Hood Basics

A car hood may seem like a simple element of a car’s body structure, in most cases its function is to cover the engine compartment and most importantly during a collision become part of the complete structure to dissipate energy away from the passenger compartment. Replacement hoods are readily available; many aftermarket collision parts will retain the cosmetic elements. The most important element is not seen; the internal hardware. This includes the hinge mounting points and the latch mounting hardware. It is important these components meet or exceed the original hood’s specifications.

In a collision a hood is designed to either break, or fold in order to absorb the impact. A strong hood is not desired; a hood that won’t fold will become a guillotine projecting itself in the passenger compartment. A properly designed hood will have rigid mounting points, a strong latch area and most importantly a weak center to permit it to fold. The mechanical properties of the material become important at this point, carbon fibre is generally 3 time stronger than steel, glass strength is double of steel. The geometry and design become important to compensate for the difference in material.

When designing a replacement hood the original hood needs to be analyzed and studied to determine how the new material will be able to perform the same way it was intended to. Crash testing every hood would be great, it makes it difficult in a low volume aftermarket to justify this cost, but the issue cannot be ignored. The next step is an FEA analysis or intuitive design. Given ACS’s experience with various OEM programs it has enabled ACS to develop a process that has been proven in various crash tests. Being involved with various car crash test and developing key design elements enables ACS to forecast successful results. Using the identical design elements along with the same material lets ACS design a product with safety in mind. Reinforced hardware points, crush initiators, engineered mechanical material properties in its panel compositions are a few of the critical design requirements.

A hood is generally made of two skins, the outer panel and inner panel. Steel and aluminum hoods are assembled by crimping the two skins together. Composite hoods are either made as a whole, or bonded using various adhesives. The quality and design will determine the durability and worthiness of the hood. In its life a hood will go trough thousands of cycles; road bumps, slamming, and various temperature oscillations are important while selecting the proper material composition to insure its performance on the long run.

Painting the panel. The one element customer generally ignore until the hood is sitting in their garage. No doubt OE steel hoods are the easiest to paint. The fact that most body shops deal with them on a daily basis facilitates the choice of primers and paint to use. Aluminum hoods are the most complicated, a proper body shop should be chosen as adhesion to the material is more delicate.

Composite hoods are either shipped with gel coat or primer. What ever material is used, it is important not to sand trough these barriers. The purpose of the barrier is to seal the substrate. This being said depending on the body panel quality a body shop may have to sand and prep the hood. Different makes will offer different quality levels. Prep time can range from 30 minutes up to 8-10 hours of prep. Selecting a bodyshop with experience with composites will save any surprises in pricing, quality and longevity of the surface quality.

Installation and fit; the fit and finish is difficult to quantify. Hood arc, stack height, panel thickness, hood/fender gap and flushness are all elements that need to be evaluated. In theory an aftermarket hood should replace the stock hood and be adjusted following the OE specs. Fenders and facias may need to be adjusted to properly contour your new hood. Depending how a car is built and how well the hood is manufactured this can take from several minutes to an hour depending on the hood quality. It is critical to research the manufacturer, internet forums are a great resources for customer feedback and speaking to local bodyshops can help you make a wise decision with your purchase. A general rule is you pay what you get…. a hood built with low manufacturing tolerance and hand assembled will be most difficult to properly align. Panels that are hand fabricated are only as good the labourer on a given day…. repeatability and quality control is key. In the upcoming article I will explain the various process used in composites. They may all smell the same but definitely do not perform the same.


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