Hood design-Week 2

Hood Designing:

What is hood?

The Hood here refered to bonnet. It is the hinged cover over the engine of motor vehicles that allow access to the engine compartment for maintenance and repair.

The Hood is also considered as another type of door in a vehicle as it consists of an outer panel and Inner panel. The Inner panel provides strength while the outer panel functions as the metal cover and has some styling surface touch.

Inner panel itself is the largest reinforcement part in Hood.

Hood itself is of two types:

1. Wrap around hood
2. Inlaid Hood

Wrap around hood: When endpoint of car lies on hood then it is called wraparound hood. Wrap Around Distance (WAD) is the trace of the points received on the front structure when one end of flexible tape of required length held at ground level and other end in contact with the front structure.

Side Reference Line - It means the trace of the highest contact point between straight edge inclined at an angle of 45 degree and side of the vehicle.

Inlaid hood: In Inlaid Hood the end point of the car lies on Fender.

It’s design consideration:

So, to design hood we keep many aspect in mind like pedestrian safety, force distribution or flexibility to dissipate and direct force in other direction and making less force reach at the occupants end Shape & Size of embosses, Hinge position, Latch and Striker position, Mastic points, Material, Weight etc.

Hoods are typically made out of the same material as the rest of the body. This may include Steel, Aluminium, Fiberglass or Carbon fibre. However, some aftermarket companies produce replacements for steel hoods in fiberglass or carbon fibre to make the vehicle lighter.

Opening angle of Hood:

It’s obvious when we open hood there has to be some range to it’s opening. So that range depends on the average height of person of that particular country. So, it varies from Nation to Nation. So, marketability person to be confirmed the hood head zone for Lock & striker and hood support rod accessibility.

It is important to avoid head injury which may happen through latch and Striker. So, the angle between hood’s rest position or when it is closed totally to angle up to which it will open is important to calculate. This angle varies for different region and this is important to find out.

Here in India its upper limit will be less compared to other European countries and America.

Gas stay / Support Rod (Hood stay):

Now after opening the Hood there need a support system to keep hood in elevated position. So that, one can access & work on the engine & other parts.

So, to fulfil this, Support Rod & Gas stay functions perfectly.

Support Rod - Support rod is basically long cylindrical rod. It safely keep an engine hood in an open state even if external forces are abruptly generated due to vibration & wind.

Advantages:

1. It is Cost effective
2. Simple

Disadvantages:

1. When striking with the automotive body panel it leaves scars which are very likely to cause rust.
2. The metal rod is often too hot to be gripped by hand when one attempts to keep the engine hood open for inspecting the engine room immediately after the vehicle has been stopped.

Design consideration of support rod:

1. When hood is closed, during that the support rod will stay in place & positioned where it gets enough clearance to lay down & doesn't hit & create rattling noise while motion
2. Support rod will always be placed opposite side of the battery’s position. So that it doesn't interfere with rod.

Ex- If battery is kept at left hand side then support rod will be placed at RHS.

1. Now a straight support rod could wobble as the weight it is going to support would be around 8 to 9 kg. So a little deflection will be given in rod and design engineer has freedom to give it anywhere along the rod but the deflection shouldn't be more than 35 mm.

GAS STAY:

It is also a Hood stay but it is automated or need some slight amount of force then gas itself actuates it and make it work desirably.

Advantages:

1. It is automatic in nature

Disadvantages:

1. It is costlier

1. Here below it is shown how gas stay looks like

2. So gas stay is a type of spring that unlike a typical mechanical spring that relies on elastic deformation, it uses compressed gas contained within an enclosed cylinder sealed by a sliding Piston to pneumatically store potential energy and withstand external force applied parallel to the direction of the Piston shaft.
3. In gas stay stud ball is used to connect it from the Hood and body from both ends.

4. As gas stay is an additional component so we will use a bracket also for reinforcement to improve strength

Latch and latch reinforcement:

The Hood release system is common on most vehicles and usually consists of an Interior hood latch handle, hood release cable and hood latch assembly. The Hood latch handle is usually located below the steering wheel beside the driver seat or set into the door frame.

How to open it:

1. Look for a release latch inside the cabin of your vehicle. New model of vehicles have a latch to release the hood somewhere inside the cabin. The release latch may be found in one of the following areas on your vehicle.
2. Under the dash by the driver's door
3. On the bottom of the dash under the steering column
4. On the driver side floor

Operate the exterior latch: You will only be able to lift the hood a few inches until you move the exterior Lever under the Hood to completely unlock it.

