Thread: Cross-plane cranks

The vid does a good job of explaining the whats and whys(once you get past the bike bits),,

why arent all cranks made this way?

[ame=http://www.youtube.com/watch?v=oEXUrO5wYcE]Yamaha YZF R1 crossplane crankshaft technology explained - YouTube[/ame]

V8's with the 90 degree crank, which is best for balance have a characteristic rumble, especially when fitted with twin exhaust systems and no balance pipe. There are several possible firing orders for a 4 stroke but they are all some form of right left right right left right left left. Effectively like two uneven firing motors running simultaneously.




Using a crank like those of inline 4 cylinder motors overcomes the uneven firing problem but creates another one. The motor vibrates at twice engine speed as inline 4's without balance shafts do. Note with this arrangement, crank is symmetrical around centre main bearing so motor does not rock from end to end. Pistons one and four go up and down together and pistons two and three go up and down together.
Regarding primary balance, they cancel each other out. However, regarding secondary balance, when two pistons are at top dead centre and two at bottom dead centre, the centre of gravity is exactly half way up the cylinders. When the cranks are 90 degrees from this position all four pistons are at the same level with two on the way up and two on the way down. They are all lower than half way down the cylinder though. Because the conrod swings in an arc, when a piston is exactly half way up the cylinder, looking lengthwise along the crankshaft there is a right angle between the main. big end and gudgeon.


So the overall result is the motor bounces up and down at twice engine speed. So does a 18o degree in line uneven firing twin, which rocks from end to end as well. A horizontally opposed twin simply rocks slightly due to cranks causing one cylinder to be forward of the other. However, because the pistons move the same in opposite directions there is no secondary vibration. Note how horizontally opposed flat 4's like VW and Subaru motors have a note of their own because they fire like 180 degree inline twins each side. ie right right left left etc. These twins and fours have a disadvantage that motors with V arrangement or multiple of three or five do not have. ie All pistons are stopped, accelerating or decelerating at the same time, creating a torsional vibration problem. Re this, I note in the Isuzu Defender forum it is recommended not to have engine idle speed too slow. Otherwise chattering caused by the torsional vibration is very destructive to gearboxes when not under load.
Many heavier 4 cylinder motors, especially diesels on tractors, have balance shafts to cancel the high speed secondary vibration. Two shafts in the sump which have off centre weights on them. They are timed and run in opposite directions at twice engine speed. With shafts side by side, the effect of the weights cancels out sideways. However, up and down they add together. To cancel secondary vibrations of an in line four, the weights need to be down when pistons are at top and bottom dead centres and up when 90 degrees from this and piston centre of gravity is down.
Anyway, hope those who were previously not aware of above balancing issues can readily follow the above explanations.