Interested in discussion about an Aero project.

I'm working on a personal aero project that I'd love someone more knowledgable than I to at least steer me in the right direction, I know just enough to get in trouble but not enough to answer some simple questions. The problem is I'm trying to keep some of this partially private. I'd really love some input but I'm reluctant to share all my thoughts and designs openly. Not out of fear of being wrong, but more so out of fear that the competition is watching, lol. This isn't a typical car project, I'm currently working on a kart.

I have had some concepts rolling around in my head and I have a few qustions that I feel are important, but maybe I'm wrong and I'm getting hung up on some of these things because I don't know how to answer them and I'm not sure if they're even as important as I'm making them. I've saught out professional help and honestly with current aero budget of shoe strings and bubble gum, that's pretty well out of the question. So if ANYONE is interested in even answering the dumbest questions I have maybe it would shed some light on a few things for me to at least settle in on a chassis design and aero package trial design.

This is for speedway(Oval Kart) kart chassis, that can be highly modified body wise. Typical speeds are 50-80mph, with avg corner speeds in the 50-60 range. Solid chassis, no suspension.

At what point is an under tray too shallow to even be effective? And, by under tray I mean a full designed tunnel with splitter to diffuser, with hopefully everything flowing and working correctly from front to back.

Or what's the lowest the chassis can be to the ground before it's not even worth the hassle of designing and implementing an undertray?

The karts are typically an inch to inch and a half off the surface. Is this enough to get a meaningful benefit, or a waste of time to devote to full implementation of a tunnel and diffuser? I know there are so many factors, but let's assume it's been designed well and not by me. As long as the front isn't too low and allows good flow under the front or it pulls from the correct spots, is any flow beneficial? Or would a pure flat bottom with no diffusers or tunnel built in be the simpler and better route?

If not, say the minimum distance was half and inch with as much as two inches in the center and a diffuser or around 4-6". If the chassis was designed to allow to for more, where is the threshold where it's effective? The reason I'm asking is the chassis I'm designing could potentially be made to utilize a higher floor if that was worth persuing. But, I'm not able to test and verify much, so I'm relying on the generosity of those more knowledgable than I. If you'd like to discuss more privately and are really interested in some of my designs, I can share some drawings and explanations. I'd really appreciate any help. And, this is sort of the tip of the iceburg as far as questions. I'm really trying to nail down some more solid ideas based on good information so I can finalize designs.

I'm not super well-versed in aerodynamics, but I can give you a bump for the morning folks–I'm certain someone will have the help you are looking for. 

Your current speed and chassis to ground clearances will provide a reasonable platform for underbody development. The diffuser height will need to be in the range you noted. Your design can be fairly well planned out since you do not have much chassis height variation that is typically a challenge for sprung suspension cars. The design will need to be defined to amplify the handling effects you like and damp the ones that create issues.

Well That is a huge relief. Now the bigger question will the benefits of designing a higher floor to accommodate a taller under tray section going to outweigh the design challenges. I could potentially get up tona maximum of about an inch more overall ground clearance in the chassis if I design for it. And if I could get more where do I stop? Guessing this is answered below by myself now that I reread this with my basic CFD testing if I get that done. Won't be definitive but would answer more questions I guess.

Is that worth the time to design for, or is the current clearances optimal? I wanted to do CFD testing but my 3d skills are limited and it's taking me a little longer to get some of the base components modeled. So, and I don't know how detailed those models need to be vs if I built a very basic model and tested it. I think just designing some basic shapes without the chassis and components, it would at least give me a decent baseline to improve upon? Just get dimensions and design the under tray then side panels and wings and stay within my confinement and that should at least get me a base line and tell me if I'm doing more harm than good. Right?

If anyone would like to chat and at least give me some ideas of which direction is best on a few things, I can give some of my major design limitations ansld direction I'm trying to go with my designs. Some major points. Covering the tires or leave then open with my current body work, doing full fender wells vs semi open as they are now. With the speeds listed, I'm not worried about drag, as we race on dirt and as long as that drag is producing meaningful downforce, I'm fine with it. Because we're limited on grip so I believe we're not utilizing aero even near it's potential in this sport currently and I want to increase overall chassis grip.

In reply to ABR69 :

That's an interesting use case - racing on dirt ovals. Presumably you're using stagger on relatively soft tires, correct?

Also, are you cornering sideways (right rear out with counter steer, or "slideways" as we used to call it)?

Those would have a effect on how you might implement a downforce-generating underbody. Look at the asymmetry of sprint car wings and modified bodies (and the significant offsets of NASCAR bodies years ago). 

