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Your Hood (Bonnet) worn out? » Read more about common malfunctions • Repair • Replacement manual
Protection, design and aerodynamics: the bonnet
The bonnet is a moving part of the bodywork, covering the engine and enabling quick access through its locking mechanism. Nowadays the bonnet is usually situated at the front. This was not always the case.
Function of the bonnet
The engine is the driving unit of the car. It is where chemical/physical processes occur, responsible for the car's propulsion. Despite its robustness, the aggregate should not suffer more than necessary. Therefore the bonnet protects it against all weather conditions. In addition, the bonnet is a decisive feature of the car's appearance and aerodynamics, giving the vehicle its flowing line and simultaneously guiding the airstream. Most cars have a front engine. Correspondingly, the bonnet merges with the frontal bumpers and the headlights. For a long time, several cars had a rear engine. The Porsche 911 was the last car with this construction. Super sports cars and a number of sports coupés (e.g. the Toyota MR-2) have mid-mounted engines. Their bonnet is located behind the driver's seat.
The bonnet (in American English: hood) consists primarily of a large curved metal sheet with stabilising rods welded onto the bottom side. It is fixed to the chassis with hinges. Most bonnets open at the front. For a long time, BMW had bonnets opening backwards. This was intended as a security feature to prevent the bonnet from opening during driving in case of a defective lock. However, it made working on the engine very unpractical, and therefore this solution was given up in the early nineties. Rubber dampers are glued onto the bonnet, ensuring a tight as well as flexible position of the bonnet when it is closed. They keep the bonnet in its place without rattling during driving. An opened bonnet is fixed with a rod, located in the engine. The accessory trade now also offers hydraulic dampers, opening the bonnet like the boot lid. Bonnets are usually made of steel. For reasons of weight, especially electric and hybrid cars have bonnets made of plastic. For the average combustion engine, however, the suitability of plastic is limited. Plastic cannot withstand the engine temperature for too long. The use of modern fibre-reinforced or carbon-reinforced plastic materials brings the heat problem under control, yet this is very expensive. Another option is an aluminium bonnet, though this presents another problem: bending and denting easily.
As a bodywork component, the bonnet is exposed to exterior conditions. Thus, hail, accidents and corrosion can damage the bonnet so severely that it has to be repaired or replaced. Dents and slight rust patches can be sanded, filled and painted. In case of accident damage, the bonnet is usually completely replaced. This is relatively easy: the bonnet is connected to the hinges with four to six bolts, which can easily be unscrewed. A bonnet is quite heavy and therefore this work should always be done by two or three people.
Replacing the bonnet
If you want to replace the bonnet, it is very important to look for quality. Many cheap offers do not offer the right fit of the original part. You run the risk of ending up with an awkwardly closing bonnet. The gap clearance on both sides along the wing is even more obvious. Ultimately, even when the bonnet is replaced on occasion of a dent or a rust hole, it will look like a repaired accident car. This brings down its price considerably and can even make it unfit for sale. The price for a high-quality bonnet starts at ca. 150 EUR (± £130). This will get you a fitting spare part which only needs to be painted in the matching colour. Optimally executed, the replacement bonnet looks indistinguishable from the original part.