Crankshaft modification advice needed. 2

[MNRaptor]

We are having custom rods and pistons made for a full out drag engine and are also planning to have the crank knife edged. We do this on a regular basis with the 4 cylinders we build however this engine is a V6 and requires a bit more work. The questions I have are.Is there a good way to determine how much should be removed from the counterweights to adjust for the reduction in rod/piston weight (the big end of the rod and rod crank journals will also be considerably smaller) The next question is with it being a 6 cylinder, is there a good way to determine where the metal should be removed and still maintain the balance.

I am looking mainly for a way to do this on paper rather than spending a lot of time at the engine machine shop on the balancer, I know that would give me what I need to know also but considering that the engines are designed on paper I am hoping there is another method.

Thanks in advance,

Mike

 

[motorsportsdesign]

I have good news for you. The middle counter weights, (those other than the end counterweights) on 99% of V engines are too small becuase there simple isn't enough room in the block to make them big enough.

The engine is balanced for vibration mostly by adjusting the end counterweights. You can confirm the extent of this condition by examining the end counterweight. If it is near symetrical to the centerline of the nearest rod pin, the crank has good internal balance. The more it is offset the more underweight the internal couterweights are.

When you rebalance your crank remove material frm the end counterweights. If the end counter weight becomes symetrical, them remove weight from all counterweights equally until you reach balance.

Write me if this doesn't make sense and I will explain it.

Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[Tmoose]

Got a junk crank? Some designs would allow a decent static evaluation of a front or rear 1/3 hunk.

mathematically I've taken the rod + piston change (with appropriate %) X 0.5 stroke to come up with a gram-inch figure. Then used a Cad program to get a 2D area and CG location, or the CG of arc segments from Machinery's handbook.

If you can take equal weight off at 180 degrees on a given counterweight the balance change is zero. No "windage" reduction.

Right about Now a debate will spontaneously erupt about knife-edged counterweights VS leading edges that are shaped like a small-mouth bass.

 

[magnograil]

Tmoose - I'll bite. The small mouth is better because the airflow is both across the V and lengthwise in the case due to piston displacement. An elliptical shape better handles the cross flow than a knife edge.
MNRaptor - Easiest way is to program the equations in a spread sheet and calculate the forces on the crank due to the pistons and rods then calculate the balance weight that minimizes the load on the main bearings. What almost all shops leave out is the moment of inertia of the rods, which in your case is important. They only weigh the piston assembly and wrist pin end of the rod. The difference can be a couple percent of the reciprocating weight.
Write the equations for positions of the rod and piston relative to the crank angle, differentiate once for the velocity, a second time for the acceleration. Multiply these by the masses and you have the crank forces. Sum the forces around one revolution and change balance mass x moment arm until you get a minimum. Simplest to use the rod journal throw for the mass balance arm.
Rotate the forces into the cylinder axies to get the entire crank assembly forces.
I have only done inline twins and triples but I believe balancing the individual cylinder is optimum even with a V.
Once you have the counterweight moment you will have to make weights to clamp on the rod journals. A dynamic balancer that measures both ends of the crank will tell you where to modify it.

 

[motorsportsdesign]

magnograil

On an automotive V engine you will find that after doing the calculations you describe that there is not enough room in the block to make a counterweight big enough. That is why the end counterweights on a V6 or V8 are much thicker then the middle counterweights. This is also why the 2nd and 4th main bearings wear the fastest, there simply isn't enough room to have a big enough weight fit in the block.

Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[willeng]

I suppose everybody's got the calculator out trying to find something wrong with Johnathan's reply!

MNRaptor

Use a counter balanced flywheel & harmonic balancer as well for V6 engines & you will find a (fine) balance. Many will disagree, but if it works it works. Like Johnathan is saying there's not enough meat in the middle of those things so anything you can do to help the balance---do it!!

There's balanced & there's balanced

 

[MNRaptor]

Thanks for all the info, I definately have some ideas now on how to proceed.

 

[magnograil]

Jonathan - Have you tried Dowmetal or tungsten slugs? Commercial aircraft typically use antimony to counter weight the control surfaces but as it is slightly radioactive it might be hard to come by.
The only V6's I am familiar with is the Alfa, which has large enough counter weights because it was designed as a V6, and the dodge dakota, which is a piece of crap. A V8 with two cylinders cut off that stumbles, uses more fuel with less power than a V8 and the latest, the front wheels fall off, which dodge says is not a safety issue.

