Ground sealing...

Recently I've been having these rather perverse ideas about itty bitty sports racers (think big FSAE with a LeGrand body and you're about there), and noticed something curious in the A-mod rules; movable side skirts ARE allowed, meaning some sort of ground sealing could be achieved. The more I think about it, the more I feel like flexible side skirts might be easier to set up, and strangely enough, I keep coming back to the idea of a door sweep. You know, that thing that looks like one row of bristles from a big push broom? Or, perhaps actually cannibalizing a push broom and using individual rows of bristles. It seems to me that something like that would be VERY easy to adapt to existing cars, especially ones with very little suspension travel. The real question is, would it work? I mean assuming you could get an AX car to work with VERY low amounts of body movement in roll and pitch and only an inch or so of bump travel, it seems like this would work very well, especially if you had a diffuser that extended into the sidepod area (ala Porsche 962) and made the skirt follow the outer edge of the tunnel rather than just running along the edge of the bodywork between the front and rear tires. This might also have the advantage of sweeping the course directly in front of the driven wheels, which can only help things I'd imagine. Of course, you'd have to periodically replace the bristles as they wore down, but I can't imagine that would be too expensive, just buy a pair of broom heads and you've got enough material to skirt at least one LeGrand or with some creativity, a few formula cars.

Am I starting to to diagnosably crazy, or could this actually work, at least for an AX/hillclimb application? If nothing else, your competitors couldn't complain but so much, on one hand you might have a serious aero advantage, but on the other hand, you're the autocross equivalent of a Zamboni

I thought the general rules said that only the tires could contact the ground at rest or some such nonsense making sealing vanes impracticle. Otherwise your on to something and you'll clean the course as you run.

well.. I would think about sacrficial air dams. They can sit a CM or two off of the ground at rest and as you get up to speed, the aero of the car and push eveything down into contact with the ground. Done right, it can just skim the pavement and make occasional contact on turns and bumps.

Make it out of a cheap enough plastic, like garden edging, and you do not have worry about spending a lot to replace it

Works very well until you hit a bump. Sliding skirts are incredibly sensitive to ride height. You're most likely to lose your seal when forces are highest, like in the corner. Lose the seal, you lose your grip, and usually at the worst possible time. This is why active aero and sliding skirts have been banned from all serious forms of motorsport (DRS aside).

That said, if you're allowed to do it and you have a lot of runoff room to rely on, JUMP ON IT. You have a huge body of performance you can unlock. I will be building a sucker car at some point in my life, just realize that they will snap on you.

Bristles actually allow a lot of airflow, vertical sliding skirts are your best bet. Ultra high molecular weight polyethylene is cheap, wouldn't damage the track, and would last a long time. http://www.makeitfrom.com/material-data/?for=Ultra-High-Molecular-Weight-Polyethylene-UHMW-PE

Well Warren is right about the snappy part but on an auto-x with a lot of run-off, I don't think that would be a problem.

Has anyone tried to built an inflatable skirt like the ones used on hovercrafts? That seems to work great for those, and you could have it retracted at rest and inflate it once on the move. Don't think it would be cheap though.

fanfoy wrote: Well Warren is right about the snappy part but on an auto-x with a lot of run-off, I don't think that would be a problem. Has anyone tried to built an inflatable skirt like the ones used on hovercrafts? That seems to work great for those, and you could have it retracted at rest and inflate it once on the move. Don't think it would be cheap though.

Good idea, the immediate problem that comes to mind is that an inflatable skirt in a hovercraft survives because most of it is separated from the surface by a thin cushion of air leaking out. There is a little bit of contact due to surface irregularities, but for the most part, it's hovering. With a vacuum under the car, that skirt will be sucked to the ground. The skirt will see a lot more wear, heat, and abrasion in that application than on a hovercraft.

Well I think Warren covered everything I was going to say...

Inflatable skirts would push the car upwards when inflated...not what you want. You just want the skirts resting on the ground under their own weight.

I would make sure this car has good safety from side impacts and rollover in case you lose the seal in a corner. The better the seal, the more downforce you stand to lose when you go over a little ripple etc.

Ground effect gets more dangerous as it gets better, this is why the rules in most classes basically sabotage it by outlawing anything that forms a seal and having minimum ground clearance.. In F1 they're even getting picky about the size and length of diffusers now.

I posted this last night but it disappeared...

