slackening V6 tensioner

[ethan schartman <eschartman@domain.elided>]

> From: Joe Elliott <jee@domain.elided>
> Subject: V6 belt tensioner
> 
> What is it about the mechanical detensioner that
> makes it slack off 
> if you turn the engine backwards?

in normal operation the crank pulls the belt from the
driver's side cam pulley putting both cams in tension
as there is no way to shorten the belt path. the
detensioner merely takes up the slack as the belt
passes back to the pulleys. in opposite rotation, the
crank pulls the belt from the passenger side and since
the detensioner is a spring the extra force causes it
to retract (shortening the belt path), freeing up lots
more slack which gets passed to the DS cam.

derailleur-equipped bicycles operate similarly, if you
want a visual aid. pedal backwards and you can watch
the derailleur lift up as the normally returning
chain's path is shortened (do it hard enough and you
can get it to jump sprockets just like timing belts).

as far as badness of reverse rotating engines, it is
an inherently poor design if the spring tension is
less than the force exerted by the crank on the
pulleys. because then there is no way to prevent the
crank from pulling slack into the system. and that is
definitely the case with every detensioner i've had
(all recent) which featured springs i could easily
push in with my hand.

except for the first time (possibly), i have not had
problems with reverse rotating the engine because i
never parked it in gear (once, on a slight incline i
was tempted to put it in reverse, but then someone
smacked it in the rear and caused it to roll forward
hard against the curb. i'd have been seriously bummed
if that'd happened w/ it in reverse!). i avoid parking
on hills to avoid the temptation (yea! flat NJ).

regards,
ethan schartman
'87 milano gold (engine nearing complete reassembly!)
princeton, nj
New DSL Internet Access from SBC & Yahoo!
http://sbc.yahoo.com
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