Step by step secondary balance explanation: • REVOLUTIONARY new INLI... (start at 01:08)
Audiovisual demonstrations of different engines: / @williammoser
00:00 Inline four is everywhere
01:52 What's a crossplane crankshaft
03:47 Crossplane and flat plane engine balance
12:37 Crossplane firing interval
14:50 Flat plane vs crossplane sound
18:06 Big bang engines in MotoGP and their benefits
24:22 Why no other bike went crossplane
The inline four cylinder engine is likely the most plentiful engine configuration in the world. It outnumbers other engine configurations because it offers an optimal blend of physical size, efficiency and power potential for most applications. And you can find an inline four engine in thousands upon thousands of different cars, bikes, boats, trucks, you name it - it’s everywhere. Walk through a city street almost anywhere in the world and chances are very slim that there isn't an inline four somewhere very near you.
And if you would take any of those countless inline four engines and opened it up inside you would find a crankshaft that looks like this. In fact at the heart of every single inline four you can find on the roads of the world lies pretty much the same crankshaft. There may be slight variations, different materials and manufacturing processes, a different number of counterweights. But all of them have essentially the same crankshaft.
Well all of them except one…….. This is the crankshaft in the Yamaha R1 and as you can see it’s different from all the others. So why is the Yamaha R1 inline four engine different from all the other inline fours out there and to what end? Well today I’m going to try and answer that question as thoroughly as possible with this detailed video. We will explain what the crossplane crankshaft is, how it differs from the flat plane, how this influences engine balance and what advantages and drawbacks it brings to the table and then with this information in mind we will talk about the history of big bang engines in MotoGP to see why they came into existence and what benefits they offer in the real world. We will end the video by answering the question of why other brands never adopted the crossplane crankshaft in their production bikes and why it never made it into another Yamaha fan favorite, the R6. So let’s get started.
Now, as you can see all the crankshafts inside every production inline four out there have 1 key feature in common and that is that two of the crank pins point up and the other two crank pins point down. The result is of course that pistons move in pairs. When two go up two must come down.
This type of crankshaft is called a flat plane crankshaft because all the crank pins are located in a single plane.
But starting with the 2009 model year Yamaha’s flagship 1 liter sportbike the R1 made a radical step away from this convention. As you can see its crankshaft doesn’t have two crank pins pointed up and the other two 180 degrees away. Instead each crank pin is pointing in its own direction. If we set our crankshaft so that the first crank pin points up then the second crank pin will be rotated 90 degrees from it. The third crank pin will be 180 degrees from the second and the fourth 90 degrees from the third. In more simple terms one points up, one left, one right, one down.
So why are 99.99% of inline four crankshafts out there flat plane instead of crossplane. The answer to that is that if one takes a rational objective approach towards designing an inline four engine the flat plane design offers more benefits and less drawbacks then a cross plane one. To understand why 99.99% of inline fours are flat plane we must understand how the primary and secondary engine balance of flat plane and crossplane inline fours differ from each other.
In flat plane inline four the pistons move in pairs. When two go up the other two go down. In other words piston one balances out piston two and piston three balances out piston 4.
A crossplane inline four also has an even number of pistons so we should have no problems with primary balance right? The inner two pistons are separated by 180 degrees so they balance each other out. The outer two pistons are also separated by 180 degrees so they also balance each other out. It’s the same thing as a flat plane but only with different pistons. So this means that the crossplane should have perfect primary balance just like the flat plane right? Wrong.
Yes the number of pistons is even but the problem is their distance from and relationship with the engine’s center of mass which results in the crossplane engine having something called a rocking couple.
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