# Bike Computer Tire Calculation

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**Bike Computer Tire Calculation**

I have a Sigma BC 8.12 hard wired bike computer that I just installed on my two wheel 'bent which has a 20" front tire and 26" rear tire.

According to the wheel chart that came with the computer it only has one setting for a 20" tire and the problem is that the wheel calculation is for a 20" 1.75" tire and my tire is a 20" 1.5" tire. The manual states to enter "1580" for the 20" 1.75" wheel. Since my wheel is a 1.5" how much off will it be? Are we talking about less than 1 mph at say 25mph?

Is anyone here really good at math and can do the calculation for the correct setting for the 1.5" tire?

According to the wheel chart that came with the computer it only has one setting for a 20" tire and the problem is that the wheel calculation is for a 20" 1.75" tire and my tire is a 20" 1.5" tire. The manual states to enter "1580" for the 20" 1.75" wheel. Since my wheel is a 1.5" how much off will it be? Are we talking about less than 1 mph at say 25mph?

Is anyone here really good at math and can do the calculation for the correct setting for the 1.5" tire?

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There isn't just one factor for a given size tire, unfortunately. It depends on tread (how new the tire is), tire pressure, and tire load.

If you live where there are measured mile markers, you could put in the 20x1.75 factor and go ride a measured mile, noting beginning and ending odometer readings. You can then figure out how far you were off and put in the corrected value. It's not hard to get it right to within 0.5%.

If you live where there are measured mile markers, you could put in the 20x1.75 factor and go ride a measured mile, noting beginning and ending odometer readings. You can then figure out how far you were off and put in the corrected value. It's not hard to get it right to within 0.5%.

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at what inflation? just kidding. you can research doing a "roll-out" or just use the number you have. how important is accuracy, so long as it is consistent, ride-to-ride?

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A "

The bike computer counts revolutions using the magnet and sensor, and times these. So the circumference tells it how far each turn of the wheel moved, giving distance and speed.

This is easier with two people. roll the wheel until the label or the valve stem is at it's lowest point. Mark the ground there--I use a piece of tape. Roll the bike forward -- I do two full revolutions. Mark the ending point.

Measure with a tape measure. To convert inches to mm, use a google search like this example: 160 1/4 inches in mm. It shows 4070.35. Then divide by two for the two revolutions: 4070 / 2 = 2035mm.

It works pretty well with just one revolution, a bit less accurate, but likely good enough. With your 20 inch tire, two or three revolutions should be easy to do.

I've seen recommendations to use your weight sitting on the bike to press down on the tire. I don't usually see much of a difference, and this isn't too easy to do.

~~~

The table shows 1580 for the 20 x 1.75. If you get 1524 for example, that's 1524/1580 = .964 or a little more than 3% difference.

**rollout**" is often more accurate than the tables (by a few percent, usually). The wheel is rolled along a flat surface and the distance measured. Those numbers (usually somewhat near 2000 for a 700c road bike) are the circumference in mm.The bike computer counts revolutions using the magnet and sensor, and times these. So the circumference tells it how far each turn of the wheel moved, giving distance and speed.

This is easier with two people. roll the wheel until the label or the valve stem is at it's lowest point. Mark the ground there--I use a piece of tape. Roll the bike forward -- I do two full revolutions. Mark the ending point.

Measure with a tape measure. To convert inches to mm, use a google search like this example: 160 1/4 inches in mm. It shows 4070.35. Then divide by two for the two revolutions: 4070 / 2 = 2035mm.

It works pretty well with just one revolution, a bit less accurate, but likely good enough. With your 20 inch tire, two or three revolutions should be easy to do.

I've seen recommendations to use your weight sitting on the bike to press down on the tire. I don't usually see much of a difference, and this isn't too easy to do.

~~~

The table shows 1580 for the 20 x 1.75. If you get 1524 for example, that's 1524/1580 = .964 or a little more than 3% difference.

*Last edited by rm -rf; 11-10-21 at 03:29 PM.*

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If you ride with your phone, you can use Strava a few times and compare the distance reported by Strava and your Sigma. Strava also has a distance correction function.

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I usually do a roll out. I lineup the valve dead up/down, mark the floor with a piece of chalk, roll the bike so the wheel makes a complete revolution, mark the floor where the valve is, measure and enter that info. It's not 100%, some folks state you need weight on the tire to emulate a rider on the bike compressing the tire, but jeez, it's close enough.

