Design of car roof

Design of car ROOF

What is 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.

Design of Roof includes:

• Ditch area
• Front roof rail
• Rear roof rail
• Bow roof( 1,2,3,4)
• Reinforcement

Ditch area

A roof ditch area extending in a generally longitudinal direction of a vehicle is fixedly coupled to a body member and form a longitudinal water management channel on a lateral side portion of a roof of the vehicle. It is a styling orient part, so it is produce inhouse only.

Ditch area may or may not having emboss at top (which may vary car to car depending on strength requirement). Minimum thickness of 0.7 i had made in my design.

Front roof rail

• It is a additional part, which will act as a reinforcement for the roof front.
• While designing this part, we have to consider the visibility criteria and also space nesscary for mounting windshield glass, rubber around glass, rubber paste, sealant.
• This part made from body side outer scrap material.
• Minimum thickness required are 0.65mm
• This part may having emboss to provide strength

Rear roof rail

• It is also an additional reinforcement part, which is placed in rear end of roof.
• While designing this part we have to consider rear head room clearance and rear visibility criteria.
• Rear head room clearance is minimum 80mm, which depends on car to car and region to region.
• This part also having contact with outer body side.
• Minimum thickness I had given is 0.65mm.

Bow roof

• The bow roof, which is placed under the roof to provide strength to the roof.
• Number of roof placed depends upon the length of car, there may be more than one bow roof placed under roof.
• It have a mastic point, which is used to make attached to the roof without any spot weld. This mastic seal influence over 60 to 80mm around it.
• Bow roof can be placed by two methods. One method of placing bow roof between front and rear roof rail and reinforcement center or it is placed on most flatten surface in roof.
• This flatten surface can be find by two methods depends on heat and snow condition.

1.Heat condition:

In order to statisfy heat distortion condition, we have to statisfy the following formula which predict the amount of displacement under load.

Prediction formula:

W = (1.73 × 10^-3× L)+ (1.85 × 10^-8 × R²/t) +( 1.10 × 10^-3 × I)- 2.68

Where,

L= ROOF length in x-direction

R= ROOF curvature

Where R= 2( R_x×R_Y)/(R_X+R_Y)

R_X= X curvature

R_Y= Y curvature

t= ROOF plate thickness (mm)

I= BOW ROOF span (mm)

Judgement condition:

OK< 2.7mm ≤ NG

Here is the prediction table for heat distortion condition:

SI.NO	BOW	RX	RY	R	I	L	T	R2/t	w	Ok/NG
1.	FR & 1st	4466.92	5794.89	5044.97	408.44	1931.8	0.65	3915649	1.82	OK
2.	1st & 2nd	7464.77	3919.79	5140.35	150.0	1931.8	0.65	4065107	1.57	OK
3.	2nd & 3rd	4076.8	9115.91	5633.9	150.0	1931.8	0.65	4883204	1.72	OK
4.	3rd & 4th	10910.62	4122.65	5984.14	200.0	1931.8	0.65	5509220	2.6	OK
5.	4th & RR	15449.84	3167.09	5256.61	174.55	1931.8	0.65	4251069	1.82	OK

Obtained value are less than 2.7, So the heat distortion condition can be statified for the placed bow roof.

2.Snowed condition:

In order to satisfy the roof strength performance under snowed condition, we have to satisfy the following formula.

Prediction formula:

Q_r = I_Y × t²/ α × s × [( R_X+R_Y)/2]² × 10⁸

Where,

α = M_Y × L_X² × 10^-12

M_Y = Y( L_Y – Y)

S= L₁/L₂ + L₂/2

t= ROOF Plate thickness (mm)

L_Y = Distance between front and rear roof rail on vechicle OY ( length of ROOF

panel with center points between two rails as reference point)

L_X = Distance between left and right end of roof on the roof bow (width of roof

panel exposed on the surface)

Y = Distance from front roof rail to roof bow (mm)

S = (L₁/L₂ + L₂/2) Distance for which roof bow bears divided load (mm)

I = moment of inertia of ROOF bow

R_X = Lateral direction curvature radius of roof panel (mm)

R_Y= Longitudinal direction curvature radius of roof panel (mm)

Judgement condition:

Q_r ≥ 3.1

250 ≤ s ≤ 380

Here is the prediction table for snowed distortion condition:

SI.NO	Bow	IY	α	LX	LY	Y	S	RX	RY	t	Qr	Ok/NG
1.	FR & 1st	5065.89	0.85	1153.52	1853.19	460.64	176.41	3353.51	5102.05	0.65	79.8	OK
2.	1st & 2nd	3849.66	0.66	1125.8	1853.79	350.20	176.1	4591.50	7252.23	0.65	39.9	OK
3.	2nd & 3rd	3583.79	0.64	1107.96	1853.79	350.20	201.13	4182.74	10905.31	0.65	20.6	OK
4.	3rd & 4th	3583.79	0.36	1044.28	1853.79	200.00	101.74	3824.51	10905.31	0.65	76.2	OK
5.	4th& RR	5170.86	0.55	1095.08	1853.79	298.77	150.72	3580.75	15089.17	0.65	16.9	0K

Obtained values are greater than 3.1, so snowed distortion condition are statified for placed bow roofs.

Reinforcement:

• This is an additional part given under roof for providing strength to the roof.
• Some car reinforcement are also act as a bow roof and some having next to bow roof.
• Its minimum thickness value is from 1.5mm.

Section modulus calculation:

Section modulus (s) = I/Y

I= Moment of Inertia

Y= yield strength

Here am taken a steel for ROOF and its yield strength is 360Mpa.

Therefore, S= 237.49/360

= 0.659 mm^6 N^-1

Here is the link of my car roof design,

https://docs.google.com/document/d/1AKX5lX8L5ncseXS0m2OaexKkZM2BMvT6uYHtpE8s0U8/edit?usp=sharing