Why not use crankshaft weights on any vee engine?

Uh it IS. Counterweights are used on just about every type of engine. Either on the crankshaft itself or attached to the crankshaft externally such as a weighted flywheel.

Uh that has been done since about 1964.

They are already there.

I have yet to see any crankshaft (other than a simple flat 4 or 6) that didn't have counterweights. Many even use balance shafts to cancel out the rocking coupling that occurs in the engines.

Has a bolt on external weight.

Thanks for the reply. From what I can see from the pictures you attached... The v8 crank is a crossplane crank which, by definition, has counterweights. The big inline engine, as far as I can see, has no counterweights (neat engine though).

I must be reading all the wrong articles. From what I've read, inline

4, straight 6, v12 , flat plane v8, (for example) do not use counterweights to balance the pistons. Rather the pistons get primary dynamic balancing from each other.

You mentioned that many engines use balance shafts. That is my primary question. If you can achieve primary dynamic balancing on a crossplane v8 using counterweights, why not use the weights on a v6 rather than an extra balancing shaft.

snipped-for-privacy@yahoo.com wrote in news: snipped-for-privacy@q9g2000yqc.googlegroups.com:

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The L4 and L6 crankshafts I've seen all have great big counterweights on them. Even the L6 crank from some 1940's vehicle a neighbor is using as his mailbox post (that one must have been a monster, with the size of the journals it has).

It has to do with movement of the center of reciprocating mass. If the mass is too great, this "primary couple" results in a vibration which is objectionable and must be damped via an opposing imbalance. That's the purpose of balance shafts.

The question doesn't compute. ALL inline engines, with the exception of extremely low-speed diesels, have counterweights. Look at a picture of an inline-six crank, for example a Chrysler slant-six:

An AMC (Jeep) 258 crank:

How about a Rolls Merlin V12:

How about one of the largest gasoline piston engines ever built, a 4-row radial:

About the only type of medium-speed or high-speed engine that doesn't have counterweights is a horizontally-opposed engine like a Porsche, VW or Subaru auto engine, or Lycoming or Continental airplane engine:

And even then there are small counterweights to account for the fact that each pair of opposed pistons is very slightly offset from each other and would make the engine rotate back and forth in the plane of the cylinders without some counterbalancing.

That the vast majority of engines other than opposed types and low-speed (sub-500 RPM max speed) types DO use crank counterweights.

While its true that some engine layouts allow the crankshaft to be balanced independent of the bob-weight of the pistons (inline-six for example) it is NOT done without counter-weights. See the picture of the Slant-6 and Jeep 258 inline six cranks I put in a previous post. In a v8, the crank and pistons have to be balanced together.

You're missing the point. Balance shafts are used (primarily) on inline-4 cylinder engines. Inline-4s HAVE primary balance through crank counterweights! The problem is that it is impossible to eliminate a

*second* order imbalance on an inline 4 strictly through counterweights. When you get perfect primary balance, you wind up with a residual 2nd order imbalance that causes the engine to "jump" up and down along the stroke axis at twice the crankshaft speed, because the center of mass of the rotating assembly shifts up and down slightly twice per crank rotation. The only way to damp that is with balance shafts spinning at twice the crankshaft speed.

Now, another application of balance shafts is a certain *class* of v6 engine: namely, a v6 built using the same infrastructure as a v8 for cost savings. These are usually rather low-RPM torque oriented v6 engines mainly for pickup truck applications, although there are others. Common examples in production today include the Chevy Vortec v6 based on the smallblock Chevy v8 casting, the Chrysler 3.7L v6 based on the

4.7L v8, and one of the many Ford 3.8 v6s which happens to be based on the old Windsor v8. Those engines are peculiar in that they don't use the "natural" 60-degree bank angle of a v6, and instead use the 90-degree bank angle that is natural to the parent v8. So for the v6 the crank journals have to be splayed. The splaying is always a compromise between balance and even-firing, so there is some residual imbalance that shafts can at least partially correct.

SIXTY-degree v6 engines don't use balance shafts.

I can also think of at least one inverse to the above case: The Yamaha/Ford v8 used in the last generation Taurus SHO was a sixty-degree v8, grown from a parent v6 casting. I don't remember if it had balance shafts or not, it might have been small enough to get away without them.

Lot's of the early engines had huge parts compared to current designs. They ran at much lower rpms and were usually HEAVY with lots of iron. Takes a stout crank to handle those iron pistons that were common. Working on some of them makes you REALLY think about the guys who raced them and worked on them to make them run.

And other than the throws looks like every other crankshaft out there. Counterweights are there on virtually EVERY engine.

Look at the "flywheel" it has on it. That IS the counterweight.

The balance shafts are NOT for primary balancing. They are to counter a rocking couple in the V engines and the vertical motion in the inlines.

I'm probably still not getting it, but back to the main question... A crossplane v8 would normally have a terrible rocking motion because the pistons at each end move in opposite direction. From what I understand, the weights on each pair of pistons can completely balance them. What makes a v8 special? Why can't any V (v2, v4, v6, v8) be perfectly balanced in the same way? Or are you saying that these engines can already be balanced like the crossplane v8 with weights alone?

