aerodynamic undertray

Discussion in 'Track Prep & Tech' started by RobT, Sep 10, 2009.

  1. Prawn Forum Member

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    Once finished it'll come off for more paint

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    Made a start on the air dam, this is just a test piece to get an idea of how it'll work

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    air dam will be attached to the splitter via tabs:

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    Supports layed out and washers and nuts ready:

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    Supports in the slam panel lower, metal bar behind these spreading load. I can hang my whole body weight off this, so I'm going to consider it strong enough.

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    Side view of the supports from the chassis legs:

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    I checked under the car for areas that would be most exposed to heat, amazingly, it's really not THAT exposed down there thanks to the enormous 6 speed box blocking half of it off :laugh:

    This was all I had, so for now I positioned it in the key area:

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    And it's on!

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    Bumper on:

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    Airdam on, not enough light from my iphone flash now for a proper pic:

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    I also added some skids to the underside to save the plywood from wearing away:

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    Initial impressions:

    I went out, and didn't get stuck leaving / entering my driveway:

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    Straight to somewhere with level ground to check it's sitting level:

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    Drove about 10 miles, roundabouts, country roads, the bumpy old A30, got up to some reasonable speeds, and no audible vibrating or resonating at 1XXmph, and nothing fell off, so I'm very happy so far!

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    Dave R and Dex like this.
  2. mercedeslimos Forum Member

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    That looks epic! how thick is the ply? Interested in doing this on my MK2, how strong is it... I have a PD with a weak ally sump, bad roads too. Would it be beneficial to do the whole underside of the car but in thinner material?
     
  3. Prawn Forum Member

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    Material used here is a 12mm exterior ply with 7 plys. It's pretty solid, totally independant of the bumper and you could probably stand on it if you wanted to, although I'd rather not test it with a point load like that! Total weight for the whole lot is around 8-9kg

    Height at the leading edge is 75mm on my road tyres / R tyres, which comes down to 62mm on slicks. At Castle Combe a few weeks ago this was JUST bottoming out on slicks going up Avon rise, but only on the really hot laps (Mvax 140mph+ before braking)

    Further work to the underside will follow over winter for sure, the difference this has made has been most noticeable indeed, with the car feeling far more stable and planted at high speeds, and the turn in above ~65 mph is very different also
     
  4. DEX

    Dex Paid Member Paid Member

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    Great "how to" prawn. I'm feeling all excited about splitters and airdams having just read 31 pages on MiataTurbo forum of their "DIY Aero" thread - some incredible stuff being done (including a load of CFD models of different setups)
     
  5. mec82 Forum Member

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    Nice one, that looks really good. :thumbup: Should really get around to improving mine!

    Regarding the angle, I know it seems counter intuitive but if anything it probably wants to be slightly 'nose up'. This is because you want the airflow to attach smoothly to the surface and be accelerated under the splitter to give a low pressure region. If it was nose down the air would have to try and turn the corner and would separate from the splitter, causing turbulence. A thick splitter with a nice big radius would be best to gently turn the airflow.

    Also the air arriving at the splitter won't be in a horizontal direction, it will be travelling in a slight downward direction.

    Hope that's useful, check out this Bentley GT3 for inspiration, note the upward curve at the leading edge:

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    Last edited: Oct 31, 2013
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  6. Prawn Forum Member

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    Interesting stuff there, cheers Mec.

    it's currently set dead level to within 0.1 degrees on my spirit level app, but thanks to the way the air dam passes over the front of the bumper and the threaded hangers it can be easily adjusted in a short time.

    I plan to remove it over winter and paint it with something more hard wearing, add more wear strips, and radius the leading edge too, so will set it fractionally higher when I refit.

    Cheers :thumb:
     
  7. Peter Jones Forum Member

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    Seeing as we're sharing.

    I was having big issues with front end lift at speeds over 200km/h.

    Any cross winds would nearly push me off the track.

    I added this splitter and it seems to have worked as intended. Top speed is higher and the car is more planted.

    Construction is 6mm marine ply coated with fibreglass resin.

    The spoiler and splitter have to be detachable for transport.

    The rear of the splitter/undertray hooks into the lower suspension brace using the aluminium bracketry shown below.

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    I uses T Nuts set into the plywood for all the connections, these were setup before applying the resin so the nuts are well and truly glued in place.

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    Being only 6mm I was worried about stiffness so I split some PVC conduit lengthways and fibreglassed them to the top of the tray.

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    The diagonal stays are overkill and of dubious benefit. These ones are leftovers from some balustrading I installed at home.

    The splitter / spoiler assembly is attached to the car with 6 M8 bolts for quick installation and removal.

    I'll be making another spare assembly when time permits.

    The upper edges of the splitter were sanded and spray puttied to hide the wood grain and present a gloss finish.

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  8. RobT

    RobT Forum Junkie

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    Ok, so a bit of an update - front splitter got a bit of a beating at Aintree - not too bad but enough for it to need a slight modification. Passenger side had just been touching the track (at 70mm high static), mainly on the fast and bumpy Becher's bend, and dome head bolts had been ground nearly off (leaving well cool vaporised metal trails....):

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    So decided to replace a lot of the bolts with bonded-in fasteners, and then cover the whole lot with abrasion-resistant kevlar. Looked about where to get kevlar from and found a great place in Stoke called EasyComposites. Very helpful, well into motorsport, and they suggested 2 layers of 175g kevlar stuck on with epoxy. Have tried to cut kevlar in the past and it is VERY hard to cut, so bit the bullet and bought some special kevlar scissors.

    So glued the bigheads in:

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    Cut two layers of kevlar with my new kevlar shears (this would be absolutely impossible to do without them as the cloth would go out of shape if you put much force on it)

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    Then, glue and stippling later, we have a kevlar laminated splitter that should last a bit longer. Next event will test it out....

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    Cheers

    Rob
     
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  9. StuMc

    StuMc Moderator and Regional Host - Manchester Moderator

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    Those vapour trails are pretty cool! :thumbup:
     
  10. sparrow Paid Member Paid Member

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    Just spotted this in your sig! Tell us more.
     
  11. RobT

    RobT Forum Junkie

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    Not really OT but have been looking at some 4wd machines since autumn - couple of Fabia rallycross cars, a Leon, and an A3. I fancy a Fabia, expensive but they are a right laugh but would be in sports libra in sprinting, and uncompetitive. But a right laugh. The Leon and the A3 would be modprod cars in the capacity bracket above me now. Really the tool to have would be a modified Evo or Scooby, or even a Skyline or a 911 turbo, but I like vags. Unrestricted rallycrosser would be awesome......

    Not this specific one, but this type, a mk1 fabia. They are about 500hp in restricted form and have very strong, a bit agricultural actually, transmission and running gear. Should be very reliable for tarmac sprinting/hillclimbing as its built for mixed dirt racing.

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    Last edited: May 5, 2014
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