What circumference did you enter into your Coker cycle computer?

Unicycles and Equipment
1

The tire on my Deluxe Coker has slight wear, so its circumference may be
up to a few mm smaller than a new tire. Here are my measurements:

Unloaded circumference: 2839 mm

Loaded circumference: 2789 mm

Loaded plus wobble: 2769 mm

These were all measurements of a single rotation. The first two were
done on a straight line holding onto a support.

The second was done actually riding slowing with a pile of talc on the
tire to mark the rotation endpoints and measuring from the center of one
mark to the next. I really need to find a verified measured distance to
get a more accurate effective circumference that accounts for both rider
tire loading and unicycle wobble at my cruising speed.

Please respond with your circumference measurement (please state type:
unloaded, loaded or loaded plus wobble) and your (subjective) tire wear
when you made these measurements.

Sincerely,

Ken Fuchs <kfuchs@winternet.com>

2

Re: What circumference did you enter into your Coker cycle computer?

Ken, I only did a loaded plus wobble measurement and it was very close to
yours. I got 2776mm which is what I’ve been using. This was pre-Deluxe wheel
over 3 revolutions (5 runs, throw out min & max then average the rest). I
haven’t gone to the trouble of measuring my newer wheels.

Most people I know have been using slightly higher numbers, but I prefer to
err to the low side so I never think I’m going faster than I really am.

—Nathan

“Ken Fuchs” <kfuchs@winternet.com> wrote
> The tire on my Deluxe Coker has slight wear, so its circumference may be
> up to a few mm smaller than a new tire. Here are my measurements:
>
> Unloaded circumference: 2839 mm
>
> Loaded circumference: 2789 mm
>
> Loaded plus wobble: 2769 mm

3

Re: What circumference did you enter into your Coker cycle computer?

“Nathan Hoover” <nathan_at_movaris.com> wrote:

>Ken, I only did a loaded plus wobble measurement and it was very close to
>yours. I got 2776mm which is what I’ve been using. This was pre-Deluxe wheel
>over 3 revolutions (5 runs, throw out min & max then average the rest). I
>haven’t gone to the trouble of measuring my newer wheels.

Your measurement method was much better than mine. Do you recall how
much wear your tire had when you measured it? Did you ride through a
pile of talc powder, a mud puddle, etc. for marking the rotations?
Maybe a line of wet paint perpendicular to the tire to mark the
rotations would work well, but it would be permanent.

The best method as mentioned in a previous post would be riding an
officially measured route and compare your computer’s distance to the
official distance, compute a +/- percentage and apply that same
percentage to your computer’s circumference number to get an “official”
circumference number. Do the route again and compare just to be sure
the computer’s circumference number is really “officially” accurate.

>Most people I know have been using slightly higher numbers, but I prefer to
>err to the low side so I never think I’m going faster than I really am.

Do you recall any of those numbers? Hopefully they will all be within a
few centimeters of each other (within the three types that is).

>“Ken Fuchs” <kfuchs@winternet.com> wrote
>> The tire on my Deluxe Coker has slight wear, so its circumference may be
>> up to a few mm smaller than a new tire. Here are my measurements:

The slight wear I mentioned is specifically the knobs in the middle in
two spots (I haven’t rotated the tire yet) are nearly gone.

>> Unloaded circumference: 2839 mm
>>
>> Loaded circumference: 2789 mm
>>
>> Loaded plus wobble: 2769 mm

Nathan, thank you very much for responding!

Sincerely,

Ken Fuchs <kfuchs@winternet.com>

4

Re: What circumference did you enter into your Coker cycle computer?

No Coker here but an alternative way of measuring. Mark off exactly 100
meters in a straight line. I have a 50 meter measuring tape that makes this
fairly easy. Start, noting pedal position, on first mark. Count full pedal
strokes to end mark. Estimate amount of last rotation. An average
circumference measure, taking into account rider weight, wheel wobble, etc
would be 100000mm / rotations. example 100000/36.25=2759mm. If you can
estimate that last bit to within a quarter rotation your accuracy is within
0.6% for that wheel size. Much more than meets my needs. I personally just
do the role it in a straight line next to a meter stick method. I repeat a
couple times and I put weight on it.

Even though you’ve been talking about coker tires that should come out close
to the same. Measuring down to the mm sounds like overkill. The
measurements are likely to change by several mm as the tire warms up, cools
down, wears, goes across different road surfaces, etc.

-Cubby

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5

Re: What circumference did you enter into your Coker cycle computer?

