Roof Design

OBJECTIVE:-

Design a Roof with its components by following the master section of the front roof rail with all the design parameter and do the curvature study and calculate the heat distortion study and snow load criteria, section modulus and perform draft analysis of each reinforcement.

INTRODUCTION:-

An automobile roof or car top is the part of an automobile that positioned above the passenger compartment, It covers and protect the vehicle occupants/passenger from sun, wind, rain, and other external elements.It has given some reinforcement to give strength to this part. It is made curved because if the roof is flat then it will more prone to bend if some load applied.

Reinforcement of Roof:

1) Front Roof Rail

2) Bow Roof Rail

3) Centre Roof Rail

4) Rear roof Rail

FRONT ROOF RAIL

Front roof rail is provided at the front end of the roof to provide strength to that area of roof, while designing this part we should maintain the visibility and headroom clearance. Embosses are created to strengthen the front roof rail.

BOW ROOFS

It is provided to give strength to the roof at that areas, it can be created at any number depending upon the length of the roof. Mastic points are provided to join this bow roofs to the outer panel

CENTER ROOF RAIL

It is provided at the centre of the roof to strengthen that area and it is joined to the outer panel by mastic

REAR ROOF RAIL

It is provided at the rear part of the roof and it is joined with the outer roof by spot welding. We should take consideration of rear headroom and visibility criteria while desiging this. Embosses are created to strengthen the rear roof rail

Heat Distortion Criteria

This calculation is performed to check the strength of the roof at a certain load with a certain temp. This study will predict the position of bow roofs.

W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/t] + [ 1.10x10^(-3) x l] - 2.68

Where,

L = Roof Length in X-Direction[mm](Roof dimension in 0-Y)

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

Procedure

1) We create a 0Y line on the top of the roof.

2)We sketch a -3Y which is 300m below from the 0Y line

3) We sketch 2 line on the both side of the -3Y line at 100mm distance

4) We sketch a line in the middle of the FR and BOW ROOF 1,BOW ROOF 1 and CR, CR and BOW ROOF 2,BOW ROOF 2 and RR.

5) We sketch 2 line on the both sides of the middle line.

6)we create intersection points of the line and project it on the roof then measure the Rx and Ry distance.

Here

• 'l' value is from one reinforcement centre to other reinforcement centre value.
• 'L' value is total length from one end to another end.

According to the judgement criteria OK< 2.7> 3.1, All values are satisfied.

Snow Load Calculations:

We have a data to calculate

Qr = [Iy x t²] / [α x s x [(Rx + Ry)/2]² x 10^-8]

Note: The Qr value given over here is correct. There was a typo error in the formula given in the video.

Where

α = My x Lx² x 10^-12 , My = Y(Ly-Y)

Judgement condition = Qr ≥ 3.1

250 ≤ s ≤ 380

t = Roof plate thickness [mm]

Ly = Distance between the front and rear roof Rails on the Vehicle along with 0Y[mm]

Length of Roof panel with the center point between Roof rail Front /Rear as the front and rear reference point.

Lx = Distance between the Left and Right end of the roof on the Roof BOW [mm]

Width of the roof panel exposed on the surface.

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]

Roof panel curvature Radius of the Length Lx in Front view

Ry = Longitudinal Direction curvature radius of the Roof panel X cross-section on Roof BOW [mm]

Roof panel X curvature radius of length s in Side view

Here we have to find out the RX and Ry values. To find that

• Create a sketh on top roof and drawn a horizontal line at the centre.
• Offset this line to 100mm on both sides.Convert all these line into reference lines
• Draw the verical lines at the middle of the each reinforcemnts and offset it to 100 mm both sides.
• Intersection point is created where the lines are met.
• Project these points to the outer roof panel.

To find out the geometrical moment of Inertia , we created a intersection curve and used section inertia command.

BOW 1:

CENTER ROOF RAIL:

BOW 2:

According to the judgement criteria Qr ≥ 3.1

Here one result of Qr is not fulfilling, this model will pass over to the styling team to change the curvature to improve the strength.

Draft Analysis:

Draft analysis was performed with 5 degrees value.

Front Roof Rail:

CENTER ROOF RAIL:

BOW ROOF 1:

BOW ROOF 2:

Learning:

• Here we learned how to create a rails with the given standards by using Master Sketch.
• Learned How to calculate Heat Disorsion and Snow load criteria
• Learned how to do Draft analysis.

Conclusion:

The roof was created with all required parameters with a certain rails, and calculation the heat distortion, snow load are done and performed Draft analysisfor safety concerns. The result is satisactory.