I find this thread amusing on many levels :D

Yeah, tune the cat-eye computer to YOUR wheel/tire/weight run-out. Off the chart is a shot in the dark.

In a given pace line/ride, we are all off a little... some more than others.

I think It makes sense now...

As long as the correct "tire size" was put into the cateye, it doesn't matter where on the spoke the magnent is (In closer toward the hub or out toward the rubber tire).

Correct?

generally towards the rim = further distance from sensor (on fork) = weaker signal
plus towards rim = higher linear velocity = shorter temporal pulse on sensor = possible mis-read
could underestimate your speed & distance

Correct! In reality the cateye only receives time of arrival data (and then does calcs), but conceptually we can think of the magnet as a tool to measure just RPM. RPM and radius give us distance!

The possibility of misreads can change things, but from a mathematical standpoint there is no impact.

Ah, here we go --

cshell, what do you have yours set to?

I'd give what I have my computer set to, but I haven't really measured it for better accuracy (roll for ten revolutions, measure the distance traveled, divide by 10... or roll for a hundred and divide by a hundred, whatever). I just told it to use what the chart said for 700x23.

23 is the tires width, You need to do a roll out to get it an accurate number. That number will also change as the tire wears.

A couple others have stated it correctly. v=r*w where v is linear velocity of the bike, r is the radius of the wheel, and w is the angular velocity of the wheel. 1 rpm is 1 rpm no matter the distance from the center of the hub.

I've only done a GPS/Cateye Strada comparison once, when I got my Garmin 705 and was geekly curious. Over a 60 mile loop both were within a tenth of a mile of each other. IIRC the cateye was a tenth higher than the 705.

Only check roll out as you tire wears if your really anal ;) . A "standard" 700c x 23 on a wheel is 667.5mm diameter (roll out is 2097mm). If you wear off 1.5mm of tread, you're left with a 666mm diameter wheel & tire (roll out is now 2092mm).
A mile is 1,609,344mm, so with a 2097mm roll out wheel & tire, it takes 767.45 revolutions to do a mile. If your tire really has a 2092mm roll out but your computer is set to 2097mm, over that indicated mile you traveled 1,604,795mm, 4.5m short of a mile. 4.5m is 0.28%. Can you live with your computer off one quarter of one percent???

If your running big old knobbies, thats a different story, but why would you be asking in the road forum :D

I'm not privy to the inner thoughts of my cateye, but I would imagine it isn't actually counting in miles. My guess is that it's counting in revolutions and converting to total miles at the very end of the processs, without discarding it's wheel revolution count. So yes, 300ft increments in displaying how far you've gone at any given time, but not a cumulative error of 6.25%, i.e. the farther your trip distance the smaller the percent error.

Anyway, I cast my vote for it being an error in tire circumference calculation. That would scale linearly with trip distance.

i have two magnets on mine on opposite spokes........ I'm training for a century and I dont have enough time after work to ride as far each day as I'd like, so having two help with my speed and distance!!


( :D )

RD

My catsye cadence is very accurate, and in group rides I get the same data as other people's computers (mostly garmins).

All I can add is that I did a century two weeks ago. Mapped out in Google maps, it was 100.0 miles. My Cateye recorded 100.0 miles. That's close enough for me.

The CatEye depends on mechanical action of a magnet placed somwhere along the radius of the wheel and measures how often magnet traverses certain point. GPS uses trig and triangulation. I'll bet on the trig and triangulation vs the mechanical CatEye variables.

Oh - but my CatEye cost me $9 on eBay. If it's within 1.5 miles over 100 miles that's OK with me !!!

I pulled my specs from the chart provided (too lazy to do a roll-out), and my Garmin Fortrex 101 agrees with the Cateye on my bike to within .2 mi over a 60mi trip...

Most of the error is probably on the GPS too, since it may 'cut' corners or induce random placement errors (one severely erroneous point can add hundreds of feet to your overall distance). A properly mounted, WIRED cyclecomputer rarely incurs any random error... Wireless is a different story.

...and let's not even talk about coverage, GDOP and other sources of errors with GPS...

The short answer is: it all depends

I disagree.

The precision of a commercial GPS unit is not very good. You're only precise to about 10 to 15 ft at any given moment. As such, a GPS cannot tally accurately the total distance traveled unless the route is straight within these 10-15 ft intervals. For example, if you are weaving around cones all day, the GPS cannot detect that; it will only give you the linear distance between your start and end points very accurately though, but your actual sinusoidal path (i.e., distance traveled) will be considerably longer than the GPS linear path. Most of us don't weave around cones on our bike rides, but we do make many small movements that a GPS cannot detect, but can add up to a significant chunk at the end of a day.

The Cateye's precision and accuracy is all dependent on its clock (crystal oscillator) and the wheel circumference setting configured by the user. Digital clocks are very, very accurate and precise nowadays. Even cheap ones. It's actually quite hard to find one that will be off by a minute every day and most reasonable ones are off by a minute per month. Given this level of accuracy and precision, and given that the wheel circumference is programmed correctly, one shouldn't see more than a 1% error rate in the Cateye's readings. In fact, I would expect that the limiting factor will be the measurement of the wheel circumference, which measured to the nearest centimeter for a 700C wheel, should give an error of no more than 0.3%.

I don't know if further accuracy is required for my purposes.


What does a 4-bit resolution got to do with the Cateye's accuracy?

This thread is so full of fail I don't even know where to begin...

The commercial GPS units have the same resolution as the military units, ever since they removed "selective availability". The GPS may only be accurate to x distance but the accuracy is not random, it doesn't rapidly jump all over the place from sample to sample. It does drift over time though, so you will notice if you ride in a stationary position for an hour you may get a hundred feet or so of "movement" detected. You have a far higher chance of your tire circumference being off enough to accumulate more error than with the GPS.

Maybe I'm not understanding this correctly. As I understand it, GPS cyclocomputers do not calculate a distance from point A to point B by doing a vector analysis between point A and B. If it actually does that, I would agree that if A and B were far apart, the GPS system will give a more accurate reading. Instead, these cyclocomputers tally up a series of small distances into a cumulative total distance.

However, as you said so yourself (underlined), there are fluctuations in GPS units that introduce significant errors when measuring small distances. These small errors, when added up, will give you a larger error than the tiny 0.3% error (estimated in my last post) of a conventional cyclocomputer.

Taken another way, what is the smallest distance a device can measure with any sort of accuracy? For a conventional cyclocomputer, it depends on the wheel circumference (at least for the Cateye) so about 2 meters (or less if you have smaller wheels). Every revolution of the wheel will give you 2 meters +/- 0.3%. For a GPS unit, it's significant larger than 2 meters so already you're behind in accuracy. Of course, I'm sure the GPS wizards have algorithms that can reduce this error rate, but you're still stuck with a system that's not really intended to measure small distances accurately, while being required to add up these small inaccurate distances.

1) you're cheating :)
2) you're doing what I do after I've changed the battery on 4 of the 6 Cateye Astrale's I unfortunately own - they reset odometer to zero so it takes me about 10 magnets until I catch up. Somehow the Astrale models from 1994 work just fine as does the cheaper Mity which are all able to retain their odo during a battery change.
3) Cateye Astrale can only read a max speed on 186mph.

As I said, you can measure the approximate amount of error introduced by just letting the unit sit in place for an hour. An hour at, say, 20mph in 105,600. If you have 400 feet registered in that hour (about what I have observed), that is .3% as well. So the error amount is similar.