply by pi!).
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Old 07-06-11, 07:34 PM   #3
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Wow. I really exposed my ignorant American status, didn't I? I had not a clue how big a millimeter is. If 35 of them are only 1.5" they are pretty d*mn small though.

My radius, measured with a tape measure to the middle of the bolt holding the rear tire on is 13.75".
13.75" times 2 gives a diameter of 27.5".
27.5" times 25.4 is 698.5mm.
698.5mm times 3.1416 is 2194.4.

So I should use 2194 instead of 2199 for the 'wheel factor?'
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Old 07-06-11, 07:48 PM   #4
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I'd say to use 2168. One chart I found http://www.ergomousa.com/default/ass...cumference.pdf ..

But in reality the best way overall is this. Setup your bike with the valve at a good reference point. Mark a location on the ground. With your weight on the bike move forward one tire rotation (use the reference point to get the valve at the same point). Mark this location.

Measure the distance between marks (if in inches multiple by 25.4) - that will give you the most accurate circumference.

Now all being said 2168 vs 2199 is about .05% difference... Not all that big of a difference overall... any of these numbers will work really well overall.
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Old 07-06-11, 07:57 PM   #5
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Alphageek's measurement suggestion is very good. However, the difference between 2168 vs 2199 is about 1.4% (.014) by my calculation. That's over a mile when doing a century (not that I'm doing centuries ).

Based on what you did, yeah 2194 is correct. If you really want accuracy, make sure you measure very precisely (to the 1/16" or even 1/32" or 1/64" of an inch). Any error in measuring the radius gets multiplied by 6+ by the time you get to the "wheel factor" (which is just the tire circumference). So if you are off 1/16" in measuring the radius, your circumference will be off by almost 1/4" - not huge, but when measuring over many miles, it adds up
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Old 07-06-11, 08:07 PM   #6
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This is what you want to measure. Try it with out your weight and
with and see how much difference there is and split the difference.

as in: 2188/2168, use 2178.

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Old 07-06-11, 08:10 PM   #7
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2168? 2194? 2199??? But which one is right? They can't all be right.

edit to add: ok, I see I'll have to do the rolly thing to get the true circumference. Seems like an awful lot of trouble.

Last edited by nutmegTN; 07-06-11 at 08:25 PM.
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Old 07-07-11, 08:24 AM   #8
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nutmegTN,

when in doubt or everytime just go to sheldon brown's website, you will find everything you need to know.

here is a specific part about calibrating your bike computer and it looks to have all the popular brands and models.

http://www.sheldonbrown.com/cyclecom...libration.html
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Old 07-07-11, 08:38 AM   #9
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Thanks everyone.

I'm going with 2168 for now, but I don't like the uncertainty of this whole thing. Still, it will be plenty close enough for my purposes.

Now for the next challenge. I have to install the wires, sensor, and magnet...
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Old 07-07-11, 11:49 AM   #10
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Quote:
Originally Posted by jgalak View Post
Alphageek's measurement suggestion is very good. However, the difference between 2168 vs 2199 is about 1.4% (.014) by my calculation. That's over a mile when doing a century (not that I'm doing centuries ).
Doh.. yeah I don't know what I did there, but it is about 1.4%

As for the OP and "uncertainty"... I wouldn't be too worried.. If you do a century you could be off by about a mile, but the other way to look at it for those not doing that distance - you could be off by as much as .1 mile for every ten miles you ride (or in otherwords miss counting about a city block every ten miles in the worst case).

For most of us, thats not too big a deal. If you have a phone with a gps in it or a gps you could double check on a run and tweak if that doesn't line up well (or use markers on a trail to see if you have one near).
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Old 07-07-11, 12:00 PM   #11
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Quote:
Originally Posted by nutmegTN View Post
2168? 2194? 2199??? But which one is right? They can't all be right.

edit to add: ok, I see I'll have to do the rolly thing to get the true circumference. Seems like an awful lot of trouble.
Even if I'm replacing with the same model of tire, I re-calibrate my computer with a 3x rollout average just to be sure. It's not really that much of a hassle to bust out a tape measure for 5 minutes.
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Old 07-07-11, 02:40 PM   #12
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Quote:
Originally Posted by nutmegTN View Post

Now, my problem is that when I multiply my wheel diameter, which the LBS told me is 35mm, I get 109.956 which does not seem to be a high enough number to be correct.

