Motorcycle Forum / General / Technical / June 2007

whoops my bad.  it should have been worded "i miss having a tach".  
sorry.

On some bikes, it's pretty near essential. On others, it isn't.

See above. On my little 250, I don't miss it and don't need it. On my
Honda 400 Four, I sure do.

What is your hypothetical "average" rider forking these days? I doubt if it's
a mild-mannered 400cc commuter twin with a docile power band.  

More likely he's on a V-twin cruiser, where RPM doesn't matter and he doesn't
have a tach, or he's on a shrieking sportbike where a tach is essential, even
on the street.

The tach isn't there for styling purposes, it's a tool to be used by those
who understand how engine performance is affected by RPM, as well as the
effect
of excess RPM on mechanical stress.

If you double the RPM, the mechanical parts are stressed four times as highly.

The engineers design their parts to withstand X amount of mechanical stress
and add a safety factor of perhaps 2 or 3. Performance enthusiasts are going
to challenge the safety factors with lots of RPM.

One guy I used to ride with had a $ sign in the red zone of his Honda's
tachometer.

My first Honda didn't have a tachometer at all, but I read in the owner's
manual that it put out 20 horsepower at 10,000 RPM. I thought that was really
neat, I owned a motorcycle that would rev up to 10,000 RPM.

So I took it out onto the street and revved it up in first gear until it
screamed.

About two screaming runs like that broke all the piston rings and I had to
tear the motor down and fix it. If I had a tachometer I would have known when
I hit
10,000 RPM, but all I had to go by was the engine's refusal to turn any
faster...

Sure. My sportbikes all have power bands that START around 8000 RPM and go to
11000+ RPM. Then the rev limiter kicks in and prevents engine damage by
shutting off two spark plugs.

The first time I ever rode a bike with a rev limiter, I thought that I had
floated the valves at 11000 RPM. But the owner said that I hadn't hurt it,
the staggering of the engine during rev limiter operation was quite normal.

High performance modern sport bikes take some getting used to. There is no
kick in the pants until you reach the beginning of the power band. Then the
engine accelerates to the rev limit so fast there is no way a human could
roll off the throttle fast enough to avoid wrecking the engine from excess
RPM.

Honda's 500cc oval piston GP racer started making good power at 10,000 RPM
and revved all the way to 20,000 RPM. The rider watched to tachometer to be
sure he didn't drop out of the power band, necessitating a downshift.

The 500cc oval piston racer didn't do that well, but Honda make a 750cc
version for the street. It started making good power at 8000 RPM and
continued to 14,000 RPM.

Later model sportbikes will rev up to 15000 RPM or higher. Yamaha's YZF R6
has a power peak at about 13,000 RPM, but safely revs up to 16,000 RPM so the
rider doesn't have to upshift and downshift in a turn, he can stay in the
same gear all the way around the turn.

i find a tach useful for troubleshooting carb issues.