Roof Design
Aim:-
To design a roof with all the components like front roof rail, rear roof rail, center roof rail, and bow roof rails by using the given styling surface and the given master sections. Also, perform Curvature Study.
Roof:-
An automotive roof or car top is the portion of the automobile that sits above the passenger compartment, protecting the vehicle occupants from Sun, Wind, Rain, and other external elements.
There are many other structures under the roof that contributes to its strength and stability. The number of such reinforcement structures/supports differs with respect to the size of the vehicle. They are primarily installed on the flatter areas of the roof to enhance the strength.
Fig.1: Car Roof
Ditch Area:-
The ditch area is the extended portion of the main roof which is concealed by the other BIW components followed by the other aesthetic surface. It also serves the joining purpose for the reinforcement which is joined using spot welding. Visibility and clearance for windshield and back door mountings should be considered while designing the ditch area.
Fig.2: Ditch Area
Reinforcements used in the roof:-
- Front Roof Rail:
Front Roof Rail is a reinforcement that features to back of the bracket lamp and the room lamp and it also serves the purpose of attaching the windshield. The development of the front roof rail is based on the given master section and also references from the given styling and the front ditch area are taken into consideration. Embosses and darts are incorporated for strengthening the part.
Fig.3: Front Roof Rail - Rear Roof Rail:
The rear Roof Rail is a reinforcement provided to the rear portion of the roof and it also features tailgate mountings. The development of the rear roof rail is dependent upon the rear headroom clearance and rear visibility criteria. The minimum clearance from the headliner to the roof is should be 80mm which will vary from variant and region to region.
Fig.4: Rear Roof Rail - Center Roof Rail:
Center Roof Rail is a reinforcement provided mostly on the center portion of the roof or at the B Pillar of the vehicle i.e, the most flatter region of the car which is susceptible to failure under the action of load. In cars with sunroofs, extra inner reinforcement is provided between the roof and the center roof rail to provide more strength. For the development of the center roof rail, the styling surface and side ditch area are taken as references.
Fig.5: Center Roof Rail - Bow Roof Rails:
Bow Roof Rails are the reinforcements provided on the flatter regions of the roof which are more susceptible to failure. The number of bow roof rails depends upon the length of the roof i.e, longer roofs have more bow roof rails like vans. For the development of the bow roof rails, the styling surface and side ditch area are taken as references.
Fig.6: Bow Roof Rails
Joining Process:-
The Flanges of the front and rear roof rails and flanges of the bow roofs and center roof rails are joined using spot welding, whereas mastic sealants are provided along the roof rails to improve the strength of the roof and also to avoid bad marks on the aesthetic surfaces.
Fig.7: Final Reinforcement Assembly
Draft Analysis:-
Draft Analysis is carried out to check the manufacturing feasibility of all the reinforcements rails. All the components are designed by considering at least the minimum draft angle value of 7 Deg.
Fig.8: Front Roof Rail Draft Analysis
Fig.9: Rear Roof Rail Draft Analysis
Fig.10: Center Roof Rail Draft Analysis
Fig.11: Bow Roof Rail 1 Draft Analysis
Fig.12: Bow Roof Rail 2 Draft Analysis
Fig.13: Roof Outer Panel Draft Analysis
Curvature Study:-
- Bow Roof Prediction / Heat Distortion:
Heat distortion temperature is the temperature at which the metal softens and deforms under a specific load and temperature. This study is done to check the ability of the roof to withstand high temperatures concerning the positioning of the bow roof rail reinforcements which contribute to increased strength.
Formula:
`W=[1.73xx10^-3xxL]+[1.85xx10^-8xxR^2/t]+[1.10xx10^-3xxl]-2.68`
Where,
`L` = Roof Length in X-Direction[mm](Roof dimension in 0-Y)
`R` = Roof curvature
`R=(2(Rx xxRy))/((Rx+Ry))`
`Rx` = X curvature
`Ry` = Y curvature
`t` = Roof plate thickness [mm]
`l` = Bow Roof Span [mm]Judgment Condition:
OK< 2.7
Fig.14: Projection of points for Bow Roof Prediction / Heat Distortion
Table.1: Data for Bow Roof Prediction / Heat Distortion
Sl. No. Region L Rx Ry R t l W Remarks 1. FRR - BRR-1 2056.42 5691.73 3014.26 3941.28 0.75 517.38 1.83 OK 2. BRR-1 - CRR 2056.42 4767.48 7016.51 5677.37 0.75 349.37 2.06 OK 3. CRR - BRR-2 2056.42 3927.46 10832.62 5764.83 0.75 371.30 2.11 OK 4. BRR-2 - RRR 2056.42 3283.12 14562.22 5358.20 0.75 349.33 1.97 OK
From the above results, the positioning of the bow roofs satisfies the condition for Heat Distortion Criteria. - Snow Load Prediction:
A snow load study is done to check the ability of the roof to withstand snowfall and bounce back to its original position on the removal of the snow without undergoing permanent deformation of the roof.
Formula:
`Qr=[Iyxxt^2]/[alphaxxsxx[(Rx+Ry)/2]^2xx10^-8]`
Where,
`alpha=MyxxLx^2xx10^-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]
`Lx` = Distance between the Left and Right end of the roof on the Roof BOW [mm]
`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]
`Ry` = Longitudinal Direction curvature radius of the Roof panel X cross-section on Roof BOW [mm]Judgment condition
Qr ≥ 3.1
Fig.15: Projection of points for Snow Load Prediction
Fig.16: Moment of Inertia for Bow Roof Rail 1
Fig.17: Moment of Inertia for Center Roof Rail
Fig.18: Moment of Inertia for Bow Roof Rail 2
Table.2: Data for Snow Load Prediction
Sl. No. Region Iy t Lx Ly Y My α L1 L2 s Rx Ry Qr Remarks 1. FRR - BRR-1 1987.12 0.75 1129.20 1978.99 605.66 831767.64 1.06 605.66 427.02 516.34 4309.95 2953.62 15.47 OK 2. BRR-1 - CRR 2472.49 0.75 1079.53 1978.99 1032.68 977229.93 1.14 427.02 448.60 437.81 3828.81 7005.70 9.50 OK 3. CRR - BRR-2 1998.27 0.75 1057.09 1978.99 1481.28 737240.95 0.82 448.60 497.71 473.15 4294.21 11353.70 4.71 OK 4. BRR-2 - RRR 0.75 1043.02 1978.99 1978.99 497.71 248.85 3924.19 16851.55 From the above results, the positioning of the bow roofs satisfies the condition for snow load Criteria.
Conclusion:-
Thus the roof is designed with all the necessary reinforcements and roof rails according to the given master section and the styling surface, by considering all the design parameters. Also, the curvature study is done by taking the heat distortion and snow load criteria for determining the strength of the roof.