Roof challenge

ROOF DESIGN & ANALYSIS

ROOF:

The conventional task of the roof panel is that of compartment shielding, passenger protection and connection of body sides, front and roof cross members. In case of roll over, compartment integrity is provided by the compartment frames, while the roof responsibility is only to avoid the ejection of passengers, providing protection against impact on passengers.

Correspondingly a wide selection of suitable materials is available: Steel, aluminium, thermosetting plastic sheet, glass.

Another task of roof bows is to support the interior roof liner which requires a number of fittings both in central and boundary areas to avoid deflections and vibrations. Fittings are usually plastic clips or Velcro strips. Some roof liners, mostly on less expensive vehicles are bonded directly to roof panel. 

Development of Styling surface:

Front and rear portion of the roof are determined as in the front portion the roof it needs to have larger curvature than at the rear side.

In the below image we can clearly see that the curvature is increasing from left to right. So the left portion is considered as rear and right side portion is considered as front.

The given surface needs to be developed for further use by creating flanges in the front, rear and sides following the given master sections using sweep operation as follows.

Clearance need to be provided at the corners for proper joining of A, B, C-pillars to the roof.

Ditch Area

The Ditch Area is the extended portion of the main Roof concealed by other BIW panels. The rail supports of the roof are attached to the Ditch Area using Spot welds, since it is not a styling surface and remains concealed. Headroom Clearance, visibility criteria, clearance for Wind shield and Back door are some of the factors to be considered while designing the Ditch Area.

Rear portion of roof with clearance at corners created following the master section as given.

Front portion of roof with clearance at corners created following the master section as given.

Reinforcements:

Reinforcements needs to be created for the roof as from the developed surface we can see the area is larger and after thickening the roof it can’t withstand the loads or the impacts during dynamic motion of car, to neutralize this rails such as front roof rail, rear roof rail, centre roof rail and bow roof rail are created for proper support.

Front Roof Rail:

It is the support which is joined with the wind shield glass, body side outer and the inner panels. A draft angle of 3-5 degrees is provided through the deep drawing operation. The wind shield end flange is the exact offset of the roof surface where spot welds are applied at wind shield end of the rail and the other end of front roof rail and roof is joined using mastic sealants which also contribute to increased strength.

The front roof rail is designed according to the master section given and the placement of front roof rail is placed with the roof as in the below image.

Rear Roof Rail:

It is the support which is joined to the back door and the body side outer and the inner panels. A draft angle of 3-5 degrees is provided through the deep drawing operation. We have to take into consideration the rear headroom clearance and the rear visibility criteria while designing the rear roof rail. Inputs from the backdoor design are also taken to accommodate the required clearance. Spot welding is applied at the back door end of the rail and the other end is joined with the roof using mastic sealants.

The rear roof rail is designed according to the master section given and the placement of rear roof rail is placed with the roof as in the below image.

Centre Roof Rail & Reinforcement:

In centre roof rail and reinforcement creation we have to consider the headroom clearance and the visibility through back door criteria while designing. Spot welding is applied for joining centre roof rail to the roof at flanges at the two ends of the rails and mastic sealants are introduced at both sides of the rails since centre roof rail and reinforcement are fully positioned under the styling surface of the roof. Reinforcement rail is attached to centre roof rail using mastic sealant.

The centre roof rail and its reinforcement rail is designed according to the master section given and the placement of centre roof rail is at the centre portion of roof.

Bow Roof Rails:

Similar to centre roof rails we have to consider the headroom clearance and the visibility through back door criteria while designing. Spot welding is applied for joining bow roof rails to the roof at flanges at the two ends of the rails and mastic sealants are introduced at both sides of the rails since bow roof rails are fully positioned under the styling surface of the roof.

The bow roof rails are designed according to the master section given and the placement of bow roof rail are placed in between the centre roof rail & Front, Rear roof rail.

 The final assembly of reinforcement rails with the roof outer panel are as follows:

DRAFT ANALYSIS:

For outer panel draft analysis was carried out and the outer panel was clearing a 7 degree draft except at front and rear panel flange portion.

For front roof rail draft analysis was carried out and the front roof rail was clearing a 7 degree draft.

