From: markk@tcs.com (Mark Kromer)
Subject: Re: Harley Outperforms ZX11, Film at Eleven
Originator: markk@sparcplug
Reply-To: markk@tcs.com (Mark Kromer)
Organization: Teknekron Communications Inc.
Date: Tue, 13 Oct 1992 21:47:13 GMT

zeleny@husc10 (Michael Zeleny) writes:
>In article <1992Oct13.020852.7884@tcsi.com> 
>markk@tcs.com (Mark Kromer) writes:
>>zeleny@husc10 (Michael Zeleny) writes:

>MK:
>>There is nothing wrong with putting two rods on one crankpin.  It is
>>common practice in V-engines and is responsible for the compactness
>>and light weight of V configurations.
>In principle, it is better to have one bearing per journal, not in the
>least because of the need to manage the thrust loads that would be
>inevitably generated otherwise.

It has nothing to do with thrust.  There is no difference in the
forces generated on the crankshaft along its length or the ability to
contain such forces.

>MK:
>>DOHC is not the be-all and end-all of cylinder heads.  For a V-twin
>>engine you would have four camshafts and two independant cam drive
>>systems for two cylinders.  This is obscenely complex and would only
>>be considered where valve float (or the need for a marketing buzzword
>>like DOHC) becomes the limiting factor in the design.  It is
>>excessivly heavy and requires larger, taller, and heavier cylinder
>>heads.  It requires pressureized oil feed to the heads.  It is likely
>>to require more difficult maintainence, more complex and expensive
>>replacement parts, and be less realiable than a push rod design.
>
>Nonsense.  The pushrod valve actuation system is far more complicated than
>the rockerless DOHC variety.  Count the number of the moving parts, if you
>are in doubt.  

HD big twin valve train:

1 camshaft (geared to crank)
4 lifters
4 pushrods
4 rockers

Typical DOHC valve train (two valve per cylinder V twin):

2 camchain drive sprockets
2 camchains
2 camchain tensioners (spring, block, rachet mechanism + critical wear
	items like chain guides)
4 driven sprockets
4 camshafts
4 buckets

>As for the need for pressurized oil feed to the heads, it is
>always a good idea to have it, if only for the sake of the rocker
>arms.  

Only necessary where high speed plain bearings are used and pointless
otherwise.  It's just another complication and potential leak (My
Ninja and FZR route the oil to the head through external lines and
banjo fittings!)

>If your concern is with top-end weight, look at motocross engines,
>which by and large have abandoned the weight-saving SOHC
>configuration.
>The fact of the matter is, pushrods set an inherent limitation on
>engine speed by introducing extra slop and flexibility.  Look at the
>BOTT race class, the last one where pushrod engines still compete with
>overhead camshaft designs, and see what the added expense of racing
>the former is, in terms of extracting the greatest performance out of
>an engine.

They're all playing the limited displacement game.  HP/liter is
crucial for them but not for Harley street bikes.

>Due to their long stroke, for Harley engines, piston speed is a far greater
>limitation than valve actuation rate.  Which just goes to show that they
>are not built for power.  Still, it is a damn shame that their design puts
>such prohibitive limitations on any possibility of sporting use.

I agree.  Harley doesn't build the bike I want either (at least not
without $3000 in mods), but that doesn't mean that the bikes they
build are inferior or that they're poorly engineered, it's just that
they're designed to a different specification.

--
)V(ark)<               markk@tcs.com                   DoD #400
1988 FZR400            1985 ZX900 Ninja             1977 RD400D
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