There's something about driving with the wind flowing over
you that makes you feel more in touch with the road and
landscape. A convertible car captures this open feeling, while
maintaining most of the practicality of conventional hard-top
cars by using roofs that are easily raised and lowered.
In this edition of HowStuffWorks,
we'll see how the convertible top works and check out a really
neat retractable hardtop. But first, let's take a look at some
of the challenges faced by designers of convertible cars.
Cutting off the Roof Designing a convertible
car is not as simple as just cutting off the roof and
installing a folding cloth top. There are other issues to
Structure The roof of a
conventional car is essential to the stiffness of the chassis.
The roof helps keep the car from twisting and bending.
Building a car without a roof is a bit like building a
suspension bridge without the cables. As such, the bottom part
of the structure of a convertible has to be stiffened
considerably. Heavy reinforcing brackets have to be added to
the body of the car. This is why convertibles often weigh more
than their fixed-roof counterparts.
If you were to cut the roof off of your car, you'd find
that the body would twist a lot, especially if you were to do
something like, say, drive diagonally over speed bumps.
Eventually, the car would develop all sorts of squeaks and
rattles and exhibit some poor handling characteristics. It's
important for convertibles to have an especially sound
structure to make up for the lack of a roof.
convertibles with their top down are much less aerodynamic
than similar cars with permanent roofs. A long, flowing roof
smoothes the airflow over the car, resulting in less drag.
However, careful attention to detail can result in a
convertible that is almost as aerodynamic with the top down as
with it up.
One of these details is the small, glass shield located
behind the headrests. Have you ever seen people driving a
convertible with their hair blowing forward? This happens
because the fast-moving air coming off the top of the
windshield encounters the slow-moving air inside the cabin.
Some of the fast-moving air is decelerated by the slow-moving
air. As it slows down, it becomes turbulent, and vortexes
form. These vortexes of turbulent air are like little
They spin in such a way as to blow air forward into the
cockpit. This can be uncomfortable for the occupants, and can
increase the aerodynamic drag. The glass shield behind the
headrests blocks this air, making the cabin quieter and more
comfortable, as well as improving the aerodynamics.
The Lexus SC430 is almost as aerodynamic with the top down
as it is with the top up. We'll take a closer look at this car
later. First, let's see how a conventional cloth-top
Soft Top The roof on the Honda S2000
roadster is fairly typical of convertible roofs. It is power
operated, but requires the driver to manually latch and
unlatch it from the windshield.
One switch on the dashboard operates the roof. To lower the
roof, you release the latches and then press and hold the
switch in the open position. If the windows are up, they will
roll down before the roof starts to move. The roof will then
fold itself back into a compartment behind the seats.
A motor that
turns a gear on each
side of the car powers the mechanism that raises and lowers
the roof. The gear engages a bracket that has gear teeth cut
into it (much like the mechanism used in power
windows). This bracket is connected to the main structure
of the roof. As the gear turns, it moves the roof into
Part of the Honda S2000's roof
The motion of the roof and the positioning of its different
parts is completely determined by the geometry of the roof
structure. A scissors-like linkage is formed by a set of metal
arms and brackets that are linked together by pins. The
linkage folds down into itself when the roof is open, and
expands to form the structure of the roof when the roof is
The roof on the Honda S2000 is soft, and the rear window is
made of clear, flexible plastic. On the S2000's roof, the
window actually folds to make the roof more compact. Some
larger convertibles have glass rear windows that can't fold.
On some convertibles, the rear window even has a defroster.
In the next section, we'll take a look at a different kind
of convertible roof -- a retractable hardtop.
Retractable Hardtop The Lexus SC430 is a
totally different beast. This car is a hardtop
convertible. As a hardtop, the car makes no compromises:
The ride is quiet and the structure is stiff. With a push of a
button it becomes a convertible. You don't even have to undo
The roof on the SC430 folds in half and stows in the trunk.
When you press the button to lower the roof, the windows roll
down and the trunk opens. It opens the opposite way, with the
front of the trunk (nearest the front of the car) lifting up.
After the trunk is open, the roof starts to open, folding in
half as it heads for the trunk. The roof folds over until it
is fully inside the trunk, at which point the trunk closes.
The cool thing about the trunk is that it can still open just
like a regular car trunk -- although with the roof down, there
isn't much room in there.
Since this roof is fully automatic, there are a whole lot
actuators and sensors required to make it work. The motors
that power the roof work in a similar way to those in the
Honda S2000. Since the Lexus also has to open the trunk and
latch and unlatch the roof, it has a couple of additional
motors and actuators to perform those tasks. The folding of
the roof is taken care of by a much simpler linkage than in
the S2000, though, since the SC430 roof only folds in half.
Having one convertible is (almost) as good as having two
cars in your garage. With the top up, a convertible can be as
practical as a coupe. But with the touch of a button (and
perhaps a few un-latchings), your ride to work can be a whole
lot more fun.
For more information on convertibles and related topics,
check out the links on the next page.