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Creating Negative Pressure Underneath Car to Prevent Lift

  1. Jan 24, 2016 #1
    Hello, I'm brand new to this site. I have a number of questions that I can't get answered by anybody.

    I want to some how create negative pressure under the car to suck the bottom of a car down using Vacuum fans (like the 1970 Chaparral 2J Can-Am racing car ("sucker car").

    I has some basic questions regarding a lightweight low-drag car driving in a straight-line:

    1.) Does a vacuum underneath a car cause a "drag" on the car?
    2.) What is the "perfect amount vacuum" underneath the car to keep it from "lifting" at very high speeds (210+ mph) in a straight-line?
    3.) Would too much vacuum slow the cars's acceleration & reduce the potential top speed?
    4.) What is more important when talking about "negative pressure" holding a car down...a large surface area or a large volume?
    5.) Would creating a "negative pressure" be any different than just any weight to the car?
    6.) Also would you have to increase the vacuum under the car as the car increased speed?

    Any formulas that are related to weight & down-force, acceleration, speed, lift and drag as related to vacuum would be great!!!

    Thanks a ton. This is related to a "real" project I'm doing. I'm trying to increase downforce without increasing drag (like using a wing).
    Last edited: Jan 24, 2016
  2. jcsd
  3. Jan 25, 2016 #2
    I don't know what the "perfect" vacuum under a car would be.. I would guess just as much as is needed.. Perhaps with position sensors on the shocks you could keep it at an even keel.
    Formula 1 cars can drive upside down at speed, they have so much downforce from the wings, but also from the air movement underneath the car (moving air is at a lower pressure than stationary air.. venturi effect)

    Start with a smooth underbelly to keep the air moving quickly under the car, and a large surface area is far more important than volume.. I would guess you'd want to minimize volume... Secondly, prevent too much air from getting under the car, most cars start to lift on the front end because air going underneath them, and they turn into a wing, suddenly generating lift.

    Nothing is free, if you're using a fan of sorts to suck the car down, it take power to run as well. you may be better using that power to push the car forward?
  4. Jan 25, 2016 #3

    Ranger Mike

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    Welcome Carguy...
    I admire someone wanting to add a new wrinkle to the whole racing situation but can we pull her into the pits for a second and discuss this??
    I think you are looking to create down force with a vacuum to make more traction, thus better handling. Before you get to this point the car has to be handling good to excellent. Can you go into a little more detail on the race series or class you are running?
    Or is this a drag car or Bonneville ride?
    There are a lot of things to consider before bolting on the old vacuum fans and jumping in ad driving..
  5. Jan 25, 2016 #4
    A general equation for downforce would be surface area x pressure difference... so if you generate -0.1 PSI and the underbelly of the car is 6x10ft 8640 sq in) you'd have 860 lbs of downforce.
  6. Jan 25, 2016 #5
    Hello, to clarify my first post, I'm re-write some of it and posting again below:

    I have been wondering about ways to "suck" the bottom down at high speeds. Everything is this post is related to creating a body and modifying the chassis to run in an "open class" Bonneville Salt Flats Speed Trial. The main goals are highest top speed possible in a straight line without the loss of vehicle control. Everything I'm asking is related to Straight-Line Racing Only.

    I want to figure out a way how to create "negative pressure" under the car to suck the bottom of the car down. I’m thinking about first making a "small-scale model" version for testing purposes only, before I decide what direction I'm going to go with in regard to the full-size body and design. One thought was possibly using electric High-suction fans (similar to the 1970 Chaparral 2J Can-Am racing car ("sucker car").

    I has some questions regarding drag, lift and top speed, etc. with doing something like this:

    1.) Will creating a vacuum (negative pressure) underneath the car cause a "drag" on the car when it's moving?
    2.) How would I figure out the "correct amount of vacuum" to create underneath the car to keep it from "lifting" at very high speeds?
    3.) Would too much "Negative Pressure" slow the car's acceleration & reduce the potential top speed?
    4.) What is more important in regard to "negative pressure" sucking the car down...a large surface area or a large volume?
    5.) Would creating a "negative pressure" be any different than just any weight to the car?
    6.) Would you have to increase the vacuum under the car as the car increases speed and lower the vacuum at lower vehicle speeds?

    Any formulas that are related to weight & down-force, acceleration, speed, lift and drag as related to "negative-pressure" would be great!!!

    My main goal is I'm trying to increase the down-force without increasing drag (like using a wing).

    Any direction of how to figure some of this stuff out before I build a "scale model" of it would be very helpful.

    Thanks again.
    Last edited: Jan 25, 2016
  7. Jan 25, 2016 #6


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    That depends entirely on how you create the "vacuum". If you do so aerodynamically (e.g. with a wing-like device or by deflecting the air flow so as to accelerate it) then it will likely involve a drag penalty. If you do it using something like fans as you originally proposed, then you would likely have some degree of viscous drag penalty, but the net effect would probably be less than doing it aerodynamically. On the other hand, fans require power, so you would have to divert some engine power to the fans rather than forward motion. The operative question is how much do you gain from using fans instead of aerodynamics compared to how much you lose in powering the fans.

