Why does the back end get so disproportionately dirty?

[david4455]

So I have had my 2014 C-Max for a couple months now.

 

Very happy.

 

We got the pearl white ( only car on the lot). What we have noticed is that the back end seems to get  dirty and shows off the light film of dirt while the rest of the car stays clean relatively clean. I wondered if it has to do with design and the way water might be retained on the surface on the back coming from the angled roof compared to the rest of the car.... or perhaps poorly designed mudflaps coming from the ground.

 

I've never noticed this before on any other car I have owned.

 

 

[jmckinley]

Good question.  I can't believe how quickly it gets dirty back there.  Just be thankful you have a white one.  I would guess that the darker colors bad as soon as you drive it out of the driveway.

[S70Pilot]

Probably the design.  The relatively straight back end, while travelling, creates a vacuum and this sucks in the dust and dirt. 

[ptjones]

So I have had my 2014 C-Max for a couple months now.

 

Very happy.

 

We got the pearl white ( only car on the lot). What we have noticed is that the back end seems to get  dirty and shows off the light film of dirt while the rest of the car stays clean relatively clean. I wondered if it has to do with design and the way water might be retained on the surface on the back coming from the angled roof compared to the rest of the car.... or perhaps poorly designed mudflaps coming from the ground.

 

I've never noticed this before on any other car I have owned.

There is very little air flow down the back of the car so when dirt lands on it doesn't get blown off. It is kind of a vacuum, I have GasPods, but I don't think it makes much difference. :)

 

Paul

[Adrian_L]

It's unbelievable on the Tuxedo black model--the top of the bumper needs a wipe at least every week. 

[fbov]

There is very little air flow down the back of the car ...

And that's a very good thing in a low drag vehicle.

 

Air flow reduces air pressure. It's how airfoils work to produce lift. Air flows faster over the top of the wing, so it exerts less pressure than the air flowing under the wing. The excess atmospheric pressure under the wing is "lift." A high air velocity on the rear window and tailgate would create a pressure reduction that opposed the direction of motion. That's drag. But there's more.

 

I did a tuft test to see how that all works on a C-Max. Above the wheel wells, flow on the side and roof is very smooth, what we call "laminar" or "attached." Coming off the roofline are twin vortices you'll see in a lot of diagrams that result in a general flow down the rear window, which is slow and attached in the C-max. Tufts placed on the glass didn't wiggle much (slow flow), and they stayed mostly straight (attached flow). I was impressed that at the window's side edges, high velocity air from the C-pillar didn't affect tufts at the edge of the glass, inches away. A very clean break.

 

Below the rear glass is a completely different story. Air coming off the sides, around the tail lights, is still attached, but doesn't detach cleanly and you get turbulence. At wheel well level, there's no longer attached flow coming off the sides, so the situation is worse. Tufts below the rear glass are chaotic; you can't see them because the move fast in varying directions. This turbulence is due ot the hole the car makes in the air, and the fact that a truncated airfoil doesn't fill the hole smoothly. I get a lot of dirt at the lower lip of the tailgate, too.

 

Bottomline is this is the price we pay for a low drag vehicle. It could be fixed, but at the cost of increased drag and higher fuel consumption. Check out other aeor cars, like Prii and Volt, especially in winter when salt patterns exacerbate the effect. You'll start to appreicate that Ford did there homework with the C-Max!

 

Have fun,

Frank