Why did fork-and-blade conrods disappear post-WWII?

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z42

Senior Airman
646
414
Jan 9, 2023
Most(?) WWII inline aero engines had fork-and-blade style conrods, with perhaps the BMW VI derived Mikulins being the odd man out with an articulated conrod (similar to the master/slave style used on radials). Ostensibly the motivation for adopting this being to avoid a the rocking couple from offset cylinders with the side-by-side arrangement. (The engine would also be about half a crank journal shorter, but hard to see this mattering much.)

However, it seems that post-WWII this was more or less universally abandoned (Harley-Davidson V-twin motorcycle engines being one of the very few exceptions, perhaps because that engine originally dates back to the bronze age and was inspired by then contemporary aero engine design?) in favor of the simpler side-by-side arrangement. One can understand the attraction of the side-by-side arrangement, in that there's only one conrod to design, analyze and manufacture so it's cheaper. But it seems even the highest performance racing engines, where cost isn't much of a concern, like those used in Formula 1 use the side-by-side arrangement. Why is that? I guess it could be lighter weight than an equivalent fork conrod, and perhaps oiling the crank journals might be easier if you have a crankshaft nose oil feed?

This seems to apply also to the VAG/Audi/Bugatti W-engines. Though I guess you can really call those V engines in the sense that, similar to "normal" V engines, they have two cylinders per crank journal. They just have the cylinders staggered in order to make the engine shorter. But anyway, yes, these W engines also have side-by-side conrods rather than fork-and-blade.

If the side-by-side arrangement really is better, as evidenced by current day Formula 1 and other racing engines, would the WWII aero inlines have been better off with the side-by-side layout?
 
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Well, post WW2 most large V aero engines stopped development and production, so that was most of the reason in that direction. Side-by-side rod engines had been designed and you have covered some of the reasons that can have influenced old designs. Rolls-Royce disliked the articulated rod in most applications because of its complicated effects on stroke and construction. Rolls-Royce were also usually conservative in their designs and the blade and fork rods were a fairly well understood feature that worked well enough, with suitable development on engines.
Yes, the extra length and construction complications of side-by-side rods did make them less appealing in some respects, despite it possibly not seeming to be a big issue.
I think some serious comment might be forthcoming from Calum Douglas on this, and I will add that there is some difference between modern small engines and the old 1930's early '40's with very large slow revving engines. Maybe the case of just using what worked back then was the big factor?

Cheers

Eng
 
Yes, "historical path dependence" might very well be an explanation, if a company has, back in the mists of time, started developing engines with fork-and-blade, it might make better sense to continue down that path, unless it's clear that switching to another layout has enough benefits to overcome the in-house knowledge of the existing system.

But, that being said, I'm sort of interested in the theoretical arguments for and against the different schemes disregarding the path dependence.
 
I also agree it comes down to cost. The fork rod has the outer machined surface that is rather complicated to machine due to the layout.

PXL_20230430_181917969.jpg

RR Meteor fork rod. Sorry it is so dirty, I have plans to clean it up but I am waiting for a round tuit.
 
Cost is a huge driver for most automotive engines, and fork-and-blade or articulated rods will cost more than side-by-side connecting rods. I don't know about the connecting rods for largish (> 1000 shp continuous) diesel engines, but I believe some use articulated rods.

I wonder (not feeling like doing an analysis; I don't like structures) whether the different loadings on the crankpin is an issue. With either articulated or fork-and-blade rods, there loading tends to go through the midpoint of the crankpin, while for side-by-side rods, there's a moment because the piston loads are reacted near the ends.

Articulated or fork-and-blade: __V__

Side-by-side: --v--^--

(sorry for the ascii "art")
 
Cost is a huge driver for most automotive engines, and fork-and-blade or articulated rods will cost more than side-by-side connecting rods.

Indeed, I said as much in my original post. But the question remains, why do modern high performance racing engines where cost isn't an issue, like F1 engines, still use side-by-side rods? Evidently there is some other advantage to them, but what exactly is it?

I don't know about the connecting rods for largish (> 1000 shp continuous) diesel engines, but I believe some use articulated rods.

These aren't the same kind of articulated rods as used in some aviation engines, like e.g. the Mikulins. In aero engines, an articulated rod is like a master-and-slave rod found on a radial engine, in that the other conrod is connected to the first one rather than to the crankshaft.

On large diesel engines, which use articulated rods like you say, the upper part of the conrod is AFAIU fixed to the piston, there is no crankpin where the rod moves, and then the upper conrod is supported by a bearing as it moves up and down, and the joint in the middle of the conrod is where the actual articulation motion happens. So in a way the upper conrod is like an extension of the piston. This is done to reduce piston sidewall loading. These engines are also very long stroked.