I would note that I'm making an assumption here that may not be accurate - that dirt racing means ovals. I know in other countries dirt karts race non-oval tracks. In either case, sideways cornering and the relatively non-dampened bumping across the surface pose some challenges to aero.

- Bill C

BillCuttitta said:

In reply to ABR69 :

That's an interesting use case - racing on dirt ovals. Presumably you're using stagger on relatively soft tires, correct?

Also, are you cornering sideways (right rear out with counter steer, or "slideways" as we used to call it)?

Those would have a effect on how you might implement a downforce-generating underbody. Look at the asymmetry of sprint car wings and modified bodies (and the significant offsets of NASCAR bodies years ago). 

I would note that I'm making an assumption here that may not be accurate - that dirt racing means ovals. I know in other countries dirt karts race non-oval tracks. In either case, sideways cornering and the relatively non-dampened bumping across the surface pose some challenges to aero.

- Bill C

Yes dirt oval and while our setups skew the chassis with rear tracking, the sliding is kept to an absolute minimum. There are things we've done to slightly benefit us to keep us from sliding aero wise but ultimately that's what the whole package is intended to do is keep the slideways driving to a minimum.

While track surfaces aren't perfect as it is dirt, it's honestly not all that much Different that some Asphalt tracks as far as smoothness and especially grip. So there will definitely be some variances and I know those create potential stall points depending on air flow and sealing the sides.  The bodies have more damage in the transitions entering and exiting the track than they get in a season of actual on track racing. But, with the research I've done I believe there isn't enough of a variance over some of the major Asphalt tracks to the well groomed dirt tracks we race for these low ground clearance karts. A normally prepped car DIRT track is completely out of the question smoothness wise. It's two fold, while we do use softer tires, being unsprung the overall height changes are still quite minimal in the grand scheme. Early track to later night racing the track will smooth out significantly usually for the better. I definitely took this into consideration, which is why one of my bigger questions is just how tall can that under body could be with half to three quarters of an inch variance in track surface at times. (Which I think if I czn run some super simple CFD sessions on the body and underbody panels, I can test this and potentially answer it some.) Because as I said I know a lot of the under body is speed related and cornering is when we see our biggest transitions, both height and speed. So potentially the underbody will stall if too tall without enough flow. Either from lack of speed or spillage in or out of the sides correct?

In reply to ABR69 :

Thanks for the reply. Two things are occurring to me here:

1. Understood about track conditions changing over the course of heats to the main, and things smoothing out. As you know, the main thing here would be adjustability. As conditions change, your optimum ride height - and underbody ride height - may change. You noted that you'll be doing some testing, seems to me that building adjustability into the height(s) of the underbody (front to rear, side to side, even across corners if you're using a flexible enough material), even relative to the chassis would be helpful. You seem to have a good grounding of knowledge to understand the factors related to stalling, and the need for a venturi providing adequate flow to the negative pressure sections. In a nutshell, build it cheap and fast and adjustable, and start trying things out and learning and improving. Even the F1 guys have problems correlating their CFD and wind tunnel results into items that actually work as expected on track. 

2. Is moveable aero legal in your series? A belt-driven or electric fan might be too much to ask, but maybe simple cable-operated wings or flaps? Or hell, in the old days (1979 IIRC) the Ligier F1 team had a hidden flap mechanism to stall their underbody on straightaways and then pick the downforce back up in corners. It worked until they got caught. 

- Bill C

We're completely open as long as it meets the over all height, width, and nothing completely over our heads. Besides that, we have very very loose limitations. A few safety things. But they're all completely understandable and have very little real effect on possible performance.

I was considering something material wise along thr lights of E Glass, because weight is quite high on performance limitations. I'm trying to cut weight, but it's not the end of the world if I  come out a little heavier than I wanted. But. Within reason, 10-15 lbs is fairly significant to us. In carbon for us the sides and wing weighs in around 4-5lbs. A typical off the shelf body from a manufacturer is in the 10-15lb range. So adding under body, it isn't the end of the world if 2-4 lbs added back in under body. I can adjust to get it back somewhere.

Got any pictures? Of a typical cart?

In reply to VolvoHeretic : I'll explain a little bit later, driving right now.

Wow!

In reply to ABR69 :

Well! There's a whole lot happening right there. The wing (could be as much for structure as for aero), slab sides with splitters along the bottom and right-biased fences along the top... interesting. Definitely looks like a lot of freedom to experiment. 

