Fortnine's video:    • Why the 270 Degree “Crossplane Twin” ...  
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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