Designing and Building a Custom Crankshaft

[Sparksalot]

The aluminum ladder hung on the wall was the best engineered piece in the photo above. The appearance of the shop and ignorant frame construction puts it out of contention of building a V8 by himself but I hope he does, rooting for underdogs is fun. They can learn.
































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[saltfever]

Your other biggest issue is to decide if it will be a 2 plane or single plane crank. Books have been written on the balance issues between the two. Both of them have an out-of-balance condition. Regardless what you read about a 90 degree V-8 being in balance there is a rocking couple taking place. This article simplifies some of the differences.
http://www.projectm71.com/Cross_FlatPlane.htm

And this will help with 2 plane crank design.
http://www.epi-eng.com/piston_engine_te ... issues.htm

Generally a single plane (180 degree) crank is chosen for packaging reasons. I.e., either the intake or the exhaust plumbing is much simpler, and more direct, with the firing order exactly 180 degrees from each other with no adjacent cylinders firing. When intake/exhaust plumbing is made simpler and more direct there is usually a small HP advantage to be gained. However, a single plane, 90 degree V-8 has a severe balance problem because piston acceleration is sinusoidal and not linear as usually assumed. This issue is mitigated with very small strokes. And that is why you will find single plane cranks in very short stroke, high revving, F-1 type engines. Because of the tiny strokes they can manage the balance issues that would be destructive in long stroke motors.

The whole subject is very complex and will take much time.This might be interesting:
http://victorylibrary.com/mopar/crank-bal-c.htm

Here is a flat (single) plane crank.

[Jerminator96]

Well the first thing anybody would ask is how much horse power? If you said 50 or 5,000 I would instantly have a feel for the size of the beast. The next, less subtle thing would be its efficiency. If you assume standard efficiency (BSFC of 0.5) and want 700 HP that is a bit different from a Formula-1 700HP motor. Same HP but very different package size.

But aside all of that, and especially pertaining to crank design, you must decide how you want the car to “feel” when you drive it. What is the response you want from the motor? This whole subject could be a PhD dissertation and that ain’t going to happen. Also don’t know your engineering background but since terms like “polar moment of inertia” cause me sleep apnea I’ll call it flywheel effect. Pretend you are driving a car with a 60 lb flywheel or no flywheel (actually a button flywheel). I have done both and the button flywheel was absolutely miserable on the street. So in the design of your crank you have to decide how heavy it will be because the location of the weight has a profound flywheel effect. Notice the drilled rod pins. Weight has been taken out of it. But most importantly the weight has been removed from the outside of the "flywheel".
BELOW: Notice no knife-edging (for aerodynamics), no light weighting of the counter weights, and 2-plane design. Continued . . .

Thanks for the resources, I will read up on those tonight. Heading to the NHRA Nationals in Charlotte this weekend so I should have a little down time.

As for power goals, I'm looking for around 300-350 HP out of a 2.0L V8. I have not looked into the average BSFC literbikes are acheiving, but I would expect it to be on the high side of average.

As far as the driving feel, I would expect a motor that acheives the above performace goal to be rather "violent" in its behavior. I have owned and driven a few cars with ridiculously light flywheels on the street, and though it can be annoying it is the nature of the beast. If I want to cruise in comfort I will spend my money on an old Rolls Royce or the like.

Your other biggest issue is to decide if it will be a 2 plane or single plane crank. Books have been written on the balance issues between the two. Both of them have an out-of-balance condition. Regardless what you read about a 90 degree V-8 being in balance there is a rocking couple taking place. This article simplifies some of the differences.
http://www.projectm71.com/Cross_FlatPlane.htm

And this will help with 2 plane crank design.
http://www.epi-eng.com/piston_engine_te ... issues.htm

Generally a single plane (180 degree) crank is chosen for packaging reasons. I.e., either the intake or the exhaust plumbing is much simpler, and more direct, with the firing order exactly 180 degrees from each other with no adjacent cylinders firing. When intake/exhaust plumbing is made simpler and more direct there is usually a small HP advantage to be gained. However, a single plane, 90 degree V-8 has a severe balance problem because piston acceleration is sinusoidal and not linear as usually assumed. This issue is mitigated with very small strokes. And that is why you will find single plane cranks in very short stroke, high revving, F-1 type engines. Because of the tiny strokes they can manage the balance issues that would be destructive in long stroke motors.

The whole subject is very complex and will take much time.This might be interesting:
http://victorylibrary.com/mopar/crank-bal-c.htm

Here is a flat (single) plane crank.

From what I understand of the single plane crank in a 90 degree V8, there is always a second order imbalance perpendicular to the longitudinal plane of the crankshaft. Given the firing order, this makes a lot of sense. I will read the information in the links you posted tonight in case I am not understanding something correctly.

That said, I would still prefer the single plane crank for the reasons you stated, it should be more simple, compact, and lighter than a dual plane crankshaft. With only a ~55mm stroke, I would hope the imbalance is manageable.

