Road Cycling - Need help setting my bike computer to be more accurate

Using a GPS unit I found that mine bike computer is off by .5 miles after a 40 miles ride. My riding partner's is almost dead on.

I have a Cateye Astrale 8 and 700x 23c tires. I used the default setting of 2096, but I need to adjust a bit one way or the other.

Which way do I adjust it? A higher number or a lower one?

Is it .5 off high or low?

If it's too high downward. (to around 2070)
If it's too low upward. (to around 2122)

Numbers are based on the about you say you are off.

Is it .5 off high or low?

If it's too high downward. (to around 2070)
If it's too low upward. (to around 2122)

Numbers are based on the about you say you are off.


My computer says we went 40.5 miles and the GPS says we went 40

Thanks for the info I will play round with those numbers and see what happens

Using a GPS unit I found that mine bike computer is off by .5 miles after a 40 miles ride. My riding partner's is almost dead on.

Remember that the GPS thinks the earth is an oblate spheroid. It doesn't know anything about hills. If you were going vertically up or down, the GPS would think you weren't moving (I can demonstrate this in my airplane). Calibrate your CatEye by rolling multiple revolutions with your weight on the bike and measuring the distance (divide the distance by the number of revolutions). It'll be more accurate than the GPS. The long halls in a building are great places to do the measurement.

Remember that the GPS thinks the earth is an oblate spheroid. It doesn't know anything about hills. If you were going vertically up or down, the GPS would think you weren't moving (I can demonstrate this in my airplane). Calibrate your CatEye by rolling multiple revolutions with your weight on the bike and measuring the distance (divide the distance by the number of revolutions). It'll be more accurate than the GPS. The long halls in a building are great places to do the measurement.


The course we rode today is as flat as it gets.

doing the roll out as above isn the most acurate. if like other computer I've had the 2096=209.6 cm sounds right. The longer the ride the more important the presure used for the roll out is the presure you ride with. Run the presure lower come up long run presure to hight come up short.
does the 1.25% error in the computer really that big a problem. I research we are happy ton get errors less the 3% be happy. Your computer says you are putting more miles on.

The roll-out should be with tyres fully inflated to riding pressure, and with the rider sitting on the seat. Variations in tyre pressure will influence what you actually get on the road.

G'day,

just ride......what does it matter?....FYI, most cars speedo's are only accurate to +/- 10% ...your speedo was wrong by 800m in 64 km?.......gee i wish I had that much to worry about in my life!...lol

Hitchy

G'day,

just ride......what does it matter?....FYI, most cars speedo's are only accurate to +/- 10% ...your speedo was wrong by 800m in 64 km?.......gee i wish I had that much to worry about in my life!...lol

Hitchy


ok ok so I am a bit anal about accuracy. :D

Remember to completely accurate you need to figure out the rate of tire pressure decrease over your ride, which will change and figure out how to average. You must also take into account how much time you will spending standing since that puts considerable more weight on the front tire changing the diameter. Oh and don't forget every pot whole you swerve around adds distance that GPS will not pick up. AHH science annoying and fun at the same time. Oh we could get into the Heisenberg Uncertainty Principle, and oh and remember time changes with speed, and more importantly the laws of motion don't apply on earth since we are spinning hence are an accelerated reference which you are not allowed to be.

My GPS takes readings once a second and assumes straight line travel between readings when calculating distance traveled. Because of this it underestimates distances when turning. If there are a lot of turns (such as in mountain biking) then it will record much less distance traveled than the bike computer.

If the speed readout on the GPS and the bike computer are the same then this is as close as you're likely to get.

I do a 50 mile ride, then retrace it in my car. Not as accurate as GPS if the GPS suffers no drop-outs due to things like trees, buildings and hills. Late model cars are reasonably accurate. The last time, I found only a 3% difference compared to just using the number in the Cyclocomputer manual for my size tire. I don't know which is more accurate, but I trimmed the value in the computer for consistency.

Al

Can't you just roll the wheel out 1 revolution and measure the distance covered, then enter that into the computer?

If you are satisfied with the GPS measurement of 40 miles as being 'correct' then multiply your current computer calibration code by the ratio of the GPS to the computer mileage.

New Code = Old Code X GPS Mileage / Computer Mileage.

Remember to completely accurate you need to figure out the rate of tire pressure decrease over your ride, which will change and figure out how to average. You must also take into account how much time you will spending standing since that puts considerable more weight on the front tire changing the diameter. Oh and don't forget every pot whole you swerve around adds distance that GPS will not pick up. AHH science annoying and fun at the same time. Oh we could get into the Heisenberg Uncertainty Principle, and oh and remember time changes with speed, and more importantly the laws of motion don't apply on earth since we are spinning hence are an accelerated reference which you are not allowed to be.
And whether you go around the corners on the outside, the inside or the middle. And whether you pulled off on the side of the road for a pee, or stopped to fill up a water bottle. And if you stopped at all, it would take about two rotations of the wheel to get the magnet to register on the probe and set the computer in action again.

1.25% error could be explained by some of the reasons the above. I wouldn't be worried about it myself.

Calibrate your CatEye by rolling multiple revolutions with your weight on the bike and measuring the distance (divide the distance by the number of revolutions). It'll be more accurate than the GPS. The long halls in a building are great places to do the measurement.

I like doing the rollout with a little dab of paint on the tire and measure between the spots that it leaves on the floor.

FWIW, my wife, a friend and I all rode an 80 mile day on RAGBRAI and ended the day within .07 miles of each other! YMMV



G'day,

just ride......what does it matter?....FYI, most cars speedo's are only accurate to +/- 10% ...your speedo was wrong by 800m in 64 km?.......gee i wish I had that much to worry about in my life!...lol

Hitchy

Hey, give me a break! I can’t help it, I’m an engineer :D :D

-murray

I had a friend go to a local High School running track to calibrate his computer riding the same lane for miles and miles. I just used the # in the chart. We did a 35 mile ride and by the end of the ride we were 2/10th of a mile different.


Life's too short.........Just go ride!

L8R