Calibrating Computer - I Hate Math
11-18-07 | 05:52 PM
#1
Thread Starter
Sassbucket.
Joined: Oct 2007
Posts: 114
Likes: 1
From: los angeles, ca
Calibrating Computer - I Hate Math
I just picked up my first computer. I need to figure out the circumference of my wheel to calibrate it, and It's giving me a bit of trouble. My wheel is 700 x 25.
Here's the formula I followed, as per the manual:
distance from floor to hub: 13 3/8" (13.375)
multiply by 25.4 to convert to mm (339.725)
multiply that number by 6.28 (2 x pi) = 2133.473
I had entered 2105 based on Sheldon Brown's recommendation for a 700 x 25 wheel. But this is a pretty substantial difference. When I entered 2133, it seemed like the miles were racking up really quickly. I'll be the first to admit that I don't have the best sense of distance, so I could be wrong.
Any ideas?
Here's the formula I followed, as per the manual:
distance from floor to hub: 13 3/8" (13.375)
multiply by 25.4 to convert to mm (339.725)
multiply that number by 6.28 (2 x pi) = 2133.473
I had entered 2105 based on Sheldon Brown's recommendation for a 700 x 25 wheel. But this is a pretty substantial difference. When I entered 2133, it seemed like the miles were racking up really quickly. I'll be the first to admit that I don't have the best sense of distance, so I could be wrong.
Any ideas?
11-18-07 | 05:59 PM
#2
Senior Member
Joined: Oct 2002
Posts: 8,952
Likes: 1,513
From: Southern Maine
And now for something to really mess with your head.... tires companies
do no provide precise size measurements.
Try a number, use it for a while, it will seem high or low. So change it a bit and
see if you can get closer. You can also try to measure by seeing how far the tire actually rolls. I found that more trouble than it was worth. I just want to be close, and if the mileage reads a little on the generous side, I am not one to complain.
do no provide precise size measurements.
Try a number, use it for a while, it will seem high or low. So change it a bit and
see if you can get closer. You can also try to measure by seeing how far the tire actually rolls. I found that more trouble than it was worth. I just want to be close, and if the mileage reads a little on the generous side, I am not one to complain.
11-18-07 | 06:06 PM
#3
Thread Starter
Sassbucket.
Joined: Oct 2007
Posts: 114
Likes: 1
From: los angeles, ca
I actually measured my wheel with a tape measure. As you said, I know not to trust what the tire says!
The reason I'd like to get pretty accurate is because I'm so bad with distances. I really have nothing to base myself on to judge if I'm too high or too low.
The reason I'd like to get pretty accurate is because I'm so bad with distances. I really have nothing to base myself on to judge if I'm too high or too low.
11-18-07 | 06:30 PM
#4
Senior Member
Joined: Sep 2006
Posts: 1,195
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Bikes: Kona Cinder Cone, Sun EZ-3 AX
I had entered 2105 based on Sheldon Brown's recommendation for a 700 x 25 wheel. But this is a pretty substantial difference. When I entered 2133, it seemed like the miles were racking up really quickly. I'll be the first to admit that I don't have the best sense of distance, so I could be wrong. Any ideas?
f you require more accuracy, you can do a "roll-out" test. Since the effective tire size is affected by tread thickness, tire pressure, and rider weight, the rolling circumference should be measured by rolling the bike with the rider aboard for one wheel revolution.
You may use the valve stem as a reference, starting the roll with the valve right over a perpendicular line on the floor, and ending when the valve is back at its low point one revolution later.
Another approach is to put a small dot of paint on the tire and measure the distance between the marks that the paint prints on the road. With either approach, the rider must hold the handlebars absolutely straight while an assistant balances and pushes the bike. Otherwise, the wheel may not follow a straight path.
You may use the valve stem as a reference, starting the roll with the valve right over a perpendicular line on the floor, and ending when the valve is back at its low point one revolution later.
Another approach is to put a small dot of paint on the tire and measure the distance between the marks that the paint prints on the road. With either approach, the rider must hold the handlebars absolutely straight while an assistant balances and pushes the bike. Otherwise, the wheel may not follow a straight path.
11-18-07 | 06:50 PM
#5
cab horn
Joined: Jun 2004
Posts: 28,353
Likes: 30
From: Toronto
Bikes: 1987 Bianchi Campione
I just picked up my first computer. I need to figure out the circumference of my wheel to calibrate it, and It's giving me a bit of trouble. My wheel is 700 x 25.
Here's the formula I followed, as per the manual:
distance from floor to hub: 13 3/8" (13.375)
multiply by 25.4 to convert to mm (339.725)
multiply that number by 6.28 (2 x pi) = 2133.473
I had entered 2105 based on Sheldon Brown's recommendation for a 700 x 25 wheel. But this is a pretty substantial difference. When I entered 2133, it seemed like the miles were racking up really quickly. I'll be the first to admit that I don't have the best sense of distance, so I could be wrong.
Any ideas?
Here's the formula I followed, as per the manual:
distance from floor to hub: 13 3/8" (13.375)
multiply by 25.4 to convert to mm (339.725)
multiply that number by 6.28 (2 x pi) = 2133.473
I had entered 2105 based on Sheldon Brown's recommendation for a 700 x 25 wheel. But this is a pretty substantial difference. When I entered 2133, it seemed like the miles were racking up really quickly. I'll be the first to admit that I don't have the best sense of distance, so I could be wrong.
Any ideas?
Real joy is riding.
11-18-07 | 07:48 PM
#6
Really Old Senior Member
Joined: Feb 2007
Posts: 14,629
Likes: 1,882
From: Mid Willamette Valley, Orygun
Bikes: 87 RockHopper,2008 Specialized Globe. Both upgraded to 9 speeds. 2019 Giant Explore E+3
I take a piece of string, tie a small loop in the end and then basically "lasso" the tire.
