Consider the triangular formed by the connecting rod and

Consider the triangular formed by the connecting rod and the crank of an IC engine as the two sides of the triangle. If the maximum area of this triangle occurs when the crank is 75°, the ratio of connecting rod length to crank radius to

Δ =	1	(PQ) (PR) sin A
	2

Area will be maximum when A = 90°
i.e. PQR is a right angled triangle.
∴ Ratio of connecting rod length to crank radius,

l	= tan 75° = 3.732;
r

l = 3.732 r