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A rollout as previously described is the best way IMO. I use a blob of paint from a paint pen on my tire and simply ride it down my concrete driveway. Then I measure the distance between the marks on the concrete. Then if you measure in inches, then convert to millimeters. Google is your friend here if you are mathematically challenged.... inches to mm

If you want to calculate it, then it'll only be a rough estimate as two different model tires of the same printed size are unlikely to be the same. Especially if one is knobby tread and the other little to no tread. But find the ISO or ETRTO size of the tire. Usually printed or embossed in the tiny print on the sidewall. A 20 x 1.5" tire is a rare tire according to Sheldon Brown's tire sizing page and since this is a recumbent, it might a 406 BSD tire and not the same BSD of other 20" tires.

Once you know the ISO size the know that tire width is also about the same as the tire height from the bead seat. The ISO size is in mm. Don't use the inch sizes at all, they lie. The larger number is your tires BSD in mm and the smaller the tire width in mm.

So the formula for your circumference is:

((2 x tire width) + BSD) x 3.14 = your correction factor.

If you want to calculate it, then it'll only be a rough estimate as two different model tires of the same printed size are unlikely to be the same. Especially if one is knobby tread and the other little to no tread. But find the ISO or ETRTO size of the tire. Usually printed or embossed in the tiny print on the sidewall. A 20 x 1.5" tire is a rare tire according to Sheldon Brown's tire sizing page and since this is a recumbent, it might a 406 BSD tire and not the same BSD of other 20" tires.

Once you know the ISO size the know that tire width is also about the same as the tire height from the bead seat. The ISO size is in mm. Don't use the inch sizes at all, they lie. The larger number is your tires BSD in mm and the smaller the tire width in mm.

So the formula for your circumference is:

((2 x tire width) + BSD) x 3.14 = your correction factor.

*Last edited by Iride01; 11-10-21 at 05:19 PM.*

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I got tired of doing rollouts so I switched to the old tried and true: Circumference = pi x diameter.

With the bike in a vertical position, measure straight up from the floor to the center of your axle in millimeters. That is your radius. Multiply by 2 to get your diameter then multiply that by 3.14 to get your circumference. The result will be very close and you can adjust it from there depending on your level of OCD.

With the bike in a vertical position, measure straight up from the floor to the center of your axle in millimeters. That is your radius. Multiply by 2 to get your diameter then multiply that by 3.14 to get your circumference. The result will be very close and you can adjust it from there depending on your level of OCD.

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The roll out method is obviously way better than relying on what is printed on the spec sheet. When I did it for the 20" front tires on my recumbent trike the result was 1442. The spec sheet shown above suggests 1580 for a 20" ISO 406 tire. That's nearly 10 percent off my correct input. If you want a more accurate way than the blob of paint on the wheel, tie a narrow piece of cord around the tire and apply some colored liquid such as India ink or food coloring and immediately ride the bike a couple revolutions of the wheel on a cement sidewalk. I generally take the average of three segments and use that as my input. It hasn't changed noticeably as the tire wears since the circumference of the tire (that's what you are measuring) doesn't change enough to make any difference.

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BIG WHEEL: you cannot rely on charts for the best accuracy BECAUSE TREADTHICKNESS VARIES A GREAT DEAL FROM TIRE MODEL TO TIRE MODEL, EVEN WITHIN THE SAME BRANDS FOR THE EXACT SAME TIRE SIZE!!! Yeah, the charts will get you CLOSE in MOST CASES, that the difference probably might be immaterial, but be forewarn that you won't likely be nearly as accurate as using 2 pi r , or doing a dummy style measurement from marks on your living room's hardwood floor.

Why not try going with 2 pi r ???? IT IS SUPER SIMPLE AND IT IS ACCURATE.

r is RADIUS , which is just your MEASUREMENT FROM THE CENTER OF THE WHEEL TO THE OUTER TREAD OF YOUR BIKE'S TIRE....simple right, yep!

HERE IS A SIMPLE EXAMPLE:

Lets say Bubba measures exactly 14 1/4 inches or 14.25 inches from the CENTER to THE OUTER TREAD....... converting to millimeters, oh well according to GOOGLE's calculation, 14.25 inches = 361.95mm ................SO YOUR RADIUS is 361.95mm or 14.25 inches......you measured with a tape measure to get the RADIUS. Okay simple enough.....you've got that!!

