Challenge

Roof Design

EEraiamuthan R30 Sept 2022

ROOF DESIGN

Roof is a part that covers the upper section of the car. This give protection from sunlight rain and also responsible for the body rollover in the time of accidents.So the roof should be strudy enough to withstand the impact on the accident.

The roof panal is supported by inner supporting rails called roof rails. These support rails provides a structure strength for the roof panal and also server as a place for some components to be mounted. Both these support rails and the roof panal are joined together with help of mastic sealents and also using spot welds.

The Roof rails are categorised into four groups:

Front roof rail
Rear roof rail
Center roof rail
Bow roof rail

All this rail helps to provide safety of the passengers during accidents and have itss own fuctions.

Front roof rail:

The front roof rail provides supports for both the roof panal and also for the front wind shield to fix. Also the cabin lights, sunshades, rear view mirrors are mounted in this area. The designing of the front roof rail is quit complicated involves concideration of many parameters in various aspects like visiblity, front head room, wind shiled sealent etc,. So the dimension of the master section of the front roof rail changes with various cars.

Center roof rail:

The center provides supports to the roof in the centeral rgion of the roof. This center roof have different master section as its only job is to provide strength. In some cases cabin lights are also mounted on the center rail with help of a clamp attacted with the rail by mean of screws and slots.

Rear roof rail :

This rail strengthen the rear portion of the roof and provides space for rear door to open. This rear roof rail designed in a manner that does not affect the rear visibility and the rear head room.

Bow Roof rail:

Bow roof rails adds supoort to the areas other than fornt and rear of the roof. The can be many Bow rail depending on the length of the roof. The bow rails are combained with the centeral rail in the case, if sun roof is added in the vehicle. The bow rails are often placed at the area with flat surface. The curved surface have good strength when compared to the flat surface. so the bow rails are usually palced on the flat surface which needs more strength.

All this rails are fixed to the outer panals with helps of mastic sealents in the visible ares and are spot welded in the sides which is not visible outside.

Besides these rail the outer panal are also provides with space to place the wind shield at the fornt and space at rear to open the rear door.

CURVATURE ANALYSIS:

After complition of the roof design and its rails, an sutdy is done to analys its strength. Generally there are Three important test shold be done on the roof.

Heat distortion test
Snowload test
Roof crush test

On this three test only two can be done before manufacturing and the roof crush test can be done only after manufacturing.

Heat distortion test:

This study helps us to understand whether the roof design withstand deformation due to temperature with respect to the postion of the rails. This study can be done on the folloing manner:

First the cauvature of the of roof is taken at 3Y plane( 300mm offset of xy plane ) at the region of the support rails which gives a value Ry

In a same way the cuvature of the roof is measured in the region of rail in yz plane.
Also several parameters like thiickness of the sheet, length of the roof, Bow span are taken.
The values obtained are applied on special formula

If the W value obtained from this is less then 2.7, then it is given as PASS. If not it is given as NG.

Snow load test:

This study Helps us to understand whether the roof can withstand the load of the snow accumulation on it with respect to the position of the support rails.

This is also similar to that of the Heat distortion test, where the cuvature of the roof is taken in OY plane (xy plane).
Also several factorss like section moment of inertia, distance at which the bow shares the load are taken into accout.
And with all this value the applied on a formula, Qr vaule is obtained.

If the Qr value is grater or than equal to 3.1, then it is PASS. If not it is given as NG.

The designs that got NG are futher moved into to design process for strengthening by apdating sever embosses on the roof panal. this would increase the strength of the roof, improve asthetics as well as aerodynamics of the vehicle.

Roof crush test:

This test is done after the manufacturing of the roof. Here an particular amount of load is appliedd on the roof at an some angle for limited amount of time denpending on the vehicle. In this test the depression of the roof is checked and rated for the safety.