BUILDING & MOUNTING THE STRUCTURES 3-13 E UROCARGO Euro 4/5 In the case of a rigid connection (shear resistant plates) between the chassis frame and subframe, the bending moment M f must be applied to the single section of the chassis frame - subframe.
= σ ≤ = σ max t t amm f t M I W Wt y = σ ≤ = σ f c M I W W max c c amm c y σ c = maximum static stress applied to the subframe (N/mm 2 ) σ t = maximum static stress applied to the chassis frame (N/mm 2 ) M f = static bending moment produced by the body (Nmm) I = moment of inertia of the single section of the chassis frame - subframe (mm 4 ) σ amm = maximum static stress permitted on the chassis frame (N/mm 2 ) y t max = distance from the neutral axis of stress of the outermost fibres of the chassis frame (mm) y c max = distance from the neutral axis of stress of the outermost fibres of the subframe (mm) Computational example of the stresses in case of rigid connection with the chassis frame We consider two C-shaped sections with the following dimensions 1.
chassis frame 250 x 70 x 5mm 2. subframe: 140 x 70 x 7mm and stressed at a given section by the maximum bending moment M f equal to 15000 and applied perpendicularly to the plane containing the rib of the structural member. From the calculation we find that the centre of gravity is set at approximately 28mm from the segment of contact on the side of the 250 x 70 x 5 mm section (chassis frame). Then we will have 1. y t max = 250 - 28 = 222 mm 2. y c max = 140 - (-28) = 168 mm and furthermore I x (cm 4 ) W t (cm 3 ) W c (cm 3 ) 1. chassis frame+subframe 5643 254 334 Applying the formulae we obtain: 1. σ t = M f / W 1 = 59 N/mm 2 2.
σ c = M f / W 2 = 45 N/mm 2 In order to guide and better contain the loads transversally, a slight protrusion of the brackets above the chassis is recommended. When the brackets are fitted flush with the upper flange of the side member, the lateral movement of the body structure must be secured by other means (e.g. using guide plates fixed only to the chassis - see Figure 3.12). When the front connection is of the coli springstype (Figure 3.11), longitudinal securing must be ensured even in conditions of maximum twisting of the chassis (e.g. off- road).
When the chassis already has factory fitted brackets for the installation of a box-type body, these brackets must be used for the installation of the structure. The brackets fitted to the subframe or to the body must have characteristics of strength no lower than those of the original brackets fitted to the vehicle. Connections between frame and counterframe Print 603.93.431 Base - January 2008