Story by Steve Stafford
For half a century, wings and spoilers have helped race cars stick to the track. Lately, though, another piece of equipment has joined the mix: underbody rear diffusers. Originally seen only on pro cars, these diffusers quickly made their way to the amateur ranks. Stroll through the grid at a club race, time trial or track …
Hey, I know that guy...
Thanks for letting me help out on a few articles.
Speaking of downforce this really astonished me. Hope it is sppropriate to prayer here. http://www.autoblog.com/2015/11/04/2016-dodge-viper-acr-13-lap-records-video/
In reply to stafford1500 :
Thanks for contributing, Steve!
Yes, working with Steve on that piece was fun.
Hey! I'm pretty sure I made the canards on that civic! They've did a bit of trimming, but I think those are mine! My buddy at DHP certainly made the splitter.
Since it seems to come down to drag vs downforce,why isn't it more effective to properly size a wing above the car's rear?
More effective than what? Wings and diffusers usually work together.
olpro wrote: Since it seems to come down to drag vs downforce,why isn't it more effective to properly size a wing above the car's rear?
Keith is right. They work really well together, and they're often found together.
I have zero science to back this claim up, but it seems like the underside of most vehicles would be much "dirtier" aerodynamically speaking than the shiny side. Perhaps by improving the worst area of the vehicle, you get a greater gain than what you'd see with a wing that has equivalent drag/downforce properties.
STM317 wrote:olpro wrote: Since it seems to come down to drag vs downforce,why isn't it more effective to properly size a wing above the car's rear?Keith is right. They work really well together, and they're often found together. I have zero science to back this claim up, but it seems like the underside of most vehicles would be much "dirtier" aerodynamically speaking than the shiny side. Perhaps by improving the worst area of the vehicle, you get a greater gain than what you'd see with a wing that has equivalent drag/downforce properties.
STM317: you are on the right track with cleaning up the flow under the car. Also a rear wing will help to energize the flow under the car to make the diffuser work better. Most cars have alots of little pieces hanging out in the flow under the car, which basically makes lots of little drag sources. Cleaning up the flow will remove/reduce the amount of drag from the area and allow the air to move more quickly and generate lower pressures over a greater area (more downforce). Short stubby things hanging out in the breeze have the biggest drag impact relative to size.
OldPro: the typical wing has a fixed drag to downforce ratio that will limit its performance. Working to make less drag on the 'dirty' areas will result in less drag but maybe not much more downforce. That works out to a huge increase in efficiency defined as Lift over Drag.
If anyone else has questions, post 'em up. I hang out quite a bit, but don't post unless I have something helpful to say.
In reply to stafford1500:
Thanks Steve.
Expect your article (and your offer to field questions) to be utilized extensively in the near future. 
This was a great piece, thanks! I'd love to see an in-depth article in the magazine, (or a multipart series,) including grassroots design and construction techniques.
If you could use say...a 95 Civic coupe...that would be especially helpful. 
Articles like this are why I love GRM. Having Steve in the comments is icing on the cake.
BRB going to start mocking up aero bits in cardboard and dream about having a budget.
Tyler H wrote: This was a great piece, thanks! I'd love to see an in-depth article in the magazine, (or a multipart series,) including grassroots design and construction techniques. If you could use say...a 95 Civic coupe...that would be especially helpful.
(edited, quoted the wrong post... and pics. Also thanks Steve for the contibution! just listened to the podcast.)
I designed/built an undertray and diffuser for the FSAE team I was on a few years back. It was very proof of concept with the idea design principles of: 1. has to come off easily 2. no composites, home depot level tech 3. simple to make and modify
I settled on twin-wall polycarbonate, the price has about doubled since my build... twin wall polycarbonate sheet
It's not the most rigid material as a sheet but boxing it made for some good rigid tunnels. Flat sheet could be stiffened with a bonded aluminum angle which are also good attachment points. Everything was glued up with automotive panel bonder.
All the cutting was done with a pneumatic cut off wheel and the joints were formed with a dremel router bit. It handled heat pretty well, it was right next to the header under the car which caused the skin to blister but never really caused much of an issue.
The diffuser isn't as efficient per square area as an airfoil but it has 2 huge advantages, tremendous available square area and ground effect. The drag delta with/without the diffuser is small but the downforce # can be pretty huge giving you a pretty incredible L/D compared to a conventional wing. Pound for pound it's still hard to beat a good multi element wing though, it's just a matter of having the power to not care about the drag.
Question:
On a full flat bottom/ undertray that spills into a diffuser, is the primary purpose cleaning up undercarriage airflow, or creating a downforce surface? It affects construction methods.
If it is just clean air, strength (particularly strength per square inch) is insignificant.
But if it becomes a downforce surface, it would not serve it's purpose unless it was strong enough to resist the negative pressure, and draw the car down with it.
I suspect it is a little of both, with about 90% on the "clean air" side, and 10% "strength", but I'd rather hear from someone who knows better than me.
I prefer vacuum downforce 
SVreX: sorry for not getting back to this for a few days.
The two goals you mentioned are linked. You can not get the best downforce if the surface is not 'clean'. Generally cleaning up the underbody means creating a surface that is closer to the ground and provides a surface for the pressure to act against. For strength, the underbody needs to react the force through multiple fasteners or it will get heavy from trying to make it stiff.