Mark the string where it overlaps and then hook the loop on a nail or something and stretch the string (approx same tension as when in "lasso mode") and measure it with the tape. Repeat until you feel you have a consistent measurement.
Divide that # by .03937.
As mentioned, a difference of 20+ out 2000+ is pretty small.
Or you could just enter 2111.
Mark the string where it overlaps and then hook the loop on a nail or something and stretch the string (approx same tension as when in "lasso mode") and measure it with the tape. Repeat until you feel you have a consistent measurement.
Divide that # by .03937.
As mentioned, a difference of 20+ out 2000+ is pretty small.
Or you could just enter 2111.
11-18-07 | 08:22 PM
#7
Senior Member
Joined: Feb 2004
Posts: 30,225
Likes: 649
From: St Peters, Missouri
Bikes: Catrike 559 I own some others but they don't get ridden very much.
Try riding the exact same route 3 or 4 times and see how closely your computer milages compare. Trying to program your computer to be more accurate than the variation among the repeated laps is fooling yourself.
11-18-07 | 08:23 PM
#8
Put a piece of masking tape on a tire at the bottom, matching piece on the floor. Sit on the bike, roll it forward one full revolution, measure from there back to the first tape. Removes the uncertainties about what the actual rolling diameter is, avoids using pi at all, etc.
11-18-07 | 08:40 PM
#9
The rollout method works. If you don't have a metric tape measure, you can use google to convert it.
For instance, entering 83 1/8 inches in mm in google search, you get this answer:
(83 1/8) inches = 2 111.375 millimeters
google will convert a lot of units. 4 lbs in grams returns: 4 pounds = 1 814.36948 grams
For instance, entering 83 1/8 inches in mm in google search, you get this answer:
(83 1/8) inches = 2 111.375 millimeters
google will convert a lot of units. 4 lbs in grams returns: 4 pounds = 1 814.36948 grams
11-18-07 | 08:59 PM
#10
Senior Member
Joined: Oct 2004
Posts: 335
Likes: 0
From: Iowa
Bikes: Actual 10-speed Olmo road, Bianchi BUSS, Kona A-Ha, Schwinn Moab 2 rain bike
I use the 5th thru 7th mile of the Quad City Marathon Course. I just ride back and forth, and adjust until the computer clicks off on the mile marks. I hear they measure those things pretty accurately for the runners.
11-18-07 | 09:52 PM
#11
Thread Starter
Sassbucket.
Joined: Oct 2007
Posts: 114
Likes: 1
From: los angeles, ca
I guess I don't really want to get into splitting hairs. I'll just split the difference and call it a day. Thanks for all the help!
11-19-07 | 07:18 AM
#12
No Rocket Surgeon
Joined: Jan 2005
Posts: 1,648
Likes: 6
From: Corona and S. El Monte, CA
Bikes: Cannondale D600, Dahon Speed T7
I agree with rm-rf, the rollout method is the only way to go.
Pump up your tires. Put a tape measure on the ground. Get on the bike. You may need a "spotter" for that step. Ride your bike so the front wheel goes one full revolution (I'm assuming your pickup is on the front fork). Your valve stem is a good indicator. Just measure how far your bike travels in one wheel revolution. Convert to mm (25.4 mm/inch), and enter this value into your computer.
If you are really anal (as I am) you will do this a few times and average the results.
I use this converter a lot:
https://www.convertit.com/Go/ConvertI.../Converter.ASP
Simple!
Have fun.
Pump up your tires. Put a tape measure on the ground. Get on the bike. You may need a "spotter" for that step. Ride your bike so the front wheel goes one full revolution (I'm assuming your pickup is on the front fork). Your valve stem is a good indicator. Just measure how far your bike travels in one wheel revolution. Convert to mm (25.4 mm/inch), and enter this value into your computer.
If you are really anal (as I am) you will do this a few times and average the results.
I use this converter a lot:
https://www.convertit.com/Go/ConvertI.../Converter.ASP
Simple!
Have fun.
__________________
Fewer Cars, more handlebars!
Fewer Cars, more handlebars!
Last edited by eubi; 11-19-07 at 08:13 AM.
11-19-07 | 09:25 AM
#13
Life is good
Joined: Jan 2001
Posts: 18,208
Likes: 14
From: Not far from the Withlacoochee Trail. 🚴🏻
Bikes: 2018 Lynskey Helix Pro
Originally Posted by jc808
I just picked up my first computer. I need to figure out the circumference of my wheel to calibrate it, and It's giving me a bit of trouble. My wheel is 700 x 25.
__________________
The Lord is merciful and gracious, slow to anger and abounding in steadfast love. - Psalm 103:8
I am a cyclist. I am not the fastest or the fittest. But I will get to where I'm going with a smile on my face.
The Lord is merciful and gracious, slow to anger and abounding in steadfast love. - Psalm 103:8
I am a cyclist. I am not the fastest or the fittest. But I will get to where I'm going with a smile on my face.
11-19-07 | 09:27 AM
#14
Senior Member
Joined: Jun 2007
Posts: 352
Likes: 0
From: Pittsburgh, PA
Bikes: BiknBrian brand custom 26 inch commuter trekker, Cannondale F600 Single Speeded MTB, Nashbar Cro-Mo CX, some other bikes and parts that could be made into bikes.
You can't just measure the circumfrence or diameter, you have to weight the tire. Your effective circumfrence will be significantly smaller with weight on it. Then you can just drive a couple mile route and make sure it closely mathches what the computer says.
