Roof challenge

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

Design consideration

Car roof is a 0.75mm thickness sheet which has some reinforcement roofs to increase the strength

the reinforcements are

1. Front roof

2.Bow roof

3.Centre roof

4.Rear roof

the number of bow roofs is based on the length of the roof panel, the more length has more bow roofs some big cars have 3 to 4 bow roofs as reinforcements to increase the roof strength

Manufacturing Requirements

We have to give a proper Draft angle for drawing process the minimum 7deg draft angle should be maintained for all the embosses

Placing of Reinforcement roofs

Placing of reinforcements based on the roof curvature, better curvature means it has better strength so when we placing the reinforcement roofs we placed it most flat area on the roof so we increased the strength of the roof by adding the reinforcement roof

Front Roof Rail

Front roof rail is additional reinforcements and also has some mounting features for room lamp

the front roof design master section is based on the Driver visibility criteria

Reinforcement Position

once the roof design is completed before sending it to the CAE team we have to justify the reinforcement position

initially, we placing the reinforcements as tentative then we have to justify the position by using some formulas

We have to consider the following criteria

1.Heat distortion criteria

2.Snowed Criteria

Heat Distortion Criteria

Heat distortion criteria is the which temperature metal softens and deforms under specified loads

From Heat distortion criteria roof strength can be increased by the positioning of reinforcement roofs

Roof Prediction formula

By using this formula we can validate our bow roof position

Snowed Condition

There are deadweight added in the roof which is the snow load in severe winter condition on European countries

in European countries, it has severe snow in the winter season

we have to validate how is our roof is going to behave on that condition

by using these formulas we can predict that

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

Note: The Qr value given over here is correct. There was a typo error in the formula given in the video.

Where

α = My x Lx² x 10^-12 , My = Y(Ly-Y)

Judgement condition = Qr ≥ 3.1

250 ≤ s ≤ 380

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 reference point of the front and the rear.

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

Width of the roof panel 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

Judgement condition = Qr ≥ 3.1

From the above calculation, we can conclude our bow roof positions are passed the snowed condition and heat distortion criteria

Moment of inertia of roofs at Zero Y section

Front Roof

Bow roof 1

Center Roof

Bow roof 2

Rear roof

Reinforcement Roof Joining process

The reinforcements roofs are joining to the roof by using following methods

1.Mastic seal

2.Spot Welding

Mastic seal

Mastic sealant is used to join the reinforcement roof to the roof outer panel

Front roof, center roof, and rear roof are the roofs joined to the outer roof panel by using the mastic sealant from the above image we can clearly see that there is some emboss given for mastic sealant

2.Spot Welding

Spot welding is a type of electric resistance welding used to weld various sheet metal products, through a process in which contacting metal surface points are joined by the heat obtained from resistance to Electric current.

The bow roof 1 and Bow roof 2 are joined to the other roof panel by using spot welding

The cross-section of Reinforcements

Front roof

Bowroof 1

Centre Roof

Bow roof 2

Rear roof

Roof Outer Panel

Roof Crush Test

Roof crush testis the failure and displacement of an Automobile roof into the passenger compartment during a rollover accident

By applying force on roof panel we can test our roof strength

The force applied on the roof process timelapse 120mseconds

By increasing the reinforcement thickness and mastic points we can increase the strength of the roof

Roof crush test is performed as per the below picture