Early this morning while battling a severe bout of insomnia, I stared thinking about ways to improve leverage on a short stroke engine... and in my delirium I thought why not offset the big end of the connecting rod say a 1/4", 1/2", or even an inch toward the power stroke of the engine? And as I slowly drifted into sleep land at about 4 this morning, I began to think that surely someone had thought of this before, and that there must be a reason why it has not been used before, but in the current state I couldn't think of why. This morning it came to me that an engine using a setup like this would probably be impossible to balance, but I decided to search the net a bit just out of curiosity and found this.... http://morepowerengine.com/ I was thinking of a MUCH more subtle offset than this and a zero offset for the crank, but still, I am about certain that even a very subtle offset would shake and vibrate to no end. I imagine that the engine in this drawing would work great for compacting dirt, but not much else...
I think it would change the leverage imposed on the crank, (especially near midstroke) thereby theoretically raising torque potential. Not that I think it has any real world potential....
Offset at the big or small end of the rod won't do anything for leverage. It will however have a big effect on the sideload on the piston.
The engine pictured is probably designed for low rpm,steady state operation. If you look at the way the rod is designed, the offset is centered on the hinge point of the rod: that is a common point of failure. He also makes the assumption that intake stroke is defined by 180 piston movement; it is best thought of in terms of camshaft rotation; the closing point extends charge motion well into the compression stroke. You would be affecting dwell times at TDC and BDC but it would also affect the acceleration rates and deceleration rates to and from those points which would require re-thinking cam timing and lobe centers. Henry had the crank offset about 3/16 in the flathead and a 7" rod so there have been different approaches to the same issue......it is perhaps best to keep the rod centerline centered on the crankpin for load distribution....fats
The force vector can only be a straight line between the piston pin and the rod journal and it doesn't matter how much the rod is bent. Rod length does matter however
If the rod is not offset, the force vector is centered on the journal, this is true, but with the offset wouldn't the the offset have the same effect on leverage as increasing the stroke while the center of the rod is outside the center of the rod journal? (I am asking, not saying). Seems to me that an offset would load the rod quire a bit more, but wouldn't this increase the leverage imposed on the crank?
I think you might be missing something here..... In my head I can't see how the leverage would not be effected. The picture on the website is a drastically exaggerated version of what I was thinking about
All that matters is where the pivot points are. You could tie the rod into a pretzel, and other than weakening it, you would see no diff. Offsetting the pin in the piston , or the bore in relation to the main bearings, will make some difference, most engines do incorporate that.
What about the added weight of the rod itself. Not only are you affecting side load, tdc dwell times and addin a place for the ro to bend you're adding weight to the rotational mass. The whole idea of a short stroke to me comes to having reduced the piston Speed to get more rpm. To me adding weight to the rotational mass is counter productive.
I remember hearing of drag racers boring cylinders offset from inline with the crank centerline. Dunno if it actually helped.
Hot Rod did an article on a small block ford that covered some of this stuff. Basically they said that to increase the dwell of the piston at top dead center you should run the longest rod possible with a suitable piston pin height. The increased dwell leaves the piston at TDC for a longer time per revolution thus increasing the potential that all the fuel will be burnt, plus the combustion has longer to push the piston down. The times there were speaking of was of course minute, but it the grand scheme of things, it made some significant improvements. The multi-part article was called "The 351 that Ford should have built". It was a good read. I couldn't find a link to part one. But they said it was in the Feb 98 issue. Part Two
there are gains to be had with pin offset. The gains are greater farther away from the torque peak.The Smoke documented this at length.
Think about that for a moment.... at the very least the arc that the rod travels in will change, and added length in a certain plane equates more leverage. An offset rod would create a "dynamic rod journal center" if you will.... A rod offset 1/4" would not weigh much more than a conventional rod, and would cause the same leverage advantage as a crank with a 1/2" increased stroke (at 90* from the cylinder center line). This would allow a small displacement engine with a big bore able to produce the torque of a motor with a longer stroke.
The bent rod dose nothing but help to clear the bottom of the cylinder. A longer straight rod with a higher deck height would be more efficent with less side load and lighter. http://www.camotruck.net/rollins/piston-offset.html
Offsetting the big end would have no effect as mentioned previously, all you are doing is adding weight and complexity. Take a straight piece of steel five inches long and hold one end in your hand, and push the other end against a tree. Now using a horseshoe or a curved connecting rod, take one of the ends in your hand, and push the other end against a tree? You'll probably see the horseshoe flex a tiny bit. As pointed out in an earlier post, you're not changing the "pivot points", all you're doing is making it easier for the connecting rod to bend.
Nothing about the motion of the piston changes. Nor does how the force is applied to the crank. The wristpin is still pushing straight down, and the resistance is still tangential to the crank centerline. The rod could be shaped like a letter J, and it would still apply force to the crank identically... A LONGER rod will help the piston stay at TDC longer, which applies dynamic cylinder pressure more effectively. I use the longest rods I can fit without putting the pin up into the oil ring.
To clarify. Rod length will change the motion of the piston, the piston will move away from tdc more slowly, and accelerate harder further down the cylinder. Pretty sure ex west knows this, just clarifying what is being said for some of the others. R/S ratio vs power production can be debated till the cows come home, and has been. What is indisputable is the effect on peak piston acceleration and its effects on bottom end stresses. For the most part (there are exceptions, like a hemi chamber with a big dome) put me down on the "long rod" side of the debate.
The rod as noted just tranfers its loads thru the two connection points. The crank offset is maybe the "story".....apparently the idea of offsetting the crank has been around longer than any HAMB. Its being reapplied to building engines (only 1-2mm) as it has the advantage of reducing piston side loading on power stroke (lower friction). As far as piston speed, etc this may be a wash and you probably don't need an offset rod for 1-2mm. As stated rod ration is another story
Google" De Saxe (Desaxe) engine" for some theory behind offset cranks. Flathead V8 Fords had an offset crank. Bob
I guess with an opposed flat you could move the crank vertically or maybe a 75* V maybe upward. It doesn't sound particularly useful to my interest
Offsetting the pin end is a drag thing where you get a longer rod, a longer rod means more dwell time at tdc & bdc, more dwell at tdc & bdc means that in the same time frame as a short rod motor (1 rpm type time frame) the piston accelerates faster with a stronger signal for both intake and exhaust. longer rod motors will move the torue curve into higher rpm. What does this mean? if you have 2 identical 355"sbc's one with a pin offset and a .060 longer rod it will get to the finish line 1st. every time.
Offsetting the pin in the piston, yes. Many engines were designed with that idea from the OEM. This does give the rod a slight "lead angle" at TDC to help accelerate the crank quicker. Also proven to wear the bore faster on the thrust side.
I have a set of old ISKY offset rods...they're thick aluminum rods and I believe the reason for the offset is to be able to clear the rod bolts at the pan rail...and cylinder bottoms. They're for the small journal 2" early rod journals on a SBC...pistons are a set of blower forged Venolia slugs [0.080 over 283's]so it was most definetely for high performance use. The offset must be about 30 degrees.
Take the same engine and build a rod shaped like this. What's the difference? As previously stated, the force is directed in straight line from the wrist pin to the crank pin
Look up some old timers in the dirt track world. Growing up we my grandpa and dad would have the crank journals ground off set, it was a way around the engine rules. It worked great in our small blocks.
Ground offset in what way? Longer stroke? One rod to the next? Offsetting ALL the journals ahead or behind makes no difference to anything but the timing mark... TDC is still TDC.
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