Again, I'm probably reading the wrong articles but I'm given to understand that the counterweights on i4, i6, etc are only their to balance the rotating stuff (the crank throw, part of the con rod, etc), while the weights on a crossplane v8 are bigger and are designed to cancel the pistons themselves. In other engines, this piston cancellation is simply done by other pistons moving in the opposite direction.

There are several modes of vibration which are related to unbalanced conditions.

The simplest countermeasures are those which attempt to balance the piston/rod, etc momentum versus the crank counterweight.

But the story doesnt stop here. nOrder vibrations are still possible, and can be irritating in an engine. The use of balance shafts doesnt mean that the unbalance energy is neutralized at the source.. it means that the engine as a system "sees" the vibration as a whole, and the balance shafts negate some of this energy... It can be an amelioration of the vibrational system, but not necessarily a neutralization of the forces that caused it.

Some people have suggested that all flat engines are inherently balanced.. Not so. Some engines are said to be inherently balanceable, but one must take care to realize that n-order vibrations can come into effect.

Balance is not a simple concept, for most of us. That is why the more serious balance work is entertained by experts.

"HLS" wrote in news:MF4Xk.9687$ snipped-for-privacy@nlpi067.nbdc.sbc.com:

Hypothetical question: Would a fork-and-blade, opposed, flat twin be the most inherently balanced engine possible?

No, it would be maximally IMbalanced. A fork-and-blade approach is for a pair of conrods sharing a journal and the came centerline (unlike a typical car v8 where the conrods are side-by-side on a journal, and therefore the banks are offset by 1/2 journal width). But in a 4-stroke twin of any sort, you have to have the two pistons moving in unison so that TDC exhaust/intake on one occurs at the same instant TDC combustion on the other. For a flat twin, that means two separate journals on opposite throws, meaning that the pistons move away from the crank at the same time and then toward the crank at the same time. If you tried a fork-and-blade setup, the imbalance would be HUGE because both pistons would move "left" together, and then move "right" together.

Steve wrote in news:UJqdnReDE9LRF7DUnZ2dnUVZ snipped-for-privacy@texas.net:

Silly me. What was I thinking? Of course the pistons in an opposed twin would have to follow each other if they were connected to the same crank journal.

I guess the only way you could have a balanced flat oppposed twin would be if the crank had three connecting rod journals, with one of the pistons having a forked connecting rod that attached to the two outside journals. This way the pistons would oppose each others' forces, but they'd still travel a common axis.

In a "just for thought" moment...

I think one of the balanced designs ever were the Fairbanks/Morse opposed piston diesels of the late 1940's to be incredibly well balanced. Unfortunately, the design did not lend itself to "easy" repair of the lower cylinders and proved to be quite a maintenance hassle.

These engines were used in large boats/tugs/small ships and railroad locomotives.

Balance of reciprocating weight is quite a bit more than it seems at first glance.

Al

Just to get back on the rails... Does anybody know why you cannot perfectly balance any 90 degree V engine (v2, v4, v6) with crankshaft weights alone as is done with the crossplane v8? What is so special about the crossplane v8? Thanks.

snipped-for-privacy@yahoo.com wrote in news: snipped-for-privacy@d42g2000prb.googlegroups.com:

The problem is a phenomenon called "primary couple", which as I understand it, results when the center of the reciprocating mass hops up and down (or side to side) as the crankshaft rotates.

A 90deg crossplane V8 has no primary couple, since the center of reciprocating mass does not shift during crankshaft rotation.

In addition, I believe a V2 cannot have evenly balanced firing impulses. (Harley Davidson: pah-PAH-pah-PAH-pah-PAH...)

OT, but did the old Triumph vertical twin have both pistons on the same cycle?

They were very clever alright. No valves at all, one bank of pistons uncovered the inlet ports and the other bank of pistons uncovered the exhaust ports at the appropriate times. Therefore, no crankshaft, no rockers, no valves. But you're wrong about the way they were balanced- each crankshaft and its pistons was balanced independently and did NOT interact with the balance of the opposing pistons and crank at all. That's because the cranks were timed slightly out of phase- in F-M lore its called "lower crank lead." The lower crank's pistons reached and passed TDC slightly before the opposing upper crank pistons did, so that at BDC the exhaust port (uncovered by the lower pistons) would be opened before the intake port to allow initial de-pressurization of the cylinder, and the intake port (uncovered by the upper pistons) would be covered slightly after the exhaust port allowing the blower to pre-pressurize the cylinder. IF the balance had been dependent on one crank balancing the other, it wouldn't have worked because there could have been no lower-crank-lead.

For the umpteenth time... NOTHING is special about a 90-degree crank v8.

90-degree v6 engines are the ones that are "special" because they're operating at the wrong bank angle. To get semi-even firing, you have to give up some imbalance which is then corrected with balance shafts.

Every 4-stroke vertical twin that I know of did. I'm not familiar with the Triumph, but John Deere "johnny poppers" did. :-)