Date:         Fri, 22 Jun 2007 14:03:34 -0500
Reply-To:     Matt Roberds <mattroberds@COX.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Matt Roberds <mattroberds@COX.NET>
Subject:      Re: Timing Chains, Belts, Gears, etc
Comments: cc: John Rodgers <inua@CHARTER.NET>
In-Reply-To:  <20070622135240.THNF27721.fed1rmmtai108.cox.net@fed1rmimpi04.cox.net>
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed

> From:    John Rodgers <inua@CHARTER.NET>
> Date:    Fri, 22 Jun 2007 06:52:56 -0500
>
> Just reading the post about "New Engine Conversion" - and I wonder -
> besides cost, is there any reason why engines have not had meshed gears
> to drive the cams.

I don't know for sure, but here are some ideas.  Beetles were set up
like this, but the distance between the cam and crank centers could vary
a little due to manufacturing tolerances.  There was one crank gear but
fifteen slightly different cam gears; the cam gear was chosen to mesh
well with the particular crank gear.  I'm sure VW had a slick jig at the
factory so that the measurement could be made quickly, but still, it
meant that you had to buy 10,000 each of 15 gears instead of 150,000 of
one gear, plus you had to pay a guy on the assembly line to do the work.
With a chain, you get to buy 150,000 of one gear and there is no special
procedure on the assembly line.  (The best year ever for the Type 2 was
in 1970, with 257,000+ units worldwide.  To put it in modern figures, if
you're paying the line guy $20 an hour and selecting the right cam gear
takes two minutes longer than a gear+chain design, that's another $0.67
in labor cost per car, or $171,000 in extra labor for the year.)

I think some early attempts at gear-drive cams used straight cut gears
which are inherently noisy.  I think you can still get "hot rod" sets to
replace the two gears + chain on a small-block V8 with three gears, and
these are noisy as well.  Beetles had gears with angled teeth.

On a Beetle, the crank and cam centerlines are relatively close to each
other, so gears worked well.  Some other engines have a bigger space here,
and would either need two huge gears (more weight and cost, bigger front
engine cover needed) or three smaller ones (more weight and cost, need
another bearing for the idler).

> No body wants to try to pull over and park on a cloud when a timing
> chain fails.

That's what parachutes are for.  :)

Matt Roberds