    2.) How would I figure out the "correct amount of vacuum" to create underneath the car to keep it from "lifting" at very high speeds?[/quote]

    You would need to know the flow field around the car. As long as the weight of the car plus the total downforce is enough for your liking, then you are fine. That is a complicated issue though, and there is no simple formula to come up with an answer. This is why Formula 1 teams pay engineers a lot of money to run CFD simulations.

    3.) Would too much "Negative Pressure" slow the car's acceleration & reduce the potential top speed?[/quote]

    It would depend to a certain degree on the shape of the surfaces against which all of the pressures are acting. The shape of the surface is going to play into the drag question you have already asked. The other issue is that if you have too much downforce, you are going to be adding too much friction to your motion, which will slow you down.

    4.) What is more important in regard to "negative pressure" sucking the car down...a large surface area or a large volume?[/quote]

    That was already addressed above. It doesn't matter at all what the volume of the car or the air is. It is all about the surface upon which the pressures are acting, so the larger surface you have exposed to a large pressure differential, the more downforce you will experience.

    5.) Would creating a "negative pressure" be any different than just any weight to the car?[/quote]

    Yes. Adding more downforce doesn't add any inertia to the car, so it won't affect your acceleration in that manner. The net effect on friction should be the same, though.

    6.) Would you have to increase the vacuum under the car as the car increases speed and lower the vacuum at lower vehicle speeds?[/quote]

    In order to optimize your performance, yes. At lower speeds there will be lest tendency to generate lift, so you need less "vacuum" to counteract this effect.
  8. Jan 25, 2016 #7


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    As you stated, creating a vacuum under the car would be beneficial to increasing the traction without the straight line speed drag losses incurred by external winglets etc.
    As stated above area is the important factor plus the maximum reduction of external air leakage into the vacuum zone; to achieve this, you should include a full skirt around the entire vacuum area and use a rigid suspension system to allow running the skirt bottom as close to the ground as possible. By minimizing the skirt leakage you can attain the most efficient vacuum system and thereby minimize the size and power draw of the vacuum fan assembly. One possible downside of the vacuum could be an increase in the rolling resistance of the tires; but, if maximizing traction is the goal then this isn't really a primary concern.
    With regard to increasing vacuum level vs speed, this will depend upon the aerodynamic lifting effect of the overall vehicle. Alternately, having the maximum vacuum at launch might be the best scenario.
    With regard to vacuum vs weight,the advantage of the vacuum system is that it increases down force without increasing the vehicle's inertial resistance during acceleration.
    Removing the air flow under the vehicle will be beneficial in reducing drag effects from air flow between the vehicle undercarriage and the ground.
    Last edited: Jan 25, 2016
  9. Jan 25, 2016 #8
    What is the general body style of the vehicle? bullet shaped? Stock-ish car shaped? Open wheel?
  10. Jan 25, 2016 #9
    Thanks so much for your useful information!!

    As for the sucction fans that I would use....they would be powered by a number of Li-Poly batteries and be able to in increase the down-force by a lot...

    The power to weight ratio is awesome. The overall power is very high. But lift will be an issue.

    This is example of what I don't want to happen:

  11. Jan 25, 2016 #10
    Thanks for your input. I agree, I would definitely have to use a skirt to isolate all of areas that would be under a vacuum, and suspension would have to be adjusted to allow the skirted area to be in a constant range of a 1/2 to 1 inch from the track at all times.

    Battery technology has advanced so much, that using it for what I want to do is now actually quite feasible.

    All good things to think about.

    Thanks again!

  12. Jan 26, 2016 #11


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    Even with batteries there are losses. Now you have added mass from the batteries, added volume from the batteries that will increase drag, or if no added volume, you get less engine and/or fuel. There are always trade offs.
  13. Jan 26, 2016 #12
    You have to realize YOU DON'T GET ANYTHING FOR FREE.

    Weigh the LiPo batteries and electric motor and fan, and figure that into your Hp/Weight ratio... (I'm going to guess a minimum 5 HP fan and 25-50 lbs weight)
    Then consider the power loss of driving the fans directly. and your new Hp/Weight ratio
    Then consider airfoils, even active ones that only tilt as much as needed.

    I understand you don't want to flip over backward.. but I think if sucker fans were the answer you'd see them on everything already... Sometimes you just can't reinvent the wheel and get something rounder than what we already have.
  14. Jan 26, 2016 #13


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    For a bonneville-type top speed car, I'd tend to think you would get the best results simply from designing the car to passively have no lift (you'd probably want slight downforce for stability, but not much downforce, since it adds drag). Active systems were investigated for cars designed for race track use, where having high downforce at low speed (a very difficult thing to do with passive aero) could help immensely in slow speed corners. I also suspect you're dramatically underestimating how much power you'll need to make a substantial difference in lift.
  15. Jan 27, 2016 #14
    I know there's nothing for free!! That's why I'm on the "Physics Forum", I looking for possible trade-offs! And there are always trade-offs in life.

