Crankshaft

1. A flat plane crankshaft for an in-line four cylinder engine, the crankshaft comprising:

five crank journals;

four crank pins; and

eight crank arms including:

a first crank arm being a crank arm of the eight crank arms at a first position counted from one of both shaft ends of the crankshaft;

a second crank arm being a crank arm of the eight crank arms at a second position counted from the shaft end of the crankshaft;

a third crank arm being a crank arm of the eight crank arms at a third position counted from the shaft end of the crankshaft;

a fourth crank arm being a crank arm of the eight crank arms at a fourth position counted from the shaft end of the crankshaft, the fourth crank arm including a fourth counter weight of the fourth crank arm;

a fifth crank arm being a crank arm of the eight crank arms at a fifth position counted from the shaft end of the crankshaft, the fifth crank arm including a fifth counter weight of the fifth crank arm;

a sixth crank arm being a crank arm of the eight crank arms at a sixth position counted from the shaft end of the crankshaft;

a seventh crank arm being a crank arm of the eight crank arms at a seventh position counted from the shaft end of the crankshaft; and

an eighth crank arm being a crank arm of the eight crank arms at an eighth position counted from the shaft end of the crankshaft,

each of the fourth crank arm and the fifth crank arm having a smaller width than a width of the second crank arm,

each of the third crank arm and the sixth crank arm having a greater width than the width of the second crank arm,

when viewed from the shaft end of the crankshaft, a width of the first crank arm, the second crank arm, the seventh crank arm, and the eighth crank arm are each defined as a maximum value of a dimension of each respective crank arm between a journal central axis, and one of a central axis of a first crank pin of the four crank pins and a central axis of a fourth crank pin of the four crank pins in a direction orthogonal to a line segment vertically extending through the journal central axis, and

when viewed from the shaft end of the crankshaft, a width of the third crank arm, the fourth crank arm, the fifth crank arm, and the sixth crank arm are each defined as a maximum value of a dimension of each respective crank arm between the journal central axis, and one of a central axis of a second crank pin of the four crank pins and a central axis of a third crank pin of the four crank pins.

2. The crankshaft according to claim 1 , wherein the width of the fourth crank arm and the width of the fifth crank arm are greater than a diameter of a crank journal of the five crank journals that is disposed between the fourth crank arm and the fifth crank arm.

3. The crankshaft according to claim 1 , wherein both a ratio of the width of the third crank arm relative to the width of the fourth crank arm and a ratio of the width of the sixth crank arm relative to the width of the fifth crank arm are configured to be an identical ratio within a range of 1.03 to 1.26.