- Bill C 

In reply to BillCuttitta :

Yeah, a lot to disect and yet so much more to improve upon. And yes, there are very little restrictions. Maximum height at center of front axle, and rear axle. Maximum width and length. A maximum for front wing height, and rear ward placement. As long as nothing is over the drivers head as far as covering it or restricting them then it's all fair game.(EDIT) Those are carbon panels and wing, roughly in the 4.5lbs range. The nose piece with splitter is upwards of 12lbs I think. It's molded hand laid fiber glass. But, we chose to stay fiberglass in the front because we tend to need nose wight for scale %'s. Which is more important than anything else ultimately. So the extra weight of fiberglass is actually a benefit there. My design I potentially will have to add lead ballast to the front to get the %'s necessary. But, with dynamic load maybe not as much if it generates as much downforce as I hope.

I have drawings and lots of ideas, I'm really wanting to prove some things before investing in building molds and carbon and Eglass body panels. Ultimately I have chassis designs that I'm working on to better incorporate the under body, which is essentially just flat on the kart above. Totally under utilized I think. The splitter rule is 3" from the main nose structure on all sides. The factories have started using a diffuser in the rear floor pan, but with the rules on stock karts it's hard to say if it's effective. But, then most use that same floor pan for these unlimited karts. So we did add the splitter and with the diffuser it might help some, but I think essentially under utilized as there is so much uncovered open space and it's all flat.

My main goal is better front structure, the front most are using a nose piece that is from the stock classes that have to adhere to a rule set. I want to do away with it completely and build it with a splitter and wing. Some are running front wings but they're Ultimately doing it wrong in my opinion just out of monkey see monkey do. But, the entire front end of the kart could be better utilized. Thus allowing for more in the rear and generating more downforce over all. We tend to float the front end on high speed tracks with the kart above. So we can't put more AOA on the rear wing without it trying to float the front tires down the straights. Partially could be due to bad aero all around. But, I think my design with and under body tunnel and properly designed front end would greatly improve downforce and handling. Utilizing tires better and make the kart easier to drive overall.

(Edit) in this design yes the wing is as much structural as it is functional, but in the design I have in mind I want the wing attached more directly to the chassis in the rear as well as the front. So that rigidity transfers the load to the tires faster than through the body, to the chassis and then the tires. Should be a directly mounted to the chassis, then the body panels can use the wing ends as a stop to increase side to side rigidity in the panels. So side loading isn't compromised from the current body, then also mount thr wing further back, to increase fulcrum loads. Which I will do with the nose wing as well. The kart above is utilizing a stock class chassis as purpose built ulimited chassis aren't common. So I can build a chassis to utilize my ideas. The chassis above is roughly 72", I can lengthen my chassis to a maximum of 110". But leave the wheelbase roughly the same if I choose. Which tend to help with turning ability, but the longer body panels give better straight line stability. Not to mention the current kart is 43" wide. And I can build the chassis to a maximum of 55". Not sure how long I will make it, need to better nail that down in testing. Width will be pretty close to the max.

link to "official" rules?

it's a lot easier to poke into gray areas when you can actually read between the lines.

In reply to sleepyhead the buffalo :

https://www.unlimitedallstars.org/2020UASrules.pdf

The Chassis rules are a collaboration from all the karting disciplines. Meaning, anything from Sprint, Enduro, Super Karts, and what we race on oval tracks as speedway chassis. The rules above specifically mention the least restrictive of all the governing bodies, within any of the karting disciplines. So the chassis rules are quite open.

From the chassis rules I'll give you maximums, 110" maximum length, 55" maximum width. Wheel base 35" minimum, 55-1/8 or 60" maximum wheel base. No maximum track width up to maximum width I do believe.

Not necessarily poking into grey area's, more so just actually utilizing the rules to their fullest, because that hasn't even been done yet by most. So, I'm just wanting to build something that is better than the rest, the rules arent really a hinderance.

thanks for that; reading it was helpful a well as your information about the chassis... 

ABR69 said:

In reply to sleepyhead the buffalo :

https://www.unlimitedallstars.org/2020UASrules.pdf

The Chassis rules are a collaboration from all the karting disciplines. [...] So the chassis rules are quite open.

From the chassis rules I'll give you maximums, 110" maximum length, 55" maximum width. Wheel base 35" minimum, 55-1/8 or 60" maximum wheel base. No maximum track width up to maximum width I do believe.

so, skimming that over, the rules treat the "chassis" as separate to/from the "bodywork"... and one interpretation of that is that the bodywork is not subject to the 110" maximum (and there's not really a limit other than 'bodywork can't extend behind the rear bumper'?) or, is that reading stepping too far out on a limb?

two other useful things that pop out at me are the following line:

"nose pieces [...] must provide adequate foot protection"

 plus, there might be a limit of between 0.115" or 0.25" minimum trailing edge thickness on any "front wing"?

Yes, the rear of the body panels cant not extend past the rear bumper, thats pretty cut and dry and is enforced.

