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Why are these holes made in crankshafts?

General engine tech -- Drag Racing to Circle Track

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Rob-bb
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Why are these holes made in crankshafts?

Post by Rob-bb »

I hope this a simple question, why are these holes made in crankshafts?

Just a random picture I got from the internet
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Re: Why are these holes made in crankshafts?

Post by blwilliams »

Those holes are to help keep the crankshaft in balance.
As strokes get longer and longer, counter weights have to get larger and larger.
Drilling those holes helps to allow keeping the weights on both the pin end and counter weight end manageable.
helps keeping the overall total weight of the crankshaft down.
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Re: Why are these holes made in crankshafts?

Post by mag2555 »

X2 with the above!

If that rod journal was solid then the counter weight 180 degrees opposite it would need to be larger also .

Rotating mass besides loading the block bearings and main caps more also eats up hp just like the mass of your tires, rims, brake rotors and internal transmission and differential parts do!
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Re: Why are these holes made in crankshafts?

Post by BCjohnny »

To a lesser extent, in some applications, it can also be a rotational inertia issue

'Heavy' crank engines are lazier to rev up, which will suit some, but not all .... the removed weight might be low in relation to all up crank weight, but due to it's position disproportionally affects the ability to gain rpm

Harmonics, NVH and other considerations, rarely in isolation, may also affect pin & CW design
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Kevin Johnson
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Re: Why are these holes made in crankshafts?

Post by Kevin Johnson »

There have been some epic discussions about this. I purchased a surplus text from the Library of Congress that contained the original published research from 1930s Germany wherein various crankshaft designs and materials were tested. It is the original work that was translated by the US Government and provided to their researchers (and was cited by C.F. Taylor). That design and others were part of the experiment.

The translated work (in Taylor) and numerous other engineering references were pitched by my university library a number of years ago.

So, yes, some companies use(d) it as lightening/balancing strategy which of course it plainly is. Other companies with well educated employees and retained institutional knowledge will know about the 1930s research. Now this type of analysis is done by computer simulation.

This is the German researcher:

https://de.wikipedia.org/wiki/Karl_L%C3%BCrenbaum

Cited in:

https://mitpress.mit.edu/books/internal ... d-volume-2

Here is the original work (in German):
Luerenbaum pp 128-129 (1).JPG
Luerenbaum pp 130-131.JPG
You do not have the required permissions to view the files attached to this post.
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Re: Why are these holes made in crankshafts?

Post by PackardV8 »

IIRC, the early 427" Ford race engines had forged steel crankshafts with those lightening holes drilled and then plugged. One theory was to keep oil out of there; reduce windage. Since Ford was doing extensive dyno testing in those days, may have been a thing.
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Re: Why are these holes made in crankshafts?

Post by modok »

Same reason it has counterweights.
The lighter the rod side is, the less counterweight it needs opposite to balance that force.
Sometimes it helps, sometimes it does not.
The ideal crankshaft is super stiff, takes up no space, and weighs nothing, which is impossible but.... gun drilled rod pins may help or may not, depends whichever is the bigger problem.
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Re: Why are these holes made in crankshafts?

Post by Tuner »

Boring the rod and main journals significantly increases torsional strength.
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Re: Why are these holes made in crankshafts?

Post by modok »

but not rigidity
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Re: Why are these holes made in crankshafts?

Post by Dan Timberlake »

The OP's picture looks to be an inline four, so not bob weighted while balancing.

So the holes in the rod journal throws might make the minimal counterweights less skimpy.
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Re: Why are these holes made in crankshafts?

Post by j-c-c »

Tuner wrote: Mon Jun 07, 2021 11:37 pm Boring the rod and main journals significantly increases torsional strength.
Care to explain?
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Re: Why are these holes made in crankshafts?

Post by Kevin Johnson »

j-c-c wrote: Tue Jun 08, 2021 9:54 pm
Tuner wrote: Mon Jun 07, 2021 11:37 pm Boring the rod and main journals significantly increases torsional strength.
Care to explain?
Taylor 492-493.JPG
Taylor 494-495.JPG
Taylor 496-497.JPG
Taylor 498-499.JPG
Taylor 500-501.JPG
Taylor 502-503.JPG
Taylor 504-505.JPG
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Re: Why are these holes made in crankshafts?

Post by j-c-c »

Kevin Johnson wrote: Tue Jun 08, 2021 10:46 pm
j-c-c wrote: Tue Jun 08, 2021 9:54 pm
Tuner wrote: Mon Jun 07, 2021 11:37 pm Boring the rod and main journals significantly increases torsional strength.
Care to explain?

Taylor 492-493.JPG


Taylor 494-495.JPG


Taylor 496-497.JPG


Taylor 498-499.JPG


Taylor 500-501.JPG


Taylor 502-503.JPG


Taylor 504-505.JPG
Whatever that is, I can't read/view it in that format.

Can you summarize if possible how it addresses how any solid cylinder has significantly greater torsional strength when its drilled, and effectively becomes a tube?
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Re: Why are these holes made in crankshafts?

Post by Kevin Johnson »

j-c-c wrote: Wed Jun 09, 2021 6:58 pm ...

Whatever that is, I can't read/view it in that format.

Can you summarize if possible how it addresses how any solid cylinder has significantly greater torsional strength when its drilled, and effectively becomes a tube?
I just double checked by downloading an image. They are of sufficiently high resolution to read if you download them and magnify them on a PC screen. If you are trying to use a smart phone -- I can see how that would be difficult.

These are pages from C.F. Taylor. The text is available new. If you have a university near you with an engineering program they might have it in the stacks. Mine had it but removed it. Check the library catalog before wasting a trip.

https://mitpress.mit.edu/books/internal ... d-volume-2
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Re: Why are these holes made in crankshafts?

Post by swampbuggy »

Heavier crankshafts cause an engine to be lazy building up RPM's especially in neutral. When the engine is preforming the task it is asked to do in the race car be it a drag or stock car the weight of the parts rotating, from the hormonic balancer all the way to the rear wheels is important. On a short track stock car, this weight is extremely important, especially on deceleration into turns 1 and 3. Mark H. :wink:
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