Roof challenge

ROOF DESIGN AND ANALYSIS USING CATIA V5:

Objective:

Design a roof using the provided skin data and also position bow roof rails in accordance with heat distortiorn and snow weight formula also find section modulus of the roof and roof rails and check the draft of the designed parts.

Software:

CATIA V5

Introduction:

Roof:

An Automobile roof is the portion of thautomobile that sits above the passenger compartment and protects the occupants from sun, wind, rain and other external elements. While designing a car roof forces due to impact, falling objects, vehicle rollover, temperature, roof loading, aesthetics etc. must also be taken into account. There are different types of roofs, the most prominent are as follows

• Hardtops
• Soft tops
• Targa tops
• Glass roofs
• T-tops

Some roofs have a window on them and are called sunroofs, while convertibles have special mechanisms that allow the roof to be retracted and be stowed away through a wide array of methods depending on the design. The roof of the car is made up of multiple parts that provide support and strength to the roof component. The actual roof sheet and the roof rails which support the roof sheet. There are 3 primary roof rails in a car the front, the centre and the rear, while additional roof rails are called Bow roof rails. The number of bow roof rails depends on the size of the roof. Headroom, visibility criteria and target region also play a key role in roof design.

Roof Panel

The roof is made from a sheet of at least 0.75mm. This protects the cabin from external conditions and objects. The ditch areas are spot welded to the pillars of the car and certain areas of the roof rails, welding on the upper surface is avoided in order to maintain a good desthetic.

Front Roof Rail:

The front roof rail is made from a sheet of at least 1.25mm thickness. This part acts as a support for the roof sheet on the front it also provides a mounting area for the windshield. It is designed with support requirements, visibility criteria and headroom space in mind. The roof is fixed to the roof rails with the use of mastics while the ends are spot welded.

Rear Roof Rail:

The rear roof rail is made of a sheet of at least 1.25mm thicktness. The rear roof rail acts as a support for the rear end of the roof and also supports the rear window in some cases also supports the rear door. It's designed in accordance with the support requirements of roof, window and rear door, and also rear visibility and headroom criteria. The roof is fixed to the roof rails with the use of mastics while the ends are spot welded.

Centre Roof Rail:

This is the roof rail that runs between the pillar of the car and provides support to the centre of the roof sheet they are usually 1-2mm thick and also provide a support area for the centre cabin light mount. The roof is fixed to the roof rails with the use of mastics while the ends are spot welded.

Bow Roofs Rails :

The bow roof rails provide additional support to the roof and are at least Imm thick, they are positioned in such a way that they provide maximum support to the roof, Care must be taken so that they do not intrude into the headroom requirements of the car. Bow roof rails are an optional feature provided as required by the design. Small cars may have none while large ones may have 3 or more, hey may also be integrated into the centre roof rail in some cases. The roof is fixed to the roof rails with the use of mastics while the ends are spot welded.

Roof Rail Position Analysis:

The roof rail positions are confirmed and necessary modifications are made in accordance with the snow load and heat

distortion criteria.

Heat Distortion Criteria:

The roof of the vehicle is the most exposed to changes in temperature, these changes cause deformation in the roof. In order to prevent failure of the roof due to excessive thermal stress, the roof rails must be placed in the correct locations. Improper designing of the roof may cause warping, cracking and failureof mounts.

This calculation is done to find if the roof under thermal stress deforms within set limits.

It is calculated using the formula

Heat Distortion:

W=1.73 x 10-3 x L+ 1.85 x 10-5 x+1.10 x 10-3 x l- 2.68

W<2.7m

Here we use the andalyse the spans between the roof rails.

HEAT DISTORTION:

CASES	L	Rx	Ry	R	t	l	L	STATUS
FRR-BR1	2125.322	1777.003	10141.688	3024.12572	0.75	375.1	1.635002024	OK
BR1-CRR	2125.322	3210.223	4971.405	3901.257469	0.75	434.5	1.850179036	OK
CRR-BR2	2125.322	4084.071	3981.185	4031.971757	0.75	434.1	1.875318034	OK
BR2-RRR	2125.322	4604.691	3247.259	3808.639712	0.75	328.2	1.715635226	OK

from the value 'w', we can see taht the all value are with in the disortion limits, therefore , the roof design is cleared to the next stage

Snow Load Condition:

The roof of a vehicle is normally not designed to hold up weight but for regions with snowial, it must be properly reinforced to hold up the weight of accumulated snow on the roof, if done improperly it may cause the roof to bend and intrude into the passenger compartment which is dangerous. This calculation is done in order to check if the location and strength of the roof rails are enough to avoid plastic deformation of the roof. The formula for the snow load analysis is as follows.

formula:

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)

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

Judgment condition

Qr ≥ 3.1

250 ≤ s ≤ 380

CASES	ly	t	α	Lx	Ly	My	Rx	Ry	L1	L2	S	Y	Qr	status
FRR-BR1	3563.588	0.75	0.972038261	1119.54	2125.322	775539.8702	2474.863	2717.488	467.927	511.893	489.91	467.927	62.45141115	clear
BR1-CRR	4392.134	0.75	0.965246616	1081.072	2125.322	825903.0111	6105.495	2406.178	511.893	511.496	511.6945	511.893	27.61717181	clear
CRR-BR2	3563.588	0.75	0.928573141	1060.617	2125.322	825465.5437	4162.321	4272.763	511.496	467.123	489.3095	511.496	24.8022692	clear
BR2-RRR		0.75	0.871334817	1060.617	2125.322	774582.8915	4783.316	3950.566	467.123	_	_	467.123

From the value of "Qr, we are able to see that all values are above 3.1 except bow roof .

Draft Analysis:

The draft of the part is the amount of taper of the edges to the striking mould of the part. The draft callows for easy removal of the part from the striker while an improper draft or negative draft causes the part to deform or get stuck on the mould.

Here using software we check if all the parts have a proper draft and are formable, green denotes positive draft and red denotes a negative draft.

Section moment of inertia is used too predict the amount of deformation and deflection of a part. A higher moment of inertia is preferable as it reduces the amount of deflection for a given force. The thickness of the part and addition of flanges and embosses increase the section inertia of the part. Here the master sections of the roof and roof rails and their section analysis are given.

ROOF OUTER PANEL:

FRONT ROOF RAIL:

REAR ROOF RAIL:

BOW ROOF 1:

BOW ROOF 2:

CENTER ROOF RAIL:

Conclusion:

Thus the roof of the car is designed as per the given skin data and necessary reinforcements are given. The roof is also analysed as per the heat distortion and snow load conditions and is found that the snow load condition of centre roof and bow roof is not within limits, So we added embossings to the roof panel for more strength. Draft analysis is also carried on all the components and there are no issues in this regard. Therefore the roof design may be sent to the next step of the design process.