As far as the nose piece, this front bumper is adequate for foot protection. So, thats all that is needed, and I can openly design around that. As long as it doesn't cover the feet and legs. But, thats somewhat relaxed as there are a few running bodies that clearly come too far back. But, I'm not looking for that direction. (This is a stock class body being mounted on the kart.) Still legal for our class if we chose. But, not many do.

And yes, they just don't want the wing to be a potential cutting hazard, so it cant be too sharp. As long as it doesn't look dangerous, (mainly because it's in the front) they are fairly relaxed until someone really calls it out. But, most don't complain too much unless they feel like they're getting beat by that detail.

ABR69 said:

Yes, the rear of the body panels cant not extend past the rear bumper, thats pretty cut and dry and is enforced.

As far as the nose piece, this front bumper is adequate for foot protection. So, thats all that is needed, and I can openly design around that. As long as it doesn't cover the feet and legs. But, thats somewhat relaxed as there are a few running bodies that clearly come too far back. But, I'm not looking for that direction.

Yes, I agree, it sounds like it's "easy enough" to make rear downforce... the trouble is balancing that out. so extending rearward isn't helpful.  but there's effectively no restriction on how far forward the front nose piece can extend, other than from the structure required to mount it rigidly to that 'front foot bumper' structure (other than the 3.25" splitter extension).  and the further forward the nose bodywork can extend, the less downforce you need to generate to balance out the rear.

also, based on the above, you could make the nose bodywork could be shaped to interact with a front wing to scoop and accelerate flow 'up' in a beneficial interaction.

there's also no restriction on the splitter being flat, so the back edge of it could curl up and help the front lip accelerate more air in the gap between it and the dirt/asphalt.

the lack of suspension movement could also mean that you could mount bodywork around the suspension and channel the air either out the side(?) or back (either under the flat floor as you've asked about; or through a tunnel to an opening above/below the bumper).

ABR69 said:

And yes, they just don't want the wing to be a potential cutting hazard, so it cant be too sharp. As long as it doesn't look dangerous, (mainly because it's in the front) they are fairly relaxed until someone really calls it out. But, most don't complain too much unless they feel like they're getting beat by that detail.

it's something to keep in mind, since a number of the "very high Clmax" wing sections tend to have thin trailing edges (although, that could potentially be overcome by integrating a gurney flap into it)

In reply to sleepyhead the buffalo :

Most wouldn't really notice it unless it looked like a razor blade. But, because the rest of the chassis and everything will be pushing some norms that most will consider outside the limits of legality (Even though, they honestly have no clue what the limits are.) I'd say the wing designs we've been running are pretty sufficient, so not a boundary I'm worried about pushing currently. Just getting a decent performing wing there will be more than enough for me.

That rule is mainly there to discourage people from wrapping a wing core with sheetmetal and leaving sharp edges more so than to hinder performance.

In reply to sleepyhead the buffalo :

Yes, rear down force is easy. Which is why I referrance the total length. I can push the front further out. With a full width front wing pushed forward to the max, as the rear is pushed rearward to it's max. That coupled with a well designed splitter and a full under body tunnel should balance the down force much better. I shouldn't necessarily say the max on them, but being able to push my chassis longer than a normal chassis, I have better control over fulcrum lengths and changing bias front and rear with down force by moving the wings either front or rear for a better balance.

As far as the splitter, a rounded leading edge, with a rolled up floor leading to the tunnel is exactly my design plan. Which is why one of my main questions was how far I should push the chassis as far as height to determine my tunnel roof. Because I know there are several factors that attribute to stall, and trying to figure that out.

PS, I can only reply once an hour, so if this is allowed I'll give my discord and email and we could further discuss some options, or if a mod would be happy to take the 1 hour limit away, I'd gladly post more in depth pics/drawings and discuss further design. At this point I want asnwers more than I'm worried about others stealing ideas. They'll see most of them eventually.

sleepyhead the buffalo said:

the lack of suspension movement could also mean that you could mount bodywork around the suspension and channel the air either out the side(?) or back (either under the flat floor as you've asked about; or through a tunnel to an opening above/below the bumper).

So, this is another one of the questions I have. I don't have any suspension at all, so I was curious if making fender wells completely closing off the area around the tires would be beneficial. I could even completely close off the front from the outside as well, like the rears of the black kart I posted. Because, we only need access to them off track. So, if that's a benefit I'd consider that in the design as well. As you said the the wheel area could be vented out the side or just up or behind the tire into the turbulent air through the center section Or completely out the back. Would it also be a benefit to cover the rear tires?

Sadly  these things I'm afraid I'll only answer with crude CFD testing through fusion if I can ever get it halfway modelled.