Roof Design

ROOF DESIGN

Aim:

To design a roof with its components by following the flanges of front roof rail, two bow roofs, center roof rail and rear roof rail with all the design parameter and to perform draft analysis for all the parts and to perform includes the curvature study on the roof based on the Bow roof prediction calculation as well as the manufacturing techniques.

Objectives:

• To create a flanges of roof outer panel
• To create front roof rail
• To create rear roof rail
• To create bow roof 1
• To create center roof rail
• To create bow roof 2
• To perform a draft analysis with all the parts
• To calculate a heat distortion and Snow load for bow roofs.

Roof:

An automobile roof or car top is the portion of an automobile that sits above the passenger compartment, ptotecting the vehicle occupants from sun, wind, rain and other external elements.

Components of the Roof:

• Front roof rail
• Rear roof rail
• Centre roof rail
• Bow roof rails

DESIGN METHODOLOGY

Master section of the 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 the spot welding. Visibility, clearance for windshield and back door mountings are some of the factors needed to be considered while designing the ditch area.

• Front roof rail:

Front roof rail is a reinforcements 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 FRR 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.

• Rear roof rail:

It is a reinforcement provided to the rear portion of the roof and it also features the tailgate mountings. The development of RRR 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 varient and region to region.

• Bow Roofs:

These are the reinforcements provided on the most flatter regions of the roof which are most susceptible to failure. The number of bow roofs depends upon the length of the roof i have longer roofs have more bow roofs like vans. For the development of the bow roofs, the styling surface and side ditch area are taken as references.

Bow roof 1

Bow roof 2

• Center roof rail

These roof rails are positioned mostly on the center portion of the roof or at the B pillar of the vehicle i have the most flatter region of the car which is susceptible to failure under the action of load.

Final Assemply of Roof:

The joining process in which the flanges of the front and rear roof rail and flanges of the bow roofs and center roof rail are joined using the spot welding, where as mastic sealants are provided along the roof rails to improve the strength also avoid bad marks on the aesthetic surfaces.

CURVATURE STUDY

Heat distortion criteria:

Heat distortion temperature is the temperature at which the metal softens and deforms under a specific load.

This study is done to check the ability of the roof to withstand high temperatures concerning the positioning of the reinforcements which contributes to increased strength.

Procedure:

• Create a center line at the 0Y region of the roof and then offset the same to -3Y.
• Create a symmetric line from -3Y region at a distance of 100mm.
• Draw a vertical line at the midpoint of the line drawn from the center points of the FRR and BR1 and followed the same till the RRR.
• Offset on the vertical line to a distance of 100mm on each side.
• Create intersection points and project them on the roof.
• From these projected points, make a point arc and the Rx and Ry values.

Formula:

W = [1.73*10^(-3) * L] + [1.85*10^(-8) * (R^2)/t] + [1.10*10^(-3)*l] - 2.68

Where,

• L = Roof length in X - direction[mm](Roof dimension in 0Y)
• R = Roof curvature

R = 2(Rx*Ry)/(Rx + Ry)

• Rx = X Curvature
• Ry = Y Curvature
• t = Roof plate thickness[mm]
• l = Bow roof span[mm]

Judgement Condition

OK < 2.7

HEAT DISTORTION CRITERIA

S.NO	BOWs	Rx(mm)	Ry(mm)	R(mm)	l(mm)	L(mm)	t(mm)	R^2/t(mm)	W(mm)	OK/NG
1	FR & Ist	5858.7	2948.3	3922.6	341.2	2132.35	0.75	2.05*10^7	1.76	OK
2	1st & CTR	4899	6361.8	5535.4	501.4	2132.35	0.75	4.08*10^7	2.3	OK
3	CTR & 2nd	3926.8	10664.4	5740	385.5	2132.35	0.75	4.39*10^7	2.24	OK
4	2nd & RR	3307.5	14456.7	5383.3	364.4	2132.35	0.75	3.86*10^7	2.12	OK

From the above results, the positioning of the bow roofs satisfies the condition for Heat distortion criteria.

Snow load criteria:

A snow load study is done tro check the ability of the roof to withstand snowfall and bounce back to its original position without undergoing permanent deformation upon the removal of the load.

Procedure:

• Create a center line at the 0Y region of the roof and then offset the same to 100mm on each side.
• Draw a vertical line at the midpoint of the line drawn from the center points of the FRR and BR1 and followed the same till the rear roof rail.
• Offset the vertical line to a distance of 100mm on each side.
• Create intersection points and project them on the roof.
• From these projected points, make a 3 point arc and Rx and Ry values.
• Project the extreme edges of the styling surfaces from measuring the width at the respected sections.

Formula:

Where,

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 Bow roof[mm]

The width of the roof panel to Bow roof[mm]

Y = Distance front roof rail to Bow roof[mm]

s = Distance for which Bow roof 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 = Lateral direction curvature radius of roof panel X cross-section on Bow roof[mm]

Roof panel X curvature radius of the length s in front view

Judgement Condition

SNOW LOAD CRITERIA

I have take the value moment of inertia in Bow roofs

• Bow roof 1

• Center bow roof

• Bow Roof 2

S.NO	BOWs	Iy(mm4)	L1(mm)	L2(mm)	s(mm)	Y(mm)	Lx(mm)	Ly(mm)	My(mm)	alpha(mm)	Rx(mm)	Ry mm	t mm	Qr mm	OK/NG
1	FR & 1ST	861.4	436.6	578	507.3	436.6	1136	2063	710086.2	0.916	4421	2714.6	0.75	8.2	OK
2	1ST & CTR	3.15*10^3	578	462.2	520.1	578	1082.1	2063	858330	1	3677	3211.1	0.75	28	OK
3	CTR & 2ND	874.23	462.2	485.7	474	462.2	1053.6	2063	740530.1	0.82	4245.7	11055.5	0.75	2.16	NG
4	2ND & RR	-	485.7	-	242.85	485.7	1039.7	2063	766094.6	0.83	1105.5	4016.7	0.75	-	-

From the above results, the positioning of bow roofs is not satisfied in the snow load criteria. One last condition is NG. So i have creating the embosses is outer panel because the increasing the strength of the outer panel.

Draft Analysis:

Draft analysis is carried out to check the manufacturing feasibility of the roof and all the reinforcements rails. All the components should be designed by considering at least the minimum draft value is 7deg.

• Roof outer

• Front Roof Rail

• Rear Roof Rail

• Bow Roof 1

• Centre Roof Rail

• Bow Roof 2

Conclusion:

The roof has been successfully designed considering all the design parameters, Master section, Manufacturing feasibility, Roof prediction calculation and Draft analysis.

Thank you