Maths question about setting up a Cateye

[jay clock]

[{START}NERD ALERT]

OK, so anyone better at maths than me?

I have put new batteries in my Cateye Micro Wireless and the chart says my tyre size equates to a circumference of 2023mm

I put this in, and then rode today. The Garmin reads consisently a bit lower. My experience is that the Garmin matches very closely to RWGPS and I happy to trust it.

When the Garmin read 40.00km the Cateye said 40.18 (I think!)

What is the new figure I need to put into the Cateye to get it to read a little bit less, and align with the Garmin?

I think it is 2.01393 so 2.014

And ideally the calculation, so I can do some test rides and adjust if needed

[{END}NERD ALERT]

 

[I like Skol]

No need to mess about with percentages or witchcraft, it is really simple.

IF you are confident/happy that the Garmin/RWGPS figure of 40km is the 'true' figure then adjust your wheel size setting as follows

New wheel size = 2023x40.00/40.18

 

[I like Skol]

I did the math and you are right, the corrected wheel size setting would be 2014mm

Just for anyone else who needs it;

Corrected wheel circumference setting = Current wheel size value x true distance/recorded distance

 

[Bollo]

Unfortunately the difference might not be due to the wheel size alone and could be very different next ride. TBH, an error of 0.18/40 ~ 0.27% without GPS (Cateye doesn't have GPS I assume) is pretty good! I doubt you'll get them as close that often.

Next time we're out on a ride I'll treat you to a lecture on signal estimation, Kalman filters, dead-reckoning, sampling, statistical and systematic errors. Or you could move house now. Or set fire to mine.

 

[Brandane]

I also ride with both a Cateye (Strada wireless) and a Garmin. The Cateye always registers a slightly higher mileage than the Garmin. I have always assumed that the reason for this is that the Garmin is set to "auto pause" at anything below 4mph; therefore it isn't going to record for a few yards as I stop and start at junctions and the like.

 

[jay clock]

Thanks! I must admit the start stop thing did strike me as a possibility. I will have to do all rides as non stop time trials

I will get working on trying to optimise it.

 

[jay clock]

[QUOTE 5134583, member: 43827"]I have sometimes wondered, in moments of madness, about the margin for error of Cateye. I understand the theory of distance based on perimeter of wheels/tyres, but confuse myself by wondering about the impact of issues such as tyre pressure and weight of rider. Is the radius of the wheel, and therefore the perimeter, smaller when ridden by a 15st rider at 80psi compared to to a 10st rider at 110psi? [/QUOTE]
I think it is

 

[deptfordmarmoset]

Another factor might be the fact that the front wheel continually does the little balancing adjustments, meaning it travels further than the bike itself. It simply goes in less of a straight line. If you can manage to get a wireless signal - or if the wire is long enough - for a rear wheel mounting, you might get closer to the actual distance using the Cateye wheel size measurements.

 

[I like Skol]

 

[Shut Up Legs]

[QUOTE 5134583, member: 43827"]I have sometimes wondered, in moments of madness, about the margin for error of Cateye. I understand the theory of distance based on perimeter of wheels/tyres, but confuse myself by wondering about the impact of issues such as tyre pressure and weight of rider. Is the radius of the wheel, and therefore the perimeter, smaller when ridden by a 15st rider at 80psi compared to to a 10st rider at 110psi? [/QUOTE]
I don't believe I've ever inflated myself to 80psi, much less 100!

Oh, sorry, you meant the tube, didn't you?

 

[Tim Hall]

[QUOTE 5134618, member: 9609"]I always use Bike Hike as my calibration tool for the bike, I also use it for the car, van, truck mileometers etc. it gives a level playing field for fuel consumption.
I reckon it is probably as accurate as you can hope for
http://www.bikehike.co.uk/mapview.php

I know my cateye non wireless shows a little extra, the wheel is 2136mm and my wheelsize options are 213 or 214[/QUOTE]
So that's within 5/2136*100= 0.23%. Over 100km you'll have an error of 230m. That's cock all in the great scheme of things probably less than other errors.

 

[Siclo]

Don't forget to adjust for the reducing circumference as the tyre wears

 

[nickyboy]

Then you have to deal with that massive issue of the fact that GPS measures linear distance (I think), whereas a Cateye will measure actual distance your tyres have travelled. All those ups and downs....basic trigonometry innit

 

[Globalti]

I find Bikehike agrees with my Cateye to within a tenth of a mile, which is good enough. My Cateye also agrees with my cycling pal's Cateye so we always compare distances and averages after a ride - he is a little faster than me and sometimes has to wait so he usually averages about 0.5 mph faster.

 

[confusedcyclist]

Even the army can't precision strike a target, that's why carpet bombing is a thing

Then you have to deal with that massive issue of the fact that GPS measures linear distance (I think), whereas a Cateye will measure actual distance your tyres have travelled. All those ups and downs....basic trigonometry innit

In a similar vein, if you measure the perimeter of the coastline of the UK, the finer detail you look at the coast, i.e. do you just look at the line on a map, or go way down to the grain, or even subatomic, the longer the perimeter becomes. And that's before you take into account that grains of sands and organic materials are constantly moving around.

The lesson here is that if you try to reduce reality to down to abstracted absolutes, you're doomed to make errors, and what's true in one moment, will not be true in another. Reality is far more flexible than we dare to imagine.