Motorcycle Savvy? I got a Question about spoofing a Tach

[LI-ghtcycle]

I'm looking to buy one of these Trail Tech Vapor Digital Gauges, but I want to be able to keep my pace from my crank, (RPM's of the crank) and the Tach function on this gauge uses the electromagnetic pulse from the ignition system of a motor cycle, and I'm thinking that this is an EMP (Electromagnetic Pulse) No not the nuclear kind :lol: but just simply an electro magnet (the primary/pulser/magneto coil I forget which would be the proper term) causing a magnetic field that is picked up by the sensor, much the same way a normal cycle computer has a magnet going by a sensor to track RPM (pace) of the crank, and Speed at the wheel.

I'm wondering if someone smarter than me (that means about 99% of you on this board ) can tell me if my idea of using a normal magnet might work to spoof, or fool the Tachometer sensor by using just the magnet that came with my cycle computer to track the pace or RPM's of my bicycle cranks. My main concern is that maybe the magnets that come with the cycle computer aren't as strong as the ignition pulse it normally receives from a motorcycle's ICE.

Thoughts anyone?

Thanks! 8)

 

[heathyoung]

The sensor that came with the bike is a reed switch - if you ran it to the 12V battery (or a 12V source) ie. +12V via a 1.2K resistor, other side to ground, and hooked up the tach wire to the side that is connected to the 1.2K resistor, you would have pulses very similar to the points on an ignition system. Ie. 12V when open, Gnd when closed. The 1.2K resistor limits the current through the reed switch contacts (otherwise you would be shorting the supply).

The RPM would measure pretty low though. If you set the Tach for 2 stroke (one pulse per revolution), it would be 1 X Cadence, 4 stroke 2 X Cadence (one pulse per 2 revolutions). Measuring RPM this low is hard to do accuratly. Buy some more magnets to stick onto your chainwheel Or gut an optical mouse and get clever.

Assuming you can pedal at 100 rpm (achievable) then 10 magnets will be 1000 rpm on 2 stroke setting - To get your cadence divide RPM/10. Easy, and the resolution should be a LOT better.

 

[liveforphysics]

The sensor that came with the bike is a reed switch - if you ran it to the 12V battery (or a 12V source) ie. +12V via a 1.2K resistor, other side to ground, and hooked up the tach wire to the side that is connected to the 1.2K resistor, you would have pulses very similar to the points on an ignition system. Ie. 12V when open, Gnd when closed. The 1.2K resistor limits the current through the reed switch contacts (otherwise you would be shorting the supply).

The RPM would measure pretty low though. If you set the Tach for 2 stroke (one pulse per revolution), it would be 1 X Cadence, 4 stroke 2 X Cadence (one pulse per 2 revolutions). Measuring RPM this low is hard to do accuratly. Buy some more magnets to stick onto your chainwheel Or gut an optical mouse and get clever.

Assuming you can pedal at 100 rpm (achievable) then 10 magnets will be 1000 rpm on 2 stroke setting - To get your cadence divide RPM/10. Easy, and the resolution should be a LOT better.

I very much doubt it's a reed switch, or any sort of mechanical switch. Something like a hall sensor switched transistor could look and act just like a reed switch, but have no moving parts to wear out or trigger from bouncing around.

A magnet or magnets mounted to the spinning part, with a hall sensor to do the triggering is what you're looking for.

 

[LI-ghtcycle]

Kewl, thanks for the info guys, I haven't got the MX computer yet, going to be ordering it on the 1st, but from what the guy from the manufacturer told me I got the impression it was just something external that measured the pulse of the ignition firing, so maybe it's attached to the spark wire ... he didn't seem to be really technically informed, and of course my application is way out of the norm for them I'm sure!

Maybe I should just get the next model down and do w/o the tach, I can tell when I am spinning to fast, I'm just spoiled now (WHEN I can read my cycle computer :lol: ) being able to see my cadence and have the most efficient pedaling cadence for me.

I like the idea of the hall sensor though, how expensive are they anyway? Is it something I can buy through digi-key?

 

[graemebc]

Nice dash would look good in my race miata. Have you looked at this thread too? http://endless-sphere.com/forums/viewtopic.php?f=28&t=15488
Eagle logger will give you rpm

 

[LI-ghtcycle]

Nice dash would look good in my race miata. Have you looked at this thread too? http://endless-sphere.com/forums/viewtopic.php?f=28&t=15488
Eagle logger will give you rpm

Yes, that is a nice one too, however I'm looking for my RPM as in my pedaling RPM rather than the motor's RPM I already have a Turnigy Watt and Power meter, i just want MPH and trip meter etc. that I can read in low light conditions, my current cycle computer is pretty sad, can't even read it after a litte rain sprinkles on it.

I'm also wanting to be able to ride at night/early morning once I also have some considerably better headlights, taillight, and blinkers. And now even in good lighting it takes me a lot longer than it should just to find the current speed I'm going.