You remember Pi from your school math class forty five years ago right.................Pi = 3.14 but the decimal placement is said to go on to infinity but 3.14 is Pi with just two decimal places......... but you could express Pi as 3.141592653589793 if you wanna go out fifteen decimal places

2 x pi

2 x 3.141592653589793 = 6.28318530718

So you see that 2pi = 6.28318530718

YOU REMEMBER THAT THE FORMULA IS 2 pi r which translates to 2pi TIMES radius , or 2pi X r

Remember that we know that our measured radius is 361.95mm

r = 361.95

So doing the formula calculation 2 pi r which is 2pi TIMES radius or 6.28318530718 X 361.95

okay 6.28318530718 X 361.95 = 2274.19892193

so then 2274mm IS OUR CALCULATED TIRE CIRCUMFERENCE FOR THE SPEEDOMETER.

No joke, it is much simpler than my lengthy example as if I was trying to explain it to 4th Grade ten year old kids.

MEASURE the RADIUS.

(6.28318530718 ) is the figure you can use for 2pi

Multiply your measured RADIUS figure X 6.28318530718

Obviously, you want your RADIUS figure to be expressed in millimeters, so depending on what unit of measurement that you did choose to get a reading with the tape measure, you'll want that figure converted to exact amount in millimeters for your calculation

Again it is simply this:

Your measured RADIUS figure X 6.28318530718 = Speedo's needed wheel circumference in mm

this is " TWO PIE ARE " and it is extremely accurate assuming your wheel is round and your Radius measurement is good.

As you know, it shouldn't matter one damn bit as to where you measure the RADIUS as in a true circle or perfect wheel, every location would be exactly the same.

Your wheel should be round enough that it shouldn't have any material differences as tiny variations should be negligible and not significant at all if your wheel is anywhere close to true.

Why not try going with 2 pi r ???? IT IS SUPER SIMPLE AND IT IS ACCURATE.

r is RADIUS , which is just your MEASUREMENT FROM THE CENTER OF THE WHEEL TO THE OUTER TREAD OF YOUR BIKE'S TIRE....simple right, yep!

HERE IS A SIMPLE EXAMPLE:

Lets say Bubba measures exactly 14 1/4 inches or 14.25 inches from the CENTER to THE OUTER TREAD....... converting to millimeters, oh well according to GOOGLE's calculation, 14.25 inches = 361.95mm ................SO YOUR RADIUS is 361.95mm or 14.25 inches......you measured with a tape measure to get the RADIUS. Okay simple enough.....you've got that!!

You remember Pi from your school math class forty five years ago right.................Pi = 3.14 but the decimal placement is said to go on to infinity but 3.14 is Pi with just two decimal places......... but you could express Pi as 3.141592653589793 if you wanna go out fifteen decimal places

2 x pi

2 x 3.141592653589793 = 6.28318530718

So you see that 2pi = 6.28318530718

YOU REMEMBER THAT THE FORMULA IS 2 pi r which translates to 2pi TIMES radius , or 2pi X r

Remember that we know that our measured radius is 361.95mm

r = 361.95

So doing the formula calculation 2 pi r which is 2pi TIMES radius or 6.28318530718 X 361.95

okay 6.28318530718 X 361.95 = 2274.19892193

so then 2274mm IS OUR CALCULATED TIRE CIRCUMFERENCE FOR THE SPEEDOMETER.

No joke, it is much simpler than my lengthy example as if I was trying to explain it to 4th Grade ten year old kids.

MEASURE the RADIUS.

(6.28318530718 ) is the figure you can use for 2pi

Multiply your measured RADIUS figure X 6.28318530718

Obviously, you want your RADIUS figure to be expressed in millimeters, so depending on what unit of measurement that you did choose to get a reading with the tape measure, you'll want that figure converted to exact amount in millimeters for your calculation

Again it is simply this:

Your measured RADIUS figure X 6.28318530718 = Speedo's needed wheel circumference in mm

this is " TWO PIE ARE " and it is extremely accurate assuming your wheel is round and your Radius measurement is good.

As you know, it shouldn't matter one damn bit as to where you measure the RADIUS as in a true circle or perfect wheel, every location would be exactly the same.