I wonder (not feeling like doing an analysis; I don't like structures) whether the different loadings on the crankpin is an issue. With either articulated or fork-and-blade rods, there loading tends to go through the midpoint of the crankpin, while for side-by-side rods, there's a moment because the piston loads are reacted near the ends.

Articulated or fork-and-blade: __V__

Side-by-side: --v--^--

(sorry for the ascii "art")

Yes, this is the "rocking couple" issue I mentioned in the original post.
 
Indeed, I said as much in my original post. But the question remains, why do modern high performance racing engines where cost isn't an issue, like F1 engines, still use side-by-side rods? Evidently there is some other advantage to them, but what exactly is it?



These aren't the same kind of articulated rods as used in some aviation engines, like e.g. the Mikulins. In aero engines, an articulated rod is like a master-and-slave rod found on a radial engine, in that the other conrod is connected to the first one rather than to the crankshaft.

On large diesel engines, which use articulated rods like you say, the upper part of the conrod is AFAIU fixed to the piston, there is no crankpin where the rod moves, and then the upper conrod is supported by a bearing as it moves up and down, and the joint in the middle of the conrod is where the actual articulation motion happens. So in a way the upper conrod is like an extension of the piston. This is done to reduce piston sidewall loading. These engines are also very long stroked.



Yes, this is the "rocking couple" issue I mentioned in the original post.
I think you may have conflated crossheads with articulated connecting rods. Articulated rods would not be used on inline engines. Iirc, Brons-Werkspoor used articulated rods on the V12s used in, among other places, the tripartite minesweepers.

As for racing engines, my supposition would be that designers are following automotive practices.

For the last, I was not referring to engine vibration; I was referring to the stresses within a crankpin.
 
I think you may have conflated crossheads with articulated connecting rods.

You're entirely correct, my bad.

For the last, I was not referring to engine vibration; I was referring to the stresses within a crankpin.

I understood that, but assuming the engine is correctly designed so that the deflection would be well within the elastic limit (below the yield point), then the potential remaining issue would be a vibration problem, no? But yes, maybe the crankshaft and the crankshaft bearings would need to be slightly sturdier with a side-by-side arrangement due to the firing loads not being evenly distributed over the two nearby bearings. OTOH I would suspect fork-and-blade conrods are heavier than equivalent side-by-side ones, so I'm not sure which would win out in the end. Again, if this would be a major advantage I'm sure high end racing engines would be using fork-and-blade setups.
 

Thanks. I recall seeing that thread earlier when trying to learn about the issue on my own. It's an interesting thread, with nice pictures of various historical conrod designs, as well as some explanation of current practice. However, infuriatingly, it doesn't actually answer my question why the fork-and-blade design disappeared after WWII even in high performance engines where cost isn't a concern.
 
Thanks. I recall seeing that thread earlier when trying to learn about the issue on my own. It's an interesting thread, with nice pictures of various historical conrod designs, as well as some explanation of current practice. However, infuriatingly, it doesn't actually answer my question why the fork-and-blade design disappeared after WWII even in high performance engines where cost isn't a concern.
Discussing car and motorcycel practice ignores the huge difference in size and power of the engines. Moto Guzzis Ducatis and BMW twins all had plain conrods side by side. But look at the size of the crank pin and the power output, weight of pistons. The Merlin was supercharged and using exotic fuel and lasted 250 to 1000 hours depending on use, less if WEP was used often. A road going M/C even a sprts type lasted for 10s of thousands of miles.
 
Discussing car and motorcycel practice ignores the huge difference in size and power of the engines. Moto Guzzis Ducatis and BMW twins all had plain conrods side by side. But look at the size of the crank pin and the power output, weight of pistons. The Merlin was supercharged and using exotic fuel and lasted 250 to 1000 hours depending on use, less if WEP was used often. A road going M/C even a sprts type lasted for 10s of thousands of miles.

I was thinking more of pure racing engines like e.g. used in Formula 1: Formula One engines - Wikipedia
Current F1 engines are maybe a bit boring due to regulations specifying pretty tightly what can be done, but it used to give quite a lot more freedom to the designers. And IIUIC top end teams expected a qualifying engine to last a few laps, there were no restrictions how many engines a team may go through in a season etc. Sure they were all a lot smaller in volume than the Merlin, but at some points they did provide similar levels of power. E.g. the mid-1980'ies turbocharged 1.5L engines produced up to 1500 hp with 5 bar boost at 11krpm (for qualifying only, significantly detuned to last through a race). Crazy! Or after turbochargers were banned, we had the naturally aspirated engines turning at up to 20000 rpm. All with side-by-side conrods (well except some engines that were straight ones and not V).

