Bicycle Speedo On Scooter

[sidthesloth]

Kymco speedos seem to be quite optimistic. When I held my GPS unit on a ride, 70kmh indicated was 61 by the GPS, 60 was 56.
So I went and bought a bicycle computer to put on the ZX. Now, I have rolled out the wheel and come up with 1422mm. The speedo unit I bought stated as being for 20" to 27" wheels. I put in my circumfrence and this speedo worked Quite well, it appeared to be in line with what I got with the GPS.
BUT BUT BUT,
81.9kmh, she dies, goes backward, 79 75 69 57 then hovers around until I start to slow down. This unit is suposed to go up to 99.9 kmh or 99.9 mph. My thinking was that the wheel is only 12" diameter, so maybe it is too many rpm for the little computer to cope with, but if so, what about 90mph, I thought maybe too much clearance between the magnet and pickup, so I closed that up to as close as I can safely, still the same.
Anyone have any ideas, or should I just accept it as a dud and buy a different brand? Not a big issue, just an annoying curiosity.

[Rianna]

Hmm...so it works...then it stops.

Could be a defective one or it could be an overstated claim by the manufacturer?

This is interesting, though, since it behaved more accurately. I would like to see this one work, sid.

[omfgdotcodotuk]

Send it back sid as its like opening a packet of crisps to find you got sardines (not as described) i have used bicycle computers on mopeds before with great results,but this one sounds a lil dodgy to me bud,in the famous words of the king"return to sender"ps i dont like elvis what so ever

[mrrmutt]

I think your 12" wheel has too many RPMs for the bike computer to keep up with. It works well until you pass a threshold. Remember that the bike computer is actually counting wheel RPM and calculating speed based on the 1422mm you entered. A larger wheel would have less RPM to achieve the same speed.

[sidthesloth]

I dont think I have grounds to return it as it stater as being for a 20 inch wheel or larger, but, having said that, how many rpm would it be doing at 99 mph? what I forgot to mention in my original post was that I had read of someone putting the magnet on the brake disc, so that would come round pretty quick.

[Agent Bob]

Perhaps if you re-orient the magnet so the north and south poles are in a different position, or maybe a bigger magnet. Seems like you touched all the bases otherwise.

[sidthesloth]

I have put the magnet on in the holder provided but, as I have glued it into position I would rather exhaust all other options before removing it. The magnet is a disc, would you suggest turning it 180° or flip it over? I have checked the paper that came with it and it does state 99kmh and 99mph.
I have sent an email to the distributor this morning and am waiting for a response.

[mrrmutt]

The 99.9 kph/mph upper limit is most likely because of the ability to display the output, not the ability to calculate it. Orientation and position, provided that they are within specs, shouldn't affect the outcome. I just senses the magnet once a revolution of your wheel.

You could double the value that you input for circumference of your wheel and divide the speed in half. This would not be a practical solution, only a way to confirm malfunction is that wheel RPM is out of the range of the sensor. It should work fine up to an indicated 51-52 MPH. (you will actually only be traveling half that, 26 MPH or 41 kmh)

Personally, I'd just ride as fast as I could and not worry about the numbers, but thats just me.

[sidthesloth]

My kymco speedo would suggest that I am in the 110-120kmh range, just trying to get a realistic figure, I could ride at 60 in the 50 zones and 70 in the 60 zones, but then I have failed in a simple task of fitting a speedo.

[Agent Bob]

I did a little math on the speedo issue and found your speedo dies at the front wheel doing 963 rpm at the 81.9 kmh (51 mph) mark. I took that rpm number and used it to calculate the speed that that a bicycle with a 20 inch wheel would be traveling at. It was 69 mph. At that speed I'd be looking at my will and not the speedo. That 963 rpm would provide a 16 cycle per second (hertz) signal which is a breeze to handle by an electronic device. I think the magnetic sensor is probably maxed out at the frequency.

Here's a link to some dude who had the same experience.

http://www.ibmwr.org/ktech/trek-speedo-fix.shtml

[mrrmutt]

I did a little math on the speedo issue and found your speedo dies at the front wheel doing 963 rpm at the 81.9 kmh (51 mph) mark. I took that rpm number and used it to calculate the speed that that a bicycle with a 20 inch wheel would be traveling at. It was 69 mph. At that speed I'd be looking at my will and not the speedo. That 963 rpm would provide a 16 cycle per second (hertz) signal which is a breeze to handle by an electronic device. I think the magnetic sensor is probably maxed out at the frequency.

Here's a link to some dude who had the same experience.

http://www.ibmwr.org/ktech/trek-speedo-fix.shtml

What he said

[sidthesloth]

Thanks for your help agent bob, I will have a look at that link when I get to the computer, (on mobile phone now).

[sidthesloth]

Thanks Agent Bob, I had a look at the link you posted. He describes the same problem at a higher speed, but unfortunately, no fix.

[Agent Bob]

I don't know where on the wheel you glued the magnet, but it might make a difference. My theory is that closer to the hub, the magnet travels slowly past the sensor, and the pulse it generates is wider than if the magnet and sensor were mounted at the outer edge of the rim where the wheel travels faster. Near the hub, the pulses become very close together to each other and eventualy overlap, hense bad readings at higher speeds. Narrower pulses allow more pulses close to eachother. Just a theory, and it might be on the rim already.

[omfgdotcodotuk]

I did a little math on the speedo issue and found your speedo dies at the front wheel,Omfg!! are you like a computer or somethin thats well impressive.