From: zeleny@husc10.harvard.edu (Michael Zeleny)
Newsgroups: rec.motorcycles
Subject: Re: Harley Outperforms ZX11, Film at Eleven
Date: 14 Oct 92 06:27:56 GMT
Organization: The Phallogocentric Cabal

In article <1992Oct13.214713.16509@tcsi.com> 
markk@tcs.com (Mark Kromer) writes:

>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.

MZ:
>>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.

MK:
>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.

There is, however, the issue of forces generated on the conrods running
side by side.  This is hardly a mere theoretical speculation, -- ask anyone
who has worked on roller-bearing Ducatis.

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.

MZ:
>>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.  

MK:
>HD big twin valve train:
>
>1 camshaft (geared to crank)

Don't forget to count the gears separately, as you do below.

MK:
>4 lifters
>4 pushrods
>4 rockers

Either include the adjusters, or count the parts that constitute the
hydraulic lifters.

MK:
>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

Real men use gears, -- cf. Manx Norton, MV Agusta, and bevel-drive Ducatis.
Also, don't forget the valve springs, retainers, etc.

MK:
>4 camshafts
>4 buckets

Now, if you have DOHC, you might as well have 4-valve heads, which add a
lot less complexity in this case, compared to that of a pushrod valve
train.

MZ:
>>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.  

MK:
>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!)

Very few engines have roller-bearing in-head camshafts, or rocker arms.  As
for external oil lines, I thought that went out with the pre-war "plumber's
nightmare" Vincent.  (Sadly, this repugnant practice is followed on all
current Ducatis...)  Just put the oil feed into the engine castings, where
it belongs.

MZ:
>>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.

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

All good engineers avoid profligate expenditure of resources.  I'd rather
ride a 400-pound Ducati than a 660-pound Harley making half the power with
nearly twice the displacement.

Harleys started out as race bikes.  Please don't tell me they went out of
racing voluntarily.  The machines are simply no longer competitive; and the
factory doesn't have the wherewithal to make them go.

MZ:
>>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.

MK:
>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.

OK, say they are superbly engineered to a very stupid design standard, --
it really makes no difference in the end.  If the machine weighs nearly
twice as much, and goes a bit more than half as fast as the best street
bike with the same engine configuration, I find it hard to see just what
its alleged superiority may consist in.  Oh, it may be great for cruising
for burgers or picking up chicks, sorry, impressing the MOTOS, -- but for
me, Italian sport bikes do at least as well.

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

cordially,
mikhail zeleny@husc.harvard.edu
"Un de mes plus grands plaisirs est de jurer Dieu quand je bande."

1977 1200 Mirage	1980 900SS	1986 1000 LeMans (later today!)