Long rod vs short rod - Harley Davidson Forums: Harley Davidson Motorcycle Forum
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post #1 of 29 (permalink) Old 11-07-2019, 05:55 PM Thread Starter
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Long rod vs short rod

What is the advantage of a long rod compared to a short rod...I'm building a 113 twin cam
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post #2 of 29 (permalink) Old 11-07-2019, 06:13 PM
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What is the advantage of a long rod compared to a short rod...


Seriously?




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post #3 of 29 (permalink) Old 11-07-2019, 07:45 PM Thread Starter
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Seriously
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post #4 of 29 (permalink) Old 11-07-2019, 07:57 PM
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I don't know that I would name myself a builder if I didn't know how to build. And I am no engine builder, but basically, to over simplify as best I can understand it, changing the stroke is a way of increasing displacement without increasing bore size because the piston moves further, but you gotta have other changes too.

There's no real advantage to just slapping a different length rod in, you'll just ruin it, you need the right length for what you're doing. Changing the rod length, from what I understand, requires also changing the case and maybe flywheels. You can't just willy nilly change the rod length. Unless you're ordering a 113" engine and trying to decide between a 113" that's a stroker or a 113" that's a big bore. So are you building an engine, or building a bike around a particular engine?

It's actually more complicated than that.

Also, displacement (like 113 ci for example) is determined by the bore size multiplied by the stroke length... well, technically, it's π x ( x bore) x stroke x number of cylinders. For it to be 113, for a given bore size, it needs a specific length rod. So if you're building a specific size, you need a specific length rod for a specific bore size.

Quick Google search... https://en.wikipedia.org/wiki/Stroker_kit:
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A stroker kit is an aftermarket assembly that increases the displacement of a reciprocating engine by increasing the travel of the piston (that is, the piston moves farther up and/or down in the cylinder). This is done by replacing the crankshaft with one where the crank pin is moved farther away from the center of the axis of rotation of the crankshaft. While this increases displacement and torque it can potentially lower the limit to which the motor can rev safely compared to the stock configuration.

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post #5 of 29 (permalink) Old 11-07-2019, 08:06 PM
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Difference?
Seem`s women prefer long.

(Come on. Somebody had to see that coming.)
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post #6 of 29 (permalink) Old 11-07-2019, 08:59 PM
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Ha - this is a fun-loaded topic name.

Engine rod length is actually a quite interesting engineering topic. Consider two engines of the same bore and stroke, but with different rod lengths. Due to the geometry of the crank throw the rod moves out to a certain angle every time the crank is at 90 or 270 degrees.

The long-rod engine has less angle of the rod under the piston. The short-rod engine has more angle. This causes the piston to dwell near TDC a bit longer every revolution. That can improve performance slightly in some engines.

The other benefit is that the long-rod engine causes a bit less side force on the piston skirt. Thus, the piston skirt may wear a bit less.

I recall this from an ancient Smokey Yunick article I read in the olden days. Accuracy is not guaranteed.
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post #7 of 29 (permalink) Old 11-07-2019, 09:20 PM
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Difference?
Seem`s women prefer long.

(Come on. Somebody had to see that coming.)
Some width also. Beat me too it Fred
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post #8 of 29 (permalink) Old 11-07-2019, 09:24 PM
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@SD CHOPPERS You in Brazil by chance?

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🇺🇸 R.E.D.

People are why I love Dogs and my Harley.
💔In Loving Memory💔
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🐾No longer by my side. Forever in my heart.🐾
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post #9 of 29 (permalink) Old 11-07-2019, 09:26 PM
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Ha - this is a fun-loaded topic name.

I recall this from an ancient Smokey Yunick article I read in the olden days. Accuracy is not guaranteed.
The man the legend Smokey Yunick. NASCAR banned his shrunkin 66 Chevelle from competition.
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post #10 of 29 (permalink) Old 11-08-2019, 07:59 AM
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Originally Posted by Thunder~Struck View Post
Difference?

Seem`s women prefer long.



(Come on. Somebody had to see that coming.)


See post #2.




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"There's no sensation to compare with this
Suspended animation, a state of bliss
" --Pink Floyd
** This line seems to come to mind often while I'm riding.

"Men do not quit playing because they grow old. They grow old because they quit playing." --Oliver Wendell Holmes

"And miles to go before I sleep
...and miles to go before I sleep." -- Robert Frost

If you give up drinking, you don't live longer...it just seems longer
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