 

[MNRaptor]

The engine I am working with is a DOHC Mitsubishi 6g72 originally from a 3000GT twin turbo. It has a very strong shortblock from the factory with 4 bolt mains and a heavy forged crank. Destroked with custom Ti rods, it will end up having a rod ratio of 2.14:1 and be 1.2:1 oversquare. The center counterweights on the crank are huge as are the ends and was fairly well balanced from the factory. With the different rods and stroke, I want to get the crank well matched and cut down some of the windage as well. The 4 cylinder cranks we do now end up like a plane wing, rounded on the leading edge and tapered to a point on the trailing edge. The four cylinder crank ends up a little over six pounds lighter when finished.

 

[swall]

Magnograil--don't you mean depleted uranium, rather than antimony? Antimony is less dense that steel.

 

[magnograil]

Swall, Possibly since there is a readily available supply of depleted uranium from reactors. Antimony less dense than steel? Sb @ 121.8 aw, Fe @ 55.8 aw + C @ 12.0 aw + Ni @ 58.7 aw. It has been a long time since Boeing and longer since chemistry.
MNRaptor, I would not think a pointed trailing edge is best. Depending upon the block configuration, the cylinders are typically vented to each other below the crank mains so the cross flow would be past the sharp trailing edge causing more turbulence than a rounded profile.

 

[swall]

Magnograil--atomic weight just tells you how much one atom weighs. Density depends on the size of the atoms and the crystal structure (i.e. how the atoms are packed into a given space). Density of iron is 7.87 g/l and antimony is 6.62 g/l. Reason I mentioned depleted uranium is because it has a history of usage in aircraft counterweights and is slightly radioactive.

 

[MNRaptor]

Thats good to know for the rest of the cranks we do. It is common for them to be like that from other venders, if it is better to use a radius, I need to look into that. Any suggestions as to how large a radius in comparison to the leading edge? Would it be optimum to match them or go a little smaller on the trailing edge?

The engine I originally asked the question about will not have this issue as the dry sump is compartmentalized into separate areas divided by the mains.

 

[magnograil]

Swall, Sorry for the confusion. I remembered the radioactive stickers (on the inside of the control surface so not to worry the passengers) and less clear on the material. If it is uranium, they probably do like businesses in california that need lead ballast - send it to Arizona for cutting to aleviate the ire of environmentalists. california prefers to export its polution rather than handle it themselves.
MNRaptor, I do not know enough about the geometry to give an answer. On my triple cranks the periferal speed is about 170 mph at 10,000 RPM. The profiling on the leading edge is mostly for the oil mist impact drag. The larger the radius, the greater the tolerance to angle of attack. If there is no cross flow the aerofoil shape is best.
Your dry sump engine has separate oil pickups for each cylinder? I have not seen that before. Typically they have a common sump with a windage tray.

 

[motorsportsdesign]

I have used mallory metal (a trade name I guess) to increase the weight of middle and end counter weights when nessesary. The net increase in weight is about 90~100% for the metal replaced.

The key to understanding V8 counterweight design is to understand that the bearing loading will be least when the 1st, 4th 5th and 8th counter weights are symetrical to the nearest rod pin.

The combination of the 2nd and 3rd weights should be symetrical about a plane between the 1st and second rod throw. If you are having trouble imagining this the plane, it will be on the rotation axis of the crank and rotated 45 degrees from the 1st rod throw toward the second (cly 3&4)

In practice you will see cranks that are very different than that. There are a few reasons for this.

1. It is difficult to forge the counterweights to the ideal shape.

2. A crank can be made lighter by making the middle CWs lighter and using the end counterweights to balance the crank for vibration. This is done at the expence of bearing loads. The dynamic balance of a crank has nothing to do with bearing loading.

3. On engines where the bob-weight is so high that the end counterweights can't balance the engine, the middle CWs are rotated in line with the end CWs. This is horrible for bearing laoding but does make an engine that doesn't vibrate.

4. Too many people make cranks that look like other cranks that successful racers used with no understanding of the design issues.

I have seen fuel racing cranks made in the 50's that indicate a good understanding of CW design and some today that are clueless.



Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[motorsportsdesign]

"I suppose everybody's got the calculator out trying to find something wrong with Johnathan's reply!"

Calculator? If you want to do it the easy way, I can model it in 3D CAD in a couple hours and balance it. Just cut the CWs to the dimensions and it will be very close.

If anyone wants to do this the trick is to cut the crank in half (front/back) then adjust the CWs until the center of gravity (with bob-weight) goes through the rotation axis.

Make the front and back the same and you have it.

Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[MNRaptor]

Magnorail, the dry sump design I am using is divided at the main bearing webs, with a scavenge stage for each compartment - one for every 2 cylinders.