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Old 03-18-05, 06:21 PM   #1
HaagenDas
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Cut me some slack please! New Computer.

Okay, so I went out and bought one. K-mart had a cyclo computer going for a lousy Aus$10.00. I figured I couldn't go wrong. Well, I set it all up just peachy and it seems to work quite well.


.
.
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However When I put in my tyre size as indicated I got an unrealistic distance. I have a GPS temporarily set up on the bike to check the computer. And for every 1km of riding the computer was adding around 150 metres. So I wound it back from an input of 2055 to 2015 and the reading is fairly close to the GPS which by the way I trust highly.

Have I set something up wrong? or do I just weigh so much the tyres are much smaller than advertised? What is going on?
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Old 03-18-05, 06:49 PM   #2
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Haggy, my man, the problem is not your weight. Your problem is that you have two measuring devices. You are like the fellow I knew who had one clock and knew what time it was. Then he got a second one, and from that moment on was never quite sure.
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Old 03-18-05, 06:55 PM   #3
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I knew my arch nemisis would be back to haunt me. I hear what you're saying. However, by varying the calibration on the computer, the readings fell in line with the GPS. I just wonder if the computer is set up correctly - and I think it is. It could also be a combo of weight, tyre pressure and size.

Really, I don't want to take my GPS with me everywhere I go, it's too expensive. I would like to have a good idea of how far I go just so that I can actually measure the distance of my improving health if that makes sense. I don't want to believe I can bike 12K when I can really only bike 10.
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Old 03-18-05, 07:12 PM   #4
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There is no standard for tire sizes. Even if there were, I think the same guys wouls still run large or small. You did the right thing. Just take a long ride with our gps, to check the accuracy of the new figure. It may need to be slightly adjusted.
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Old 03-18-05, 07:16 PM   #5
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As a PR man in my professional life, I usually just make up some plausible sounding baloney. But I can't even begin to guess at a suggestion as to whats up with your computer/GPS situation.

However, Haagy, I'm always ready to help a pal. Simply forward the GPS unit to me by overnight express. I'll have it analyzed and calibrated and promise to return it to you sometime within the next eight to 10 years.
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Old 03-18-05, 07:22 PM   #6
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Thanks, it all of a sudden worked itself out
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Old 03-18-05, 07:23 PM   #7
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Ive never used a cheap computer like that,but on Cateyes you can measure the wheel circumfrence then add that into the computer and it works perfect. Although ive never really tried testing it w/ GPS....but ive tested mine against others and they always right there.

I would say spend a little extra cash and youll be happy for a long time.
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Old 03-18-05, 07:28 PM   #8
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I would say spend a little extra cash and youll be happy for a long time.
Have you been talking with my wife? That seems to be her mantra too.
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Old 03-18-05, 07:55 PM   #9
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Have you been talking with my wife? That seems to be her mantra too.
hahaha,i wish my girlfriend was like that.she says im cocky because i know a lot about bikes and I always tell people not to buy cheap bikes and components.she is crazy about that i think hahahaha
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Old 03-18-05, 08:31 PM   #10
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I would say spend a little extra cash and youll be happy for a long time.
Rubbish, all cylocomputers that are sold work off the wheel circumference. So if you don't have a good number to put in, do a roll out test.

http://sheldonbrown.com/cyclecomputer-calibration.html
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Old 03-18-05, 08:48 PM   #11
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I've heard, but don't really know, that handheld consumer GPS units usually under read distance by "staightening" out curves and corners due to averaging. Bike computers, when calibrated properly, measure actual distance traveled by the wheel.

We have a licensed land surveyor on staff. Maybe I'll pose this question to him on Monday.
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Old 03-18-05, 08:54 PM   #12
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I've heard, but don't really know, that handheld consumer GPS units usually under read distance by "staightening" out curves and corners due to averaging. Bike computers, when calibrated properly, measure actual distance traveled by the wheel.

We have a licensed land surveyor on staff. Maybe I'll pose this question to him on Monday.
Cyclocomputer distance travelled will always be more accurate than GPS.
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Old 03-18-05, 08:56 PM   #13
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i was talking about a roll out test,because i also assumed that he didnt do it
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Old 03-18-05, 09:37 PM   #14
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I did do a roll out test but I still use the lower figure of 2014 rather than 2040.

As far as a GPS goes, their accuracy is fantastic. However they will always under read a cyclo computer. The reason for that is they only count distance in a flat plane. When you go up or down a hill the computer will count the length of the road, the gps will only count distance moved, hence the gps distance will show less.
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Old 03-18-05, 10:14 PM   #15
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Is the computer certified for operating in the southern hemisphere?
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Old 03-18-05, 10:32 PM   #16
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Is the computer certified for operating in the southern hemisphere?
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Old 03-18-05, 11:43 PM   #17
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I did do a roll out test but I still use the lower figure of 2014 rather than 2040.
When you did the rollout test were you riding the bike? Your extra weight on the bike will give a slightly lower circumference.