Your wheel should be round enough that it shouldn't have any material differences as tiny variations should be negligible and not significant at all if your wheel is anywhere close to true.

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Good advice on roll-outs.

My favorite is to put a dot of paint on the tire and roll or ride the bike in a straight line. I usually get two dots on the pavement and just measure between the dots. I've even used catsup instead of paint. .

My favorite is to put a dot of paint on the tire and roll or ride the bike in a straight line. I usually get two dots on the pavement and just measure between the dots. I've even used catsup instead of paint. .

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You asked the question is it within 1 mph at 25 mph? That is a 4 percent error tolerance, you are likely within 4 percent for speed which many would find good enough.

Many bike computers round off to the nearest half mph. Some round up to the nearest half mph, thus if you are going 14.1 mph some computers would read 14.5. So, computers already have a lot of error built into the readings.

But most people would want better precision for distance traveled. If you rode 26 miles and it said you only rode 25, would that be good enough? You decide. If you only look at it for speed and not distance, then distance error does not matter to you.

I use a tape measure and measure starting with valve at lowest point, as described above. There are 25.4mm per inch.

I like to keep track of my longer rides to the nearest tenth of a mile. Thus, I want more precision. But, I am an engineer, and engineers like precision.

Many bike computers round off to the nearest half mph. Some round up to the nearest half mph, thus if you are going 14.1 mph some computers would read 14.5. So, computers already have a lot of error built into the readings.

But most people would want better precision for distance traveled. If you rode 26 miles and it said you only rode 25, would that be good enough? You decide. If you only look at it for speed and not distance, then distance error does not matter to you.

I use a tape measure and measure starting with valve at lowest point, as described above. There are 25.4mm per inch.

I like to keep track of my longer rides to the nearest tenth of a mile. Thus, I want more precision. But, I am an engineer, and engineers like precision.

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If the chart says to put in 1580, subtract about 25 from that and that's about what what their chart would say if it had a spot for 20x1.5. Catsup and a tape measure, riding your bike at the tire pressure you run might give you a better number. Your real number might be more like 1500.

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Small wheel issue

My Magene sensors for my 20" cumbie that I have set up as a stationary trainer , give speed and distance wildly different from my PowerTab hub (Pro+)

This was brought out when I used the https://www.bikecalc.com/speed_at_cadence

It correlates well with the Powertap, but is 20% off with Magene using a Karoo K1 head unit.

The tire is Kenda 20x1.5 that is C/W a Kwest but not model labeled.

Measured circumference with calibrated tape was 154 cm. (1540mm)

Charts all seem to have been copied from an unknown source give 1490, except Cateye which uses 1513.

I have Lots of variables still to work out, but I was wondering if small wheel folks (cumbies and foldies) were having an issue with speed and distance accuracy?

As tire/wheel circumference may be off by a enough to make a difference.

Doubt if foldy folks would go down this rabbit hole,

but cumbie trainers are more likely to have seen this, if it is a real thing.

My Magene sensors for my 20" cumbie that I have set up as a stationary trainer , give speed and distance wildly different from my PowerTab hub (Pro+)

This was brought out when I used the https://www.bikecalc.com/speed_at_cadence

It correlates well with the Powertap, but is 20% off with Magene using a Karoo K1 head unit.

The tire is Kenda 20x1.5 that is C/W a Kwest but not model labeled.

Measured circumference with calibrated tape was 154 cm. (1540mm)

Charts all seem to have been copied from an unknown source give 1490, except Cateye which uses 1513.

I have Lots of variables still to work out, but I was wondering if small wheel folks (cumbies and foldies) were having an issue with speed and distance accuracy?

As tire/wheel circumference may be off by a enough to make a difference.

Doubt if foldy folks would go down this rabbit hole,

but cumbie trainers are more likely to have seen this, if it is a real thing.

*Last edited by bikebikebike; 10-07-22 at 09:48 PM.*

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I'd use the 1540; that's what you measured. You can tweak the number any way you want to match whatever else you think is more accurate.

The problem with charts is 20" wheels have two different sizes: 406 and 451 and some generic 20" size that I don't know the erto for. Add in different widths and the circumference varies widely.

The problem with charts is 20" wheels have two different sizes: 406 and 451 and some generic 20" size that I don't know the erto for. Add in different widths and the circumference varies widely.

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