For rear roof rail draft analysis was carried out and the rear roof rail was clearing a 7 degree draft.

For centre roof rail & reinforcement draft analysis was carried out and the centre roof rail and reinforcement were clearing a 6.9 degree draft as for 7 degree a negative draft was observed on the centre roof rail.

For bow roof rails draft analysis was carried out and the bow roof rail was clearing a 6.9 degree draft as for 7 degree a negative draft was observed on the bow roof rail.

CURVATURE STUDY ON THE ROOF BASED ON THE BOW ROOF PREDICTION CALCULATION:

Curvature Study

It is done to justify the positioning of the Bow Roofs. There are mainly three criterions to fulfil the design of roof.

1. Roof Crush test
2. Heat Distortion Criteria
3. Snow Load Criteria

Roof crush testing is done only after the part or prototype is manufactured in physical conditions and heat distortion and snow load can be found doing some calculations based on the formulae and design of the roof and its reinforcements.

Bow–Roof Prediction Formula

Heat distortion can be found using the below Bow-roof formulae and if the judgement condition satisfies than the roof withstands the distortion created due to heat.

W = (1.73*(10^-3)*L) + (1.85*(10^-8)*(R^2/t)) + (1.1*(10^-3)*I)-2.68         

Where,

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

R = 2(Rx*Ry) / (Rx+Ry)   [Roof curvature]

Rx = X curvature

Ry = Y curvature

t = Roof plate thickness [mm]

I = Bow Roof Span [mm]

Judgment Condition

If the value of W is less than 2.7 (<2.7), than it satisfies the condition.

CALCULATION:

For this calculation 4 conditions were taken and in these 4 conditions we need to calculate the curvature of roof in x and y direction (Rx, Ry), for that a sketch on the roof is created as:

The curves are created by projecting points on the roof by creating a sketch as below and the intersection of lines were taken as points

The roof length (L) along in the direction of X taken as from the measure as below.

The bow roof span (I) is the distance between the front roof rail and bow roof for condition-1, the distance between the bow roof and centre roof rail for condition-2, the distance between the centre roof rail and bow roof rail for condition-3, the distance between bow roof rail and rear roof rail for condition-4.

The thickness (t) is the thickness of the outer roof panel after thickening.

As the value for Bow-roof prediction formulae for 4 conditions were satisfied as the values are less than 2.7, hence the curvature of roof withstands the distortion created due to heat.

Snow Load Prediction Formula

Snow load prediction can be found using the below formulae and if the judgement condition satisfies than the roof withstands the snow.

Qr = (Iy*t^2) / (α *S*(((Rx+Ry)/2)^2)*10^-8)

Where,

α = My * Lx^² * 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 0-Y[mm]

        Length of Roof panel with the centre point between Roof rails 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 Roof Rail to bow roof [mm]

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

        S= L1/2 + L2/2

Iy = Geometrical moment of inertia of bow roof (Y cross-section) [mm4]

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

        Roof panel X curvature radius of length s in Side view

Judgment condition

 If the value is greater than or equal to 3.7, then the roof satisfies the snow load criterion.

CALCULATION:

For this calculation 4 conditions were taken and in these 4 conditions we need to calculate the curvature of roof in x and y direction (Rx, Ry), for that a sketch on the roof is created as:

The curves are created by projecting points on the roof by creating a sketch as below and the intersection of lines were taken as points

Moment of Inertia (Iy) is calculated for all the reinforcements created for different conditions different moment of inertias were taken

Moment of inertia for centre roof rail, from the measure inertia we need to take the lowest value and the value is in metres we need to convert in to mm.

Moment of inertia for bow roof rails, from the measure inertia we need to take the lowest value and the value is in metres we need to convert in to mm.

Distance along the 0-Y between front roof rail centre point to bow roof rail centre point for condition-1, distance between bow roof rail centre point to centre roof rail centre point for condition-2, distance between centre roof rail centre point to bow roof rail centre point for condition-3.

As the value for snow load prediction formulae for 3 conditions were satisfied as the greater than or equal to 3.7, the roof can withstand in snow conditions.

ROOF ASSEMBLY: