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  • FIRST POST
    • craig1123
    • By craig1123 17th Mar 17, 8:20 AM
    • 134Posts
    • 56Thanks
    craig1123
    Why are the rear ends of cars always the dirtiest part?
    • #1
    • 17th Mar 17, 8:20 AM
    Why are the rear ends of cars always the dirtiest part? 17th Mar 17 at 8:20 AM
    The back of my car is always filthy yet the front is relatively clean. Why is this?
Page 1
    • peter_the_piper
    • By peter_the_piper 17th Mar 17, 8:26 AM
    • 25,555 Posts
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    peter_the_piper
    • #2
    • 17th Mar 17, 8:26 AM
    • #2
    • 17th Mar 17, 8:26 AM
    Because the movement of the car through the air lowers the air pressure at the rear end. Effectively the car sucks the dirt onto the rear.
    Millies next back in July.
    • AdrianC
    • By AdrianC 17th Mar 17, 8:38 AM
    • 13,937 Posts
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    AdrianC
    • #3
    • 17th Mar 17, 8:38 AM
    • #3
    • 17th Mar 17, 8:38 AM
    Basic aerodynamics.


    Some manufacturers pay more attention to the airflow around the rear than others, and so some cars keep the rear cleaner than others.
    • DoaM
    • By DoaM 17th Mar 17, 9:17 AM
    • 2,221 Posts
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    DoaM
    • #4
    • 17th Mar 17, 9:17 AM
    • #4
    • 17th Mar 17, 9:17 AM
    Hatchbacks, people carriers, vans etc. will tend to create lower pressure at the rear due to their shapes (less streamlined), and so "suck" dirty air onto the rear of the vehicle. The faster you go, the lower the effective pressure so the greater the issue.

    I notice this in my Golf when it's wet and I'm on the motorway - the rear wiper has to be used a lot to keep the rear screen clear.

    Obviously the dirt is coming from wet, grimy road surfaces.
    Diary of a madman
    Walk the line again today
    Entries of confusion
    Dear diary, I'm here to stay
    • Strider590
    • By Strider590 17th Mar 17, 9:32 AM
    • 11,063 Posts
    • 6,152 Thanks
    Strider590
    • #5
    • 17th Mar 17, 9:32 AM
    • #5
    • 17th Mar 17, 9:32 AM
    Mostly as above, but exhaust plays a part too, diesels get dirtier faster. The above effect traps exhaust gases in that vacuum behind the vehicle.

    This is why on 4x4 with opening rear windows (Vitara and Freelander spring to mind), the manufacturer will normally state not to drive with only that rear window open, as exhaust gases could be pulled into the car. Opening a side window + the rear has the effect of changing the aerodynamics, removing the vacuum effect by pulling air through the car.
    Last edited by Strider590; 17-03-2017 at 9:34 AM.
    Having the last word isn't the same as being right.......

    "Never confuse education with intelligence"
    • craig1123
    • By craig1123 17th Mar 17, 10:19 AM
    • 134 Posts
    • 56 Thanks
    craig1123
    • #6
    • 17th Mar 17, 10:19 AM
    • #6
    • 17th Mar 17, 10:19 AM
    Thanks everyone
    • Sleazy
    • By Sleazy 17th Mar 17, 10:22 AM
    • 4,908 Posts
    • 3,871 Thanks
    Sleazy
    • #7
    • 17th Mar 17, 10:22 AM
    • #7
    • 17th Mar 17, 10:22 AM
    My rear end is also my dirtiest part
    Signed Sleazy
    Otherwise known as the Lounge Lizard
    • mrmot
    • By mrmot 17th Mar 17, 12:15 PM
    • 180 Posts
    • 121 Thanks
    mrmot
    • #8
    • 17th Mar 17, 12:15 PM
    • #8
    • 17th Mar 17, 12:15 PM
    My Sierra hatch doesn't get anything on the rear window if you keep it moving, the subtleties of vehicle aerodynamics.
    • Strider590
    • By Strider590 17th Mar 17, 12:46 PM
    • 11,063 Posts
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    Strider590
    • #9
    • 17th Mar 17, 12:46 PM
    • #9
    • 17th Mar 17, 12:46 PM
    My Sierra hatch doesn't get anything on the rear window if you keep it moving, the subtleties of vehicle aerodynamics.
    Originally posted by mrmot
    That's because it's a bloody jelly mould, nothing, not even dust would be seen dead on it
    Having the last word isn't the same as being right.......

    "Never confuse education with intelligence"
    • adonis
    • By adonis 17th Mar 17, 12:48 PM
    • 647 Posts
    • 555 Thanks
    adonis
    Mostly as above, but exhaust plays a part too, diesels get dirtier faster. The above effect traps exhaust gases in that vacuum behind the vehicle.

    This is why on 4x4 with opening rear windows (Vitara and Freelander spring to mind), the manufacturer will normally state not to drive with only that rear window open, as exhaust gases could be pulled into the car. Opening a side window + the rear has the effect of changing the aerodynamics, removing the vacuum effect by pulling air through the car.
    Originally posted by Strider590
    And you shouldn't drive with the rear boot or hatch open to carry long stuff for the same reason and it usually says so in the car handbook
    • Jonners85
    • By Jonners85 17th Mar 17, 1:05 PM
    • 109 Posts
    • 8 Thanks
    Jonners85
    Is it a soot chucker?
    • Strider590
    • By Strider590 17th Mar 17, 1:08 PM
    • 11,063 Posts
    • 6,152 Thanks
    Strider590
    And you shouldn't drive with the rear boot or hatch open to carry long stuff for the same reason and it usually says so in the car handbook
    Originally posted by adonis

    Indeed....

    Always keep the rear access closed in order to avoid toxic fumes and offensive odours
    Having the last word isn't the same as being right.......

    "Never confuse education with intelligence"
    • ElefantEd
    • By ElefantEd 17th Mar 17, 4:33 PM
    • 512 Posts
    • 937 Thanks
    ElefantEd
    I always drive in reverse so the rear end is always clean.
    • stator
    • By stator 17th Mar 17, 4:53 PM
    • 5,521 Posts
    • 3,508 Thanks
    stator
    My rear end is also my dirtiest part
    Originally posted by Sleazy
    no skid marks I hope
    Changing the world, one sarcastic comment at a time.
    • Gloomendoom
    • By Gloomendoom 17th Mar 17, 5:30 PM
    • 12,156 Posts
    • 16,309 Thanks
    Gloomendoom
    And you shouldn't drive with the rear boot or hatch open to carry long stuff for the same reason and it usually says so in the car handbook
    Originally posted by adonis
    All you need to do is equalise the pressure by winding down the front windows.
    Advice; it rhymes with mice. Advise; it rhymes with wise.
    • EssexExile
    • By EssexExile 17th Mar 17, 8:09 PM
    • 2,117 Posts
    • 1,396 Thanks
    EssexExile
    The front wheels are the dirtiest part of my car.
    Tall, dark & handsome. Well two out of three ain't bad.
    • Robisere
    • By Robisere 17th Mar 17, 9:57 PM
    • 1,456 Posts
    • 2,192 Thanks
    Robisere
    Mention of the Sierra reminded me of the Mk. 1 when it was first sold. The "jelly-mould" shape caused airflow problems at the rear and an unsafe "twitch" from the rear of the car around bends. This was investigated and a very simple cure was effected by small plastic strakes to the rear of the small rear side windows. ("C" pillar)

    This broke up the airflow into a vortex at each side and stopped the "twitch". A similar but reverse solution applied to aircraft, with "winglets" at the ends of wings, breaking up a vortex into an upward spiral, instead of creating a vortex.
    There may be more than one way to skin a cat.
    But the result is always inedible.

    • seatbeltnoob
    • By seatbeltnoob 18th Mar 17, 9:24 AM
    • 161 Posts
    • 39 Thanks
    seatbeltnoob
    its the front wheels that disturb the dirt/mud off the floor and kick it up. that is why front allows are always dirtier. the dirt become airborne and spreads out and sticks at the back of the vehicle.

    the bonnet and bumper tend to get clear as the high speed wind hitting it tends to lift off dirt abd debris from it
    • Strider590
    • By Strider590 18th Mar 17, 10:00 AM
    • 11,063 Posts
    • 6,152 Thanks
    Strider590
    A similar but reverse solution applied to aircraft, with "winglets" at the ends of wings, breaking up a vortex into an upward spiral, instead of creating a vortex.
    Originally posted by Robisere
    Those things help to reduce the length of the wing and increase fuel efficiency. I read somewhere that without those the wing would need to be upto another 50% longer.
    Having the last word isn't the same as being right.......

    "Never confuse education with intelligence"
    • iolanthe07
    • By iolanthe07 18th Mar 17, 10:00 AM
    • 4,786 Posts
    • 4,536 Thanks
    iolanthe07
    My MX5 doesn't have a rear window washer, but strangely the rear window never gets dirty, either the hardtop or the ragtop. I guess it is due to the shape of the car.
    I used to think that good grammar is important, but now I know that good wine is importanter.
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