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Old 05-03-04, 08:15 PM   #1
Person
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How Accurate is your Computer?

How accurate is your cycle computer? I just put on a Sigma BC1200. I road a mile and marked exactly on the road where it came up a mile. I then jumped in my truck and drove the same route. It probably came out to be about 1/15 of a mile off.

My truck could be off or the computer could be off; I am not sure. Is it impossible to get them to read perfectly accurate?

Thanks,

Person
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Old 05-03-04, 08:37 PM   #2
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This is from one of my posts last year.

An easy way to do a roll out measurement it this:
1) Pump tyres up to pressure
2) Find a line to follow, gutter, concrete groove in driveway etc.
3) Place valve at bottom of tyre
4) Sit on bike and roll out 5 complete rotations. Measure the distance.
5) Repeat 2 more times.
6) Add the 3 measurements together and divide by 15. This should give a very accurate measurement.

Even with an accurate measurement it can still only get to within +/- 1% as when we ride we rarely ride in a perfect straight line and we take different lines through corners than other cyclist's.

CHEERS.

Mark
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Old 05-03-04, 09:26 PM   #3
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Quote:
Originally Posted by Person
How accurate is your cycle computer? I just put on a Sigma BC1200. I road a mile and marked exactly on the road where it came up a mile. I then jumped in my truck and drove the same route. It probably came out to be about 1/15 of a mile off.

My truck could be off or the computer could be off; I am not sure. Is it impossible to get them to read perfectly accurate?

Thanks,

Person
Is your computer calbriated to your wheel size? If so, it shouldn't be that inaccurate.
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Old 05-03-04, 09:33 PM   #4
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I was wondering that too. Theoretically, the faster moving part of the wheel is higher up, but then again, no matter where on the wheel you put the sensor, it still is going to pass the sensor the same amount of times...Anyway, my CatEye enduro seems to be pretty right on. It once registered that I was traveling at 37 mph in a 20 mph zone on m mountain bike(steep downhill in a park), and my favorite local trail, which is 2.5 miles one way, came up as 2.3 miles on the computer, but I carried over a muddy area.
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Old 05-03-04, 10:00 PM   #5
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I believe mine to be pretty accurate, since it's based on a satellite signal I get in real time that tracks me as I'm moving.

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Old 05-03-04, 10:05 PM   #6
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Quote:
Originally Posted by Koffee Brown
I believe mine to be pretty accurate, since it's based on a satellite signal I get in real time that tracks me as I'm moving.

Koffee
Oh my!!!
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Old 05-03-04, 10:06 PM   #7
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How much did THAT cost koffee
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Old 05-03-04, 10:13 PM   #8
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Quote:
Originally Posted by Koffee Brown
I believe mine to be pretty accurate, since it's based on a satellite signal I get in real time that tracks me as I'm moving.

Koffee
heehe do you have to pay a monthly service charge
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Old 05-03-04, 10:17 PM   #9
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Quote:
Originally Posted by Koffee Brown
I believe mine to be pretty accurate, since it's based on a satellite signal I get in real time that tracks me as I'm moving.
GPS isn't always super-accurate either. CEP can be as low as 12ft. but can be as high as 300ft. or more... especially if SA is in effect. Now of course you may be able to get to the 1ft. accuracy scale using dGPS or WAAS but that's also subject to SA. So the problem is that while GPS may be (most of the time) more accurate than a standard wheel-sensor based recording device, it is not reliably accurate. The amount of jitter can be considerable. The best system is a hybrid one. That is to say that like those found in aviation units where an INS uses GPS with WAAS/dGPS to checkpoint itself.
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Old 05-03-04, 10:19 PM   #10
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Very Accurate.

Don't use one.
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Old 05-03-04, 10:45 PM   #11
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Quote:
Originally Posted by Koffee Brown
I believe mine to be pretty accurate, since it's based on a satellite signal I get in real time that tracks me as I'm moving.

Koffee
mine has a rocket launcher on it to remove obstacles or chubby hikers.
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Old 05-03-04, 11:04 PM   #12
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Assuming the organisers of the ride did accurate measurements themselves, I'd say my bike computer isn't entirely all that accurate... well... at least not on the second day.

First day was supposed to cover 100 miles....



Second day, 106 miles..



Also keep in mind that stray EM can sometimes screw with the readings too. Here's a maximum recorded speed on one descent that ended with me standing on top of a traffic sensor. The EM pulses that the sensor (essentially a metal detector) uses to induce an EM response field in order to detect the metal of cars can play havoc with your wheel pickup and create anomalies.
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Last edited by khuon; 05-03-04 at 11:14 PM.
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Old 05-03-04, 11:08 PM   #13
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146.1 miles/hour...hmmm...New York to Chicago in.....