Yeah, I love my GPS too, but it goes nuts under trees and in valleys. Can't really rely on it. Eats AA batteries too...
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Old 03-18-05, 11:57 PM   #18
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Did some more testing.

Did a roll test with me on the bike. Measured distance was only 15 metres. Tested at both 2040 and 2014 and the 2014 was more accurate. I do believe you'd have to test it over at least 100 metres to get an honest reading.

Then went for my longest ride yet. 6 Kms and the GPS and Computer were duelling the whole way. When I got back the computer under read by about 5 metres. Reset the computer to 2020 which should give the best reading. Still that's less than 20 on the "recommended" figures.

Yes, according to the Chinese manual, the computer can be used in Australia and other large wheat export countries.

Got the two wheeler up to 46 Kph, not bad for someone in my condition.

Gawd, the sheila has just pumped up the tires on her old critter.
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Old 03-19-05, 12:10 AM   #19
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When I started riding, I was pretty picky about getting accurate numbers. I quickly found that the approximate numbers the instructions usually give just weren't good enough to suit me.

I sat on the bike while rolling to get the circumference, but after checking it several times for one revolution, I also checked it for 3 revolutions to be sure.

I also found that different brands/models of tires give different results, but now I know exactly at what point on my typical ride I should hit 10 miles and so I use that to make final adjustments.

I was really surprised to find that fluctuations that certainly must occur when riding from one day to the next apparently pretty much even out because it really is almost exactly the same each time.

Personally, I would not treat the GPS as the more accurate item. Aside from possible lapses in signal, you have the problem of how it handles hills someone else mentioned and possible errors based on how it handles curves.
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Old 03-19-05, 12:11 AM   #20
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Mate, just take the bloody front wheel off, get a tape measure, put a dot of whiteout corrector on the middle the tread and measure around the circumference of the tyre until you come back to the dot. Subtract 5mm if you wish to account for your weight on the bike. The figure you arrive is the one to key into the computer.

If you use a measured kilometre as per the signs on highways, there may well be some variation in what your computer reads and the distance between the two signs. If it falls to about 1% you are doing well.

I organised a century ride last weekend, and the variation on the computers of the 106 bikes at the finish ranged from 96 (???!!!) to 102. Of course, as the organiser, MY computer was the most accurate

Are you riding an MTB bike or a hybrid with 700C wheels?

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Old 03-19-05, 12:30 AM   #21
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I'm on a MTB with 26x1.95

Fluctuations would easily be caused by road temperature which would change the pressure in the tyres. I'm happy now with the fact that my GPS and cyclo computer read pretty close.

Believe me, the GPS is one of the most accurate measuring tools around. I've seen the video of a DGPS equipped trawler with the wheel house boarded up pull into it's own berth. The aviation industry which is years behind in navigation instruments is finally granting permission to use the GPS IFR procedures. VOR and radio beacons are 1930s technology. Now with GPS IFR pilots have been able to reduce minima to the lowest levels ever. Whilst aviation panel mounted GPS must use 5 satelitte signals excellent accuracy can be acheived with three. On the Great Barrier Reef they have DGPS now and ships can blast through the reef without slowing down at all.

Yes, when the signal gets ratty you lose accuracy but even when the satelittes were being fed error it was still an excellent tool.
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Old 03-19-05, 03:04 AM   #22
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Sir Les, do a quick google re: gps speed accuracy you will find their accuracy is known to be out by at least 5%, military testing suggests an error of .49 mph real world.
If you enter the correct circumference your bike computer does simple maths to get the readout, it's sensor/transmitter is mm's away not kilometers as with satellites and it also measures in 3d, commercial gps don't.
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Old 03-19-05, 04:28 AM   #23
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Originally Posted by Blackberry
Haggy, my man, the problem is not your weight. Your problem is that you have two measuring devices. You are like the fellow I knew who had one clock and knew what time it was. Then he got a second one, and from that moment on was never quite sure.
That's a terrible misquote of Benjamin Franklin.

Haagen, just be glad you didn't get the wireless with HRM and all that. Nothing like stopping for a cuppa and forgetting to turn it back on!
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Old 03-19-05, 06:02 AM   #24
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This is all so deep and I'm such a simple mind.
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Old 03-19-05, 07:56 AM   #25
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That's a terrible misquote of Benjamin Franklin.
:
Just doing what I do best. I work in journalism. My job is to mangle other people's quotes.
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