Fortnine is Wrong. 285 Crossplane Twin Engines Do Not Have Better Balance than 270 Twins

time stamp is 07:26

Today I'd like to correct a mistake in a video made by one of my absolute favorite channels on youtube, and that is Fortnine, who produce some of the best motorcycle content out there, so before we continue Id like to say that this video is in no way meant to diminish the credibility of the fortnine channel or in any way tarnish their reputation. We all make mistakes, I make mistakes, universities, governments, scientists, you, everyone else. To err is human. But I believe it is important to identify, correct and learn from our mistakes.

So let's see where the problem is and learn about inline two cylinder engines in the process.

In their video Fortnine make the following claims:

A 285 inline twin cylinder engine has better secondary balance than a 270 degree inline twin

They imply that because of this better balance of the 285, Phil Irving, who can be called the father of the crossplane inline twin engine, originally proposed the 285 offset and not the 270

And finally number 3. They claim that manufacturing 285 degrees offset inline twins iis more expensive and that KTM is the only company that builds 285 inline twin engines because …..

Unfortunately all of these claims are wrong. First let us address the balance. In every reciprocating piston engine that is composed of a crankshaft, connecting rod and piston we have two types of balance to worry about. Primary balance and secondary balance.

Primary imbalances are created by the piston. Secondary imbalances are created by the rod.

If we plot the forces generated by a piston on a graph where force magnitude is the y axis and engine rotation degrees are the x axis we have peak upward force at top dead center and peak downward force at bottom dead center. This force shakes the engine up and down as it’s running and to restore primary balance all we need is another piston offset by 180 degrees from the frist piston. The forces of the piston can now cancel each other out and this is the logic we find behind a 180 degree inline twin.

Next tup we’re going to explain secondary forces and balances. Now the singular culprit behind secondary engine imbalance is the connecting rod and the fact that the relative length of the rod changes in relation to the distance between the piston and the crankshaf. Now the absolute length of the rod of course, hopefully stays the same but as we can see when the engine is at top and bottom dead center the rod is fully upright when the engine is at 90 and 270 degrees of rotation the rod is fully angled. Now a fully upright rod is longer in relation to the piston and crankshaft than a rod full angled.

This means that as the rod transitions from fully upright to fully angled it pulls the piston down by an additional little distance, this distance equals the difference in height between the rod fully upright and the rod fully angled. This additional distance, this action of the rod on the piston is what causes the piston to reach peak velocity before 90 degrees of rotation. Because moving from 0 to 90 degrees the rod is becoming shorter so it pulls the piston down.

First primary balance. In a 270 degree twin we have 270 or 90 degrees of offset between piston 1 and piston 2. So as piston 1 is at top dead center piston 2 is at 90 degrees of rotation. This means that piston 1 is exerting a maximum upward force whereas piston 2 is at peak velocity and is exerting zero force. So we have maximum upward force on one side of the engine.

Now in the case of the 285 engine when piston 1 is at tdc the other piston will be 75 degrees after tdc, so just 15 degrees before peak velocity. In other words piston is not at zero, it is still producing a small upward force. Which means that the engine as a whole produces an upward force which is overall greater in the case of the 285 because the other piston is also producing a small upward force. So the 285 has a worse primary balance compared to the 270.

Now secondary balance. First the 270 twin. Piston 1 is at tdc, rod is fully upright, so we have upward secondary force on piston 1. Piston 2 is at 90 degrees, rod fullz angled, so we have a downard secondary force. The two forces of equal magnitude and opposite direction cancel out.

Now in the 285 twin when piston is at tdc and exerting and upward secondary force piston 2 is at 75 degrees so it’s not exerting a maximum downard secondary force. In other words the secondary force of piston 2 is not sufficient to fully cancel out the force of piston 1 and we have secondary vibrations. The 285 degree twin does not have better secondary balance than the 270 degree twin, it has a worse secondary balance.

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#d4a #enginebalance

[da]

Just in case someone is still confused. In the beginning I say how peak velocity is 75 but then show a graph where it's at 90. To understand this you have to keep watching because at 08:45 it's made clear how 75 is with the rod, 90 without the rod. When we speak of primary balance alone we imagine that the piston magically moves up and down, we pretend it's a free piston because we are isolating it from the rod to observe its forces. When we add the rod we get the total. In the beginning I speak of the total and then about the primary, but I don't think I stressed this properly so maybe that's why a few people got confused?

[alcobra]

An intelligent and thoughtful man respectfully and succinctly peer-reviewing another intelligent man's work, all while acknowledging the FortNine channel's brilliance and success. This is A+ content.

[v]

As a fortnine viewer, this should be interesting.

EDIT: The respect you show to him as a viewer and also a creator in a similar space is NOT TO BE UNDERMINED. Its so different than just going at each others throats. You two are awesome!

[Daidc]

As a trainer/lecturer I must congratulate you on the clarity of your explanations and the use of visuals to illustrate the concepts

[mehmetgurdal]

wow.
a professional and respectful critism to my favourite motorcycle channel.
I no longer have a favourite motorcycle channel anymore, from now I have 2.
:) thank you very much for taking your time and correcting their mistake.
Please consider collaborating with them in their future engineering related content.
now if you excuse me I'll be binging your videos :D

[blar]

Life hack:

Hydrolock one of your KTM cilinders to fix it back to 270º

[r.j.bedore]

Yeah, I did think the 285° thing sounded a bit off from previous videos and research I had seen about engine balance and crossplane engines, but I didn't have the time to check F9's work, so I'm glad you did.

