From: SFPhysics <SFPhysics@aol.com>
Date: Thu, 30 Apr 1998 21:34:08 EDT
To: pinhole@exploratorium.edu
Subject: Re: Automobile Cooling Noises
> On a different note, does anyone know why a car engine quietly creaks after
> it's turned off. Obviously it has to do with the metal contracting, but
> what causes the regular 'ticking' pattern that comes from the metal?
>>
These are thermal contractions caused when the hot exhaust is no longer going
through the exhaust manifold, the catalytic converter, the muffler, and the
tailpipe. As the overall length of the cooling exhaust system shortens, the
various components stick then slide over the (mostly rusty) hangers attached
to the car frame or body. Why don't you hear them do the opposite when you
are starting up? Simply put, you are in motion and the system is jarred
enough to make smaller adjustments (even sitting still with just the motor
running). Though a different driving force, if we could only make the San
Andreas Fault have smaller quakes by injecting water as lubricant as some have
suggested....... it wouldn't make those 8.5 creaks either. ;-) Same
stick and give-way action? Many think so!
Al Sefl
SFPhysics@aol.com