Electronic Speedometer made into a tachometer

GoldenMotor.com

ocscully

New Member
Jan 6, 2008
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0
Orange County, CA
It sure seems simple enough but I can't think off the top of my head where you can get the magnet and sensor located directly to the out put shaft of the motors most people use?

ocscully
 

KCvale

Well-Known Member
Feb 28, 2010
3,966
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Phoenix,AZ
I was just searching for some stuff and ran upon this site. Anyone ever thought of trying something like this? Pretty neat and I think it just might work. So freakin' simple!

Tachometer made from a bicycle speedometer (cyclocomputer)
Thanks for the link, I was looking for the 'magic number'!
That number is 268mm for wheel size.

You don't need to mount the sensor to the motor, just mount it as speedometer on a wheel as usual if you have a fixed gear single speed normal bike kit.

The gear ratio from the motors 10 tooth sprocket to the 44 tooth wheel sprocket is 4.4, so just multiply 268x4.4 = 1179.2mm wheel size 'magic number' for single speed fixed gear.

That will make your speedometer give you your motor RPM in 100's.
For example 4500 RPM would display as 45 MPH.

That simple way however is not what I need this for this current project...

I am using a JackShaft and I need to measure the RPM of the pedal drive sprocket so I have the motor RPM before it goes to the gears.

Why? Because the 'magic number' would be different for each gear ratio on a shifter and it is going on a NuVinci hub with ~900 gear ratios!

No problem, the jackshaft ends at the pedal sprocket so I just have to mount the magnet to it, and place the sensor on the bike just as if that sprocket were the back wheel and do the math.

Note this should work with any JS'ed Back Wheel Gearing!.

I'll just do the math here as there are different sized sprockets here too but this is them for a standard SickBikeParts jackshaft.

So we start with the 268mm 'magic number' then:
10T motor to 17T JS left gear is 1.7 x 268 = 455.6

Then the 10T JS right gear to the 48 tooth pedal chainring is 4.8x 168= 806.4

455.6 + 806.4 = 1262.2mm 'magic number' wheel size for a JackShafted bike, which means a slightly larger back wheel as the JS drops the drive gear ratio for better low end performance.
Or is that the other way around for more top end?

Now to find a clever way to mount a magnet on the inner side of the inner pedal sprocket and pick-up sensor.
When I get it done I'll post up how I did it ;-}