Noise and Hearing (was Re: [rescue] Re: Throw anotherpacketon the server...)

Sheldon T. Hall shel at
Sun Aug 24 11:57:24 CDT 2003

Sammy Ominsky wrote ...
> On Sunday, August 24, 2003, at 12:27 AM, Phil Stracchino wrote:
> >>> was on a Triumph Bonneville 650, with the pistons running in parallel
> >>> (like a single-cyl thumper with two cylinders),
> > All the big Triumphs were parallel twins, except of course for the
> > Trident.  They didn't normally run in "thumper" mode though.
> Hmmm, note I said "like a". I just know it was in parallel, but don't
> know exactly how it works. I'll freely admit that, too. Both pistons
> were in the same position at the same time.

The Triumph twins are 4-stroke engines, so each cylinder fires
every-other-time it gets to the top.  With two cylinders in parallel, one
cylinder fires on each stroke.  You have even firing pulses (one every 360
degrees of crank rotation) but a lack of primary balance.

Most 4-stroke vertical twins are built this way, certainly all the British
twins I've worked on (Triumph, BSA, Norton/Matchless, Royal Enfield, Ariel).
In the sixties, Honda made a version of their famous 305cc engine that had
the pistons on crankpins set at 180 degrees (rather than 0).  The result was
much better balance of the rotating parts, but the uneven firing sequence
made for a different kind of vibration.

If you want a vertical parallel twin where both cylinders fire at once, look
at the Puch "twingle" of the sixties ... but it was a 2-stroke, so you still
got a power pulse every 360 degrees, although not much of one.


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