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Discussion Starter · #1 ·
Anyone have a quick/easy tool or formula for calculating tire circumference?

Don't know how I didn't realize it sooner, but the prior owner had put a 160-60-17 on the rear on my bike, trying to figure out how exactly that might be affecting the ride and speedo etc.
 

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The circumference of a circle can be found by multiplying pi ( π = 3.14 ) by the diameter of the circle.

Your speedometer shouldn't be affected since it's driven off the front hub. Your RPM's vs speed might be affected though.
 

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Put a chalk mark on your tire sidewall, and a matching mark on the driveway [point 'A']. Roll the bike forward til the tire chalk mark is once again perpendicular to the pavement [point 'B']. Measure the distance from point "a" to point "b". That is the tire's circumference.

Since all tire manufacturer's tire casings are so vastly different, even within identical sizing, this method is the easiest way to get a handle on tire circumference between different tires/brands.

Only problem with your situation, is that you might have trouble finding out the circumference of the original tire.....perhaps an email to brand X's customer service dept. is in order.
 

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Put a chalk mark on your tire sidewall, and a matching mark on the driveway [point 'A']. Roll the bike forward til the tire chalk mark is once again perpendicular to the pavement [point 'B']. Measure the distance from point "a" to point "b". That is the tire's circumference.

Since all tire manufacturer's tire casings are so vastly different, even within identical sizing, this method is the easiest way to get a handle on tire circumference between different tires/brands.
your high-school math teacher is crying right now, you jerk! :green_lol:
 

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2 pie R is the formula. 2 times pie (3.141593) times the radious of the object.

It may effect the speedo a bit, but it is already 10% off so a little more isn't going to mean much. This will depend if you are riding a wee or a Vee.
The ride qualities are probably just fine, but that tire may effect the handling some.
 

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Discussion Starter · #6 ·
I might be missing something here, but the original tire is the Bridgestones, which run 150/70-17 at the rear. The current Dunlop Sportmax is a 160/60-17.

As to the weeping of a high school teacher, I can attest to the fact that most tires are a little off-size. I have measured bicycle tires over the years and found that given makers tend to err on one side or another relatively consistently... Contis are always tough to jam onto a bicycle rim, for example.

Any idea of how the altered tire profile might affect handling?
 

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The stock 150 is (nominally) 105 mm tall (rim to tread, roughly), the 160 is only 96 mm tall, so your radius is reduced by 9 mm. The wider tire will be a bit slower to lean, but I wouldn't imagine much impact on handling.
 

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2 pie R is the formula. 2 times pie (3.141593) times the radious of the object.
You math majors kill me:
Please explain the difference between "2pieR" and "Dpie" as stated in an earlier post?????

Acually the marks on the ground way is the only way to get it right because the tire flexes (flat spot on bottom) unless you get involved with a large calculus equation.
 

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your high-school math teacher is crying right now, you jerk! :green_lol:
The only thing I learned from math class was WWII war stories. He was shot down on a bomber run and spent the rest of the war in German hospitals and POW camps. Those tales beat ANY variation of Pi calculations.

Math? We don't need no steenking math.
 

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If you want the actual circumference then use the method from post #3 as it will be the most accurate.

however.... for a wee front tyre 110/80 19

The tyre diameter (19) is actually the rim diameter so you can't use that.
The 110 is the width of the tyre in mm
The 80 is the aspect ratio of width to height as a percentage.

So for the front wee the 110/80 size means that it is 110 mm wide and 80% of 110, high

So theoretically a 110/80 19 tyre is .....

Rim = 19 inch
Tyre height = 0.80 x 110 mm = 88 mm = 3.46 inches

Thereforr the overall diameter is 19 + 2 x 3.46 = 25.92

Therefore the circum is 81.43 inches.

However the measurements vary greatly, so roll it along the ground.
 

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TM is the answer. Not only low-tech, but even used by truckers.
TM = Tape Measure (not Transcendental Meditation).

Or perhaps I'm just missing the joking in this thread.
.
 

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If you want the actual circumference then use the method from post #3 as it will be the most accurate.

however.... for a wee front tyre 110/80 19

The tyre diameter (19) is actually the rim diameter so you can't use that.
The 110 is the width of the tyre in mm
The 80 is the aspect ratio of width to height as a percentage.

So for the front wee the 110/80 size means that it is 110 mm wide and 80% of 110, high

So theoretically a 110/80 19 tyre is .....

Rim = 19 inch
Tyre height = 0.80 x 110 mm = 88 mm = 3.46 inches

Thereforr the overall diameter is 19 + 2 x 3.46 = 25.92

Therefore the circum is 81.43 inches.

However the measurements vary greatly, so roll it along the ground.
Oh NEGATORY BIG BEN. The aspect ratio only gets you to the outer edge of the sidewall. The center of the tires diameter is greater than that of course. How come nobody thought to just take a frickin tape measure and wrap it around a tire? Sheesh.
 

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Oh NEGATORY BIG BEN. The aspect ratio only gets you to the outer edge of the sidewall. The center of the tires diameter is greater than that of course. How come nobody thought to just take a frickin tape measure and wrap it around a tire? Sheesh.
Nope,

I've double checked and I was right. I've checked several sites and all say the same. Here is one with pictures...

DennisKirk.com - Tire Sizes Explained
 

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The manufacturers & tyre sites state the nominal sizes - which is okay for a very rough approximation.

But if you want the actual size, you'll have to measure it.
Tape measure to the rescue.
[ My original front Bridgestone radial TrailWing - I think that was the name - measured 105 mm in width, despite its nominal 110/80 size. Sorry, I haven't bothered to measure any circumferences. ]
.
 

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Here is what I have always done with bicycle tires (all bike MPH computers need the circumference and, as mentioned, it is bit different model to model):

Put a small grease dot on the tire. Sit on the bike. Roll the bike straight of level concrete. Measure the distance between the grease dots you just left.

This method takes into account tire flex, etc. If you want to be more accurate roll the bike farther, measure the distance between grease dots that represent two rotations. Measure and divide by two.
 
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