Roof challenge
INTRODUCTION:
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. Because the earliest automobiles were designed in an era of horse-drawn carriages, early automobile roofs used similar materials and designs.
Now roof plays important role in Crash test also and it is main parameter during roll over crash as too much deformation can cause inury in head and also it can create obstruction to come out after crash. Roof postioning based on the head space and crash requirements and other factors like Heat Distortion and Snow Load conditions decide th curvature of roof
Types of roof:
1. Covertible roofs
2. Hardtop roof
3. Sunroof
4. T Tops
5. Targa Tops
6. Vinyl Roof
Generaly automobile roof contains following parts
1. Roof Panel
2. Front Roof Rail, Reinf IRVM nountings
3. Central Rof rail, Reinf Lamp bracket
4. Roof Bows
5. Rear Roof Rail
Some Reference Standards For roof Design
Master Section Study
After receiving CAS from styling team we begin preapring master sectoin and packaging study with reference to our surrondings we start preparing ditch area for roof and then rails. In below picture main pramters are highlighted which decides the impotant dimensions of master section
As we can see from above section how the main parameters decide the dimensions and shape of master section tool opening angle in ditech area is also taken minimum for meeting visibility criteria.
Now with the help of this master section we construct front roof ditch area and then front roof rail.
Roof Panel
Front and Rear Ditch Area Roof
Bottom View
Section View
Front Roof Rail
Rear Roof Rail
Bottom View
Section View
Rear Rail
Central Rail with Reinforcement
Bottom View
Section View
Central Reinforcement and rail
Reinforcement and Central Rail
Front and Rear Roof Bows
Section View
All Roof Bows and rails
Now we have prepared the intial models as per the master sections and we will now do analysis for Roof we will fcous on two main analysis here that are:
1. Heat Distortion Criteria
2. Snow Load criteria
Heat Distortion Criteria:
The material softens under increased temperature conditions. While applying load at this state makes the material much vulnerable to static loads. So to formulate this scenario the heat distortion criteria is introduced. The formula for heat distortion criteria is
W = [1.73 x 10^ (-3) x L] + [1.85 x 10^ (-8) x (R^2)/t] + [1.10x10^ (-3) x l] -2.68
Judgement Condition: OK< 2.7> 3.1
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]
we have standard mehtod to calculate Rx and Ry
As we can see that our Roof Assy is meeting the heat distortion criteria so now we can move forward with our design
Snow Load Condition:
This condition is to analyse our roof for the heavy snowfall in winters so here again we have some mathematical formula which tell us whether our Roof bows placed and their section are strong enough or not
SNOW LOAD CRITERIA
Qr = [Iy x t ] / [α x s x [(Rx + Ry)/2] x 10 ]
Where
α = My x Lx x 10 , 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 rails Front /Rear as the reference point of the front and the rear.
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 from 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
Moment of inertia of Front roof bow:
Central Roof Rail:
Rear Roof Bow:
Now we will do the calculations and plot them on table
As we can see tht our Roof Assy is satisfying Snow load Conditions so this roof will pass the snow conditions test also
we can safely go ahead with this design.
Draft Analysis
Roof Panel we take nominal draft value i.e & deegre for sheet metal parts
Front Roof Rail:
Front Roof bow
Central Roof Rail Draft analysis
Rear Roof Bow Draft analysis:
Rear Roof Rail Draft analysis
Complete Roof Assembly:
ISO VIEW
Ortho View:
