Cateye Calibration with Google Earth

nfrang

I may be being a bit anal or have too much time on my hand's but it was the bank holiday and seem to have got a pretty good result.
After a recent ride i reset my Cateye Strada and clocked the last 3.5 miles or so on the road home.
Checked this with the rulers on Google Earth and over the distance the Cateye reading was nearly 100 metres out(not massive i know but i figured if i'm gonna use the thing it could be more accurate than that).
So the maths bit went like this. Converted everything to mm cos that's how the wheel circumference is entered on the Cateye.

Clocked 5680984mm divided that by 2093mm(current wheel size inputted)
= 2714.278 Wheel revolutions.

Actual from Google Earth 5600517mm divided by wheel revs 2714.278.
= New wheel size for Cateye 2063mm!

Done a few tests since and reckon its now accurate to within 15 metres(maybe less).
Thought i'd share it cos i couldn't find anything similar on the forums.

Used this for the conversions...
http://www.onlineconversion.com/

alfablue

Hmmm, are you sure Google Earth will give the most accurate reading? I have my doubts. Your actual path along the roads may differ from the line taken in Google Earth.

Did you actually measure the wheel circumference or just enter what the book said for your tyre size? You should measure it, I found mine was quite a bit different from the expected circ.

(The cateye should be accurate when set correctly providing there are no occasions where it misses data, say if the sensor is too far from the magnet).

nfrang

Yeah i'm fairly sure Google Earth is accurate...did check the start finish line on the local schools running track and it came in bang on 100m.
I'd measured the wheel normally by marking the ground but its hard to get exactly right.

Reckon it's more accurate now anyway...most cars speedos are out by a couple of mile an hour!
GPS would be the best thing but i'm not a serious enough rider to spend £300+ :shock:

Sure i read somewhere that bike computers are more accurate if the sensor is located closer to the hub??? Any thoughts?

alfablue

Yes, I had heard that too (sensor nearer to hub), but I don't know why that should be.

A gps would be less accurate, they invariably lose signal occasionally and the track cuts corners (I use one and you can see this when uploaded to Sportstracker/Google Maps). Actually, you can get a gps for free - I got a Nokia N82 with built in gps for a contract of £22/month on Orange. Using the free Sportstracker software it is brill!

stumpyjon

Hmm

I've had similar issues. I have three Cateye Strada wirless units running on three diferent bikes, all with different tyres. I've set each one up after measuring tyre circumferance to the mm with a tape around the circumfrance. All three give me higher milage readings than my Garmin Vista HCx GPS unit and they also give a higher reading than if I plot the routes in Tracklogs on the PC. When the first one was out I assumed it was a duff unit and altered the tyre circumferance on the unit until it measured the same as the other Garmin.

When I discovered the other two also read high I started to think about it a bit more carefully. One thing I'm farily sure is that Tracklogs (and probably the Garmin) only log distance horizontally. I've drawn a line between two grid squares on Tracklogs across very steep contours and it still logs the distance as exactly 1km. As the Garmin seems to agree with Tracklogs (and it logs distance based on GPS) I'm assumiing that neither Tracklogs or the Garmin take into account distance travelled due to gradient :shock: .

However I've also tried to work out the extra distance due to gradient using trig and whilst it's not insignificant it doesn't seem to fully account for the discrepancies.

So now I base my ride distances on the Cateye units as they suggest I've been further and gone faster . Don't know if anyone else would care to comment (and yes I know I'm probably a bit anal too).

cougie

Why would the proximity to the hub make any difference ?

I used to race the local 10s - they are as accurate as anything - so if the reading was high - I'd lowe my circumference slightly. Little things like tyre pressure on the day make a significant difference - so another 10PSI in and you could be out again ?

alfablue

stumpyjon wrote:

Hmm

I've had similar issues. I have three Cateye Strada wirless units running on three diferent bikes, all with different tyres. I've set each one up after measuring tyre circumferance to the mm with a tape around the circumfrance. All three give me higher milage readings than my Garmin Vista HCx GPS unit and they also give a higher reading than if I plot the routes in Tracklogs on the PC. When the first one was out I assumed it was a duff unit and altered the tyre circumferance on the unit until it measured the same as the other Garmin.

When I discovered the other two also read high I started to think about it a bit more carefully. One thing I'm farily sure is that Tracklogs (and probably the Garmin) only log distance horizontally. I've drawn a line between two grid squares on Tracklogs across very steep contours and it still logs the distance as exactly 1km. As the Garmin seems to agree with Tracklogs (and it logs distance based on GPS) I'm assumiing that neither Tracklogs or the Garmin take into account distance travelled due to gradient :shock: .

However I've also tried to work out the extra distance due to gradient using trig and whilst it's not insignificant it doesn't seem to fully account for the discrepancies.

So now I base my ride distances on the Cateye units as they suggest I've been further and gone faster . Don't know if anyone else would care to comment (and yes I know I'm probably a bit anal too).

Don't you see the odd corner cut with the gps track? It only approximates your route, and your track will probably take the fastest route round bends from time to time. Add that to the vertical anomaly and the gps will always read less.

jmillen

Funny this topic should come up...just my 2p's worth...

