wheel circumference
 

Hi
Sorry if this has been asked before ,just need the wheel circumference for 700x23c
michelen lithion tire for recaliberating my computer and too lazy to actually measure it.
Thanks

I generally just use whatever number the computer manual gives for my tire size. That's plenty close enough for my purposes.

Thanks

they vary a lot. currently I have 2110 on one bike, 2105 on another. I have seen them as low as 2095

always from a 10x rollout.

Once you re calibrate you can check it by riding 10k (or 10 miles) according to your computer, and then mapping your route on gmap-pedometer.com to see if it actually measures 10 whatever.

Fortunately that doesn't translate to much difference in real terms. If you think your average speed was 21.1 mph, and it was actually only 21.05 mph, it's not off enough to worry about. It's accurate to 0.25%, or 1/400.

that would be approximately 4 places in some time trials I have done recently.

also your math is wrong.

calculate the rollout of the wheel per revolution * gear ratio * 100 (rpm) * minutes in a time trial

I got the following for a 30 minute time trial in 53/15 at 100rpm

at 2095mm = 13.798 miles
at 2110mm = 13.899 miles

approximately 528 feet difference at the line (pretty far at the end of a time trial.)

Would it have changed the pace you rode?

By the way how do Garmins compare? how accurate is GPS?

Or for that matter the accuracy of Map My Ride?

the point is, all my computer readings and subsequent data would be off enough to make me wonder if the official timing or distance was off. it is easy enough to peg the start and finish of a TT by watching the power chart. As long as the chart shows the correct number of seconds (as recorded by the official timing) then I know exact speed and distance.

I take you point.

I just wondered if people are monitoring their speeds that closely in a TT. I work off HR in TTs. I have tried riding without speed completely but it was a step too far.

usually the gaps are much bigger at the back of the field :o

I look at watts and sometimes cadence. speed is irrelevant until I get home and analyze the data.

EDIT: I also watch my stopwatch. It helps me track timing waypoints (If I expect a TT to last 24 minutes, then I know the 12 minute mark is important, and may also mark the turnaround. Also It helps me gauge when to fire all the rest of my bullets)

Fortunately time trial results are decided by the organizers using one set of timers, not by each individual's bike computer.

0.23%. Put another way it's the difference between 20 MPH and 20.05 MPH.

I guess you skipped math class too.

High school geometry class gives the formula for the circumference of a circle as:

C = 2(pi)R

pi = 3.142
R = radius.

If you measure diameter instead of radius, then use the formula C = piD

Since pi is a ratio, circumference will end up being in whatever unit you measure the radius or diameter in.

Do not forget to take your measurements from the center of the hub to the edge of the tire itself (for radius), or from the outer edge of the tire, across the center of the hub, then to the outer edge of the opposing outer edge of the tire (for diameter) - and not simply the edge of the rim.

Or if you do not want to mess around with math, just use a quick reference table. It will be accurate enough for what you want to find out.

My GPS bike computer uses magic to guess my wheel size by counting revolutions and comparing that to the speed as measured by the GPS. It guessed correctly, even in spite of the error in GPS signals and possible errors in circumference tables. Nifty.

Pretty sure mine rings up at 2099 pretty consistently.

Make that 2090 on one bike, 2103 on the other. Go figure. Bontragers on one bike, Conti 4000's on the other.

If you want to be precise you have to count weighted revolutions.

+1. If you are going to do a roll out, you need to test it like this at the PSI you ride with.

This is not necessarily a good way to test it... You can't ride in a perfectly straight line, as it will be measured on a map. In addition, what lane you are in will also affect the total distance when you compare it to a map(you cover more distance to make a left turn than a right turn, etc). Even though these differences will be small, they are still going to invalidate any type of precision you want.

Usually the difference between the number supplied in the manual and the rollout measured is very small. If I bother doing a rollout, I'll use that but otherwise close enough is close enough.

That's true, although I guess that raises the question of how you want to measure distance cycled. Is it more valid to measure it in miles of road distance covered, or miles of wheel track contact?

Correction: (2110-2105)/2105 = .002375 or .24% difference

Even if it was as lowas 2095 it still doesn't make any difference in riding or racing.

facepalm