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Old 09-30-2009, 08:01 AM
Fabian Fabian is offline
Join Date: Sep 2009
Location: Australia - Melbourne
Posts: 168
Default Super Simple Tacho

G'day all

I've had enough of being in the land of unknowns with lack of displayed RPM's.
Necessity being the mother of all invention has found the solution.

I've taken the liberty to make a super simple motorised bicycle tacho that can double as a spare speedo if the original speedo computer kicks the bucket.

Step 1: Go to the SickBikeParts website and buy yourself the Deluxe Jackshaft Shifter (shift) kit and optional accessories to make your Chinese motorised bicycle engine more reliable.

Step 2: Go to K-mart and buy yourself a 9 function Huffy bicycle speedo or two of them if not having a speedo in the first place.

Step 3: Go to your local electronics store and buy 2 meters of the second finest wire available (i used 1 meter red and 1 meter blue wire for easy identification).
Cut the bicycle computer speedo wire about 6 inches from the programable bicycle computer and very carefully cut the outer insulation, peeling back the two inner wires.
Carefully cut the insulation from the inner wires, maybe leaving about 10mm of exposed copper wire.
I found the original speedo did not have a long enough wire to secure the computer head to my handlebars from the Jackshaft output sprocket location.
Maybe a different manufacturer of bicycle speedo's has a longer length of wire with their product- you can report back on that if finding something more suitable but it's a question of keeping it low cost.

Step 4: Whilst at the electronics store grab yourself a length of heat shrink tubing to fit over the insulation of the individual 1 meter wire lengths; also get yourself a length of heat shrink tubing to fit over both the 1 meter wire lengths, to keep them nicely bound together.
As i don't have a soldering iron, i just twisted the exposed copper ends of the pushbike speedo wire and the extra 1 meter lengths together, slipping the 3 inch lengths of heat shrink tubing over the top of the twisted copper wire sections (make sure you put the heat shrink tubing on the wire before twisting together).
CAREFUL you get the polarity of the wires correct before heatshrinking everything.
You can connect the wires both ways and the speedo will work but if you connect it back to front, the icon in the top left hand corner, indicating if you are moving forwards or backwards will be wrong when you are moving forwards.
Try the setup with the wires connected one way or the other till you get it right.

Step 5: Once everything has been heat shrinked together, mount the sensor pickup on the Jackshaft metal plate with plain old superglue (cyano), and also the magnet on the inside drum face (face closest to the sprocket teeth) of the Jackshaft output sprocket, making sure it clears the chain and the sensor pickup.
You will have to carefully cut off the threaded stud on the magnet for it to clear the chain and Jackshaft output sprocket teeth.
NOTE, NOTE, NOTE - make sure the surfaces you are gluing onto are absolutely spotless and totally free from grease or oil - MAKE SURE EVERYTHING IS SPOTLESSLY CLEAN or the superglue or any other type of glue will fail to hold - YOU HAVE BEEN WARNED.
When finished, there should be about a 2mm gap between magnet and sensor (you can epoxy around both the sensor and magnet later for a more secure hold if you feel it is nescessary).

Step 6: Once you've mounted the magnet on the Jackshaft output sprocket and the pickup sensor on the Jackshaft metal plate, you'll need to input a wheel circumference into the computer speedo of exactly 1162mm and have it reading the measurement in kilometers.

Now you have a cheap and reliable Tacho which also stores highest RPM by scrolling to the "MAX SPEED" function and average RPM by scrolling to the "AVERAGE SPEED" function.
Just some additional info on the obvious, but if it isn't - 18 kilometers per hour (KMH) = 1800 rpm, 22.7 KMH = 2270 rpm, 34.3 KMH = 3430 rpm.
Naturally, "MAX SPEED" and "AVERAGE SPEED" reads the same way.

This method works on any of the available Jackshaft output sprocket sizes because the sensor pickup is reading revolutions, not circumference - just mount the magnet on the sprocket and you'll be a happy chappy.

Enjoy having a cheap tacho on your bike.

For extra fuel capacity, i've installed a seat mounted drink bottle holder, using o-ring sealed aluminium drink containers - I use these for fuel, giving me an extra 1.5 litres of reserve capacity.

Cheers Fabian
Attached Images
File Type: jpg Bicycle Speedo & Tacho 1.JPG (134.8 KB, 141 views)
File Type: jpg Bicycle Speedo & Tacho 2.JPG (126.1 KB, 109 views)
File Type: jpg Bicycle Speedo & Tacho 3.JPG (116.0 KB, 172 views)
File Type: jpg Bicycle Speedo & Tacho 4.JPG (90.9 KB, 141 views)
File Type: jpg Bicycle Speedo & Tacho 5.JPG (122.2 KB, 99 views)
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