What circumference for Hetre and computer?

[satanas]

Trying to figure out what number to plug in, as the one I tried (2100) was significantly out. I'm guessing the low pressure gives a smaller effective circumference, but how much?

Thanks,
Stephen

[Rick Johnson]

If you have a tape measure handy it's easy enough to get a very accurate
figure. Start on a clean section of sidewalk or driveway. Use something
inky or greasy to put a mark on the tire and ride forward in a straight
line for one complete revolution. Then measure the distance between the
two imprints left on the ground.

I've even used a half chewed raisin to get this number...

Rick

Rick Johnson
Bend, Oregon

Every revolutionary idea seems to evoke three stages of reaction...
One, it's completely impossible.
Two, it's possible, but it's not worth doing.
Three, I said it was a good idea all along.

Arthur C. Clarke

[Stephen Poole]

My worry is how much to compensate for tyre compression due to low pressure. The uncompressed circumference is easy enough to figure out...

Later
Stephen

[Brian Rowe]

How did you get your measurement?

Here is one method you could try: Measure the width of your tire on your rim at the pressure you ride. Then assume the tire is as tall as it is wide. For example if your Hetres measure 40mm wide on your rim, then the circumference should be close to (584mm + 80mm)*3.14 = 2,085.

Brian

[Rick Johnson]

That's why I say "ride" forward. The resulting measurement will be a very accurate figure for the rolling circumference based on the load and pressure at the time the test was conducted.
This method has been confirmed against GPS and can be accurate to a fraction of a percent.

Rick

Rick Johnson
Bend, Oregon

Every revolutionary idea seems to evoke three stages of reaction...
One, it's completely impossible. 
Two, it's possible, but it's not worth doing. 
Three, I said it was a good idea all along.

Arthur C. Clarke

[Stephen Poole]

That sounds reasonable, but I was getting an error varying between about 5-9%, assuming the route sheet at the weekend could be believed. This makes navigation even more difficult than it would be otherwise! Since the error % seemed to vary at different places - though it was always over - applying a correction factor didn't seem all that likely to be accurate.

Later,
Stephen

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[Brian Rowe]

Rick,

Can you give more details about how you do this? Such as, do you allow some room to get started riding in a straight line first, and take the measurement at a later point say 10 or 20 meters after you start? Also, if you mark the tire with a grease blob, do you measure the grease spots center-to-center? :)

I'm interested in setting up a computer because I got mentally fatigued on a recent brevet and had some trouble navigating by my usual method with just the cue sheet and a wristwatch.

Brian 

[Rick Johnson]

Brian,
It's pretty simple really. The more consistent and accurate your measurement the better your end result will be.

The key points are:

• Tire pressure should be set to your normal riding pressure.
  
• If you will be carrying cargo measure with the same load.
  
• Measure on the same wheel that computer sensor reads.
  
• You should be fully weighted and balanced as you most commonly ride while measuring. For some people starting from a track stand might work fine, others might need a little room to get stabilized. Do what works best for you.
• The measurement needs to be straight line distance to the most reasonable degree. 
• Measure from center to center since your mark might expand or smear. 
• It's more accurate to measure without the tape measure tip. For example measure from 100mm to 2100mm marks where you can precisely line up the graduations, then subtract 100mm to get your final number. 
• If in doubt take multiple measurements, then average them. 

Personally I mark the top of the front tire while sitting on the bike, ready to go. From that start I am usually stable in a half revolution of the tire. Then I simply I ride until I see the mark come around again.
If you use a marking method that's readable over multiple revolutions that will increase the accuracy. However the results on the legibility of that will vary. Smooth concrete (garage or warehouse floors) can help with seeing the marks more clearly.
Other marking solutions to consider are "Sharpie" markers (which make a nice crisp line), "tire crayons" (softer than a typical art crayon - such as they use in automotive tire shops to mark customer tires), even soft chewy candies or raisins can be used in a pinch.

Good luck,

Rick

Rick Johnson
Bend, Oregon

Every revolutionary idea seems to evoke three stages of reaction...
One, it's completely impossible. 
Two, it's possible, but it's not worth doing. 
Three, I said it was a good idea all along.

Arthur C. Clarke

[Harald Kliems]

I'm usually calibrating my bike computers via a GPSr: Make sure you have ideal GPS reception and a fairly straight road. Start out with whatever nominal value your tire is. Reset GPSr and computer at same time. Ride 2.00km and compare and adjust. So if you start out with 2100 and your GPSr shows 2.00km and the computer shows 2.20km, then reduce the computer setting to 2100 - 2100*.1 = 2079. Lather, rinse, repeat until the two values match up.