Or for something more contemporary to WWII aviation engines, the crazy pre-war Auto Union V-16 and V-12 engines: Auto Union racing cars - Wikipedia E.g. the Type D produced about 3x the power/volume than the Merlin. And all with side-by-side conrods (I wasn't able to find a picture of the conrods, but from a top picture of the engine block one can see that the cylinder banks are offset to each other, which I think is enough proof that it uses side-by-side conrods).
 
A special con rod from my $15 1948 Plymouth. In the early 60s on the way to AF Reserve drill I spotted a 48 Plymouth along side a gas station. I watched for it the next month, and after drill, I stopped. The gas station owner said a Marine had owed his $15 and was transferred so left the 48 (not running) for his bill. The man said if he could get his $15, I could have it. My fellow airman with me said, "Offer him ten. Offer him ten." I gave the man his $15 and we towed home to be with my two other 48s. The rod pictured is what stopped the engine. Whoever had put the rod cap on #6 didn't tighten the bolts. The wear pattern showed one bolt fell out (found in pan nearly worn in half) and the other allowed it to run until it jammed the crank. It must have made a terrific noise. Had to break up the piston to get it out. Used strips of emery paper to dress the crank journal. new bearing inserts, and on she went with an Edmunds aluminum head and Edmunds aluminum intake manifold with two Stromberg 97 carbs. The old girl met her end after I gave her to a friend and he parked in another guy's yard near RR tracks. Train derailed filled with gasoline, set two blocks on fire and the 48 and several other old cars were history.

IMG_6531 copy.jpg
 
I was thinking more of pure racing engines like e.g. used in Formula 1: Formula One engines - Wikipedia
Current F1 engines are maybe a bit boring due to regulations specifying pretty tightly what can be done, but it used to give quite a lot more freedom to the designers. And IIUIC top end teams expected a qualifying engine to last a few laps, there were no restrictions how many engines a team may go through in a season etc. Sure they were all a lot smaller in volume than the Merlin, but at some points they did provide similar levels of power. E.g. the mid-1980'ies turbocharged 1.5L engines produced up to 1500 hp with 5 bar boost at 11krpm (for qualifying only, significantly detuned to last through a race). Crazy! Or after turbochargers were banned, we had the naturally aspirated engines turning at up to 20000 rpm. All with side-by-side conrods (well except some engines that were straight ones and not V).

Or for something more contemporary to WWII aviation engines, the crazy pre-war Auto Union V-16 and V-12 engines: Auto Union racing cars - Wikipedia E.g. the Type D produced about 3x the power/volume than the Merlin. And all with side-by-side conrods (I wasn't able to find a picture of the conrods, but from a top picture of the engine block one can see that the cylinder banks are offset to each other, which I think is enough proof that it uses side-by-side conrods).
I used to be an avid F1 fan. The whole thing started turning sour with the turbo era. They could make an engine that would just make it through qualifying and the race, but frequently they didnt, with a massive number of engines blowing up and sometimes only around 8 finishers. Same with Auto Union, it was bankrolled by the state, the engines only had to last 2 hours or so.
 
In NACA Technical Memoranda No. 290 "AVIATION ENGINES IN THE ENDURANCE CONTEST". By G. Lehr. From "L'Aeronautique,' July, 1924.
Aviation Engines in the Endurance Contest

It raises the issue of differences in expansion, which I had not realised could be an issue with fork & blade rods.

The other V engine, the Panhard, has a forked connecting rod which enables accurate rotation around the crank pin. This is more difficult to make than the preceding one and the wear of the connecting rod bearings in the endurance tests sometimes leaves something to be desired. In order to eliminate the effect of differences in expansion, the bushing carried by the forked connecting rod, which assures the turning on the shaft, was not made of bronze but of lined steel. The connecting rod turns exteriorly on the middle portion of this bushing. In Fig. 2, BB is a section of the connecting rod. The bushing is cooled by a copious flow of oil, produced not by oil grooves, which have the disadvantage of reducing the friction surface, but by interior perforations. A median orifice GG receives the oil which leaves the crank pin under pressure. The oil then passes through channels following the generatrices of the bushing. The oil, after passing through the bushing, flows back to the bottom of the crankcase, through perforations 00 in the large end of the connecting rod.​
The lubrication of the cylinder depends on the play left in AA sufficient cooling circulation can thus be obtained, without lubricating the cylinders to excess.​

1683570083956.png
 
I understand that people modifying RR Merlin engines for air racing typically replace the original connecting rods with modified versions of those used for the V-1710. The Allison rods are stronger.
 

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