“Cubby01” <cubby01@lycos.com> wrote:

>No Coker here but an alternative way of measuring. Mark off exactly 100
>meters in a straight line. I have a 50 meter measuring tape that makes this
>fairly easy. Start, noting pedal position, on first mark. Count full pedal
>strokes to end mark. Estimate amount of last rotation. An average
>circumference measure, taking into account rider weight, wheel wobble, etc
>would be 100000mm / rotations. example 100000/36.25=2759mm. If you can
>estimate that last bit to within a quarter rotation your accuracy is within
>0.6% for that wheel size. Much more than meets my needs. I personally just
>do the role it in a straight line next to a meter stick method. I repeat a
>couple times and I put weight on it.

Thanks Cubby! This is an excellent method and description.

For those of us who do want that elusive extra measure of precision, we
could have someone else estimate the final fraction of a rotation by
putting a strip of masking tape on the sidewall parallel to the valve
stem. Doing a flying start, one person could estimate the initial
fractional rotation with respect to the masking tape mark on the tire
and a second could estimate the final fractional rotation.

>Even though you’ve been talking about coker tires that should come out close
>to the same. Measuring down to the mm sounds like overkill. The
>measurements are likely to change by several mm as the tire warms up, cools
>down, wears, goes across different road surfaces, etc.

It truly is overkill for anyone who is primarily interested in a
computer record of their daily and total mileage. However, when one is
interested in serious training for time trials or other racing, it is
very important that their computer be calibrated as precisely as
possible.

We should know that riders that demand precision will have to
recalibrate their computer periodically as the tire wears down.
Cubby also makes an interesting point about the circumference changing
as the tire heats up and cools down. Would be interesting to compare
the circumference of a Coker tire at -20 C versus 30 C for example
(could be done in the winter where outside temperatures dip below -20 C
versus the warmest room in a house, 30 C).

Concerning road surfaces: Touring unicyclists may ride on paved,
gravel, sand, clay, etc. Unicyclists doing time trials will likely
restrict themselves to primarily smooth paved surface, which a computer
should accurately measure if precisely calibrated on the same surface.

Cubby, thanks also for your points about temperature related tire
expansion/contraction and varying road surfaces effects on computer
accuracy!

Sincerely,

Ken Fuchs <kfuchs@winternet.com>

6

Re: What circumference did you enter into your Coker cycle computer?

Hi Ken: I don’t know if I get the difference between “loaded” and “loaded with wobble”. My circumference is 2756. Measuring technique was to hop on with the tire valve stem straight down (6 o’clock position), have my wife mark the ground there with sidewalk chalk, get myself one full rotation forward using her SUV for support, mark the new spot, then measure from the centers of the two chalk marks.

Primitive I know, and dependent on another human and her vehicle. I like the pile of talc in the road idea…

7

Re: Re: What circumference did you enter into your Coker cycle computer?

What PSI did you have in the tire? From the number you got it looks like the tire may have been a little low.

Many Coker riders like to ride with the tire over inflated to 50+ PSI. It would be interesting to see what the calibration number is for 30 PSI, 40 PSI, 50 PSI, 60 PSI and 70 PSI along with the riders weight. The calibration number is going to be different depending on the PSI of the tire and I’m curious about how much of a difference the tire pressure makes in the calibration number.

8

My Method for Recording Rollout

Brian’s easy method for measuring loaded wheel circumference:

Take a piece of string, cotton or something absorbent, not nylon.
Tie the string around the tire & rim.
Put some ink on the string. Black ink will show well, even on (old) black paving.
Mount with the string at the top of the wheel and go several wheel turns.
Measure the distance from the first mark to the last and count the marks.
Do the math.

Most accurate method:

Get Bikini Bikelog and install it on your Palm PDA.
Install your PDA on your cycle.
Ride a large measured distance, like a mile or so. Don’t use your car to measure the distance, car speedos are not good for better than 10% accuracy, if that.
Download the information to your PC and load it into a spreadsheet.
Do the mat . . . uh, have the spreadsheet do the math.

The last one, while accurate, would take a little work to use on a unicycle. And a little confidence that you’d not do a real good UPD with your PDA involved.

9

Re: My Method for Recording Rollout

Brian, how do you determine the “measured distance”? If you already have a known distance of a mile or multiple miles (Km’s for those that use 'em) then correcting the cyclometer is fairly straight forward. Just change the circ millimeters by the amount needed to correct for the distance error. It’s only simple ratio math but you still get to have a little intellectual fun :slight_smile: After the change ride the distance back and it should be dead on…assuming no variance in wobble and all the other little things that can come into play.

By the way, I like the inky string method. Seems it would be more accurate than talc.

-Cubby
Who runs across a nifty use for a PDA several times a year but still can’t bring himself to own one.

10

Re: Re: My Method for Recording Rollout

Here, in the US, some roads have “Mesaured Mile” sign posts. I use a surveyed distance that just happens to be on the sidewalk near my house–they were rebuilding the road. You could use a GPS, too. Any longish distance that you can verify to the accuracy that you desire.

