Tachometer (RPM) gauge using Bike Speedometer!

ZnsaneRyder

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It would be nice to actually know what RPM's your engine is spinning to prevent overrevving it, or know if your belt, or friction drive is slipping, or to set your idle speed.

This was brought to my attention on a DIY go karting forum:

http://www.instructables.com/id/Tachometer-made-from-a-bicycle-speedometer-cycloc/

It says by entering the correct number into the bike computer, that it makes a good and simple tach. For Example, 2850RPM would read 28.5Mph.

Here is the calculation...
1 mph = 1.61 kph.
1.61 kph / 60 minutes in an hour = .026833 kilometers per minute
.026833 kpm* 1,000,000 millimeters per kilometer = 26,833 millimeters per minute
26,833 mmpm / 100 (scale factor) = 268 mm

*The only issue I see, is that they mount a magnet to a high speed shaft, which IMO is a bit risky if the magnet flys off, or it could cause some weight imbalance on a smaller High-RPM engine like a HappyTime.

Well, I'm planning on doing this, but use the white or yellow ignition wire to control a transistor to open and close the connection for the bike speedometer. I figure that the pulses on that wire would be a good indicator of RPM and would eliminate the magnet.

So, what do you folks here say about that? My goal is to make life easier for us bikers, and hopefully make our engines last longer, and a tach would do just that.
 
Nice trick. I have an old cheapie 5 function laying around somewhere... I think I'll put it to work someday.
 
I was informed that you can also put the mag sensor under the engine shroud cover, next to the flywheel, and the magnet already in the flywheel triggers the sensor.
 
has nayone tried the "tiny-tach". there about $40 and use just 1 pickup wire that simpley winds around the spark plug cable and is battery operated. advertised 5 years battery life and is also a hour meter. my buddy has one on a jet ski, visited there site and they have one for single cylinder 2 strokes.
 
has nayone tried the "tiny-tach". there about $40 and use just 1 pickup wire that simpley winds around the spark plug cable and is battery operated. advertised 5 years battery life and is also a hour meter. my buddy has one on a jet ski, visited there site and they have one for single cylinder 2 strokes.

Lots of MBC members have used this product and it's been rated highly.
It does have the weakness of having a non-replaceable battery though and many cyclecomputers are less than $15.
 
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