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

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Nice bike you have it all. Lots of info on the tac build too thanks.

G'day Fabian! Phow i got i bit fuzzy on step 3 and 4 but i will figure this out yet!

whats the max rpm ya gettin?

Good job

steve

I wonder if there's any number that can be input for use with magnet shaft or engine output sprocket that will work for those of us that don't have shift kit. Maybe don't need the extra magnet.

Yeh i second that. thats a $250 tacho for me

Hi Steve

I'm getting 1900 rpm at idle and the engine revs to 3500 when serious vibration starts.
Basically, these motors will give good service life using plain old cheap mineral oil at 20:1 using 98 octane unleaded to compensate for the octane lowering properties of 20:1 oil/fuel ratio, but avoid the 100 octane fuels that have ethanol - it attracts water.

I've written an extensive post on the other site about this and cover castor oils ect.
One simple tip for good reliability - apply a verly light application of grease to the helical drive gears every 100 kilometers, DO NOT OVER REV PAST 3500 RPM and use 20:1 together with 98 octane fuel.

Cheers Fabian

Massive points fer ingenuity!!! You rock man - if I had a jack shaft I might just do this for the project of it

For the rest of us/lazy bastids I'll pimp my thread again lol
EGT/CHT and other instrumentation

The "sender" for the tach in that thread is simply a wire wrapped around the plug wire a few times, same as TinyTach BTW.

Hi xlite

You can do the same thing with your single speed drive system - you will need to attach the magnet to the rear drive sprocket and have the pickup sensor fixed in a position where it can read the magnet.
From there you will have to go back to your high school maths books to work out your drive ratios back to the crankshaft to work out rpm's and then work out what circumference you will need to tell the bicycle computer to work with so it can accurately display correct rpm.

Hi Bandito

You haven't seen nothing yet - let me post some more pics of my bike.
I've already got the design up and running for a 350 to 500 watt DC generator to run off the jackshaft to supply a pair of 100 watt driving lights, front and rear indicators for the bike and brake lights as well as brake lights and indicators for the trailer.

The DC generator will keep a 7 amp hour battery charged and a button on the handlebars to allow power to flow back to the generator to turn it into an electric motor, feeding an extra 350 to 500 watts of power into the jackshaft system for emergeny overtaking maneuvers of men in lycra tights on their overly expensive road racing bikes.

I'll post a few pics of my trailer

Fabian

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