I then went with the high probability that the LBS was wrong and tried using 700 instead. That yields 2199.12, which seems more likely to be the correct answer.

So, is 2199 the right answer? and why isn't "700x35" on the damn chart????
No.

"700C" tires have a bead seat diameter of 622mm.

Assuming accurately stated width (manufacturers sometimes lie because a 25mm wide tire that actually measures 23mm is lighter than a competitor's 25mm tire so it looks better in parts catalogs) and approximately equal tire width and height (even clinchers have a roughly circular cross-section), 700 x 35 tires would have a diameter of about 622 + 35 * 2 = 692mm and unloaded roll-out of about 2174mm.

Actually measuring is obviously better.

Last edited by Drew Eckhardt; 07-07-11 at 02:43 PM.
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Old 07-07-11, 02:45 PM   #13
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You won't go to far wrong if you use "27" (700x32c) ---------2155" from the chart.

This ain't rocket science so it doesn't need to that damn precise to work.
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Old 07-07-11, 03:55 PM   #14
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Quote:
Originally Posted by CliftonGK1 View Post
Even if I'm replacing with the same model of tire, I re-calibrate my computer with a 3x roll out average just to be sure. It's not really that much of a hassle to bust out a tape measure for 5 minutes.
This is the most accurate measurement that you can get (3x roll out)
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Old 07-07-11, 04:27 PM   #15
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I was confused when I bought my computer and had the LBS install it for me. $10 later and 20 minutes I was rolling away happy and not frustrated. That seemed to be the best way to handle the situation for me.
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Old 07-07-11, 08:49 PM   #16
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I'd rather do it myself so I know it is right. And in this case I can pick how inaccurate I want to be.

I rode with my computer for the first time today and it seemed to work fine. When I head to the bike trail I'll check it again the mile markers and see how close it comes.

I'm a little worried because the zip ties provided with the computer don't seem to stay very tight. I think the sensor is going to shift around and get out of range of the magnet. Maybe I need a better quality zip tie?
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Old 07-07-11, 09:58 PM   #17
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Quote:
Originally Posted by nutmegTN View Post
I'd rather do it myself so I know it is right. And in this case I can pick how inaccurate I want to be.

I rode with my computer for the first time today and it seemed to work fine. When I head to the bike trail I'll check it again the mile markers and see how close it comes.

I'm a little worried because the zip ties provided with the computer don't seem to stay very tight. I think the sensor is going to shift around and get out of range of the magnet. Maybe I need a better quality zip tie?
If slippage is a problem, I use a dot of hot glue under the sensor and when it sets, I add the zip ties. It doesn't move.
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Old 07-08-11, 11:32 PM   #18
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If you back calculate, using the 700 x 32 of 2155. Divide 2155 by PI and the result is 686mm diameter.
Subtract the 700c rim diameter that is 622mm you get 64mm
Divide the 64 by 2 and you get 32mm tire size.

If you work this forward
Multiply your 35mm tire size by 2 to get 70mm add this to the rim diameter 622mm and you get 692mm as the total diameter.
Now multiply by pi and the answer is 2174 this is the wheel factor.
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Old 07-09-11, 07:05 AM   #19
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2174, 2168, 2199, 2194 ......

So far two of you have said 2174. That sounds like the one to use if I don't want to do the roll out thing.

The great thing about having this computer is that I no longer have to map every little ride on mapmyride.com to find out how far I went.
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Old 07-09-11, 02:17 PM   #20
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nutmeg:

You have spent way more time worrying about it and posting than
it would have taken to do the "rollie" thing!
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Old 07-10-11, 08:38 AM   #21
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nutmeg:

You have spent way more time worrying about it and posting than
it would have taken to do the "rollie" thing!
That's always been my problem: too much thinking, not enough action.
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