Michael Fernie profile picture Michael Fernie 5 years ago 75
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Do Sunroofs Actually Compromise Rigidity?

Many avoid buying cars with sunroofs due to fears over reduced rigidity. But do they make any difference, and do they compromise safety?

Remind me later
Do Sunroofs Actually Compromise Rigidity? - Blog

A sunroof can completely change the environment within a car, giving you some of the benefits of a drop-top without the drawbacks. But some car lovers gloss over a sunroof on a spec sheet - or discount a used sun-roofed motor - as they believe that a car’s rigidity is affected due to the gaping hole cut in the roof.

Who would want to spend hours building their perfect racecar for it to be compromised by a lack of stiffness? With another aspect being driver safety, is a sunroof actually something to worry about?

Do Sunroofs Actually Compromise Rigidity? - Blog

The most obvious scenario associated with sunroof safety would be if a car is rolled. But, there’s no need to worry. Car companies are scrutinised heavily by automotive governing bodies to make sure their products can cope with extreme crash situations. One of the procedures is a roof-crushing test, with a car only passing if it can withstand four times its own weight before deforming no more than five inches.

Cars these days are specifically designed to absorb huge amounts of energy, spreading it throughout the body shell to reinforced areas, especially the A, B, C and D pillars. These pillars are made from ultra high-strength steel and do a fantastic job of absorbing any forces being applied on the roof.

This diagram is a fantastic representation of a car's structure, with the red areas of this Volvo showing the locations of the highest strength steel along with the yellow cross members for added rigidity
This diagram is a fantastic representation of a car's structure, with the red areas of this Volvo showing the locations of the highest strength steel along with the yellow cross members for added rigidity

Smaller lateral strengthening beams also span the car’s roof for added structural integrity but have to deal with very little force due to the dissipation to the main pillars. It is between these beams that a sunroof sits, essentially just replacing what would be sheet metal with a pane of retractable glass.

With this in mind, a sunroof has no tangible effect on the structural rigidity or strength of a car, even during a rollover. The pillars and beams absorb almost all of the forces induced during a roll. This also means that even in the hardest of cornering manoeuvres, the stiffness of a car is not affected by a sunroof in the slightest; no tangible amount of flex will occur in the body to warrant not having one.

Or just be this guy, he seems to have things covered...
Or just be this guy, he seems to have things covered...

Now, one could argue that glass is less strong than a metal like aluminium, which would be true. A material’s overall strength boils down to its Young’s Modulus; a unit that describes a material’s brittleness when deforming.

The Young's Modulus is found by comparing the stress and strain that the material can cope with before breaking. If you can understand this graph, take the day off, you deserve it
The Young's Modulus is found by comparing the stress and strain that the material can cope with before breaking. If you can understand this graph, take the day off, you deserve it

Glass has a small Young’s Modulus of 65 GPa, while a metal like steel is much stronger, boasting a Modulus of 200 GPa. This means that a glass section absorbs less energy before it fractures compared to a metal section.

Do Sunroofs Actually Compromise Rigidity? - Blog

So if a car were to roll and you were unlucky enough to land the roof on top of a rock or bollard (right between the support beams), then in that instance a glass sunroof would become dangerous. However, if you’re travelling at a speed that is enough to roll your car, a bollard through the roof is probably going to have you for dinner, be it puncturing glass or metal.

In conclusion, don’t be afraid of speccing your new 911 or F-Type with a panoramic sunroof. Let the sun bathe your interior in a stream of light and experience a less compromised alternative to a full convertible.

Just don’t land roof-first onto a bollard.