Speedo Accuracy

AussieSteve

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I've fitted a (Cateye) bike speedo/computer to my bike and originally set the wheel circumference to the figure in their chart for my tyre: 26" x 1.95" MTB - 205cm.

Today, I got around to checking the actual circumference, under load - 202cm.
(I set the bike with the valve at the bottom and marked the spot, then sat on the bike with my normal weight distribution, rolled forward one turn and measured the distance.)

I also have a cheap Chinese speedo on my electric bike and it was even worse - 208cm recommended, 200cm actual.

I only weigh 50kg, so most people would have a much greater error. Each brand of tyre would also be different. Pressure makes a fair difference too.

I've noticed some of the extraordinary speed and calculated RPM claims for these bikes and wonder if this has something to do with it?

... Steve
 
I've noticed some of the extraordinary speed and calculated RPM claims for these bikes and wonder if this has something to do with it?

... Steve

I tend too agree, unless the speed is GPS certified i treat all claims with a certain degree of scepticism. Cateye are ne of the better quality brands too i hear...There SHOULD be an option to enter in your exact tire size IMO.

KiM
 
Sorry, I didn't make myself totally clear. There is an option on even the cheap speedos to enter the exact wheel circumference, in increments of 1 cm, (which I've now done on both bikes).
The problem only occurs if the values in the supplied charts are used.
As I discovered, it also pays to measure it out by rolling the wheel, fully loaded. A simple measurement of the diameter multiplied by pi will be a fair way off. (Several cm)
Yesterday, my top speed was 36kph. Tomorrow on my next ride, with the accurate circumference?? (I could calculate it, I guess, but I'll wait until tomorrow)

... Steve
 
If you have a GPS. Still, for those like myself with only a speedo, nothing wrong with having it as accurate as possible. (Can't justify the price of a GPS purely for speed measurement)
Incidentally, many of the cheap Chinese speedos don't work on these bikes due to RF interference with the micro-controllers. (See my thread in 'Introduce Yourself') :

http://www.motoredbikes.com/showthread.php?t=23922

The Cateye was the first one that worked reliably for me.
... Steve
 
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totally agree was just mentioning it buddy, i have been using a cheapo brand myself and question its accuracy, i now have a Drain Brain for my new build that measures all the battery info it also has a speedo incorporated hopefully this will be more accurate than my cheapo speedo...The one i have is also wireless & drops out periodically i think due to the length of the fork i use, on old bike with shorter fork it was flawless...

KiM
 
I'd say that even the cheapies would usually be fairly accurate if set up correctly, since the micro program only needs to count turns and add the circumference each time, unlike analogue (mechanical and electric) speedos.
Having said that, the cheap one on my electric bike recorded a max speed of 85.7 kph one day, on a road with heaps of bad potholes. That could only have been caused by chatter of the reed switch sensor. If a power supply was available, a hall effect sensor would be more reliable, like those used for CDI triggering on most small engines.
... Steve
 
I thought it was best to use an actual tape measure on an inflated tire&wheel. I tape the 1" mark to the tire, all the way around with cheap electrical tape, then turn the wheel to measure all the way around the center tread, pulling it taut. Works for me. All the speedos I've owned let you enter in mm for example 2033.

If your speedo bugs out, check the battery with a test meter and replace if necessary, then check your magnet and sensor set up. (or do that first if its obviously moved.)
 
I thought it was best to use an actual tape measure on an inflated tire&wheel. ....... Works for me.

Hello happycheapskate,
It might work for you, but it's not accurate. The rolling diameter under load is noticeably smaller than unladen. As I mentioned earlier, pressure makes a major difference, too. (Do some tests - I did.)

All the speedos I've owned let you enter in mm for example 2033.

Both of mine work in cm, a Cateye and a cheap Chinese one. That's irrelevant, though.

... Steve
 
If the circumference of the tire is 2033, and you enter 2033 on a cycle computer asking for the circumference, how is that inaccurate? How would it be less accurate than measuring wheel roll-out unless you have some really knobby tires, perhaps?
 
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