General Cycling Discussion - Computer Calibration

Please check my figures and tell me if my thinking is correct.

I recently installed my new cycle computer (Cateye Astrale 8). I set the wheel circumference to 2136. The next day I went on an organized ride with cue sheets. By comparing my odometer to the 2 cue sheets that I used, I determined that my computer gains about one tenth of a mile every 10 miles. (These cue sheets are reliable.)

I figure this way:
My computer odometer shows a 1% increase over reality; therefore it ‘thinks’ that my wheels are 1% larger tan they are. I have to find the correct setting by using this formula 2136 = 1.01 * X (where X = the new setting). Transposed the formula is X = 2136/1.01 This gives me 2114.85 which I round off to 2115.

Did I goof anywhere?

Did you actually use a rollout circumference measurement to enter in the computer? Perhaps you did and I am not picking that up from your post.

I did a rollout circumference and found that my wheels differed quite a bit from what the programming instructions said to enter as my wheel circumference--it measured my mileage short, like yours is doing.

No - this time I didn't. I just went by the table in the instructions. I guess this is what caused the 1% error. The tires are probably a little flater with me on the bike.

I forget how much difference it made, but there was definitely a difference when I made a measurement rolling the bike with me on it and not on it. I would also expect differences based on how much air pressure is in the tires.

Bob

Don't forget. Only do the "rollout" testing on "even numbered" calendardays with both water bottles mounted and filled.

WHAT SIZE ARE YOUR TIRES 700X23?

An easy way to do a roll out measurement it this:
1) Pump tyres up to pressure
2) Find a line to follow, gutter, concrete groove in driveway etc.
3) Place valve at bottom of tyre
4) Sit on bike and roll out 5 complete rotations. Measure the distance.
5) Repeat 2 more times.
6) Add the 3 measurements together and divide by 15. This should give a very accurate measurement.
Even with an accurate measurement it can still only get to within +/- 1% as when we ride we rarely ride in a perfect straight line and take different lines through corners than the people that made the cue sheets.

CHEERS.

Mark

A lot of motorcycle manufactures think it's totally acceptable to have as much as a 6% error.
I doubt you're going to find anything better than +/-1%.

Just get a tape measure, turn the bike upside down, place a pen mark on the tyre tread, place the start of the tape measure on the mark and "walk" the tape along the tread until you come back to the mark. Simple. Some might say you sitting on the bike alters the outcome, but I don't think it matters that much.

If you use the straight-line-on-pavement method, use a small mark of paint on the tread to mark the start, intermediate and finish points.

+/- 1% is about it. Variations occur with differing tyres pressures, differing loads, and as Dutchy says, different ways of riding. And, as the tread wears down (which has more of an influence on altering readings than anything).

Otherwise, if you use the route sheets created by the same people all the time, just increase/decrease the your error by the percentage you have worked out. Which means I don't think you goofed anywhere, although I am slow and would have done this:

Original reading = 2136
1% (or 1/100th) of original reading = 21.36
Subtract from original reading = 2136 - 21.36 = 2114.64

You have rounded to 2115, so we have the same result! Phew!!

1% error certainly sounds good to me. I wouldn't fool with it for that small an error.

You say that the tip sheets are reliable. How do you know that? What was used to measure the distance printed in the sheets?

If you really want to try to improve the accuracy of your computer, you need to measure against a known distance. The rollout test described above is probably the best indicator.

WHAT SIZE ARE YOUR TIRES 700X23 I think they are 700x28c, but I would have to go home and check.

You say that the tip sheets are reliable. How do you know that? What was used to measure the distance printed in the sheets? I don't know the method they used but they have used those cues for a few years and the other riders I spoke with were satisfied with the cues accuracy. So I guess you could say it was 'general consensus'.

1% error certainly sounds good to me. I wouldn't fool with it for that small an error. I guess you're right. Although I already changed the number by 1% according to my calculations mentioned in the first post. I should go and do the same rout again to compare my results, but it is not close by and I am not likely to get down there soon.

Allow me to stray somewhat from the topic, but a few months ago I had my POLAR Xtrainer's batteries replaced by my LBS.
Both me and the LBS forgot to recalibrate the POLAR after this replacement. I cannot remember what the default setting is for the wheelsize, but I remember clearly that it is substantialy lower than the real circumference of my wheel...
I cycled 3 weeks believing that I somehow must have lost my form from last year, or maybe I was just getting older. I just could not get up to a reasonable average speed. It was only until I trained with my club, and compared other riders speeds, that I realised what a fool I've been!1

Originally posted by poululla
Allow me to stray somewhat from the topic, but a few months ago I had my POLAR Xtrainer's batteries replaced by my LBS.
Both me and the LBS forgot to recalibrate the POLAR after this replacement. I cannot remember what the default setting is for the wheelsize, but I remember clearly that it is substantialy lower than the real circumference of my wheel...
I cycled 3 weeks believing that I somehow must have lost my form from last year, or maybe I was just getting older. I just could not get up to a reasonable average speed. It was only until I trained with my club, and compared other riders speeds, that I realised what a fool I've been!1
OK see if I have this figured in my head right:
So a small wheel size setting made you computer ‘think’ that your wheels were smaller than reality - right? That means you computer thought that it took more revolutions to go a mile and therefore it indicated a slower average speed and shorter distances than reality. Is that right?

OK...here is my garanteed method of calabration.

1)Take your bike to a local High School Track.

2)Set your calabration a few digets low (or high) so that you know which way to change it to make it more accurate.

3)Do a couple of laps on the inside lane. Raise the calabration ONE (maybe two) number(s). Check the calabration. If you did two laps the the distance should read 1/2 mile.

If it does not then repeat step 3.

Some High Schools frown on bikes on their track so you might have to do this at 2AM.

Wes

OK see if I have this figured in my head right:
So a small wheel size setting made you computer ‘think’ that your wheels were smaller than reality - right? That means you computer thought that it took more revolutions to go a mile and therefore it indicated a slower average speed and shorter distances than reality. Is that right?

WRONG! lower numbers create "high" speed and distance readings.

But the absolutely most "RICHARD CRANIUM" POINT of this TOTALLY USELESS THREAD IS: all you are doing is playing with the accuracy of "front-wheel travel". Since you dont really travel in straight lines, this measurement is always approximate....... Race courses and maps all use rounded "straight line measurements for their readouts-- so it's useless to play with computer "wheel" rollouts, it's such a small variation, it's always inaccurate.

The roll around the track idea is good, but just driving somewhere ten miles and then making the computer match it is bettter.........Then you have averaged in ten miles of "real world" bicycle operation, not 10 miles of front-wheel rotation........ AURGGHHHHH, (can I say that?)

WRONG! lower numbers create "high" speed and distance readings. Yep - you're right. I stated it wrong - but adjusted the computer correctly.

Thanks to everyone for their opinions.