Automotive engineering is a perpetual tug-of-war between performance and safety. Cars need to crumple like accordions in crashes, yet remain supremely stiff and strong while cornering. They need to be light enough to accelerate like rockets yet maintain enough structural mass to protect occupants during impacts from any direction. They need to channel collision forces around passengers yet still allow them to feel at one with the machine on the road. Balancing these needs is an immense challenge but, when successful, an enormously satisfying engineering triumph.
Then there are pedestrian accidents—the most infuriatingly unbalanced of all, and a realm carmakers have been paying more and more attention to in their designs. Cars can now detect pedestrians and automatically brake to avoid striking them, or even detonate small explosive charges in the rear of the hood to lift it into the air by just a few inches prior to an impact, giving a better cushion for pedestrians to land on. But the tug of war between performance and safety continues at intensely granular levels to make pedestrian accidents both less deadly, and even less injurious.
During a visit to Honda’s R&D lab in Ohio recently—during which I also previewed a radical new passenger airbag design and observed a variety of crash tests—I spoke with Adam Mihm, a senior engineer in the company’s pedestrian protection effort, about how the company balances pedestrian protection with the obvious need for vehicle strength and durability. Below, his thoughts on threading this particular needle.
EA: I imagine pedestrian protection engineers want to build their cars out of Nerf. But you can’t do that, so where do you start?
AM: There's an extreme amount of design work and iteration and iterative processing going on using the simulation technologies we have. If you take the hood as an example, I'm trying to make it very soft, so that it bends a certain way in a frontal crash. But in high speed driving, you don’t want the hood bouncing around in the wind because it’s too soft, thus distracting the driver. Also, from a general marketability standpoint, you can’t make it so soft that you dent it while washing your car. So we simulate all the areas of movement and stiffness before we actually make the part, and then test all those areas using our sensor-equipped crash dummies.
EA: Is there a trick to balancing those needs, or a new tuning of materials where, say, the softness of a hood will vary at different points across its surface.
AM: We can separate those modes—strength and flexibility—in our design work, and yes, we’re already applying different levels of stiffness in critical areas. For example, we talked about the high speed driving mode. So obviously, the components need some lateral stiffness so they don’t act like an airplane wing and start lifting at high speeds. But we can increase the stiffness of the frame components on the inside to try to shift the load around and prevent that kind of lifting scenario. In that case we’d also be able to have additional space under the hood, which I'm protecting so I can provide energy absorption before the impact forces reach a different, stiffer structure.
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EA: Do you also factor in the ways materials break and fracture, potentially causing even worse injuries? Obviously windows are designed to not break into shards any more—what about the rest of the car?
AM: One example of that might a bolt on the underside of the hood. When you collapse that surface that bolt might start pushing on the hood material. If it were to get to the point where it starts to push through the material, we’d investigate applying a shorter bolt or expanding the clearance space around it. We do take those things into account. It's not regulated or anything, but it's something that we consider.
EA: Are there any optimal configurations for different components when it comes to pedestrian safety?
AM: Yes, take the bumper system. We spoke earlier today about leg injuries. If we can package more energy absorbing space in front of the structures, in the bumper beam, that protects the occupants in high speed and low speed collisions. But we also want the bumper to be lower to protect pedestrians. It's better to have that surface low so you don’t have a situation where the pedestrian goes underneath the car. That's one of the challenges with SUV developers, where they have to balance that against ground clearance. So that is a big challenge for us, as well.
EA: Are there any places on a car that are better than others to impact in a collision?
AM: There are a lot of considerations, of course, but it really depends on where your head might contact a vehicle. If you were to be struck by an SUV, because the vehicle is higher your head is more likely to land on the hood area, but in a smaller car you’re more likely to impact the windshield. It's still very much case-by-case, but it's a little bit of a misconception that if you hit glass, that's likely to be extremely severe. In reality, the center of the glass area on the windshield—because there's nothing supporting it right behind it—is actually a reasonable area to impact, all things considered. You’ll see in our data that you’ll have high initial g-forces but it’s for a very short duration because once the glass goes, it’s essentially a cushion with a significant amount of space and material, including a membrane that keeps the glass together.