The Chaparral sucker car used Lexan skirts that were actually tied to the suspension with a mechanism to maintain a constant 1/4" gap. The front seal had a different mechanism, I've seen talk of an air spring powered by the pressure differential but never seen diagrams.

Bristles might be a good option, actually. They don't seal as well as a real skirt, but they may also be much more tolerant of momentary interruptions because of it. It doesn't take much pressure differential to make a really big difference to downforce when you consider the amount of area you're working with.

We've had a sucker car of our own at GRM. What did the 'Vette use?

Keith Tanner wrote: I posted this last night but it disappeared... The Chaparral sucker car used Lexan skirts that were actually tied to the suspension with a mechanism to maintain a constant 1/4" gap. The front seal had a different mechanism, I've seen talk of an air spring powered by the pressure differential but never seen diagrams. Bristles might be a good option, actually. They don't seal as well as a real skirt, but they may also be much more tolerant of momentary interruptions because of it. It doesn't take much pressure differential to make a really big difference to downforce when you consider the amount of area you're working with. We've had a sucker car of our own at GRM. What did the 'Vette use?

The Cheaparral 2J2 used vertical sliding plastic skirts and a snowmobile motor spinning a tank's cooling fan. P&G brought it to Georgia Tech as a recruiting tool a few years back. I remember it having twin turbos, too. Thing was incredible.

Found the website. WARNING: COMIC SANS! http://www.cheaparral.com/index.html

Somehow this thread got split into 2 threads... Here is what I posted on the other version.
A solution like a row of bristles could provide you with some downforce. It would be less than actually sealing with solid materials, but still most of the gain is there.
As far as how to keep different systems sealed: You could use something like a soft door sweep mounted at an aggressive angle to reduce the skirt loading, but it would wear quickly (faster with more downforce trying to pull it under the car).
Rows of bristle type material will flex back and laterally with car motion, so unloading the rest of the car is less of an issues than proposed above. Bristles would work better on sides than the front. The wear of the bristles will be minimal and if the car is setup the same for most events any wear would be limited to roughly the same max roll/heave.
Lexan skirts (or other plastics) can be made to slide and pivot in channels on the sides of the bodywork and diffusers. That gives the advantage of a complete seal, wear adjustment and body motion all in one package.
The low speeds of AX do not necessarily mean low downforce. If you were able to generate even 0.1 psi of vacuum under a typical BMod size car you are talking about nearly 500 pounds (96in50in0.1psi=480pounds) of downforce. Keep in mind that a little suction spread over a large area adds up quickly.
Steve Stafford

I never thought about the bristles leaking air, but I suppose it does make sense... perhaps I could get something like a bicycle inner tube and cut it up/attach it to the sides of the bristles (sew it around a row of bristles with fishing line or nylon thread or something?), having the rubber extend the full length of the bristles, to form a basically airtight layer that was still flexible enough to deal with an inch or so of body movement (relative to the ground) as well as surface irregularities, and still provide some "sweeping" capability.

Do you all think it would greatly increase the effectiveness of the underbody if I found some way to effectively seal the front as well as the sides? maybe fasten some bristle material to the undertray slightly forward of the front edge of the front fender openings the full width of the car (inc. in front of the front wheels), fasten the bristles angled forward at about 30deg, and ensure the bristles are about 2x as long as the anticipated ground clearance, so they naturally sweep towards the rear of the car when it rolls forward, but have a decent amount of tension on them keeping them on the ground? if nothing else, this would brush debris away from the front tires quite effectively, even if it didn't seal terribly well. On a similar train of thought, if I made the sideskirt bristles about 2x longer than the anticipated ride height and found a way to make them angle away from the car's centerline, wouldn't the bending forces in the bristles as well as the suction under the car do a LOT to seal them to the track surface, as well as give me more leeway in terms of body movement before I lost seal?

I am becoming a true GRMer, I'm devising ways to out-downforce most of A/B-Mod using cannibalized broom heads, bicycle inner tubes, and fishing line! If I can find some way to incorporate duck tape, bailing twine, and gorilla glue, I might just come away with a bachelors degree in redneck engineering

The inner tube idea is a good one. It will retain flexibility and offer a bit more sealing. My experience with bristles, vacuums, and irregular surfaces comes from wood CNC machines. They work to contain air, but the pressure differential will be significantly less than sliding skirts. Still worth trying.