re: Connecting rods

[Godfrey DiGiorgi <ramarren@domain.elided>]

Olli,

>I don't think the original question about connecting rods in
>v-engines refers to articulated rods (as in radial engines).

If you read the first part of my response, replicated below, the original
poster responded to me in private that he was indeed speaking of
articulated connecting rods as in radial engines, thus my response
addressed that issue. 

>As I understand it,  Harley angines have one rod with a 'forked'
>big end with two bearings side by side. The other rod has one
>bearing narrow enough to fit between the bearings on the the
>other rod.
>The original question remains:  why is this set-up not used in
>car engines?

This is called a "fork and knife" connecting rod design. Harley-Davidson's
45 degree V-Twins have used this design since the turn of the century. 

The reason it's not used in automotive engines is that it's both more
expensive to manufacture and it's also not as strong, nor as suitable for
high rpm engines. The H-D motor was designed to be inexpensive to
manufacture (you can use the same machining jigs as a Single) and the
notion was to keep the engine very slim, the cam followers in a single
line, and to allow for the motorcycle leather belt drive and later
primary/transmission drives. It also keeps the cylinders in line to
eliminate rocking couple vibrations caused by offset rods (not that H-D
motors aren't famous for shaking your fillings out anyway when not
carefully set up...). The H-D crankshaft is a built up affair of roller
bearings and ball bearings. High rpm, racing versions of the H-D motor
require exacting care in building and very expensive connecting rods ..
the forked rod has a lot of stresses in it at the big end where it forks.
They continue with this design because of tradition, etc, rather than for
any significant benefit.

Actually, the modern H-D road racing engine, the VR1000, departs radically
from this design to a more modern single crankpin/offset rods layout with
60 degrees included angle between the cylinders. It's an all plain bearing
engine with many other modern features. While the 750 and 1000cc versions
of the traditional pushrod motor with the forked rod layout can be pushed
to around 110-130 hp in racing trim and peaks at rpm in the 7000 range
(and last two/three weekends), the new DOHC VR1000 motor makes closer to
160 hp and will rev safely to over 12,000 rpm. They've run happily for far
more hours between rebuilds with no bearing distress. The offset
connecting rod design is just much stronger in every way and much less
expensive to build. The tradeoff of a longer crankshaft can be compensated
in other ways, such as thinner connecting rods supported by more oil
pressure, etc. 

Godfrey

>>Date: Sun, 1 Aug 1999 12:30:34 -0700
>>From: Godfrey DiGiorgi <ramarren@domain.elided>
>>Subject: Re: Vee engines
>>
>>>> >I have often wondered why automotive manufactures don't use ascissoring
>>>> >connecting rod setup on V engines to save weight, length and on moving
>>>> >parts.
>>>>
>>>> I'm not sure I understand what you mean by a "scissoring connectingrod".
>>>
>>>On radial engines where you could have up to nine pistons acting on one
>>>crank shaft journal there is a master rod with other individual rods
>>>connected to a boss on the master rod.
>>
>>Ah, you mean an articulated connecting rod ...

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