Yes, GPS is not perfectly accurate, but for me this works well enough. I can look up the values for my CdlV's later.

 Harald.

[Dave Johnston]

Hetre, Synergy rim, 25lb bike, 140lb rider, 30-35 psi, avg of 2 rollouts = 2056mm

YMMV,
Dave

[Stephen Poole]

Thanks Dave, very similar to me - there's a lot of Daves here! Will try 2056 and see what happens. :-)

--

[Ken Freeman]

Yees the tires should be weighted.

GPS is not very accurate as an absolute measurement, error can be up to 5 meters.  If your method involves differences over time it can be very accurate.

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Ken Freeman
Ann Arbor, MI USA

[Ken Freeman]

Another approach to an estimate is as follows: inflate to a high pressure, have an assistant sit on the bike and keep it in balance, and measure the wheel radius to ground front and rear.  You may want to measure on both sides and take the average.  Let some air out to achieve your "softer" pressure, and measure radius again, just as carefully.  Multiply each radius by 6.28 or by 2*pi to get the front and rear circumferences, which you can then compare.

This is only an estimate, since I have not checked it against a rollout.  But it should be a little easier than a rollout.

[g...@footils.org]

Hi;

you ask: by how much? I answer: by 15 Percent. :-)

Unlike most cycling enthusiasts I never bother with roll-out tests or measurements and instead used pure math and a bit of common sense.

Assuming all numbers on paper are correct and I use a Frank Berto tire pressure with 15 % drop, I get this theoretical circumference for Hetres:

math.pi *  (2 * 42 * 0.85 + 584) = 2059 mm

(the 0.85 is 1 minus 15%)

Actually I even use 2060 in my computer because it's easier to input (one digit less different from zero).

Now this seems like I'm a very sloppy guy, but as I see it, I'm just being realistic in accepting, that I will never be able to get the exact same pressure into my tires twice. So the pressure I had while doing a roll-out test will most certainly be different from the pressure I will actually ride with later. And this difference in pressure will most certainly have a bigger error margin than my rollout distance measurement had. So it's useless for me to try to be too exact with the rollout.

Some numbers: Assuming I have a tire drop not of 15 but of 20 percent. This gives the circumference:

math.pi *  (2 * 42 * 0.80 + 584) = 2046 mm

And a drop of just 10 percent would give:

math.pi *  (2 * 42 * 0.90 + 584) = 2072 mm

With 2060 as the 100% value, these 5 percent drop differences result in just about 0.6 percent differences in roll-out. Even the full no-drop theoretical circumference of 2098.58 would just be 1.8 percent off the 2060. That error is just 18 meter on every kilometer! This is not much!

Now we all know that manufacturers tire sizes vary, and although Hetres are pretty well known to indeed have 40 - 42 mm, some roll-out or GPS verification for these values can be useful. But you don't have to do it on your own: Almost every value for Hetres found on the web and in this group is in the vicinity of 2060 mm (give or take 10 mm).

So while your 2100 value indeed seems to be too large, even this should never lead to the 5 or 9 percent deviation you got from the route sheet you mentioned in another post.

So I would guess the route sheet had errors. Or maybe you were weaving too much - might be a trail issue. :-)

All the best,
--
Frank

[Stephen Poole]

The route sheet definitely had errors, but distances were always over. I have a few more similarly complicated rides coming up, so the more accurate the odometer the better!

(I agree that basic maths should be close enough, but it didn't work in this case.)

Later,
Stephen

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[g...@footils.org]

Hi Stephen,

just don't overdo it: To correct for the 5 or 9 percent overshoot, your 2100 mm would need to go down to 2100 * 0.95 = 1995 or even 2100 * 0.91 = 1911 mm, and these values definitely would be too small for Hetres. Sheldon Brown's chart lists 1913 mm as roll-out for skinny 26 inch tires (25-559), see here: http://sheldonbrown.com/cyclecomputer-calibration.html

I'd not go lower than 2050 mm.

Good luck
--
Frank

[Chris Cullum]

I think I got 2055mm input in my cyclocomuter. IIRC. It's pretty accurate.

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Chris Cullum
Vancouver, BC

[Stephen Poole]

Thanks guys. I've got it set at 2056, and on a new computer (VDO rather than no name), which is simpler and should be more reliable. The test will be Saturday, when I have a 300 km randonee...

Later,
Stephen