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Re: cranks

To: Bill Babcock <BillB@bnj.com>
Subject: Re: cranks
From: "Michael D. Porter" <portermd@zianet.com>
Date: Fri, 21 May 2004 01:01:21 -0600
Bill Babcock wrote:

> We're kind of saying the same thing--it's the usual problem of language
> imprecision.  
> 
> -----Original Message-----
> From: owner-fot@autox.team.net [mailto:owner-fot@autox.team.net] On Behalf
> Of Randall Young
> Sent: Thursday, May 20, 2004 11:25 AM
> To: fot
> Subject: RE: cranks
> 
> 
>>Long stroke engines have high torque for an obvious reason--the length 
>>of the lever arm (the distance of the rod centerline from the crank 
>>centerline).

It's an interesting thread, at least in the sense that one is 
dealing with what is pretty much a production crank--there can be 
only incremental improvements to the original design, given the 
package into which it must fit. After all, despite changes in 
materials in aftermarket parts, the four-banger is still a 
three-bearing crank, which means there's a goodly unsupported 
distance, by design, between bearings. That encourages all sorts of 
torsional loading and unloading in an inline-4, and that is the 
origin of many vibrations. The inline-6 has the same problem, in 
that it is a four-bearing crank, rather than seven-bearing. Lots of 
room for it to twist and turn under load.

For those inclined to textbook explanations, I still recommend 
Philip Smith's _The Design and Tuning of Competition Engines_ (5th 
edition is probably the best of the bunch, if it can be found) for 
explanations of the effects of crankshaft torsional wrapping and 
unwrapping and how that encourages vibration. Good book. Serious 
reading. Not for the timid. Interesting anecdote in the midst of the 
subject about the BRM 16-cylinder H-configuration engine, and why 
torsional loading was the undoing of that engine.

Cheers.

-- 
Michael D. Porter
Roswell, NM

Never let anyone drive you crazy when you know it's within walking 
distance.

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