As far as the forging goes, yes it is cheaper and easier to make a 270° crank than a 285° crank. When forging a 270° crank, you can set up the 2 halves of the forging dies so that you forge in one journal then turn the crank 90° or 270° and forge in the second journal without affecting the first journal. Due to how the two forging dies come together, you can't really do that if the journals aren't either parallel or perpendicular to each other. Thus, for a 285° crank you would first forge in the journals for a 270° crank, then use a separate machine to twist the crank that additional 15°. Since this requires a completely separate machine along with an extra step, most companies just stop at forging in the 270° crank.

[HorizonOfHope]

Where is the Fortnine / D4A collab we all want?!😊

[liver.flush.maestro]

Each time you re-explain secondary balance, you do it better 🙂

[operator]

I'm a machinist who works with fixtures and dies, and I have a lot of manufacturing experience. I came to the same conclusion you did about the manufacturing process after watching Fortnine's video. While I may not have much experience in an engine manufacturing plant (very little and none relevant to the question at hand), It seems to me that creating a die and platen that can stamp out a 270 crank would mean that you could do the same for a 285 crank just as easily. From my experience, the time and cost difference to make the tooling would be negligible. Again, I have no experience with heavy forging like what you would need to create a crankshaft, but I'm having a hard time believing that it would take any extra steps to create a 285 crank vs a 270. Even the finishing machine work would be very similar.

[brunocarmona]

It is always said that the main reason the KTM LC8c parallel twin has a 285 degree offset is to mimic the feeling of the bigger KTM LC8 V-twin engine which has a 75 degree opening of the V. And thank you for this explanation, it was great!

[minklmank]

Nice! Two of my favorite youtubers in constructive and respectful debate! This is so insanely wholesome compared to the usual primitive beef amongst communities. My inner scientist is happy

[leek]

Phil Irving's original driver for proposing 76 degree crankpin stagger (referred to here as 285 degree), was to use the maximum inertia of one piston to help the other over TDC; in other words, to reduce the size and weight of flywheel required for low-rpm running. He was grappling with the need for more flywheel effect on the AJS Porcupine, and had vast experience of the assistance of one piston for another on V-twins.
As the presenter here notes, crankpins at 360 degrees, 270 degrees and 285 degrees are all have their advantages. All can be fully balanced with balance shafts, but 285 degrees maximises the free flywheel effect.
By the way, Irving's 76 degrees (effectively 284 degrees) was based on conrod to stroke length ratio of 2:1 - common at the time.

[ChooseBrian]

As someone who does not have any background in any types of engineering, I found this video and explanation to be extremely enlightening. If you can bring Normie’s along for the ride in such a way, I think you’re both winning.

[kOlbThreeTimesOne]

as a mechanical engineering student who has done a semester long project developing a MATLAB model simulating exactly what is discussed in this video (acceleration and force modelling, advanced kinematics and kinetics of mechanisms) i have to say your style of lecturing is WORLD CLASS, you never lost me once during this video and I imagine even someone who has never taken high school physics can follow along the fundamental concepts explained in this video, even though you make no sacrifices in justifying your claims. Well done!

[kannermw]

The 285 degree parallel twin most commonly mimics KTMs 75 degree V-twin. Turns out it is a KTM brand character thing and nothing else. Great presentation!

[qvlrsyo]

I am one of those 50% so I will correct you :D
TL:DR The reason KTM went with the 285 is experience. What once used to be a necessity for creating a compact V-Twin has helped KTM in creating a well performing Inline 2 without having to start from scratch

Back then KTM only made single-cylinder bikes. The idea for a KTM two cylinder idea came from a university student project; a V-Twin made from two KTM dirt bike singles. The students didn't want to use a 90° cylinder angle because the engine would be too long. So they decided to move the cylinders closer in a 75° arrangement with a multipurpose shaft between the two cylinders which balances the engine and drives the water and oil pump aswell as the the alternator. Shortly after Pierer started developing the LC8 (Liquid Cooled 8-Valve)
The basic arrangement stayed until today. KTM spent more than a decade on perfecting the 285° V-Twin so when they wanted to join the midpriced segment they designed the engine in a way that they can use their knowledge from LC8 development to make the LC8c. (c for compact)

[lemster]

Awesome to see a video response from you to the F9 video. To me the only thing that felt weird originally watching the F9 video was your last point on the production of the crankshaft. Instinctively it feels there shouldn't be a difference. Yamaha for example also makes the CP3 engine with 120° between cylinders so it's not like Yamaha can only do variations on 90° affordably.

[gchoquette]

Your physics are spot on and so are your conclusions. A couple of minor points. (1) the angle of the peak velocity can be different depending on the length of the connecting rod relative to the stroke. The specific equation can readily be developed with a little trigonometry, it can also be found in Taylor and other engine textbooks. (2) a primary method to reduce the first order vibration are the counterbalances on the crankshaft itself.

Now open the Pandora box on balance opposed (e.g. Boxer) engines. Perhaps an upcoming video of yours

[DltaRL]

I legit was like "wth is fortnite doing with engines?"