When I was setting this unit up I wondered how the unit knows where the sensor is positioned. For example:

You input a "standard" wheel diameter, lets say 762mm (26" diameter + 2" for tyre).

The diameter that the sensor on the spokes spins at is different to the above diameter (because the sensor isn't placed on the extents of the tyre).

When the unit uses the diameter to work out the distance and speed, parts of this are going to be incorrect. If you set the unit to use the larger diameter (the outside of the tyre)and left the sensor as close to the outside of the wheel as you can, then although the circumference was correct, the number of revolutions the wheel has made will be slightly out (it'll be slightly higher due to being closer to the hub), so therefore so will the distance and speed.

If you set the diameter to that of the actual sensor on the spoke, rather than the tyre, then there would be no problems with the number of the revolutions, but the circumference of the wheel will be wrong, meaning so will the distance and speed.

Seems you can't really win...I wonder if the unit accounts for the difference between the edge of the tyre and sensor location...

Or of course, I could just be talking sh*te?

cougie

I think you are talking shi*e ?

You tell it the circumference. So one click as the magnet goes past is one revolution. doesnt matter where the magnet is surely ?


Unless its me ? :?:

jmillen

Lets take it to the extreme...

If the magnet were right down by the hub, it will register one revolution alot quicker than it will measure one revolution if the magnet were right up by the tyre as it has less distance to complete a full circle.

Once anything to do with the revolutions are wrong, then so is the distance and speed as thats what it uses to work those 2 things out.

Granted, it may only be a small difference. But over a long ride, it'll soon add up.

cougie

But look along the spoke - you'll find that the top of the spoke goes past the sensor at the same time as the bottom of the spoke does ? Unless your wheel routinely falls apart ?

I've used bike computers like this for decades - its not like its witchcraft or anything ?

GPS is less accurate as it does cut corners - I can see it on my own GPS recordings - the standard bike computer is best.

jmillen

This is also true....yeap I'm talking sh*te then

So basically bang in your overall diameter and off you go...suits me and also means I've actually gone as far as my computer is telling me

Nice one cougie !

cougie

Good - you almost had me doubting myself here ! ;-)

alfablue

jmillen wrote:

This is also true....yeap I'm talking sh*te then

So basically bang in your overall diameter and off you go...suits me and also means I've actually gone as far as my computer is telling me

Nice one cougie !

nope - circumference.

nfrang

One revolution is one revolution regardless of where the sensor is....the only reason i can find(discussion in another thread) for it being nearer the hub is because it's travelling a lot slower so less chance of it miss-reading. Not that important though imho.

cat_with_no_tail

I use GE to plan my routes all the time, I think it's fantastic, but I would never use it as an exact measurement of distance.
Perhaps I am being a bit of a spaz here, but how were you able to use the rulers on GE to *accurately* trace your path home? 100m on a level, straight track is one thing, but 3.5miles with (presumably) corners is a different kettle of fish. suppose you run wide round a corner, if you follow the curb with the ruler on GE, you will already be a few meters out, just for 1 corner. Even if you were to ride on the kerb all the way home and then use that to measure on GE, it's still not going to be accurate because the ruler will not follow the EXACT line your wheels took. Unless you ride down the middle of the road, following solid white lines
I am not saying that computers are always going to be accurate to the mm on every ride, but I personally qwould be more inclined to believe what my Micro Wireless is telling me than GE.

cougie

You can use Gmap pedometer - that has a facility to follow the road - be a bit easier than straight lines everywhere.

nfrang

It's not an exact science i know...more a bit of fun. Only changed the circumference by 30mm so at worst it's gonna tell me i've done slightly less distance(could make it huge and feel like a hero after every ride

I'm suprised people dont find bike GPS systems too accurate. Never used one but i know the technology is used in construction/surveying...that sort of thing and it must get them to within a cats c*ck hair. I guess the signal will drop off occasionally though especially on cheaper models.

cougie

Different needs. GPS tracking devices on bikes dont need to be massively accurate - Surveying stuff does need to be.

benneally

As for the question to why they advise you to place the sensor as close as possible to the hub, the only thing i can think of is that the magnet, attached to the spoke, will feel less centripetal force the closer it is to the hub (as it is moving slower than it would be if placed near the edge of the wheel) and therefore is less likely to slip/move up the spoke when you are gunning it.

I would always trust my bike comp to a GPS. STUMPYJON you are completly correct. The GPS/Maps dont take into account distances due to altitude gain. These are quite large over a long ride. Also as its been said before corners are also a cause of discrepancy. Also when you ride, you dont ride in a straight line, you weave to find the best line. Especialy going uphill.

stumpyjon

benneally wrote:

Also when you ride, you dont ride in a straight line, you weave to find the best line. Especialy going uphill.

Yep, that's me, weave all over the place (and that's on a flat fireroad). Nice to have my half feverish musings confirmed (usually end up mulling it over on some horrible steep climb somewhere). Will now start using the Cateye for distance and the GPS for route logging and finding.