crankshaft counterweight shape

[naukkis79]

Bays 2 and 5 have differently shaped counterweights

BMW inline-6 engines have perfectly symmetrical counterweights at all throws, as is case with most modern inline engines. That angle of view makes they look different but they aren't.

Yes, I walked out to my shed and inspected an Eta crank and it does have symmetrical counterweights but it is not correct that all BMW inline-6 engines have perfectly symmetrical counterweights.

Here is a link to an OEM BMW crankshaft for sale in Germany; note the asymmetrical counterweights:
https://www.ebay.de/itm/BMW-E46-M3-S54- ... 0009.m1982

BMW Kurbelwelle.jpg

Ok, I stand corrected. For long time BMW used mainly fully counterweighted cranks in its inline-6 engines which are as good as best aftermarket ones. Forging fully counterweighted inline-6 crank is pretty expensive so seems that newer engines are back to using partially counterweighted cranks.

That S54 crank is little in between one, it has full 12 counterweights but it still isn't fully counterweighted crank - japanese manufacturers did use those kind of designs in 4-cylinder engines in 70 and 80's but switched later to fully counterbalanced versions. Odd that BMW put partially counterbalanced crank in high revving, long stroke S54 when same time M52 versions got fully counterweighted cranks. Maybe they tried to lessen engine inertia but less counterweighting means more bending force to crank which leads unwanted torsional vibrations which greatly shortens all moving parts life.

[SchmidtMotorWorks]

Sorry Newold,
You were probably making a point but I can't envision how a lathe will cut the leading and trailing edges.
There really isn't anything wrong with the manual Cuttin N Carvin method.
I also have done them this way before such things could be ordered.
I have also used the mill to rough them in and then blend them to final shape by hand.
Sometimes you just have to get dirty.
No floors were harmed in this exercise.

The first 100 or so cranks made at Bryants had the counterweights and rod-throws shaped on a Bridgeport style mill.
With regards to aesthetics, they were some of the best cranks made there.

[jed]

The first 100 or so cranks made at Bryants had the counterweights and rod-throws shaped on a Bridgeport style mill.
With regards to aesthetics, they were some of the best cranks made there.
[/quote]

Do you rember what made them the best crankshafts made over today's technology crankshafts?
Also do you rember the science used to place the counterweights?
John

[inline__engine]

would you design a crank different in a wet sump application compared to dry sump?

so we are at that there is no benefit to streamlined counterweights?

[SchmidtMotorWorks]

The first 100 or so cranks made at Bryants had the counterweights and rod-throws shaped on a Bridgeport style mill.
With regards to aesthetics, they were some of the best cranks made there.

Do you rember what made them the best crankshafts made over today's technology crankshafts?
Also do you rember the science used to place the counterweights?
John
[/quote]

Sure, if you are very good with a manual milling machine you can sculpt with it. In the manual case it is possible for a skilled person that cares to machine the shape of the rod through so that it flows into the faces on the counterweight.
Of course it is possible to model a nice continuous shape like that in cad and then have as associative cam program that updates to each design but that is not how cranks are generally made in practice.
Instead the rod throughs and counterweights are formed by seperate programs and then the shapes are simply smoothed a little or nicely blended by manual hand and polishing.
In the end there is no public data that I am aware if that convinces me that any particular shape has significantly better aerodynamics than any other.
What I do know is that the organizations with the most testing and success in the field don't seem to follow the knife edge or aerodynamic shapes on crankshaft. These are professional operations that treat engine development and production as they would any other industrial machine.
We that invest emotion into the craft tend to do things for reasons that satisfy us because they validate our intuitions by doing work to fulfill them. From that point forward objectivity is rare.
If there is any one shape that tends to show up where research is the deepest, it is counter weights that are rounded on the intersect in of the leading face and the circumference so the center axis is parralel to the axis of crank rotation.
If you give it some thought you might imagine why that might work. But don't take that intuitive thinking as good engineering. That shape could be a byproduct of economical forging, and nothing more.

[SchmidtMotorWorks]

The first 100 or so cranks made at Bryants had the counterweights and rod-throws shaped on a Bridgeport style mill.
With regards to aesthetics, they were some of the best cranks made there.

Do you rember what made them the best crankshafts made over today's technology crankshafts?
Also do you rember the science used to place the counterweights?
John
[/quote]

Sure, if you are very good with a manual milling machine you can sculpt with it. In the manual case it is possible for a skilled person that cares to machine the shape of the rod through so that it flows into the faces on the counterweight.
Of course it is possible to model a nice continuous shape like that in cad and then have as associative cam program that updates to each design but that is not how cranks are generally made in practice.
Instead the rod throughs and counterweights are formed by seperate programs and then the shapes are simply smoothed a little or nicely blended by manual hand grinding and polishing.
In the end there is no public data that I am aware of that convinces me that any particular shape has significantly better aerodynamics than any other.
What I do know is that the organizations with the most testing and success in the field don't seem to follow the knife edge or aerodynamic shapes on crankshaft. These are professional operations that treat engine development and production as they would any other industrial machine.
We that invest emotion into the craft tend to do things for reasons that satisfy us because they validate our intuitions by doing work to fulfill them. From that point forward objectivity is rare.
If there is any one shape that tends to show up where research is the deepest, it is counter weights that are rounded on the intersect in of the leading face and the circumference so the center axis is parralel to the axis of crank rotation.
If you give it some thought you might imagine why that might work. But don't take that intuitive thinking as good engineering. That shape could be a byproduct of economical forging, and nothing more.

