This article was first published in 2005.
These days, most cars are aerodynamically pretty quiet. Whistles and rustles
from the A-pillars and exterior mirrors are subdued – the result of many hours
of development work in aero-acoustic wind tunnels. However, the windscreen
wipers often escape the aerodynamic analysis and so contribute more than their
fair share of wind noise.
When not being used, the best position for wipers is below the
trailing edge of the bonnet – that way, they’re completely shielded from the
airflow and so are quiet and don’t contribute to aero drag. However, taking this
design approach requires a separate ‘parking’ position for the wipers, which
adds cost to the mechanism. The result is that many cars use a design that
leaves their wipers exposed to the airflow all the time.
In some cases, the shape of the plenum area ahead of the windscreen, or the
trailing edge of the bonnet, is used to direct air over the wipers, leaving them
in what’s called a separation bubble. In other words, the airflow unsticks from
the surface of the car just enough to pass over the wipers, re-attaching on the
windscreen a little above the wiper line. The result is much reduced aero noise
from the wipers.
Hmm, but what if your car (a) doesn’t run wipers with a separate
below-the-bonnet park position, and (b) doesn’t appear to do anything fancy to
direct airflow over the wipers? Chances are, you’ll hear whistles and rustles as
the airflow collides with the aerodynamically ugly assemblies. (These noise are
often most apparent on freeways, where the turbulence created by other cars can
cause local wind gusts way above the average airflow speed.)
So are you stuck with the noise? Not at all – it’s easy, cheap and
straightforward to make a small deflector that directs air over the wipers. The
result is a quieter car.
In this case, the car is a ’99 Toyota Prius. Despite having an excellent drag
coefficient of 0.29, little attention seems to have been paid to the wipers.
They don’t park beneath the level of the bonnet and as shown here, if anything,
the bonnet flattens out before the wipers. At speed, aero noise can be clearly
heard from the wipers.
To see what the air was doing around this part of the car, small tufts of
wool were temporarily stuck to the car with masking tape. The car was driven at
60 km/h and these pics taken. As can be clearly seen, there is attached flow up
the bonnet and then onto the windscreen. (Attached flow is shown by the tufts
lying flat along the body and pointing in the one direction – ie not whirling
Looking closer, it can be seen that reattachment occurs very low on the
windscreen – even directly behind the wipers, the airflow is reattached at or
below the black line on the glass. In other words, the full strength of the
airflow is impacting the wiper assembly.
This photo shows very clearly how the wipers are exposed to the air
It’s dead easy to make a prototype deflector out of thin plastic and duct
tape. That’s just what has been done here, with an old Penfolds Wines plastic
sign cut with sharp shears and bent into the required shape. The trial deflector
was left on the car for a week while several hundred kilometres of freeway
driving was undertaken.
Two effects were noticeable: wind noise from the windscreen wipers could no
longer be heard, and the cabin ventilation system tended to breathe hotter air.
The latter point is important: most cars pick up their cabin ventilation air
from directly in from of the windscreen and so any change in the aerodynamic
pressure at this point can affect ventilation.
Building the Real One
The final deflector was constructed from 4mm ABS sheet. This plastic is tough
but can be bent (if heated first, the bend is retained), cut and filed. ABS is
much more resilient than acrylic, for example. The sheet was bought from a
Using tape, the plastic was marked out to the approximate shape.
Putting it temporarily into position allowed the shape to be assessed. It’s
easiest to install if the deflector projects out from under the trailing edge of
the bonnet. Look out for wiper clearance – both with the bonnet closed and
To angle the deflector upwards, it needed to be heated and bent along its
length. This was achieved a small section at a time, using a heat gun and few
pieces of particle board to create a clamp and a lever. It’s tricky to get the
bend even all the way along, but ABS can be re-heated and re-bent as often as
you like. Practice on a scrap bit first!
The deflector is held in place with double-sided tape. As can be seen in this
view, unless you knew what you were looking for, you wouldn’t even see it.
This side view shows the angle of the deflector.
Wool tuft testing was undertaken after the deflector had been fitted. As can
be seen here, the flow reattachment is higher up the windscreen – the area below
the black line is now within the separation bubble. And therefore, so are the
wipers! However, the separation bubble is still very small - which is important
for low drag.
Sure, the wool-tuft pics show the changed airflow behaviour, but what about
the practical outcomes? In short, the deflector works very well. Aero noise from
the wipers is now non-existent, while the final version of the deflector (which
is a bit shorter than the white plastic prototype) doesn’t appear to change the
behaviour of the cabin ventilation system.
After the installation of the deflector, highway fuel consumption has
remained the same – indicative of the change in drag being very small or
It’s no wild mod that will knock 2 seconds off your quarter mile, but as
something that’s easy to trial and not much harder to do, if you have aero noisy
wipers, making a small deflector is worth exploring.