Wiswell

Hello, we just bought the above c-computer and set the calibration according to the manual chart for the wheel size of 700x23. However, the speed, and average speed is off by 0.4 mph over a 30 mile ride, for example (reads low). My computer is accurate based on past use with cue sheets. In the past when my husband and I rode together, using identical c-computer, the difference in readings was not this significant. This computer uses circumference in millimeters to set - see(https://sheldonbrown.com/cyclecompute...n.html#formula). Sheldon Brown's site suggests that I should set the calibration at a number (2097) higher than that recommended by the manual (2096). This seems like it will help (thinking about geometry and circumferences). Any thoughts? Also, are the wireless less accurate than the wired?

FIVE ONE SIX

the average speed could be off for a few reasons. one thing that very few people realize is that different computers have different settings for when the computer pauses due to rider speed. some computers pause when the rider completely stops, some computers pause when the rider speed drops to 3 mph, ect. that may not seem like a lot, but if you stop for a good amount of traffic during that 30 mile ride, it could either exaggerate (if it's set to pause at 3mph or higher) or diminish (if it's set to pause at a complete stop) your real numbers. and that's just one of the reasons...

take your results with a grain of salt, i know if i ride with a group of riders, everyone in the end will get different results. and the reason for that is more than just the tire size calibration. i say change it to 2097 and see what happens, and take it from there...

and what exactly is off? just the average speed on your computer, or the average speed on both computers? the distance on your computer, or the distance on both computers?

as far as wired vs. wireless accuracy, i can't help you there, i've never use a wired computer...

and i used to use the Micro Wireless too, and i used to get lower numbers than almost everyone else i rode with too, but it didn't really bother me.

Wiswell

My computer, the wired one, is reasonably accurate, relative to cue sheets I've used. (within 1/2 mile over 60 miles, for example).

The wireless is running about 1/10 off per ten miles. Doesn't sound like a lot, but the greater issue is that the speed and average speed are way off, by 0.5, as compared to mine. And we are riding right behind one another. Also, it seems to be longer in terms of time. Could be the point of when it stops reading, as you describe. We'll keep tweaking, and perhaps try the rollout method and actually measure the circumference.

Also, will the relative weight of the rider (due to tire compression extending the circumference), also affect the measuring?

FIVE ONE SIX

also know that if it is because of when it pauses, that will also shorten (if it pauses while you're rolling to a stop) or lengthen (if it pauses when you come to a complete stop) the distance of the ride on the computer too, so that's one way to notice it too...

and i never realized this until i used a Garmin, because it allows you to set the speed that you want the computer to pause, when you come to stop. also, it says when it's paused, and the Micro Wireless doesn't. so, that makes it impossible to see when it actually does pause, unless you call them and speak to someone about it...

geraldatwork

The differences probably due to the reasons the other posters have mentioned. The difference is approximately one percent and that is assuming your other computer was the one that was accurate. I wouldn't worry about it. As far as bike computers go I have them on all 3 of my bikes and find they work very well with very little or really almost no interference from outside sources. If I got a 4th bike I'd put one on that one too.

Wiswell

I just noticed that the tire size B is being used on this computer. On my computer, the instructions indicate that the B size is designed for low speed use. Could this affect the accuracy on a road bike? I will try tonight to reset it at A and see if that makes a difference.

FIVE ONE SIX

it should effect the accuracy, considering the default setting for a A is 2096, so i am guessing the default setting for B is different. and you don't have to reset anything, just switch it from B to A, and make sure A is either 2096 or 2097...

niz16

im using mine on the a setting, but unfortunaly ive got no one to ride with to check if its out, but im only using it as a guide, ride time, check speeds down hill and so on, plus it gives me a rough idea of how far ive riden,

RussB

It you want to increase the accuracy of the computer, try doing a roll-out. You do this by measuring the actual distance traveled by your wheel. Mark a spot on the pavement. Position the valvestem of the wheel with the computer sensor on the mark. Now with your weight on the bike. go forward say 5 full revolutions of the tire. Mark this new spot and measure the distance traveled. divide by 5 (the # of revolutions), now convert this measurement to millimeters. This is a more accurate number to put into the computer.

I found the number in the charts to be pretty close, but this will allow for tire construction and the combined weight of you and the bike.