11

My method is very similar to Brians. I use a red “Sharpie” marker. I lay down a very heavy mark on the tire, by tracing back and forth across the tire tread for 20 seconds or so. This builds up a lot of ink at this one spot on the tire. Then I mount the unicycle and ride at least on total revolution across my garage floor. This leaves two very precise marks to measure from. I measure from the leading edge of one to the leading edge of the other mark.

Note: I have my wife or a daughter hold my hand so I ride perfectly straight while taking this measure.

Note #2: Wobble greatly affects this measurement, as does air pressure in the tire. --chirokid–

12

Re: What circumference did you enter into your Coker cycle computer?

>Ken Fuchs wrote:

>> Please respond with your circumference measurement (please state
>> type: unloaded, loaded or loaded plus wobble) and your (subjective)
>> tire wear when you made these measurements.

tomblackwood <tomblackwood.tefk4@timelimit.unicyclist.com> wrote:

>Hi Ken: I don’t know if I get the difference between “loaded” and
>“loaded with wobble”. My circumference is 2756. Measuring technique was
>to hop on with the tire valve stem straight down (6 o’clock position),
>have my wife mark the ground there with sidewalk chalk, get myself one
>full rotation forward using her SUV for support, mark the new spot, then
>measure from the centers of the two chalk marks.

If the Coker tire moved in a straight line, then the above measurement
is the loaded (rider on seat) and no wobble type.

Loaded with wobble would have to be done without external support, so
the wheel freely wobbles in response to one’s leg, crank and pedal
masses moving as one pedals.

Your circumference of 2756 mm is significantly less than mine at 2769
mm. My tire was new except for minor wear in two spots. How much wear
did your tire have when you measured its effective circumference?

>Primitive I know, and dependent on another human and her vehicle. I
>like the pile of talc in the road idea…

In my first attempt at measuring the Coker tire circumference, I just
rolled the wheel unloaded on my kitchen floor and got 2839 mm!

Thanks tomblackwood!

Sincerely,

Ken Fuchs <kfuchs@winternet.com>

13

Re: What circumference did you enter into your Coker cycle computer?

>tomblackwood wrote:

>> My circumference is 2756.

john_childs <john_childs.temqk@timelimit.unicyclist.com> wrote:

>What PSI did you have in the tire? From the number you got it looks
>like the tire may have been a little low.

>Many Coker riders like to ride with the tire over inflated to 50+ PSI.
>It would be interesting to see what the calibration number is for 30
>PSI, 40 PSI, 50 PSI, 60 PSI and 70 PSI along with the riders weight.
>The calibration number is going to be different depending on the PSI of
>the tire and I’m curious about how much of a difference the tire
>pressure makes in the calibration number.

I keep my Coker inflated at the 32 PSI recommended value and my
measurement for loaded and wobble was 2769 mm (very slight wear).

Tire pressure sounds like an important factor. Thank you John, for
pointing that out.

Sincerely,

Ken Fuchs <kfuchs@winternet.com>

14

Re: Re: What circumference did you enter into your Coker cycle computer?

Thanks for the clarification, Ken. That makes total sense.

From what I’m reading, I probably botched the initial roll-out I did, although it doesn’t seem to have had a big effect. Prior to getting the computer, I measued a number of my 4-8 mile local neighborhood rides in my car, and my post-computer rides of these loops measured out pretty much the same. Although since I used my car initially, that goes back to my debate with Klass, and I still don’t know how the “yell the speed out the car window” speed compares to the loaded with/without wobble speed.

As soon as my ankle allows me to get back in the saddle, I’m going to re-measure using the talc on the sidewalk technique. Then I’m going to measure one mile of relative straightaway in my car, and ride the same thing on the Coker (without water in support of showard) to see if they match. That’s a week or more out, so will revive the thread then to report my findings.

15

Re: What circumference did you enter into your Coker cycle computer?

“brian.slater” <brian.slater.tewgl@timelimit.unicyclist.com>

>Brian’s easy method for measuring loaded wheel circumference:

>Take a piece of string, cotton or something absorbent, not nylon.

>Tie the string around the tire & rim.

>Put some ink on the string. Black ink will show well, even on (old)
>black paving.

>Mount with the string at the top of the wheel and go several wheel
>turns.

>Measure the distance from the first mark to the last and count the
>marks.

>Do the math.

Sounds great! Thanks Brian!

>Most accurate method:

>Get ‘Bikini Bikelog’ (http://home1.swipnet.se/~w-51358/pilot/) and
>install it on your Palm PDA.

>Install your PDA on your cycle.

>Ride a large measured distance, like a mile or so. Don’t use your car
>to measure the distance, car speedos are not good for better than 10%
>accuracy, if that.