[Kevin Johnson]

It has been a few years (almost 13 years) but I believe this article was written and researched by David Vizard. It includes careful dyno data on the effects of modifying a specific crankshaft in a particular way for both aerodynamics and coatings.

https://www.hotrod.com/articles/ctrp-06 ... echnology/

https://www.hotrod.com/articles/ctrp-06 ... -aero_face

[SchmidtMotorWorks]

It has been a few years (almost 13 years) but I believe this article was written and researched by David Vizard. It includes careful dyno data on the effects of modifying a specific crankshaft in a particular way for both aerodynamics and coatings.

https://www.hotrod.com/articles/ctrp-06 ... echnology/

https://www.hotrod.com/articles/ctrp-06 ... -aero_face

A claim of 1.4% gain toward the high rpm, yet one of the worlds most refined engines (R1 by Yamaha) with decades of highly funded R&D that runs roughly 2X the rpm doesn't have any obvious aerodynamic shape consideration.

[Kevin Johnson]

A claim of 1.4% gain toward the high rpm, yet one of the worlds most refined engines (R1 by Yamaha) with decades of highly funded R&D that runs roughly 2X the rpm doesn't have any obvious aerodynamic shape consideration.

I spoke with one of the engineers on an engine that I made some windage control parts for. I asked him why some rather simple/obvious things had not been done (oil draining from the head was dumped directly into the rotating assembly, for example). He explained the major reason was that a target power level was sought and was reached in the first design iteration and it was unnecessary and undesirable to add further improvements that could cause taxation problems in certain markets with a higher power output per liter.

Edit: Here is a link for a discussion on the BMW S1000RR crankshaft which does have aerodynamic profiling:

https://www.s1000rrforum.com/forum/111143-post21.html

[SchmidtMotorWorks]

A claim of 1.4% gain toward the high rpm, yet one of the worlds most refined engines (R1 by Yamaha) with decades of highly funded R&D that runs roughly 2X the rpm doesn't have any obvious aerodynamic shape consideration.

I spoke with one of the engineers on an engine that I made some windage control parts for. I asked him why some rather simple/obvious things had not been done (oil draining from the head was dumped directly into the rotating assembly, for example). He explained the major reason was that a target power level was sought and was reached in the first design iteration and it was unnecessary and undesirable to add further improvements that could cause taxation problems in certain markets with a higher power output per liter.

Edit: Here is a link for a discussion on the BMW S1000RR crankshaft which does have aerodynamic profiling:

https://www.s1000rrforum.com/forum/111143-post21.html

I would not conclude that the shape is designed for aerodynamics, it has no difference from the shape one would design into a forging die to extend its life. Large draft angles and rounded pocket floors.

[modok]

And where is the block in this discussion?
The air flows as a result of the pistons going up and down, and that would be the first thing I'd be looking at as far as improving the flow.
On a conventional four or six, it's handy just having counterweights at the ends and middle, as that does not interrupt the flow path.

[Kevin Johnson]

I would not conclude that the shape is designed for aerodynamics, it has no difference from the shape one would design into a forging die to extend its life. Large draft angles and rounded pocket floors.

I think (hope) that the teams of engineers for the two areas would both have taken fluid dynamics and could communicate with each other. Cost accountants in an engineering firm might also be hoped to have an engineering background and point out the economies of considering both. There may be cultural differences due to the location of the firms and suggestions for changes in a different area might need to be more or less obliquely couched so as to allow saving face.

A couple of my relatives actually were cost accountants for large engineering firms. One told a story of noticing a higher than normally expected failure rate in a component. He went to the factory floor and observed the installation of the component over a period of time. Because of the awkward/difficult installation procedure the component was regularly dropped by the technician. The accountant then made the inference that if the component were more robustly made and tested to withstand drops from the work height the failure rate could be reduced. Certainly there were other options but the company did ultimately save a significant amount of money by doing this redesign.

[inline__engine]

It has been a few years (almost 13 years) but I believe this article was written and researched by David Vizard. It includes careful dyno data on the effects of modifying a specific crankshaft in a particular way for both aerodynamics and coatings.

https://www.hotrod.com/articles/ctrp-06 ... echnology/

https://www.hotrod.com/articles/ctrp-06 ... -aero_face

thanks for the link, I'm going to practice on a lesser crank. the engine will rpm more than that one so the surface speed will be higher so may be something to gain.

[Kevin Johnson]

thanks for the link, I'm going to practice on a lesser crank. the engine will rpm more than that one so the surface speed will be higher so may be something to gain.

Another thing that pops into my head is that it is worth paying more money for flap disks with higher quality abrasives (if you choose to go that route). They last significantly longer and cut much more quickly and cleanly. Excellent idea to practice.

[inline__engine]

And where is the block in this discussion?
The air flows as a result of the pistons going up and down, and that would be the first thing I'd be looking at as far as improving the flow.
On a conventional four or six, it's handy just having counterweights at the ends and middle, as that does not interrupt the flow path.

im not at the same place as the engine to take photos but there are no holes like shown in pg 15 of the pdf. is that what you meant? mine is a different configuration as it is a non bed plate design so maybe not as important?

https://www.motor-talk.de/forum/aktion/ ... tId=730180