The dilemma is that the front structure should deform under high impact force. However, this force must be lower than the maximum that the safety cage surrounding the occupants can handle. This means that collision performance is highly dependent on just how strong the safety cage can be made.
In a traditional front structure, incoming impact forces are transferred from the side members down to the sill and the floor. During this transfer of the impact force, large bending moments are created. These bending moments limit the maximum force that the side member can be designed to withstand. Therefore, in order not to crush the back-up structure, the force in the side members has to be kept relatively low.
The energy that the side member can absorb is a function of how long it is and the force level it can handle.
Low force capability gives a long side member – creating an additional challenge when designing a smaller car.
New, patented front architecture
The all-new Volvo S40 and V50 have a new front architecture for which Volvo has the patent.
In the Volvo S40/V50, the forces from the deforming side members are transferred into
a web of longitudinal and lateral members that form the front part of the safety cage.
This web can be compared to an old-fashioned railroad bridge where zigzag members absorb all forces in the optimum way.
With this architecture, we can allow the side members to deform at a higher load.
This means that the same amount of energy can be absorbed with shorter side members. The complete front end can therefore be made shorter.
We have used High-Strength Steel in the body area that absorbs the collision energy.
This grade of steel is optimal for deformation at a very high load.
In the areas surrounding the occupants, where we want very little or no deformation
at all, we have used Extra-High-Strength Steel. This steel grade is very strong but still ductile enough to make sure that the safety cage deforms in a controlled manner.
Each part knows where to go in a crash
The Volvo S40 and V50 are thus designed to absorb incoming collision energy and to take the front-end deformation in a controlled way. The only problem is that we have a five-cylinder engine in the way.
“Normally when you design an engine, you make sure that it fits into the car. When we designed the engine for the new compact cars, we made sure it would be slim enough to fit into the engine compartment even after a collision,” says Ragnar Crona.
The five-cylinder engine is new and it is made extremely slim to allow the biggest possible crumple zone. Therefore, the new engine gives the occupants more of a survival zone compared to previous Volvo engines.
Everything you find under the bonnet of the Volvo S40 has a predetermined position after a frontal impact, piling up in an organised way ahead of the safety cage.
More space inside
We have also increased the space inside the car during impact. In a crash, the
re-engineered steering column will collapse more than in previous models. This allows
the driver to move further forward inside the car during the crash without compromising safety. As a result, the belt and airbag are better able to help limit the forces imposed on the driver, thus bringing his or her body to a “softer” stop.
Compact car safety
Altogether, the patented front structure, the engine bay packaging, the combination of different-grade steel and the re-engineered collapsible steering column give the Volvo S40 and V50 a remarkably good front crumple zone. In fact, in terms of frontal collision protection, this clever engineering puts the new compact cars on a par with the current Volvo S80, recognised worldwide as one of the safest cars on the market. It is not possible to change the laws of physics, but new technologies can certainly be harnessed to make smaller cars even safer.