Re: More Help on RPM Calculations?

On Thu, 07 Feb 2008 17:43:34 -0600, Mark Olson <olsonm@xxxxxxxxxxxx>
wrote:

David T. Ashley wrote:
"Sean" <no.spam@xxxxxxx> wrote in message
news:jdLqj.4422$FA.970@xxxxxxxxxxxx
Here's what I would do -- get (or borrow) a dual trace oscilloscope
or a frequency counter. Wrap some wire around a spark plug lead and
run that to one scope input. Then fix a magnet to your wheel somewhere.
Wrap some wire around a chopstick and hold it near the wheel. Whenever
the magnet goes by you'll see a blip on the scope's other trace.

Great post! I'm guessing you're drinking and watch MacGyver?

I actually have an MSEE, so I understood every nuance of what you are
suggesting. I have a 4-channel digital scope sitting on my desk.

The idea is pure genius. It makes great use of standard equipment, and I
never would have thought about using a scope as a tachometer or wheel
rotation timer. It should also be extremely accurate, as a modern scope
probably has a timebase well within a half percent, and I'm sure the overall
calculation would be within 5% (figuring one might not be able to use the
full screen width when one has to get 4 pulses on two traces displayed
simultaneously).

It also doesn't require running the bike fast, even on a stand. All that
would be required is a stable speed for a second or two to get the ratio.

Amazing idea!

In case you weren't aware already, some recent automobiles have used a
Hall-effect sensor and a magnet on the driveshaft to measure vehicle speed.

The idea is pure genius. It goes beyond "out of the box" thinking.

Even better is a suggestion to use your PC's sound card, it is
essentially a low-spec dual channel digitizer... perfectly
adequate (and quite accurate) for what Sean is suggesting. Not
everyone owns an oscilloscope or can borrow one from work
(luckily for me I can, if I need one).

In fact I have used a laptop in exactly this fashion more than
once to analyze a motorcycle engine noise, and to measure RPMs
of a homemade electric motor.

http://olsonm.dsl.visi.com/~olsonm/concours/noise/index.html

Brilliant !


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Relevant Pages

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