As for the first pic, COMEON! for a 206mile ride your bound to be a bit off! For that distance I would give or take 10 miles
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Old 05-03-04, 11:15 PM   #14
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Quote:
Originally Posted by mindbogger
146.1 miles/hour...hmmm...New York to Chicago in.....

As for the first pic, COMEON! for a 206mile ride your bound to be a bit off! For that distance I would give or take 10 miles
Actually, that's what another rider at the finish line said to me when I pointed out the mileage on my computer. He showed me his and it was off by about 8 miles (reading high). I guess what I should have said was that it wasn't entirely unexpected to see a discreption but that there is one.
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Old 05-04-04, 06:03 AM   #15
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I calibrated both of mine (roadie and MTB) by measuring the rollout while on the bike and found that there's about a 2-3% difference from what is given in the wheel sizing charts. Not a big difference, but enough to notice on longer rides like khuon mentioned.
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Old 05-04-04, 08:22 AM   #16
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Quote:
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I calibrated both of mine (roadie and MTB) by measuring the rollout while on the bike and found that there's about a 2-3% difference from what is given in the wheel sizing charts. Not a big difference, but enough to notice on longer rides like khuon mentioned.
Well, I calibrated with the rollout method too using an average of several back-to-back rollouts. Also note that several factours will effect tyre dimensions slightly across the course of a ride. In addition, the computer itself may not have as fine a precision when setting the dimensions. My Shimano FlightDeck has 5mm granularity for tyre circumference.
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Old 04-26-06, 06:57 PM   #17
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Any recommendations as to where to mount the Cateye Micro Wireless sensor (distance from hub) to attain a fairly accurate measurement. The manual says at the top of the fork, but this seemed a little unclear...any tips?
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Old 04-26-06, 07:16 PM   #18
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Suggestion ...

Quote:
Originally Posted by deuce_nuts
Any recommendations as to where to mount the Cateye Micro Wireless sensor (distance from hub) to attain a fairly accurate measurement. The manual says at the top of the fork, but this seemed a little unclear...any tips?
It doesn't matter. One trip around is a single rotation, it doesn't matter at what distance from the hub it is measured.

My hunch for making a unit more accurate is to get an extra sensor and put the second one on the exact opposite spoke. Than you input a circumference half as much as you roll out. This will keep the speedometer from jumping around and making large jumps.
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Old 04-26-06, 07:36 PM   #19
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Shouldn't it be better to get the magnet as close to the sensor as possible?
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Old 04-26-06, 09:40 PM   #20
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... oh yeah, thanks! Sometimes my fingers type before I think...

Happy trails!
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Old 04-27-06, 07:34 PM   #21
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Quote:
Originally Posted by deuce_nuts
Any recommendations as to where to mount the Cateye Micro Wireless sensor (distance from hub) to attain a fairly accurate measurement. The manual says at the top of the fork, but this seemed a little unclear...any tips?
The distance from the hub doesn't matter - what does matter is the distance of the sensor from the computer. Cateye says keep it within 70mm to not compromise reception and get crappy readings, hence the top of the fork.

I have mine mounted about 8-10" above the axle to and it's well within the 70mm specified. Remember, the sensor mount doesn't have to sit square on the fork - the sensor swivels in the mount so you can get it close to the magnet and still have the face of the sensor square to the magnet. If you like, I'll take some pics later on tonight to illustrate what I mean.

Edit: BTW - the manual says to have the magnet within 5mm of the sensor as it goes by...
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Old 04-27-06, 08:19 PM   #22
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Quote:
Originally Posted by Person
How accurate is your cycle computer? I just put on a Sigma BC1200. I road a mile and marked exactly on the road where it came up a mile. I then jumped in my truck and drove the same route. It probably came out to be about 1/15 of a mile off.

My truck could be off or the computer could be off; I am not sure. Is it impossible to get them to read perfectly accurate?

Thanks,

Person
Perfectly accurate? "No" Werner Heisenberg. Close enough? "Yes" any engineer. A local agency put 1/2 and 1 mile marks on a local flat, straight MUP. My bike was 0.4% low. I went back the other way dodging and weaving like there were a lot of other path users. Now it read 2% high. Close enough.
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Old 04-27-06, 08:36 PM   #23
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there's also a lot of discussion in this thread from the mechanics forum:

Circumference Question
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Old 04-28-06, 04:29 PM   #24
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my 1200 was dead accurate after doing the wheel measure thing. I started at the valve stem down and marked a spot then rolled the wheel until it was down again and made another mark then measured. different tires use different settings, even ones with the same width.
I have done the same thing with my 1600 and its just as acurrate
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Old 04-28-06, 09:47 PM   #25
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I just got the same computer today as the poster did. After calibrating with the instructions, I found that mine was about 6/100 of a mile off (under). That's good enough for me. I might recalibrate based on what I've been reading here, but i don't think i'll get or need much better.
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