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Thread: Build my own sucker car

  1. #1
    I'm thinking about playing around with a sucker car. I'd like to build a setup and test it out for fun. I realize they're banned from just about every form of racing, but that doesn't mean I can't have fun with one, does it?

    I'm curious what ideas any of you might have as to the design/construction of such a system. It would be applied to a street car (probably a unibody), but obviously not intended for street use. Kindof a build it up and screw around with it over a weekend type of thing.

    For sucking power, I'm thinking either electric or self-fueled gas leafblowers, nothing directly coupled to the car's powertrain. Or possibly an industrial fan/pump/compressor that would better suit the intended use. Cutting large holes in the floorboards for airflow will be acceptable for this project, the test car would likely be a $100 beater that I'd junk afterwards.

    For the skirt, what kind of material would work best? How should it be supported? I'm envisioning a more rigid structure around the car, that has a flexible material attached for contact w/ the ground. I'd be driving the car on asphault, although obviously I'd like to feel the effects of downforce around the turns, so a good deal of body/skirt motion will be needed. But again, this will be a weekend kind of thing, so wear should not be a MAJOR factor. Figure maybe 5-10 miles of driving, max.

    So that's it. What kind of 'suckers' & 'skirt' would you suggest?
    __________________________________________________
    UMich-Dearborn '04-'06
    Carnegie Mellon '99-'03
    [url=http://eVileNgineering.com][b]eVil eNgineerin

  2. #2
    Hmmn, there's a good writeup on the Brabham fan car in one of Allan Staniforth's books (I think it's Race & Rally Car Sourcebook).

    They went with an engine coupled fan.

    I would think that the second engine option would add a lot of weight. Some sort of PTO off the main powerplant might be a simpler, I can picture something with a VW or Corvair style fan.

    As for skirts, UHMW polyethylene sheet is cheap, self lubricating and moderately abrasion resistant.

    Cheers, Ted

  3. #3
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    The two most successful racing sucker cars were the Chaparral 2J (1970) and the Brabham fan car (1978?). The Chaparral 2J raced four times, never won (various glitches), but set lots of fastest laps etc., then was banned (in the "free-thinking"!! Can-Am series). The Brabham raced once (Swedish GP?), won, and was banned. Obviously sucker cars work - a bit too well!

    For the Chaparral see "Chevrolet - Racing?" by Van Valkenburgh, or "Can-Am Racing Cars 1966-1974" (a compilation of magazine articles) published by Brooklands Books. The Brabham was a simplified version of the Chaparral.

    I see two main approaches to this problem: 1) Low suction over a large area (the whole underbody). 2) Higher suction over a smaller area.

    The Chaparral used option 1 with two axial flow fans powered by a twin cylinder 45hp two stroke motor (so you will need more than a leaf blower). The skirts were Lexan (polycarbonate) connected to the suspension so that they always hung just (1/2"?) off the ground. See books for more details.

    I might try option 2. A good centrifugal fan (industrial size - check catalogues) can suck harder than axial fans. Then build two "suction cups", about 1 to 2 square metres area each, and attach these directly to (under) the front and rear suspension. This way you don't compress the normal suspension springs and "skirt height" is easier to control. Make "skirts" out of 2x4" softwood (they will grind themselves to the right profile). Possibly make the suction cups diamond shaped in plan view for better sweeping away of gravel, etc. Then a separate 50hp motor and centrifugal fan with ducts to the two suction cups...

    Then lots of fun!!!

    Oh, maybe some heavy duty truck tyres to carry the loads...

    Z

    PS. How much suction can you get? The limit is atmospheric pressure which is 1 kg/sq.cm, 1 ton/square foot, or 10 tons/square metre. So with, say, 1/10 atmosphere over 4 sq.m that's 4 tons! (Make that BIG truck tyres!)

  4. #4
    Z's got the right idea. A few years ago Road & Track (I think) ran a skidpad challege, and the Bowlands showed up (Todd went to VT around 1991 and they build their FSAE chassis to be a sucker until Cornell got it outlawed ) with their tube frame autocross terror (big wings) and two leaf blowers hooked up to a sucker system. Ended up riding on the jounce bumpers I might have it in my archive somewhere. Their skirts were lexan 'flaps' I think that just dragged on the ground.

    I'm designing a sucker car for my own enjoyment as well, and the research I've done says you need a huge planform area and relatively little leakage (10mm gap) to develop significant downforce on a 1000 kg car (ie 1g additional). Ends up having to move a lot of air, and I think two-stage axial flow fans will actually do a better job than a centrifugal (try the Multi-Wing Optimizer to size a fan, you can get it for free @ download ) for the flow rates you'll need. Of course, if you can get a better seal - then the flow rate goes down.
    John Bucknell

    FSAE since 1990 - Design Judge since 2003
    Scrutineer: SCCA ProRally/Formula One
    General Know It All
    /Performance Development Engineer

  5. #5
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    Quote by John "if you can get a better seal - then the flow rate goes down."
    ----------------------------------------------------------------------------

    Yes, that's my intention with the smaller "suction cup" approach. Firstly, it minimises circumferential length. Secondly, you should be able to run it closer to ground, maybe 3mm (1/8") on average. Having the perimeter of the cup somewhat flexible, and supporting it every 1 foot or so(?) by castoring skateboard wheels would help sealing over uneven roads. The "roof" of the suction cups carry most of the load so they should be strong and attached to the outboard ends of the suspension. A beam rear axle would be good here (and a beam front if possible) with the roof of the suction cup attached solid to the axle.

    Another approach on an open wheeler car would be to build a small "dog house" around each wheel, this "suction-cup-per-wheel" being carried by the wheel's axle. Sucking the air out of these wheel-enclosure-suction-systems will provide downforce to each wheel, and also clean the road in front of and under the wheel of any sand or water!

    A third option is like the "dog house per wheel" but the skirt only goes as high as the waist of the wheel - ie. the top half of the wheel sticks out of a hole in the roof of the dog house. This lets the atmos. pressure (ie. the column of air from ground to space) push directly down on the top of the tyre, rather than having to act through the wheel bearings. So smaller wheel bearings, but extra leakage around the waistband.

    Z

  6. #6
    Since I don't plan on driving it any considerable distance, I was thinking I could design the skirt to be in constant contact with the ground. I was even thinking I could make a rigid frame with castor wheels that stayed on the ground, then couple it to the car with a flexible material. (and secure it with a couple of links)

    Thanks for the ideas so far. Dirt cheap parts are prefered, as this is just for fun. I'll have to do some research on fans.

  7. #7
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    Last point (then I've got to do some real work!).

    Power-required = Pressure-differential x Volume-flow-rate x Fan-efficiency.

    IE. Less leakage (= less Volume-flow-rate) = smaller motor.

    Z

    (Edit: fixed equation for usual usage of "efficiency"...)
    Last edited by Z; 11-03-2013 at 09:48 PM.

  8. #8
    Or less leakage = MORE DOWNFORCE!

    This will be fun if I can pull it off.

  9. #9
    this isn't for (one of) the 944s is it?
    Mike Miles
    Carnegie Mellon SAE/Carnegie Mellon Racing -- Formula SAE 2003, 2004, 2005, 2006

  10. #10
    No way man, my 944 can pull 3Gs stock!

    No, this would be on a much crappier car, if thats even possible. Seriously just for kicks.
    __________________________________________________
    UMich-Dearborn '04-'06
    Carnegie Mellon '99-'03
    [url=http://eVileNgineering.com][b]eVil eNgineerin

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