Roof Design

Title- Design of Roof

Objectives-

1. For the Given Roof styling, Develop the essential flanges and reinforcements, provide appropriate dimension and check for Draft analysis and also submit your section modulus report on the 0-Y section.
2. Start with the creation of a roof ditch area the tool opening angle is only 3 degrees.
3. Do a curvature study on the roof and perform calculations to determine the position of the Bow-roof

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.

Types of Car roofs-

• Convertible roofs
• Hardtops
• Sunroofs
• T-tops
• Targa tops
• Vinyl roofs

Safety point of view vehicle roof plays very crucial role specially roll over kind of accidents, in which vehicle tips over onto its side of roof. And vehicle rollover crashes are the causes of many fatalities and several head, neck and spine trauma around the world. Therefore, passenger safety is also very important parameter in front of OEM’s. It is not only a marketing strategy but also it is a obligation stipulated by international standards that are now place in several countries, as well as a governmental requirement.

Rollover crashes, especially in the country, are usually very destructive events. Vehicle damage often includes deformation of the roof and its supporting structures. Head and neck injuries are common, and associated with roof deformation. Strengthening of the roof is suggested as an appropriate countermeasure for such injuries.

Types of Rollovers

1. Tripped-NHTSA data show that 95% of single-vehicle rollovers are tripped. This happens when a vehicle leaves the roadway and slides sideways, digging its tires into soft soil or striking an object such as a curb or guardrail. The high tripping force applied to the tires in these situations can cause the vehicle to roll over. E.g. Soft soil, Guardrail, Steep Slope
2. Un-Tripped -Un-tripped rollovers are less common than tripped rollovers, occurring less than 5% of the time, and mostly to top-heavy vehicles. Instead of an object serving as a tripping mechanism, un-tripped rollovers usually occur during high-speed collision avoidance manoeuvres.

For this reason, strength is very important criteria for designing vehicle roof.

FMVSS 216 Roof crush standards-

FMVSS 216, Roof Crush Resistance, establishes strength requirements for the passenger compartment roof of passenger cars, multipurpose passenger vehicles, trucks and buses with a GVWR of 2722 kilograms or less. The purpose of the standard is to reduce deaths and injuries due to the crushing of the roof into the passenger compartment in rollover accidents. The standard does not apply to school buses and passenger cars that conform to the dynamic rollover test requirements of FMVSS 208, Occupant Crash Protection, and S5.3 by means that require no action by passenger car occupants. It also does not apply to convertibles, except for optional compliance with the standard as an alternative to the rollover test requirements in S5.3 of FMVSS.

The prescribed static loading device is a rigid unyielding rectangular block 762 millimetres by 1,829 millimetres. It shall not move more than 127 millimetres to achieve the specified resistive load when applied to the forward edge of a vehicle’s roof – see Figures 1 and 2. The resistive load to be achieved is 1½ times the unloaded vehicle weight (UVW) of the test vehicle or 22,240 Newtons, whichever is less.

The force shall be applied in a downward direction perpendicular to the lower surface of the static loading device at a rate of not more than 13 millimetres per second until reaching the compliant resistive load. The test shall be completed within 120 seconds. The static loading device shall be guided throughout the test so that it moves, without rotation, in a straight line with its lower surface oriented as follows and as shown in Figures

Following are the design considerations of the roof-

1. Visibility criteria
2. Head clearance
3. Heat distortion and snow load criteria
4. Draft analysis
5. Safety parameters

Input data from styling team-

We will be having following data which is provided from styling team

1. A- surface of the roof along with railing rack.

2. Master sections for Front, Bow 1&2, Centre & Rear rails.

Design of components of Roof-

1. Design of ditch area flange

We will start first by extracting surface from input data solid roof part. Using this surface we design ditch area flanges.

DITCH AREA FLANGES AREA MADE

Design of front roof rail

For designing front roof rail we need to consider some parameter which are as discussed below

a. Visibility of the driver and passenger

We always keep in mind while designing the front roof we do not reduce the visibility available to driver. While designing front roof one must stick to the master section dimensions because little change in dimension will have impact on the visibility of the driver (e.g. small change in angular dimension will lead to large change in linear dimension)

b. Head room

There should be enough space between roof rail and head position of driver so when accident will happen head will not hit to the front roof rail.

FRONT ROOF RAIL

FRONT ROOF RAIL SECTION

2) Design of Bow roof rail (1 & 2)

Bow roof is sheet metal component which is used to add more structural strength to the roof. On the basis of the curvature of the roof bow roof is provided.

To justify the position of the bow roof there are two criteria are used-

a. Heat Distortion Criteria

b. Snow Load Criteria

Bow roof rail (1 & 2)

Bow roof rail 1 Section

Bow roof rail 2 Section

3) Design of Centre Roof Rail

Following same design process as front roof rail and master section we design centre roof rail.

CENTRE ROOF RAIL

CENTRE ROOF RAIL SECTION

4) Design of Rear roof rail

Rear roof rail is also designed same as front roof rail.

Rear roof rail

Rear roof rail Section

Assembly View of Roof

Bow roof rail calculations-

1. Heat Distortion calculations

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]

Procedure to measure the parameters-

a. On plane parallel to the top plane draw sketch in which 0Y line is offset by 300 mm (3Y) on both the sides of 0Y line.

b. Draw lines perpendicular to 0Y line passing through point exact midpoint of the two adjacent parts (e.g. midpoint between front rail and bow roof 1)

c. Offset these reference lines on both sides by 100 mm

d. Mark intersection points (red points) of reference line (Yellow lines) and 100 mm offset lines(Black lines).

Take all necessary measurements required to calculate W value as per mentioned formula.

2. Snow load criteria calculations

Snow load criteria is useful to justify the bow roof rail position in snow fall regions. Due to snow fall snow will get collected on the top roof which will tend to bend the roof. Bow roof will be at such position which will provide more strength to roof against bending.

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 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 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

Procedure to calculate necessary parameters

a. On plane parallel to the top plane draw sketch in which 0Y is projected alog the direction perpendicular to top plane

b. Draw lines perpendicular to 0Y line passing through point exact midpoint of the two adjacent parts (e.g., midpoint between front rail and bow roof 1)

c. Offset these reference lines on both sides by 100 mm.

Take all necessary measurements required to calculate QR value as per mentioned formula.

Rendered Image

Conclusion-

1. Using all input data we have successfully designed Ditch area flanges, Front roof rail, Bow roof rail 1&2, Rear roof rail as per the requirement.
2. As per heat distortion criteria we come to know all bow roof positions are OK.
3. As per Snow load criteria we have experienced that still improvement is required.
4. All roof rails draft analysis are found OK.