Now once hood will get open to hold it in open position use the support rod or gas stay if available

Latch Positioning:

Here hood latch will be placed accurately in the middle of the Hood and slightly down from front end of Hood.

So here the positioning of latch totally depends upon hinge axis because when striker hits the latch system built in body compartment it must be tangentially perpendicular to the circle made from hinge axis or the moment it hits the latch system it inserted or get locked smoothly because slightest deviation may result in improper closing of hood.

Here in this I have made striker tangentially perpendicular and have 90.04 degree of angle

Latch reinforcement:

Latch reinforcement is provided because of latch emboss, strength in that area was compromising so to balance that, latch reinforcement with thickness up to 1.5 mm has been given.

Rubber pad:

Rubber pad is used at the middle of the Hood to improve NVH as during motion hood may produce noise so to compensate that we use it but somehow we achieve good NVH value. So this pad is mostly removed at the end as cost reducing entity.

Inner panel:

Inner panel is that part of hood which provides strength to Outer panel. Inner panel itself considered as biggest reinforcement in hood apart from hinge and latch reinforcement.

Design of inner panel consist:

1.Flanges 2. Embosses 3.Split surfaces 4.Mastic point 5. Hinge 6.Latch space

Flange: Following the master sketch we will made flanges for inner panel.

Embosses - Embosses has been given to create extra strength in inner panel and embosses design needs to be inspired by force dissipation phenomena like here I have made embosses in a way if any object hit from front most of the force will get dissipated by its design towards the upper left & right corner so that less force reach at occupants end.

Some split or cut is given on embosses to reduce head impact result.

Mastic points are also given to apply mastic sealant. A white paste like sealant to join inner panel and outer panel with each other by providing some heat to it. In heat it gets hardened.

While designing inner panel one must take care of spaces to provide on panel for latch and hinge.

Mastic data:

It is basically an NVH topic mastics are point / surfaces given on embosses for applying mastic sealant to join outer and inner panel and also give good value of NVH.

So mastic sealants are joining material used to join inner and Outer panel and at the same time reduces the NVH value.

One spot of mastic sealant provides strength locally at around 80 mm so we need to put these mastic points in such a way that it covers the most of the hood region except reinforcement area of latch and hinge because that alone provide good NVH so no worries there.

Here I have spotted points where I have applied mastic data when mastic sealant will be applied and while undergoing hot bath it gets hardened & make the joint strong.

We use mastic sealant because we cannot provide spot welding on outer panel as it won't look good from outer side and car’s aesthetic will compromise in doing so.

Pedestrian Safety Measures:

A lot has been done in this field by far-

Euro NCAP the European car assessment program is European car safety performance assessment program based in Leuven and founded in 1997.

They published safety reports on new cars and awards star ratings based on the performance of the vehicle in a variety of crash test including front side and pole impacts and impacts with pedestrian. The top rating is 5 star

So here in Hood we separate the whole area of hood into several small part / region during frontal crash. Hood is the major portion a person is gonna come in contact with. So Euro NCAP specified some guidelines under which we distribute hood region into several parts and try to get more green region.

Here below I have pasted an image to get an overview.

This region is basically distributed in relation to the height of children and adults who come in contact from front while accident. So, here the main motto is to achieve more green region and for that we provide splits in embosses so that during head impact the consequences of collision come less effective. So here I have given splits accordingly to improve head impact zone effectiveness in providing less damage to the person who hit it.

Outer panel:

Design of outer panel is mainly inspired by aesthetic but there is some engineering also to it. Here main purpose of outer panel is to cover the inner panel to provide aesthetic look at the same time Panel must be curve & sleek so that it behave aerodynamically and air hitting from front reflect from it efficiently. So here rather than using seam and spot welding we use hemming method to join outer and inner panel. Hemming is a type of folding or wrap-around process in which roller bend and press the extrapolated flange in a way that it holds the inner panel firmly, then a sealant is also provided in between them under folds to get water tightening.

While hemming we provide relief around corner as it helps in easy hemming or undistorted hemming while roller took turn around any corner.

It’s thickness is also 0.75 mm maximum

It is typically of Steel and Aluminium image

Hinge and hinge reinforcement:

Hinge is a mechanical bearing that connects two solid objects typically allowing only a limited angle of rotation between them it has one degree of freedom. So here hinge is providing rotation to hood about it’s own axis and also keep it connected to the fender portion.

So while creating space for hinge we need to provide emboss there, because of emboss and hole its strength got affected so to compensate that we provide hinge reinforcement.

Hinge reinforcement has thickness around 1 to 1.5 mm to provide reinforcement to hinge embossed region.