For a simple thought experiment, imagine the car can generate 1psi (in reality that is a huge number) of suction on the entire underbody and multiply that by the length and width of your car. You will have a seriously big downforce number. If you try to support all of that with one, two, three, or even four bolts/rivets you will get lots of deflection which decreases the effectiveness or rips parts off the car.
The strength of a diffuser comes from the vertical walls and trying to use lightweight but thick sections. The double walled polycarbonate in the post above is a good example. Lastly, shape matters. Smoother shapes will get better results than defined angles between major flat sections.
olpro wrote: Since it seems to come down to drag vs downforce,why isn't it more effective to properly size a wing above the car's rear?
Another consideration is rules. Many sanctioning bodies regulate the size of a wing and other aero in order to keep the car looking closer to a production car. Items under the car are out of site and out of mind.
Obviously the article is focused on diffusers and underbody aero in racing applications, but what about street driven vehicles where more ground clearance is needed? Would there be any performance benefit if the diffuser were say 6 inches off the ground?
I'm guessing the drag reduction would still be present which would probably help fuel economy, and perhaps acceleration/top speed, but there wouldn't be much downforce generated. Does that seem reasonable or am I off base?
In reply to STM317:
Why would someone need more downforce on the street? The extra only helps when you need to increase the grip limits on the car.
Having driven cars at their physical limits, I've never come close to that on the street.
For that matter, if you raise the top speed 5mph, I don't see that as a help on the street as well.
Maybe the acceleration. Maybe.
For sure, the fuel economy.
I read an old review of one of my cars--a '92 NSX--and the writer mentioned that the weight distribution is exactly the same at 168MPH as it is standing still. Apparently, the engineers tuned the shape of the car to maintain the same dynamics at any speed. I have confirmed this on my home-built Grassroots rolling-floor wind tunnel that I made using 64 attic fans, some plastic sheeting, and a staple gun. (not)
Anyhow, you can go out and buy what look like pretty respectable diffusers for the car off the shelf for not much money. What I am wondering is whether it is really a good idea to tinker with the work of the gods? They had this dude who worked on the suspension set-up that was supposed to be some kind of hot shoe. Ayrton Senna or something? Sounds like an illegal, if you ask me, but people seem to think he knew his stuff.
But seriously, do you mess with this knowing that you are going to have to try to work out a more effective splitter or something to help the front of the car? Or am I just trying to improve something that can't really be improved?
You can do some interesting things with aero and balance. Usually, you try to set it up so that you have the F/R downforce the same as the static weight distribution - this keeps the handling balance consistent regardless of what's going on with the air. But I've got my car set up to generate a bit more rear downforce. This means that it's more prone to oversteer on tight corners to let me rotate the car, but moves towards understeer at high speeds for a bit more stability. It's easy to tune that with the wing.
Of course it's a good idea to tinker. You'll never really learn anything if you don't. When Honda raced the NSX, they altered the aero to make it work better. And I'll bet they changed the suspension as well.
The guys at Slick Auto put a lot of time into aero tweaks with their Miata. It's not fully documented, but you can see quite a few of the tidbits on their blog. Definitely attainable by the garage builder. http://slickauto.net/?q=blog&page=1
stafford1500 wrote: If anyone else has questions, post 'em up. I hang out quite a bit, but don't post unless I have something helpful to say.
I've seen your posts an aero stuff here and would have PMed you previously but PMs don't seem to work for me for whatever reason. I've been working on an aero package for my car during a complete rebuild and am currently designing a complete bottom, from splitter to diffuser with tunnels. I don't race the car in any particular class so no regulations to follow. I plan on different setups for road tracks, LSR's, drags, open road races. The full project thread is here https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/70-firebird-the-14-car-performance-therapy-project/72356/page1/ and the last couple pages of the thread cover some of the aero modifications. If I start a thread on the just the aero stuff would you have time to offer advice?
As for a question I've been unable to find an answer to. Is there an advantage/disadvantage to using a squared off roof tunnel as opposed to a round roof? I understand the bottom edges being square to create a rolled vortex in the tunnel but why the top?
I gave away my drafting table when I moved to FL, and never learned Solidworks or other CAD programs so I'm designing GRM style using DIG (Draw In Garage). Old school, but it'll work.
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alfadriver wrote: In reply to STM317: Why would someone need more downforce on the street? The extra only helps when you need to increase the grip limits on the car. Having driven cars at their physical limits, I've never come close to that on the street. For that matter, if you raise the top speed 5mph, I don't see that as a help on the street as well. Maybe the acceleration. Maybe. For sure, the fuel economy.
I wouldn't be tickling the limits on the street. I was thinking about a dual purpose vehicle that spends time doing some track work/ autocross as well as being driven on the street on nice days. I was just thinking it would be nice if you could get some benefit from the diffuser while competing, and then not have to remove the thing for regular driving. Obviously, it wouldn't be the most effective tool for creating downforce, but any improvement in that area would be welcomed during motorsports events, and the fuel economy gains would be nice on the street. If it could have some usefulness during each driving situation that would be ideal, even if it's a compromise in one specific area.
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