[Jerminator96]

I guess I can cheat and just buy one of these...

http://greenville.craigslist.org/pts/4929382798.html

[Sir Yun]

this is the Hartley crank btw.

[Jerminator96]

So I got a rod and piston in the mail yesterday, since $10 shipped on eBay was the cheapest way to get accurate measurements.

Looking at the important dimensions (stolen from the 10k rpm v12 thread):

Bore spacing- 3.23
Bore- 3.03
Compression height- .9
Rod width and clearance- .852, the rod bearings are .6764" wide
Main web width and clearance- ???
Stroke- 2.165
Bearing dimensions- rod bearing ID 1.417, main bearing ID 1.358
Counterweight radius- 3.56 max to bottom of piston interference?
Rod length- 4.125
Bank Angle- 90

It looks like I have .368" minus side clearances to work with to package the counter weights. Is this even possible? It doesn't seem like it would be unreasonably thin, and through rough calculation I think I can fit enough counter weight in.

[SchmidtMotorWorks]

So I got a rod and piston in the mail yesterday, since $10 shipped on eBay was the cheapest way to get accurate measurements.

Looking at the important dimensions (stolen from the 10k rpm v12 thread):

Bore spacing- 3.23
Bore- 3.03
Compression height- .9
Rod width and clearance- .852, the rod bearings are .6764" wide
Main web width and clearance- ???
Stroke- 2.165
Bearing dimensions- rod bearing ID 1.417, main bearing ID 1.358
Counterweight radius- 3.56 max to bottom of piston interference?
Rod length- 4.125
Bank Angle- 90

It looks like I have .368" minus side clearances to work with to package the counter weights. Is this even possible? It doesn't seem like it would be unreasonably thin, and through rough calculation I think I can fit enough counter weight in.

0.368 for a 3.23 bore spacing is too thin unless you have large overlap between the rod and main.

You will be unlikely to use any stock bearing widths, but it is easy to narrow bearings.
Also keep in mind a good racing crank will have large blend radius on the journals so the bearing needs to be clearanced and actual bearing contact will be even less.

[saltfever]

Short stroke designs are extremely strong due to rod pin overlap. IOW, the throw is so short the centerlines of both the main and the rod are very close together. When the centerlines are far apart the counterweights are all that are holding the thing together and strength has to be built into the web thickness. If your dims are correct above, the rod journal is bigger in diameter than the main diameter. With a throw distance of 1.082 (0.5 x your stroke) it looks like your rod material is overlapping into the main material roughly 3/8 of an inch. That overlap is adding a lot of strength. Plus you are going with a stronger single plane design. Mr. Schmidt has a valid point but I wouldn’t reject your design just yet. However, you are asking for around 40-43 HP per cylinder. That is a lot of stress and it is now time to start looking at those forces and calculating material strengths. My sketch is illustrative only. Your rod diameter is bigger than the mains.

[saltfever]

Look at the thin counterweights on this Bugatti crank. See the pin overlap in the red circle? That is adding a lot of strength. At this point I think your design is ok because you have overlap. But I don't have the FEA or CAE to back that up. Now may be the time to start outsourcing that help.

[Jerminator96]

Look at the thin counterweights on this Bugatti crank. See the pin overlap in the red circle? That is adding a lot of strength. At this point I think your design is ok because you have overlap. But I don't have the FEA or CAE to back that up. Now may be the time to start outsourcing that help.

Wow, that does look thin. Though I'm not sure of the scale, I'd imagine it matters more with such a long crankshaft. Looking at the Hartley crankshaft Sir Yun posted, those counter weights seem kind of thin, and I believe they offer a 1000hp turbocharged version of that motor...

I should add, that though my end goal would be to use stock rods and rod bearings, there is no point in keeping the mains stock. I can increase the diameter and thin them if necessary.

Also, the outer two main journals will be outside of the bore spacing, so I can make them SBC sized if I want, right?

Maybe it is time to think about modeling this crank. I have a copy of Solidworks (maybe 2011?), but I don't remember the included FEA being very helpful outside of testing very basic designs.

[Kevin Johnson]

Wow, that does look thin.

Do a search for Porsche boxer engine crankshafts -- thin counterweights.

If you get discouraged you could always move to a low rpm engine like they used in the Tiger, with roller bearings on disc counterweights acting as mains.

[SchmidtMotorWorks]

Notice the cranks that live with thin counterweights have small spacings between the mains proportionally.

If you do and FEA, the most revealing thing to do is just put torque on the main before the arm and fix the main after it.
Adjust the bore spacing and run the simulation again, and you will see why it matters so much.

[Jerminator96]

Thin Indeed, this one caught my eye.



Isn't the flat-6 inherently better balanced than the 90 degree V8? That has to count for something.

[SchmidtMotorWorks]

Notice how short the length of the rod pin is.

[lada ok]

Another Kiwi trick, would be to look for a 4 cyl crank with widish big ends, then you could fit .......