Bonneville America Forums Forums Technical Forums General Tech Thoughts on Tachs

Hey, so here's an idea (I still get a fresh one once in awhile). I've been thinkin' about a lighted, handlebar-mounted tach that I could position just southeast of the America speedo. The problem is, the tachs I like most aren't designed for the Triumph electrical system. After mullin' this over, I started to wonder why the tach has to be wired to the ignition at all. When a cylinder fires, it creates a strong vibration at subsonic frequencies. That vibration travels through the entire motor block. If a tach were designed to "hear" the motor through an electret microphone, and the mic was attached to the block, no electrical connection to the motor would be required. Taking it a step further, if the tach was calibrated for motors with 1-2-3-4-5-6-7-and 8 cylinders (a DIP switch), it would be truly universal. Has anyone here ever heard of something like this?

nope. Easy enough to try though with something like an arduino. If i were going to try to roll my own tach though i'd probably try "listening" to the coils rather than the block. It would be even easier to directly grab igniter pulses which is what they do.

Btw i think most electronic tachs would work fine with the america.

I would think that using sound or vibration would be a lot less accurate. Sounds don't always have a definite moment where they begin. Your measurement could slide around quite a bit. Ignition pulses I think would be a lot more defined.

OK, good ideas. It would be easy enough to try it both ways. So, let's say rather than an entire tach, I develop a li'l module that allows tachs designed for HDs to read our bikes correctly. Again, there wouldn't be an electrical connection between the tach and the bike, other than +12V. Would that be useful? Is anyone else lookin' at third-party tachs? There's some really fine looking instrumentation out there.

are you sure an hd tach wouldn't work?

This one works fine right out of the box. Can be made to fit in the dash. Comes with a clamp to mount on bars. There are instructions here as far as what wire goes where, but I can't find them.

I was thinking more along the lines of something like this, so I could mount it off to the side of the speedo. The price on Eddy's is much better though. Something to think about...

Sun used to make a 4-6-8-12 cylinder tach. A tach with a counter driven by an inductive pickup on one plug wire driving a counter that was, in turn, reset by a timer would only need to have a switch to select 2 stroke/dual fire - 4 stroke single fire so it would know that sparks occure every revolution or every other one.

Thought it might be worth resurrectin' this for one more post. A buddy with a Sportster has informed me that the optional tach he just bought for his bike doesn't run off the coils or ignition circuit at all. It gets its signals from a serial data link in the harness that also runs some of the other instrumentation and whatnot. Sooooo... if ya wanna use a tach for a HD on yur Trumpet, it might be smart to confirm its functionality before plunkin' down the green.