Does anyone know of a formula to calculate the minimum big end rod size? I can calculate force, piston speed, etc., and am having custom pistons made, but I want to specify the smallest big end size I can without compromizing the reliability of the rod.
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I'm, along with many others, designing and building a 1200cc air cooled motor loosly based on the current air cooled Ducs. The goal is 100+ ft/# of torque.
We know a lot, but I try not to be so arogant as to act like I know everything and I like to get other qualified opinions.
If you have any experience or know of a source for the information, please post.
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A man who works with his hands is a laborer.
A man who works with his hands & his head is a craftsman.
A man who works with his hands, his head, & his heart is an artist.
Reading "Internal Combustion Engine Fundamentals" by John B. Heywood (start on pg. 42) proves there's a lot to calculate. Things like bore, stroke, rod length, crank radius, crank angle, etc. I would think piston speed, mep, crankcase volume and room and even rod bearing material would or should, come into effect.
Props to you for going for it, though. Keep us informed of the progress!!
Yeah, that's pretty much the same things I'm coming up with. I remember the Coswoth guy saying that after doing the engineering of a motor it should run perfectly, without the need for further modification. I don't suppose anyone has his home phone #?
I have a rod guy (steel H beam, BTW), a piston guy, and a crank guy but was hoping for some universal "this is how it's done" so I could educate myself a little more.
I can run almost any bore size on the motor but want to get as much stroke as possible. By reducing the size of the crank pin, I can add a little more stroke. The Harley motors have tiny crank pins but I don't know how relevant that is. They use a whole different system.
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motomaxy.net
A man who works with his hands is a laborer.
A man who works with his hands & his head is a craftsman.
A man who works with his hands, his head, & his heart is an artist.
The HD stuff uses a roller bearing rod, so yes, very different. Also low rpm, inertial loads at least grow at the square of RPM.
What revs are you going to turn? I'd suggest on the order of a 21mm pin, 11/12mm ID, 43-44mm crankpin should do it for the pin dia, not sure on rod width. NOSE OIL FEED THE CRANKSHAFT! Journal feed sucks. Use 50% overbalance (inertial loads are at minimum there).
As for 'this is how it's done' stuff you need to know the cylinder pressures (and therefore the gas loads), as well as an idea of the masses of components to determine inertial loads. Once loads are known, the rest is relatively easy.
In general increased stroke will increase inertial and piston side loads. Increased bore will likely lead to fun with knock on an air-cooled motor.
Do you think Keith Duckworth made his living because his motors needed no further modification?
You are dealing with a lot of variables. As in a math problem you need to set some of them to constants to be able to solve the problem.
Set a goal, ie horsepower, RPM limit, bore, stroke parts availability etc and use that as a start to design the rest of the system to it.
For example Rods are available on E-Bay from old Formula 1 cars. Cheap and available. That would set your Rod length and big and small end size and the weight of the rod. This would enable you to solve the rest of the equation more easily.
Unless you have a big stack of cash, designing from scratch is not the answer. See Moto Cyzz (sp). You are going to have to use readily available parts for part of the engine.
The force exerted on the piston pin is duh me,,, the same on the crank pin.
I'm not an ingineer, but my simple assumption is that the crank pins are larger diameter in a plain bearing engine to provide sufficient surface area as to not exceed the psi load carrying capability of the soft material in the bearing shells which would otherwise induce deformation and failure. If you are in the designing mood, maybe a copper/tin bearing would hold up better as the diameter shrinks appreciably.
I have worked on really large aircraft radial engines, bore diameters to 6.5", and supercharged for tremendous cylinder pressures. The pin bore and crank pin diameters are almost the same diameter.
Thanks for the tips guys. This is a fun project because I love to learn and develop new things, but money is ALWAYS tight and I am definatly not qualified to do this. Hopefully I'll end up with a great, inovative motor, or..........a nice decoration.
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motomaxy.net
A man who works with his hands is a laborer.
A man who works with his hands & his head is a craftsman.
A man who works with his hands, his head, & his heart is an artist.
big end rod size
from a wanna be engineer.
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