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I bought a used road bike that had a Polar CS200 computer already installed. I’m curious how accurate the speed and distance functions are on these non-GPS units. Has anyone taken the time to actually test how accurate (not just the Polar, but biking computers in general) these things really are?
 

· Adorable Furry Hombre
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If it is not using GPS--speed/distance is as accurate as the wheel rollout measurement (measurement of the circumference of the tire under rider load). The speed sensor only counts how many times the magnet passes. You multiply the number of wheel passes by the wheel rollout and you have distance, you divide that by time and you have speed.
 

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I fully understand who it works, my question is how accurate are they. The manual tells you how to enter the tire size, that which is stamped on the tire. If they are setup per the manual with proper tire inflation, how accurate are they?

If they tend not to be accurate I know I'll need to tweak the tire size.
 

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missin44 said:
I fully understand who it works, my question is how accurate are they. The manual tells you how to enter the tire size. If they are setup per the manual with proper tire inflation, how accurate are they?
It depends on how much you weigh, inflation pressure, and the tires you use.

How much you weigh and PSI alters tire deflection. You sit on your bike-you deflect the tire shape changing the rollout length a bit. No two tires are the same circumference to start with either. The inaccuracy in wheel-sensor-based computers derives from those factors. There is no universal answer therefore as to how inaccurate they are.

Generally, your bike computer will be a bit more accurate than your car's speedometer presuming a reasonably accurate rollout measurement. Of course, your car is not measuring wheel rotation it is measuring transmission speed.
 

· Resident Dutchbag
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If the tire size is correct they are as accurate as their clock.
 

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rogger said:
If the tire size is correct they are as accurate as their clock.
I have cateye wireless 7 and at the most it has been off by a tenth of a mile go enough for me and sometime right on too based on my car mileage. All i really want is some idea of distance of where i went. hope this helps.
 

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francoaa said:
I have cateye wireless 7 and at the most it has been off by a tenth of a mile go enough for me and sometime right on too based on my car mileage. All i really want is some idea of distance of where i went. hope this helps.
Your car mileage is more inaccurate than your bike computer, assuming a decent rollout on your bike.


Your bike computer takes a wheel circumference and calculates distance from that. It actually measures the distance you travel from known and measured quantities.

Your car measures the transmission speed. Assumes appropriate inflation. Assumes weight of load for deflection calculation. It also assumes tire AND wheel size. Your car odometer/speedometer NEVER actually measures distance. It infers it-albeit good enough for most purposes. You also are unable to correct for flagrant flaws in all the assumptions in your car.
 

· turtle killer.
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Most cars base speed/odometer on a gear in the transmission. If your tires are not inflated at the manufactured spec, not the original size, have any casing deviation etc.. they can be off too. The method that bicycle computers use is the same as a measuring wheel, which are pretty accurate.
 

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Avocet used to claim 99.9% accuracy with their cyclometers, presuming you performed a rollout measurement. I think that's accurate enough.

I went on a ride today with some friends. Afterwards, we compared distance numbers. Between the two of us, the variation was 2.8%. Close enough. I have no idea whether the friend used a rollout measurement; nor can I remember if I did.
 

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missin44 said:
I fully understand who it works, my question is how accurate are they. The manual tells you how to enter the tire size, that which is stamped on the tire. If they are setup per the manual with proper tire inflation, how accurate are they?

If they tend not to be accurate I know I'll need to tweak the tire size.
Although I've used the much-touted "roll out measurement" to calibrate computers in the past, the last 6 or 7 Ive done (many bikes in this family) I've simply used the pre-programmed code for the tire size. I've found it to be plenty accurate. When checking with my Garmin gps, it's generally within a few 1/10 mile in a 20-30 mile ride. I have absolutely no need to be any more accurate than that. I could care less if my ride is 24.8 or 25.3 miles.... It's "about 25".

I remember once checking against the mileposts on a local highway on a 30 mile ride and it was within 1/10 mile every 10 (similar to the above GPS comparison).

My advice is to just use the pre-programmed code and you'll be fine. The roll-out measurement is a waste of time and only for those who think precision is much more important than it is ... and they probably don't get significantly more accurate measurements from that method anyway.
 

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I use a Cateye Microwireless (the best computer I've ever used) and I think it's VERY accurate. I was within 2/10 mile of someone with a GPS on a century. For all I know he might have looped around the parking lot once.

In other words, bike computers are as accurate as car speedometers, maybe more so, so long as you're putting in the right wheel measurement.
 

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Inaccuracies would really lie in tire differences. I'm assuming the original calibration by the factory would be based on a "typical" 700x23c tire. There's really too many differences between brands and sizes to get the 100% correct readings. Some computers allow exact, manually-inputted measurements.

The value of such inaccuracies is little. It will get larger over longer distances, but quite frankly, you can't always squeeze in the extra mile to make up for it on the route just like that. For speed, you can use whatever numerical values as references for pacing.
 

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It's only as accurate as your roll out measurement. You must weight the bike, so you get the proper "tire drop". One way to do it is to lay a tape measure on your driveway, get on the bike and push with your toes, three revolutions of the front wheel ) or back, if you have a rear wheel pickup)
Divide this number by three, convert to mm, and this is your circumference. If you want to be dead on, make this measurement, three or four times, and take an average.

Wireless bike computers aren't as accurate as "wired" computers, because power lines can interfere with the signal.
 

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I'd guess they are very close to 100% accurate for measuring the way they were programed to.

I have one 50 mile ride I do quite often and I always pump the same tires to the same pressure before the ride and the measurements are always within 2/10th of other times and even that small variance I would explain by not going in a perfect straight line and not taking corners the same every time.

So who cares as long as it's consistant.

The average speed is all messed up on those things though. It's generally way to low so I add a few MPH before I post about it on the internet.
 

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the police here place radar-calculated speed displays in some residential areas.

my ancient Cateye Vectra always agrees exactly with what the device shows.

my guess is that most computers are pretty accurate overall is the correct tire factor is used.
 

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ok i have been wounder this also
does it matter here on the spoke you put the magnet if it lover on the spoke it would read faster is what i thinking
is this true or not
 

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The average speed is all messed up on those things though. It's generally way to low so I add a few MPH before I post about it on the internet.
LOL!! The average speed is your average speed and is just as accurate as your measured distance is, it's just that when you are starting and stopping a lot it has a big effect on the average.

I use average speed over particular routes to measure performance and improvement, and although it varies from day to day according to wind and how often I need to stop, I know that when my average speed over a certain route is 0.5 - 1mph faster consistently (i.e. from week to week) I am fitter. This is well within the range of error on the cycle computer if I have not done a roll out test. I get a 2% difference on the roll out test from the standard recommended figure for the tyre size (the recommended figure overestimates my speed and distance).

The method I use for the test is to pedal the bike slowly for one tyre revolution indoors while using a wall for support (if you get the distance from the wall just right you can be almost, but not quite, balanced on the bike the way you would be when riding with your shoulder touching the wall). Repeat several times and average.
 

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ok i have been wounder this also
does it matter here on the spoke you put the magnet if it lover on the spoke it would read faster is what i thinking
is this true or not
Not... the computer just counts the number of wheel revolutions and uses this to measure distance and speed based on the tyre size. The number of revolutions is the same whether you are looking at a point on the spoke nearer to the hub or nearer to the rim. The magnet will be traveling faster if it its nearer to the rim, but this isn't what the computer is measuring.

Does the hour hand of a clock point to a different time at the tip from what it does at the bottom? :)
 
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