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Keep on Ridin’
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weird member
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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 (½ x bore)² 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:
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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Wayward Son
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Difference?
Seem`s women prefer long.

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

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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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Wayward Son
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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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Keep on Ridin’
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Retired citizen
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IMO, considering the rest of the drivetrain the long rod engine produces more torque at lower RPMs. Stump pulling power. The short rod engine revs up quicker and produces power in the upper RPM range.
1/2 " breaker bar is a long rod, 3/8" ratchet wrench is a short rod. You are the piston !
 

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IMO, considering the rest of the drivetrain the long rod engine produces more torque at lower RPMs. Stump pulling power. The short rod engine revs up quicker and produces power in the upper RPM range.
1/2 " breaker bar is a long rod, 3/8" ratchet wrench is a short rod. You are the piston !
A short stroke (ie crankshaft) motor revs up quicker and produces power in the upper RPM range.

Rod length doesnt effect this.

Understanding Rod Length, Piston Compression Height, and Crankshaft Stroke
 

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Retired citizen
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Well, the stroked crank requires the longer rod. They wouldn't just install a longer rod on a stock stroke crank.
 

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Well, the stroked crank requires the longer rod. They wouldn't just install a longer rod on a stock stroke crank.
Got that backward. Long stroke either a shorter rod or the pin placed higher in the piston.

Actually they would install a longer rod on a stock stroke if the engine builder was looking for a specific result. I seriously doubt anyone would notice the difference on a street engine.
 

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Actually the SE stroker crank comes with shorter connecting rods than stock crank.

Stock (4.000" and 4.375") crank = 7.667"
Stroker (SE 4.625") crank = 7.575"

Rod length affects where the wrist pin location will need to be for a given compression ratio and crank.

Stroker cranks would need a shorter connecting rod as the stroker crank is already pushing the piston higher at TDC.
A shorter crank would off set this, but would also pull the piston down further in the cylinder at BDC.
 

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I give up !
I'm not an engine builder or high performance anything. I'm just remembering from how stroking was done back when.
To stroke a Sportster or Shovel Head the crank needed offset pins and longer rods with stroker plates under the stock cylinders to compensate for the rod length.
The result was a slower revving engine but much more torquing engine. Stump pulling power but harder to kick over ! Worked best on the heavy bikes.
I explained the easy answer to the OP's original question, long rod v/s short rod. Long wrench v/s short wrench !
 

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weird member
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What concerns me is the op's name is "SD CHOPPERS", which to me, sounds like the name of a shop.

I personally don't think I'd open a shop around building choppers before I learned how to build choppers... part of which is understanding different kinds of motors and builds.

I mean, props for trying to learn and all that and not being afraid to ask a question, but to start a shop before knowing what you're doing isn't too bright. I've seen quite a few fold like that, guys do one bolt-on mod and think they're a builder, or they think because they can work on theirs, that they can work on any. Cart before the horse and all that.

Never mind that a quick Google search can tell you the basic difference between a short stroke and a long stroke.
 

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Wayward Son
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Barney Fife - Poser
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I give up !
I'm not an engine builder or high performance anything. I'm just remembering from how stroking was done back when.
To stroke a Sportster or Shovel Head the crank needed offset pins and longer rods with stroker plates under the stock cylinders to compensate for the rod length.
The result was a slower revving engine but much more torquing engine. Stump pulling power but harder to kick over ! Worked best on the heavy bikes.
I explained the easy answer to the OP's original question, long rod v/s short rod. Long wrench v/s short wrench !
What concerns me is the op's name is "SD CHOPPERS", which to me, sounds like the name of a shop.

I personally don't think I'd open a shop around building choppers before I learned how to build choppers... part of which is understanding different kinds of motors and builds.

I mean, props for trying to learn and all that and not being afraid to ask a question, but to start a shop before knowing what you're doing isn't too bright. I've seen quite a few fold like that, guys do one bolt-on mod and think they're a builder, or they think because they can work on theirs, that they can work on any. Cart before the horse and all that.

Never mind that a quick Google search can tell you the basic difference between a short stroke and a long stroke.
You guys both seem to be confusing long stroke v short stroke with long rod v short rod. Two different things. Stroke is determined by the crank (not the rods) and yes, if you change the crank to one of a longer stroke, you need to shorten the connecting rods, or lengthen the cylinders (or both).

Engine torque is not developed by the connecting rods or pistons, it is developed by the crank. Torque is a rotational force (the crank rotates). the pistons move up and down (linear) and the connecting rods just tie the pistons to the motion of the crank. A piston pushing with the same force at a larger distance from the center of the crank (longer lever arm) will produce more torque. That's how physics works.
 

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You guys both seem to be confusing long stroke v short stroke with long rod v short rod. Two different things. Stroke is determined by the crank (not the rods) and yes, if you change the crank to one of a longer stroke, you need to shorten the connecting rods, or lengthen the cylinders (or both).

Engine torque is not developed by the connecting rods or pistons, it is developed by the crank. Torque is a rotational force (the crank rotates). the pistons move up and down (linear) and the connecting rods just tie the pistons to the motion of the crank. A piston pushing with the same force at a larger distance from the center of the crank (longer lever arm) will produce more torque. That's how physics works.
Exactly what I was saying !
 
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