>Download the information to your PC and load it into a spreadsheet.

>Do the mat . . . uh, have the spreadsheet do the math.

This sounds good as well, but some of don’t have a PDA, so we might
prefer method #1.

Thanks again, Brian!

16

Weight factor also

Wheel circumference is an interesting problem. I was surprised when I measured the loaded circumference of my tire and compared it to the diameter of the tire (29 inch Nanoraptor). You might expect that the radius of the flattened tire when you are sitting on it correlates to the circumference, but I think it is somewhere in between that and the measured (unloaded) radius.

Those of you comparing Coker diameters don’t forget that your weight is as important a factor as the tire pressure. In fact, there should be some correlation, 120 lbs on a 40 psi tire should have about the same circumference as 150 lbs on a 50 psi tire, since the same area is needed to contact the road.

Phil

17

Re: What circumference did you enter into your Coker cycle computer?

“brian.slater” <brian.slater.tf0mo@timelimit.unicyclist.com> wrote:

>Here, in the US, some roads have “Measured Mile” sign posts.

How accurate are those mile marker sign posts? (The mile marker signs
start at one end of a highway/road route and count the miles up to the
other end and are sometimes used to report the locations of accidents or
other roadside emergencies.)

Sincerely,

Ken Fuchs <kfuchs@winternet.com>

18

Re: What circumference did you enter into your Coker cycle computer?

tomblackwood <tomblackwood.tfe23@timelimit.unicyclist.com>

>As soon as my ankle allows me to get back in the saddle, I’m going to
>re-measure using the talc on the sidewalk technique.

The ink line or string around the tire actually sounds far more precise
than my talc method. I’ll have to try it sometime.

>Then I’m going to measure one mile of relative straightaway in my car,
>and ride the same thing on the Coker (without water in support of
>showard) to see if they match. That’s a week or more out, so will
>revive the thread then to report my findings.

One can get one’s cycle computer to match one’s car odometer, but car
odometers unfortunately suffer two problems:

  1. I suspect that car odometer calibration needs to be accurate only to
    within a few percent for new tires.

  2. Car tire circumference clearly decreases with wear.

Ever, drive down a street that has speed detectors showing your actual
speed; my speedometer shows a speed that is 2-3 MPH more than the speed
detector readout, probably due to tire wear or possibly initial
calibration of the car odometer/speedometer.

An officially measured distance would be better. However, I suspect
that although car odometers may be inaccurate, they are probably precise
in that they would give distances with a consistent % error (i.e. -2%),
until the tire wears down significantly. Thus, one might have to
multiply the odometer trip number by .98 to get the true distance for
example.

Sincerely,

Ken Fuchs <kfuchs@winternet.com>

19

Wow, the science of speedometer/odometer calibration for Cokers. Lots of clever ideas here!

Not to take away your fun of determining the most precise measurement at a given rider weight, air temperature, tire pressure and tire wear, but I expect these things to change for me from ride to ride. If I ride enough, even the rider weight part. :slight_smile:

So I’m fine with a nice, median number. That’s why I would be interested to hear the results people are getting, in the form of circumference numbers, listed along with those other factors mentioned above. If several people do this, the rest of us (the lazy ones) may be able to make pretty accurate extrapolations from that data.

As for wobble, this is another factor that varys not only with the individual, but I think also with the speed you are riding. I’d rather not factor in wobble, unless it happens automatically as part of the calibration method.

Thanks for all the research!

20

Re: What circumference did you enter into your Coker cycle computer?

Phil aka schroder <schroder.tfzt7@timelimit.unicyclist.com> wrote:

>Wheel circumference is an interesting problem. I was surprised when I
>measured the loaded circumference of my tire and compared it to the
>diameter of the tire (29 inch Nanoraptor). You might expect that the
>radius of the flattened tire when you are sitting on it correlates to
>the circumference, but I think it is somewhere in between that and the
>measured (unloaded) radius.

So the distance from the rider loaded axle to the ground (compressed
radius) is smaller than the loaded radius (loaded straight roll out
circumference / pi) and the loaded radius is smaller than the unloaded
radius:

compressed radius < loaded (roll out) radius < unloaded radius

Note that wobble is not considered here, since the only roll out (to
compute the loaded radius) is done in a straight line.

>Those of you comparing Coker diameters don’t forget that your weight is
>as important a factor as the tire pressure. In fact, there should be
>some correlation, 120 lbs on a 40 psi tire should have about the same
>circumference as 150 lbs on a 50 psi tire, since the same area is needed
>to contact the road.

Thanks Phil, for this very precise and clearly true statement! It’s
important to see the relationship between body weight and tire pressure
to see it’s effect on loaded circumference.

Sincerely,

Ken Fuchs <kfuchs@winternet.com>