Do rain gutters increase drag?

**ronallandottk** · December 1st, 2009 09:02 PM

You know what I'm referring to, those plastic deflectors mounted on the tops of our vehicles' side windows.

In other markets, they call them rain guards, vent visors, or window deflectors. Here, I think we typically call them rain gutters.

We all know that increasing wind resistance and drag can significantly increase fuel consumption, especially when traveling at highway speeds.

My question is, do rain gutters increase drag?

I tried researching this but I came up with conflicting results. It seems no one has bothered to measure the aerodynamic effects of rain gutters.

http://www.car-stuff.com/acuraventvisor.htm

Vent visors are basically considered as a type of spoiler, working to improve the aerodynamics of the automobile for higher acceleration and easily reaching top speed. Their shape reduces wind drag, aiding then to achieving better fuel mileage.

http://www.expeditenow.com/news/publ...s_AirTabs.html

To reduce air resistance and drag, the first thing you need to do is eliminate anything you can which causes resistance in the first place. Side mirrors and the brackets cause quite a bit of resistance, but you don’t want to remove those. An exterior sun visor is a major factor in wind resistance, and many people have removed it for that reason. I installed a bug shield and side window rain guards on my Sprinter and noticed an immediate hit on fuel mileage, which didn’t come as a surprise. The surprise was just how big a hit it was – more than half a mile per gallon. The Sprinter’s front end is pretty streamlined, so it didn’t take much to mess it up.

What do you guys think? Rain gutters look cool and all, but if they ruin a car's aerodynamics, then they may not be worth it.

**1D4LV** · December 1st, 2009 09:39 PM

Originally Posted by ronallandottk

You know what I'm referring to, those plastic deflectors mounted on the tops of our vehicles' side windows.

In other markets, they call them rain guards, vent visors, or window deflectors. Here, I think we typically call them rain gutters.

We all know that increasing wind resistance and drag can significantly increase fuel consumption, especially when traveling at highway speeds.

My question is, do rain gutters increase drag?

I tried researching this but I came up with conflicting results. It seems no one has bothered to measure the aerodynamic effects of rain gutters.

http://www.car-stuff.com/acuraventvisor.htm

Vent visors are basically considered as a type of spoiler, working to improve the aerodynamics of the automobile for higher acceleration and easily reaching top speed. Their shape reduces wind drag, aiding then to achieving better fuel mileage.

http://www.expeditenow.com/news/publ...s_AirTabs.html

To reduce air resistance and drag, the first thing you need to do is eliminate anything you can which causes resistance in the first place. Side mirrors and the brackets cause quite a bit of resistance, but you don’t want to remove those. An exterior sun visor is a major factor in wind resistance, and many people have removed it for that reason. I installed a bug shield and side window rain guards on my Sprinter and noticed an immediate hit on fuel mileage, which didn’t come as a surprise. The surprise was just how big a hit it was – more than half a mile per gallon. The Sprinter’s front end is pretty streamlined, so it didn’t take much to mess it up.

What do you guys think? Rain gutters look cool and all, but if they ruin a car's aerodynamics, then they may not be worth it.

of course they ruin a car's aerodynamics, for if they don't they would come as stock.

imho, the drag that they may cause could be insignificant for everyday driving. but could be significant if you are on a track, and microseconds really mean a lot.

which reminds me... i have to look for rain gutters for my B14....

**vernieg** · December 2nd, 2009 07:18 AM

At speed beyond 40kph, you'll start hearing wind resistance and if you travel at an average of 60kph most of the time, significant na ang effect ng drag.
If these deflectors have similar design to vortex generators at its rear part, this will help lessen the air drag but I don't see one on the market. You have to make your own design to minimize air resistance.

**alwayz_yummy** · December 2nd, 2009 07:57 AM

question is: do you race everyday? if so...forget that window visor....

gusto ko ung most OEM design kasi di masyado malaki ang buka unlike those mga aftermarket brands na malaki ang opening...pero ang OEM kasi ay 2x the price ng aftermarket ones...

**CVT** · December 2nd, 2009 08:26 AM

OT: Never used one (although freebie ito ng 2 car purchases namin recently,- just replaced it with another freebie)... Palagi naman kasing nakasara ang car windows namin (at naka aircon).....

IMO,- negligible ang drag attributable to this, sa tipo ng city driving natin dito.

9000:snowfight1:

**ericp** · December 2nd, 2009 08:38 AM

Drag... a little bit...at high speeds.

But I like them because I can park my car with windows
partly open....cools the cabin especially when parked under the sun.

**cktlcmd** · December 2nd, 2009 09:07 AM

To be honest...I only like rain guards for only one reason...so I can leave a small opening on all 4 windows when parked under the sun. I don't think there's any other use for them aside aesthetic appearances.

Ohhh....YES they do produce drag...but I would still use it for the reason above.

**oj88** · December 2nd, 2009 09:08 AM

OEM visors have negligible impact on drag. In fact, Mugen (a Honda aftermarket parts manufacturer) makes them to provide aerodynamics and cabin ventilation for the Civic, including the Type-R.

This is what I have... also OEM.

**Memphis Raines** · December 2nd, 2009 09:55 AM

For me it depends on the Size and the Design.

On our Pajero FM before I installed a rather large one the occupied nearly 3/4 of the front door windows. Maingay at 60kph up and you can feel that a vortex is building up on the inside part that's responsible for strong vibration.

On my pick-up I installed a slim type that had good curves which helped keep the vortex and vibration down but did very little to prevent rain from coming in at 1/4 window opening.

At night medyo sagabal din sa vision particularly the dark colored type.

I don't intend to install any on my current truck.

**ronallandottk** · December 2nd, 2009 12:15 PM

Originally Posted by alwayz_yummy

question is: do you race everyday? if so...forget that window visor....

gusto ko ung most OEM design kasi di masyado malaki ang buka unlike those mga aftermarket brands na malaki ang opening...pero ang OEM kasi ay 2x the price ng aftermarket ones...

No, I don't really race...the question was more aimed at reducing drag to increase fuel economy, particularly at highway speeds (80+ Kph).

The reason this questioned popped into my head is because I recently took my week old 1.2 i10 on a 150 Km trip to break-in the engine (as well as other stuff). I was pleasantly surprised because it was pretty stable and wind noise was quite low despite 120+ Kph speeds, even though this is supposed to be a city car. I've driven a B12 and B13 Sentra at about the same speed before, and they were well planted on the road as well, though wind noise was quite higher (especially with the B12) with whistling sounds, especially around the side view mirrors and the radio antenna. I even remember an even older Ford Laser hatchback not only created a lot of wind noise, but also tended to weave left and right at high speed.

It turns out the i10 has a respectable coefficient of drag, 0.325, and the roof mounted OEM spoiler seemingly helped in reducing turbulence coming off the top of the hatch at high speed.

It was fitted with OEM rain gutters as well (casa freebie), which were pretty slim, not that much different from the pictured OEM gutters on a Civic.

Incidentally, the up and coming 2010 Mazda 3 supposedly has a .29 Cd. It would be a shame to ruin the aerodynamics of a slippery shape all because of an ill-designed rain gutter.

So I guess the consensus is that rain gutters either increase drag minimally, or even actually improving aerodynamics mildly. So along with the other benefits such as allowing you to open the windows slightly at speed or while parked in the sun, rain gutters should be reasonably useful.