    I'm going have to TOTALLY disagree with the following statement you wrote: "..but I think if sucker fans were the answer you'd see them on everything already...".

    ****First of all the "Blower Cars" got banned from the racing series because of political reasons. Every article I've road on them, claimed that it worked great after and they ironed out some of the initial kinks. They where able to add a 1000lbs of down force to a 2500lb car without increasing drag...even reduce drag by not running wings.

    "....This gave the car tremendous gripping power and enabled greater maneuverability at all speeds. Since it created the same levels of low pressure under the car at all speeds, down-force did not decrease at lower speeds. With other aerodynamic devices, down-force decreases as the car slows down or achieves too much of a slip angle, both of which were not problems for the "sucker car"..."


    Also something interesting occur. The early blower cars help to encourage aerodynamic Engineers to search for ways to generate down-force without using items that caused drag. This led them to discover that by creating a venturi effect...which gave them exactly that...and that's when the so-called "ground effects" race cars started to occur...

  16. Jan 27, 2016 #15


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    That's a road racing car though. A top speed car doesn't want or need a thousand pounds of downforce - it just needs to minimize lift and be stable.
  17. Jan 27, 2016 #16


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    Not mentioned in the Wiki article is that Formula One race cars have a flat skid plate on the bottom required by rules, but are allowed to use difusser effect to provide a spot at the back of a Formula One race for the lower pressure / higher velocity (relative to car) flow created by wing and upper body effects, a place for the flow exiting from the underbody to slow down (relative to the car) and expand (cross sectional area).


    In addition to fans being banned, movable side skirts that nearly sealed off the sides of race cars were also banned. The reason was sudden loss of downforce if anything went wrong, causing cars to go off track and possibly crash. There have been issues with air getting caught under some Lemans type cars, causing them to blow over, such as the 2 1/2 back flip with 1/2 twist Mercedes at LeMans in 1999. The car landed wheels down and the driver was OK and continued his racing career.
  18. Jan 27, 2016 #17
    As I was reading that wiki article, I read between the lines a little bit.. and thought about the pros and cons of each....

    Ground effects.. works well at speed, and the faster the better, susceptible to sudden loss of suction if the car's suspension allows it to bottom out, thus causing a sudden loss of down force.. I don't think it takes a whole lot of power

    Wings, airfoils, etc.. good for high speed, like ground effects, not very useful at low speed... good thing is the faster you go, the more it works.

    Suction fans, work particularly well at low speeds, but require sealing of the underbody. I also see them requiring more and more power to maintain effectiveness with increased speed as more air gets shoved under the car and needs to be forcefully removed.

    What I see as a serious danger with it is that should anything cause a leak in the system, you get a positive feedback loop.. That is.. if the car's attitude changes a little bit nose-up, it will break the suction, contributing to exactly a LeMans style backflip.
  19. Jan 27, 2016 #18
    rcgldr brings up some good points. Underbody generated downforce can be difficult to control and occasionally leads to catastrophic problems. As a salt flat speed run you may escape some of these but you will need much tweeking.
    On to the parts I have worked with. Some of the parts of generating downforce are in the set up and allowances. There are actually a couple of plates on the bottom of F1 cars. Metal skid plates are used on the sides of the pods to protect body parts in a bottoming out event due to rough pavement or cornering. There is also a "plank" of sacrificial material down the center of the car. This is a rules item. In the tech inspection after the race if the plank has lost more than 1mm of material due to abrasion it is disqualified.
    Cars are also set up with rake. Typical in my era was a front end ride height of 20mm and a rear of 70mm. As speed and wing generated forces were applied the suspension compressed often to the point where ride height at speed was close to or less than 10mm. I know that Williams has experimented with active ride control (currently banned) that maintains less than 5.
    As previously mentioned one major factor is a very smooth bottom. This combined with plenty of surface develops the force. Several years ago I do not remember which team but they had some very successful experiments with an inverted airfoil in the cars underbody. The high point of the chord was approximately 30% of the cars length from the front. It worked very well but was not allowed to continue racing.
    These are a few observations I had from the team I also observed that the airflow engineers worked very hard on the underbody. I can't off hand give you formulas but down force generated under the car came with less drag than wing pairs generating the same force.
    One other item to practice with from our team. As the force increased the car would settle into the suspension. Allowances also had to be made for the expansion of the tires at high rotational speeds. Our tires at full speed expanded by approximately 5.5mm. Half of whatever your value is will be added as increase in ride height.

    Just a few notes
  20. Jan 28, 2016 #19
    Thanks! You brought some very interesting points to think about...
  21. Feb 2, 2016 #20
    Thanks everybody, you guys got me thinking...which can be a pretty hard thing to do sometimes...LOL!
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