DESIGN OF CAR ROOF

OBJECTIVE:

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

GIVEN STYLING SURFACE:

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. Because the earliest automobiles were designed in an era of horse-drawn carriages early automobile roofs used similar materials and designs.

• Convertible roofs. Roof modules. Roof modules.
• Hardtops.
• Sunroofs.
• T-tops.
• Targa tops.
• Vinyl roofs.

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 support the roof sheet. There are 3 primary roof rails in a car the front, the center, 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.

DESIGN OF ROOF:

ROOF PARTS:

ROOF OUTER 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 aesthetic.

FRONT ROOF RAIL:

The front roof rail is used to provide strength to the roof panel front and it also acts as a link between windshield glass, roof panel, and headliner(interior trim).

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 used to provide strength to the roof panel front and it also acts as a link between windshield glass, roof panel, and headliner(interior trim).

BOW ROOF 1:

The bow roof rails provide additional support to the roof and are at least 1mm 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, they may also be integrated into the center roof rail in some cases. The roof is fixed to the roof rails with the use of mastics while the ends are spot welded.

REINFORCEMENT ROOF CENTER:

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

ASSEMBLY OF ROOF PARTS:

HEAT DISTORSION 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 failure of mounts.

• This calculation is done to find if the roof under thermal stress deforms within set limits.
• It is calculated using the formula

S.NO	BOWs	Rx	Ry	R	l	L	t	W	OK/NG
1	FR&1ST	5684.428	3073.459	3989.8181	555.397	2033.705	0.75	2.29	OK
2	1ST&CTR	4572.538	7821.03	5771.05106	443.041	2033.705	0.75	2.169	OK
3	CTR&2ND	3694.587	12081.569	5658.717	373.468	2033.705	0.75	2.0606	OK
4	2ND&RR	3120.107	15663.29	5496.269	373.468	2033.705	0.75	1.894	OK

According to the W Values, Head Distorsion criteria is OK.

SNOW LOAD CRITERIA:

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

The roof of a vehicle is normally not designed to hold up weight but for regions with snowfall, 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.

S.NO	BOWs	Ly	α	Lx	Ly	Y	s	Rx	Ry	t	Qr	DETERMINATION(Qr)
1	FR&1ST	3.49*10^8	0.984	1131.736	2033.705	649.66	589.0685	4242.951	3080.648	1.25	3.135	OK
2	1ST&CTR	4.07*10^8	0.9266	1079.32	2033.705	528.477	493.86	3921756	7855.4	1.25	6.168	OK
3	CTR&2ND	1.24*10^8	0.809	1057.794	2033.705	459.243	412.774	4339.559	12990.584	1.25	5.347	OK
4	2ND&RR		0.6	1048	2033.705	366.3085		3703.441	17949.779	1.25

From the above Qr values, The Snow Load Prediction Criteria is OK.

MOMENT OF INERTIA :

Section moment of inertia is used to 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 the 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.

BOW ROOF1:

REINFORCEMENT ROOF CENTER:

BOW ROOF 2:

DRAFT ANALYSIS:

The draft analysis for all parts is passing the 7 degrees minimum draft angle.