Roof Design

ROOF

An automobile roof or car top is the portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements.Roof is not just single panel. It consists of Subtypes.

• Front Roof
• Bow Roof
• Center reinforcement
• Rear Roof

Front roof: Front roof is the first and the main part of body. It joins windshield glass and front roof rail body side outer. It adds clearance to the main head room. Proper standards are maintained for this.

Bow Roof: Bow roof number depends upon the size of the roof structure. Small cars have less and large cars have more positions for bow roof. Bow roofs are placed mostly in flat surface.

Center Reinforcement: Center reinforcement is added to improve the structural strength for the roof curvature

Rear Roof: Rear roof is placed on the bottom area where it meets the rear windshield glass.

JOINING OF ROOF AND SUB ELEMENTS

Rubber: Rubber structure is added between the front roof rail and the windshield glass structure because it will damage the front roof part because windshield has high thickness and strength.

Mastic: Mastic is added between inner and outer structures for mounting and improvement in strength.

Spot Weld: Spot welding is applied for roof mountings for complete assembly of panels at the ditch area.

Emboss: Adding embossments increases the strength of the part.

DRAFT ANALYSIS

• Draft analysis of the individual components of the were done.
• The surfaces having draft angle greater than or equal to 7 degrees were highlighted in green.
• Only the outer roof part were having surfaces which didn't satisfy the condition.
• For the front and rear rail, a tooling axis was created seperately for the analysis as the existing vectors couldn't satisfy the requirement.

Front Roof Rail

Rear Roof Rail

Centre Roof Rail

Bow Roof 1

Bow Roof 2

Outer Roof

MOMENT OF INERTIA OF INDIVIDUAL PARTS

Rear Roof Rail

Front Roof Rail

Centre Roof

Bow Roof 1

Bow Roof 2

Outer Roof

BOW ROOF PREDICTION CALCULATION

The heat distortion study plays a major role in sheet metal urges. Heat distortion temperature is a temperature limit above which the material cannot be used for structural applications. This study is used to predict the heat distortions temperature at where the material starts to soften when exposed to a fixed load at an elevated temperature. To avoid bending or damage to the roof, based on the heat distortion temperature, this study will predict the bow roof position which helps to strengthen the roof.

Formula :

W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/t] + [ 1.10x10^(-3) x l] - 2.68

Where,

L = Roof Length in X-Direction[mm](Roof dimension in 0-Y)

R = Roof curvature

R = 2(Rx x Ry)/(Rx+Ry)

Rx = X curvature

Ry = Y curvature

t = Roof plate thickness [mm]

l = Bow Roof Span [mm]

Data & Calculation :

Inference :

• The required value for 'W' was obtained to be less than 2.7.
• This ensures that the locations at which the bow roofs were placed are feasible.

SNOW LOAD PREDICTION CALCULATION

Snow load is the downward force on a car's roof by the weight of accumulated snow and ice. The roof can fail if the snow load exceeds the weight the roof was designed to shoulder or if the roof was poorly designed or constructed.

Formula :

Qr = [Iy x t²] / [α x s x [(Rx + Ry)/2]² x 10^-8]

Where,

α = My x Lx² x 10^-12 , My = Y(Ly-Y)

t = Roof plate thickness [mm]

Ly = Distance between the front and rear roof Rails on the Vehicle along with 0Y[mm]

Length of Roof panel with the center point between Roof rail Front /Rear as the front and rear reference point.

Lx = Distance between the Left and Right end of the roof on the Roof BOW [mm]

The width of the roof panel is exposed on the surface.

Y = Distance front Front Roof Rail to Roof BOW[mm]

s = Distance for which Roof BOW bears divided load [mm]

s = L1/2 + L2/2

Iy = Geometrical moment of inertia of Roof BOW (Y cross-section )[mm4]

Rx = Lateral direction curvature radius of roof panel Y cross-section on Roof BOW [mm]

Roof panel curvature Radius of the Length Lx in Front view

Ry = Longitudinal Direction curvature radius of the Roof panel X cross-section on Roof BOW [mm]

Roof panel X curvature radius of length s in Side view

Data & Calculation :

Inference :

• Only 3 regions were considered due to the unavailability of L2 for the last region.
• The resultant value for both Qr & S were not satisfactory.
• This particular roof won't be able to handle snow load.

Possible Solutions :

• As the value of 's' exceeded the range (250 ≤ s ≤ 380) for all the 3 regions, it is advisable to add an additional bow roof to further distribute the load.
• The 'Qr' value can be increased by increasing the strength of the roof.
• This can be done by increasing the moment of inertia value.
• Increasing the cross section to improve the strength and MOI is advisable.