You are half right. When a car is designed to be open topped from the word go it's typically lighter than one designed as a coupe and later modified. However, the roof does wonders for torsional stiffness. For a given stiffness the open top car will be heavier even if it started as an open top car.

 

I'm not seeing half of what I said as being wrong!

 

"Cars like the s2000, Miata, Boxster, don't suffer a weight penalty for being convertibles because that's what they were designed for from the get-go."
Yes, you can make a very light convertible but when dealing with a road car and when you want a given level of torsional stiffness the roof is critical. If you put a structural roof on the Miata it would gain weight and a significant increase in chassis stiffness. If you were to design a similar car using the same suspension and driveline bits as a coupe it should be either just as light but stiffer or just as stiff but lighter. Any way you slice it, for a given level of stiffness a roof makes things better.

The 2 different Corvette chassis might server as an example. The base car has a steel chassis that was designed for a roofless car. This makes sense since all C6s with the steel chassis are either convertibles or targas (ie they don't have a structural roof). The aluminum chassis, while very similar to the steel chassis was designed to take advantage of a structural roof panel. GM knew that without the roof the alloy chassis wasn't as stiff as the steel chassis. By adding the bolt in roof panel frame the net result is a chassis that is stiffer and lighter. Certainly if GM decided to bolt in the roof panel on the steel car they could reduce the stiffness of some of the other parts to more than make up the extra weight even without going to the extreme of the alloy chassis.

In the end, for a given weight a closed box is stronger.

 

Believe me, I'm a professional structural engineer, I FULLY understand the ramifications of an open roof vs. closed. But in the end, given the design constraints of either open or closed roof road car from the start, and given some reasonable leeway to incorporate stiffnening features in the chassis of the convertible (e.g., S2000 X-bone design), there isn't really much *if any* weight penalty associated with a convertible road car vs. closed coupe. I'll stand by that remark...

 

While I agree that designing it in from the start helps, you just aren't going to do it as well as a car with a roof. Going back to the box, I can add extra structure to the bottom of my box to make it stronger but the roof is simply a more mass efficient way to close things up. Yes, you can do things like add the massive Corvette style central backbone to make up for some of this. The down side there is a huge loss in interior space. I guess that would qualify under the leeway claim. Regardless, all other constraints equal (and they almost never are) a roof is a lighter way to close up the interior for strength.

 

Put it this way: If I were designing a ground-up serious sports car with a fixed/given powerplant and power/weight target (hence weight target), wheelbase, track, etc., aimed at being trackable out of the box, I'd be setting the same stiffness targets whether fixed-roof or convertible.

Other aspects of the design would likely be impacted, i.e. convertible might have less interior space like you suggested, and/or deeper rocker structures making ingress/egress more difficult.

I have always been a bigger fan of fixed-roof coupes partly for the structural consideration, but having been through multiple (2-wheel) vehicle designs with differing configurations, you just find other solutions for achieving the weight and structural requirements.

A coupe and a convertible of the same size/weight/stiffness will evolve differently structurally, for sure.

Also worth considering that the roof, while being a killer shear panel, is at a huge offset from the firewall and aft structure. To take advantage of the roof, you have to add stiffness (weight) to its perimeter and to the pillars supporting it. It's not as structurally efficient as, say, adding a lid to a shoebox!

 

While I agree that designing it in from the start helps, you just aren't going to do it as well as a car with a roof. Going back to the box, I can add extra structure to the bottom of my box to make it stronger but the roof is simply a more mass efficient way to close things up. Yes, you can do things like add the massive Corvette style central backbone to make up for some of this. The down side there is a huge loss in interior space. I guess that would qualify under the leeway claim. Regardless, all other constraints equal (and they almost never are) a roof is a lighter way to close up the interior for strength.
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true for torsion stresses. car with roof = more of a circular structure = hollow shaft "effect" = more efficient

i call bs on the SE license. pics or it didn't happen

 

Car with a roof mounted way up on pillars is nothing like as structurally efficient as a tube. Again, it's not the same as putting a lid on a 5-sided box. A roof on top of four pillars a goodly distance above the firewall and aft structure of the car does of course give you some stiffness, but it isn't analogous to a closed-section tube or closing out an open box.

Like I said above, I'm a big fan of fixed-roof coupes, but you can design in required torsional and bending stiffness without a roof at the same weight, with only minor impact to ergos.

Degrees are on the wall in ATL, I'm in RI. Anyway, I've done structural design and analysis on primary structures on all of the following (and other classified programs):







Not that any of them is perfect, but still,

 

Is that the space shuttle fuel module?

 

Yes, Space Shuttle External Tank, ET for short...