Roof Design

AUTOMOTIVE ROOF DESIGN

OBJECTIVE:

To design the automotive roof based on the given styling surface and to analyze the designed model to check the manufacturability and the feasibility of the design.

AUTOMOTIVE ROOF

The component of an automobile that lies above the passenger compartment and protects the occupants from the sun,rain,wind and other external elements is known as the roof or car top.Some car roof types are Hardtops,Sunroofs and convertibles.Main components of roof assembly are outer roof panel,front roof rail,bow roof,center roof rail and rear roof rail.The roof has an outer panel which is the styling part.Below the outer panel of a roof are strengthning parts placed those are roof rails and bow roofs.The front roof rail provides provision for the windshield glasses.Provision for windshield mounting and inside rearview mirror is provided on the front roof rail.Bow roofs are provided in between the front roof rail and center roof rail as well as the rear roof rail and center roof rail.They are provided to enhance the localized strength of the roof and are attached directly to the bottom of the roof with either spot welding or with the help of Mastics.The Rear roof rail has hinge mounting for the back door.It is attached at the portion where the back door meets with the roof.Reinforcement center or center roof rail is used as a cross member between left hand and right hand bodysides.Both the front roof rail and rear roof rail provided with the Master section for the design.While designing of roof the Visibility criteria,Head clearance,Curvature study,Head distortion and Snow load criteria considerations have to be taken into account.The commonly used materials for the outer panel are Steel and Aluminum alloys.

Software Used - Siemens NX 12

DESIGN CONSIDERATIONS

• Thickness for all sheet metal parts 0.75mm.
• A minimun draft angle is 7 degree.
• Clearance between outer roof panel and Front roof rail,Rear roof rail,Bow roof and Reinforcement is 5mm.
• All edges should have fillets and there should not be any sharp edges present.

SECTION INERTIA ANALYSIS ON ROOF RAILS

To calculate the section inertia analysis we need to take the intersection of all the roof components individually on the 0-Y section.

DRAFT ANALYSIS OF ROOF AND ROOF RAILS

Draft analysis technics is used to improve the manufacturabily of designed components.This tool represents the draft values of component faces in different colours.Minimum draft angle used in this analysis for roof components is 7 degree.

HEAT DISTORTION CRITERIA CALCULATION

Before we calculating the heat distortion criteria,we need to create a sketch for note down the values.Create a line at the center of the roof part in the 0-Y axis.then draw another line with a distance of 300mm to the 0-Y line.Now create a line at distance of 100mm to both sides of 300mm line.Create a new sketch and draw the center line in between Two roof rails in X direction.Then draw the line at distance of 100mm to the this center line both sides. Intersect the both X direction and Y direction lines create the points and project into the outer roof panel.Use this X and Y direction points find the Rx and Ry curvature values in different bow roofs.

Formula, W=[1.73x10^(-3)xL]+[1.85x10^(-8)x(R^2)/t]+[1.10x10^(-3)xI]-2.68

Where, L=Roof length in X direction [mm]

R=Roof curvature

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

Rx=X curvature, Ry=Y curvature

t=Roof plate thickness[mm]

I=Bow roof span[mm]

Judgement condition: OK<2.7

S.NO	BOW	Rx	Ry	R	L	t	I	W	OK/NG
1.	FR-BR1	2472.66	5740.84	3456.53	2031.11	0.75	373.82	1.53	OK
2.	BR1-CR	5994.10	5022.67	5465.55	2031.11	0.75	391.82	1.99	OK
3.	CR-BR2	10016.91	4089.91	5807.76	2031.11	0.75	376.36	2.07	OK
4.	BR2-RR	14223.70	3335.97	5404.41	2031.11	0.75	445.52	2.04	OK

*All dimensions are in mm.

SNOW LOAD CRITERIA CALCULATION

This load calculation is done to know how is the roof behaving when there is a snow over the roof.The roof is normally dent due to snow weight,but the roof should be designed in such a way that when the snow is removed the roof should go it its original position.This is the basic requirements is followed in snow load criteria.For the Snow load criteria calculation we take Rx and Ry curvature in center line of roof panel(0-Y direction).

Qr=[Iy x t^2]/[`alpha`x s x [(Rx+Ry)/2]^2 x10^-8]

Where, `alpha`=My x Lx^2 x10^-12, My=Y(Ly-Y)

t=Roof plate thickness[mm]

Ly=Distance between the front and rear roof rails on the vehicle along with 0-Y[mm]

Lx=Distance between the left and right end of the roof on the roof BOW[mm]

Y=Distance from front roof rail to Roof BOW[mm]

S=L1/2 + L2/2

Iy=Geometrical moment of inertia of roof BOW[Y-cross setion][mm4]

Judgement condition: Qr`>=`3.1

S.No	BOW	Iy	`alpha`	Lx	Ly	Y	S	Rx	Ry	t	Qr	OK/NG
1.	FR-BR1	1843	0.6956	1040	2094.47	373.82	382.82	2536.18	4576.42	0.75	30.81	OK
2.	BR1-CR	6373	1.1399	1040	2094.47	839.30	384.08	3757.40	5897.35	0.75	35.15	OK
3.	CR-BR2	1854	1.1881	1073.58	2094.47	1303.84	410.93	4217.56	10395.53	0.75	4.00	OK

*All dimensions are in mm.

Snow load criteria test for this roof assembly Front roof rail-bow roor rail1, Bow roof rail-center roof rail and center roof rail-bow roof rail2 are passes the condition.Its ensures the strength of bow roof rails.

ROOF ASSEMBLY AND CROSS SECTIONS

CONCLUSION

Roof and all roof rails has been successfully designed with fulfilling given heat distortion curvature study,snow load criteria and Bow roof positions judgement conditions.For attaining more strength of roof rails different emposses is performed in required sections.