How smooth should a vertical twin be?

Again, with the 1980 KZ750 twin.

At idle it has some vibration, I expected this much. However, it feels
like some resonance at the higher RPMs, especially between 4-6k RPM. Do
twin cylinders usually feel like this? The bike has a lot of power, just
had carb work, new plugs, etc.

 

Well, your machine does have twin chain-driven counterbalancers hidden
away inside, but it also has a 360-degree crankshaft. The direction of
the rod stroke is basically up and down, aimed at a point just behind
the steering head, so you're going to feel some low frequency vibration
in the handlebars that some people might describe as a "shake" rather
than a "buzz" as a vertical inline-4 would produce.

Tell me, does the front wheel act like it's trying to jump off the
ground while the engine is idling? Old Brit bikes like Triumphs and
BSA's did that front shake at idle...

The first time I ever saw a Triumph 650cc Bonneville, the front wheel
was shaking up and down, and the front fender was vibrating, and, naive
as I was, I thought the shake indicated power instead of balance
problems...

In 1974, Yamaha built a 750 vertical twin with a counterbalancer shaft,
the XS-750. It was only produced one or two years. It wasn't popular,
maybe because CB-750 Honda I-4's were being sold for----hold your
hat!---$1700 to $1800...

The old Trumpets and Beezers had 360-degree cranks without
counterbalancers. Both pistons rise and fall at the same time, but you
get a spark on each cylinder every other turn of the engine. Big
shakers, they were. Got any paint that needs mixing?

Early Honda 250cc and 305cc engine had 360-degree crankshafts, again
with no counterbalancer. They were buzzy little engines. But, early on,
Honda started making 180 degree crankshafts.

In the 180-degree crankshaft arrangement, one piston is going up to
fire, while the other piston is going down on the power stroke. Then
the engine turned 540 crankshaft degrees without a power stroke. That
lowered the frequency of the Honda buzz (riding a motorcycle that needs
to turn 6000 RPM to go 60 mph is definitely buzzy) to a lower frquency
shake that some riders described as a "lope"...

And Yamaha came up with a vertical twin (TR-850?)several years ago that
had a 270 degree crankshaft to emulate the power delivery of a Ducati
90-degree V-twin. It seems the 90-degree V-twins are known for putting
power to the ground without breaking the rear tire loose suddenly like
an inline-4.

And the Duc has perfect primary balance, as the vertical piston is
going one way, the horizontal piston is going 90 degrees to that and
the engine doesn't vibrate.

A mad tuner on a webpage I was looking at several months ago rearranged
the firing order on his XS-650 vertical twin to emulate that more
modern Yamaha...

 

Thanx for the elaborate reply. Great info.

The front wheel doesn't seem to want to jump, its just a vibration at
idle that feels a little like a go-kart or my 80cc Yamaha. It's the
revving that brings out the unpleasant stuff.

 

Got foam handgrips or gel handgrips?

 

TX750. The XS was the triple.

It also had a terrible reputation for reliability.....

Much...... ;-) You stil get a bit of rocking couple because the big
ends are side by side.

You can also use offset crankpins (a la Honda V-twin series like the
Hawk) on vertical twins. Someone once built such a crank for a Trumph
twin - I saw it - but whether it ever ran is another matter.

 

Ehhh...Not sure, probably foam, they're old and kind of rotted.

 

Was "rocking couple" the source or is it an imbalance 45 degrees after every moment of "perfect
balance"?

Now *there's* a rocking couple.

 

Well. slice the old grips off with a sharp razor blade and install some
gel grips or foam grips and find yourself a gel pad for the seat to
protect your bottom from "monkey butt" on longer rides, and that's
about all you can do to quell the vibration.

It seems that motorbikes are rather light vehicles in comparison to the
weight of the rider and passenger they must carry. The engineers can't
spring the machine perfectly to suit everybody, and turning down the
spring preload to make the ride more compliant just results in a
feeling that the machine isn't as precise and controllable as it was
when the springs were cranked down tight.

So, the front tire transmits road vibration to the handlebars and the
rear tire transmits road vibration to the seat, at the same time the
engine is aiming all those pulses and tingles at the handlebars, and
the best thing you can do is isolate yourself from the vibration with
foam or gel...

 

It's because you have the crankpins (big ends) side by side, so they try
and 'rock' the crank. Same with all V-twins (and flat twins) except for
Harley, which uses a forked conrod to get round the problem.

Or used to. I dunno if they still do. And on a 45 degree vee twin it's a
waste of time anyway.

 

You've never seen a Ducati crank, then?

 

Ducati cranks don't have forked conrods.

Next question?

 

What about a "BarSnake"?