The following diagram shows typical frame dimensions for a road bike and the location of forces experienced by the frame when the bike is at rest.

The rider produces a force W₁ on the seat and W₂ on the pedals (the weight of the bike is small compared to the rider and is lumped with these forces). Reaction forces at the wheel hubs must balance these weights so that:
(R₁ + R₂ ) = (W₁ + W₂ )
This balance ensures that the bicycle does not sink through the road! Also the moments
(Force x Distance) about the front axle must be zero so that:

From: McMahon & Graham,
"The Bicycle & the Walkman," Merion (1992)

(W₂ x L₂) + (W₁ x L₁) = (R₂ x L₃)
where L_i is the horizontal distance of the force R_i or W_i from the point D. Similar moment balances must apply to any other point in the diagram in order to ensure that the rider does not take a header!