Possible idea for a cheap/easy tachometer?

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motor_bike_fanatic

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Jul 26, 2011
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Please bear in mind that I know very little about electronics and please dont flame my post. I just happen to have creative ideas. Sometimes these ideas are right, sometimes they are wrong. This idea could go either way, or it could be half right. That being said, here is the idea.


take any WIRED bicycle computer and disconnect the wire from the sensor. Then, measure the diameter of your engine's magnet. Enter that value as the wheel value in the computer. Then take the end of the sensor wire, strip it, and connect it to the spark plug between the hex and the seat.


Possible limitations/concerns with this setup:

1: The sensor wire could draw voltage from the spark plug, possibly overloading the computer or underloading the engine.
2: Does a bicycle computer give readings above 999?
3: The sensor wire could interrupt the ignition completely, causing the engine not to run at all.
4: This could cause other damage to electrical components or to the engine itself.


So, thats my idea. I would like to hear from those with electrical knowledge before I decide to test myself. Thank you.
 
The_Aleman

The_Aleman

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Jul 31, 2008
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Overspeeding a bicycle computer will cause it to rapidly drain it's battery. And no, most of them don't even go to 999.

If you want a cheap/easy tach, a TrailTech TTO or TinyTach fits the bill. They are small and under $40.
If you want an analog tach, an Equus 8068 with inductive pickup will do. But you also need a power source for it.
 
M

motor_bike_fanatic

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so other than draining the computers battery and the computer not going over 999, my idea was theoretically correct? not asking this to argue my idea further. just curious if my basic theory was correct.
 
Kiwegapawa

Kiwegapawa

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May 2, 2011
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Hooking up a Bike Computer to you spark plug will permeantly burn it out. Most any Bike computer is actually a integrated circuit (Ic), they work in the Millivolt range or below. The spark plug receives full amp's. It operates on the other side of the decimal point in full numbers.

You would literally / physically smoke it by hooking it directly to the spark plug. I do believe your engine would go on running till enough of the computer melted or insulated wire leading to it. At that point, you just may start dancing in your seat yourself till the engine fully died and the flywheel stopped rotating. zpt


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L

Lazy Dog

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Oct 13, 2011
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The wire sensor picks up a magnet that's on your spokes to read RPM's, Not the wire pulses like a real tach.

Does make me wonder tho, I know the computers will never let set the wheel that small. I no very little about electronics, But there's already a magnet spinning in there. So I would imagine it could be made to work kinda easily, if you could reprogram/replace the prom maybe, or a few resistor's or what ever's in there. I know just needs a 555 timer then just make it show what ever RPM you feel comfortable running :)
 
tooljunkie

tooljunkie

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Apr 4, 2012
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stihl tachometer works very well,i borrowed mine from work until i get grubee dialed in.top revs 6300 right now.book calls for idle speed around 1000,i prefer it higher,though.
 
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Gr4yb3ard

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Mar 31, 2014
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Just to be sure, these little tachs will work with 2 and 4 strokes, as the four stroke mag (should) be delivering a waste spark on the exhaust stroke........ right?
 
MotorBicycleRacing

MotorBicycleRacing

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Jul 28, 2010
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Gr4yb3ard said:
Just to be sure, these little tachs will work with 2 and 4 strokes, as the four stroke mag (should) be delivering a waste spark on the exhaust stroke........ right?
Click to expand...
You got it.
I like the TTO tach units out of all of them.

Works on all gas engines (including fuel injected 4-cycl engines
Tracks total hours on motor, non-resettable
Measures tach up 20,000 RPM in real time
Remembers max RPM, resettable
Spinning arrow activity icon
Adjustable display update smoothness (fast/slow)
Adjustable tach pulse sensitivity (high/low)
Click to expand...
 
rocksalt27

rocksalt27

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Oct 24, 2013
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Echo lawn equipment makes a few models. Not sure if they're available to customers or just dealers but you can always ask. I believe they make a wired one and a handheld inductive.
 
Davezilla

Davezilla

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Mar 15, 2014
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This might work... if you got a small 12v battery to power it and keep your tune under 13k rpm... I just gotta find out if it can read a wasted spark single cylinder 4 stroke, if it does, then it can read a single cylinder 2 stroke without modifying... http://www.ebay.com/itm/Motorcycle-...Parts_Accessories&hash=item27d211fbba&vxp=mtr

I'm still just looking into it but thinking this one might be best for those who prefer the analog reading.
 
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