Overheating,
Excessive Idle Time, ETC.
of your BMW Airhead Motorcycle
excessive.htm-41
© Copyright, 2011, R. Fleischer
There is a relatively common misconception that
even moderate idle time will
cause overheating on our air-cooled bikes. Not so. There are of
course limits,
even to our BMW Airhead motorcycles. Truly excessive idle time can certainly
overheat the top
end and cause problems from warpage to loose valve seats to
frozen-up parts.
Our Airheads have a large amount and area of quality cooling fins,
and the cooling
is quite good due to these.
In addition, the cylinders are outwards
and have good
convection cooling.
A few minutes of idle time at a stop light,
even in the
hottest weather, is not going to do any harm. This
assumes you will ride off a bit after that.
Human beings know what outdoor temperatures are comfortable to
THEM, and may unconsciously transfer this to what they think about our engines.
Engine's are generally more
'concerned' about the first minute or so of starting-up in cold
weather, where engine clearances are decreased,
and wear is high.
Idle and low power levels produce vastly less
heat than
cruising down the road at highway speeds. The faster you go, the
more heat is produced inside the combustion
chamber, which is gotten rid of by the exhaust, and by the
oncoming wind and the turbulence effects of that oncoming air as
it passes very irregularly around the fins, etc.
Airhead owners who have the BMW oil-cooler radiator and thermostat might have
thought something
was wrong with the thermostat if the oil-cooler radiator did not feel hot
to their hand,
after they
stopped. The thermostat will usually NOT open for the additional
oil cooling when
driving slowly in town, or moderate stop and go traffic.
I personally think that the "ten minutes" as being OK for idle, as has been
cited from BMW's literature, CAN
be excessive. I think 3 or 4 minutes OK, but nearing
the SAFE limits, and if
there was not a mile or or so of riding between multiple long stops, I'd pull off the road.
I would be more worried about lack of timing-chain oiling.
This is particularly so if your engine is worn some, and/or your
idle is below 900 rpm, a BAD idea anyway.
This is not to say I would not be concerned about head warping
and bad effects at the valves, if the
idle time was truly
excessive. The head could, if idle was quite
prolonged,
even cause loosening of valve seat inserts, etc, previously
mentioned.
The timing
chain is oiled by the high oil pressure release valve located at
the crankshaft
sprocket and timing chain area. It is set for ~75 psi
actuation. Oils thin
out as
it gets hot, some oils having a poorer viscosity index
than others so are worse at this,
and although thinner oils get through the system faster, there might
be much lower
oil
pressure, particularly on an higher mileage Airhead, where
bearing
clearances, etc., have opened/worn. Thus, there may be
very little,
or no oil, at idle to the timing chain
and sprockets, etc. when the engine is idling slowly.
I think the worst situation
would be either a
VERY long wait in stop and go driving in the city.....or;
worse....an
excessively long stop right after quite
a few miles at warp
speed.
BTW....oil, particularly dino-based oils, deteriorate faster, on
a very steep
deterioration (with temperature) curve, above very roughly 250°F or
so. Most
synthetics are better, and good quality part-synthetics, such as
Golden Spectro 4, 20W50, and pretty darn good too.
The hottest area for oil is not inside
the main engine
case, but around the valve stems in the heads.
There is also a tendency, perhaps carried over from the /2 days,
to idle the
Airheads too slow. Not only does this make for more critical
carburetor
adjustments, but there is less oiling to the
timing chain/sprockets/guides, and in
addition, the chain and even the sprockets will jerk around more,
which is
harder on the sprockets, etc. Because of these things, I have
long recommended
that the ideal rpm for idle is likely about 1025. Those with
dual-plugged
engines
and the Bing CV carbs have further reasons for having this
slightly higher idle rpm, as the carburetor adjustments are more critical as the butterflies are closed more
on the
dual-plugged bikes.
Revisions:
11-24-2010: review article, make MINOR updates,
strictly for clarity; add copyright notice at lower area, check
meta-coding
05-27-2011: Clean up some, nothing major
©
Copyright, 2011, R. Fleischer