Undertray design

[skokle]

First, what are peoples opinions on having an undertray specifically. Is it worth it if it doesn't detract from more important parts?

To all the teams that have one, how did you go about designing it? Any software?

[Homemade WRX]

I'd be interested in the responses from this...bump

[J. Schmidt]

I assume you're asking about an aerodymanic undertray, since the rules require an undertray to protect the driver and harneses from road debris. ...just wanted to clarify.

[NovaCat2005]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by J. Schmidt:
I assume you're asking about an aerodymanic undertray, since the rules require an undertray to protect the driver and harneses from road debris. ...just wanted to clarify. <HR></BLOCKQUOTE>
Because you never know what you're going to end up hitting on a racetrack...
Road Debris (http://www.dc101.com/timages/article/rabbit~11.mpeg)

[skokle]

Yes, talking about an aerodynamic undertray.

[MikeWaggoner at UW]

I think undertrays are bitchin'. They usually create downforce around the center of the car (not creating a lift moment on the front tires like a huge rear wing can); Aren't affected by winds, and don't mess up your CG. If you've got a car that's designed for them from square one, you can avoid raising the car too much. At the same time, many people into aero in FSAE don't realize how small the benefits may be, or how much dev time it takes. Our '04 car ROOM for an undertray, but not much else.

[Moke]

All that and they allow you paint a "Visual Haka" on it. Which is cool.

[Joel Miller]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Moke:
All that and they allow you paint a "Visual Haka" on it. Which is cool. <HR></BLOCKQUOTE>
Yes yes, I agree. Very cool. But how much downforce and weight did it add? How did you measure the downforce?

[UTA racer rikki]

skokle,

UT Arlington has been using different methods of implementing aero into our cars for the last four years. As for te undertray design, we used fluent and a master's student (haha) to get our current design. The question is, how do you want to implement it.

On our '02 and '03 cars, the undertray is mounted to the a-arms with the wings attaching to that. This had some positive, and some quite negative effects so we changed the package.

The '04 car has the undertray mounted to the chassis, with the wings attached another in another method. Undertrays can be quite beneficial if you want to use them. Just be sure you have some data to back it up.

We have some wind tunnel data from after competition last year from Texas A&M's facilities. The results confirmed our CFD models and the track times from Aero to non-aero really show the benefit.

In any case, plan ahead and design an undertray that is appropriate for your car and package. There is a lot of time needed up front in figuring out the whole package. In the end, I think you'll find out that the ends definitely justify the means, i.e. the downforce vs weight from the undertray is a none issue.

[Nate Notta]

Could somebody maybe post a photo of an "aerodynamic undertray"? I can't picture it, does it have ducting?
We lube up our shear plate every year, is that the same thing?

[Big D]

UTA's undertray:
http://maepro.uta.edu/fsae/02_undertray.jpg

I love it.... so much carbon, so sexy

[Jon Huddleston]

The problem with the picture above...That package weighs 35+ lbs for just the undertray and mounting. The '04 and 05' undertrays are less than 10 lbs.

[Nate Notta]

So does that replace the sheet aluminum shear plate we run?

[Joel Miller]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by UTA racer rikki:
This had some positive, and some quite negative effects so we changed the package.
[...]the downforce vs weight from the undertray is a none issue. <HR></BLOCKQUOTE>
Jon, Erick, few questions...

What were the negative effects? Was it just the weight? Or was there some unwanted suspension warp mode stiffening? Or was it something else?

For the wind tunnel testing, how did you guys account for the ground effect?

The sprung undertray: is much downforce/consistency lost because of heave, pitch and roll motions?

Gotta say, I do think you're doing it the right way. Keeping the weight of an aero car low is most important.

What kind of downforce do you think you're getting? Our analyses suggest that about 12kg at 15m/s is available with an undertray. Is this enough?

Thanks.

[Jon Huddleston]

The weight was the biggest issue. The system was built with 1.5 in of up/down slop per wheel so it wouldn't act as a swaybar.

The ground affect was figured with some known equations(not by me) and the numbers we got from the windtunnel without the wing.

Good question about the moving undertray, we have not done those type of windtunnel/fluent runs as of yet.

I agree with your aero/weight comment. Our goal is to have the highest downforce to weight ratio.

I believe our downforce is 170 lbs at 60 mph for the wings including the lift of the car. I'm not sure if that includes the undertray. According to the grad student who did the undertray analysis the floating undertray contributed half of the total downforce of the car.

[Andycostin]

You think a hare is bad, try hitting a kangaroo at speed, like Richards did at Bathurst in 04. But that's Aussie v8 Supercars, most of you wouldn't be familiar. Check it out...

Kangaroo at Bathurst (http://www.mytired.net/Collection/MT04_Bathurst2004RichardsKangarooHit.jpg)

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by NovaCat2005:
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by J. Schmidt:
I assume you're asking about an aerodymanic undertray, since the rules require an undertray to protect the driver and harneses from road debris. ...just wanted to clarify. <HR></BLOCKQUOTE>
Because you never know what you're going to end up hitting on a racetrack...
http://www.dc101.com/timages/article/rabbit~11.mpeg <HR></BLOCKQUOTE>

[Nate Notta]

Video here:
http://www.bathurstafterdark.com/cgi-bin/GM/archives/00000159.shtml

[Frank]

a guy on our team investigated these a little

it is a pitty that "sliding skirts" are banned

[Andycostin]

What, undertrays or kangaroos??

[Joel Miller]

Thanks Jon.

Does the 1.5" up/down slop per wheel cause different aerodynamic loads to be fed into different tyres depending on single wheel bump or warp motions? Or does the undertray have a fair amount of warp flexibility? Or do I imagine it all wrong?

I am imagining the tray suspended at the four corners. If the tray is infinitely stiff, and one corner is moved up or down, then the tray would be suspended by only three corners. Most of the load would be fed into the corners of the upper diagonal.

If the tray has some flexibility, then the load would be fed into each corner, but not evenly (unless the tray was totally flexible). Again, most of the load would go to the higher diagonal corners.

Does this differential loading lead to undesirable characteristics, or am I seeing it all wrong?

Thanks.

[UTA racer rikki]

Joel,

The "slop" that Jon so politely referred to it as was part of the design. The design of the undertray called for it to be "infinitely" stiff so that the wings, which were mounted to the undertray, would not flex into the ground with increasing downforce.

The way it turned out, the undertray was torsionally flexible. The "slop" allowed the undertray to pick up off the perches if it needed to. We didn't want to try to calculate or predict the roll stiffness that a solid attachment from one suspension member to the other side would make. That was more undesireable than the tray suspended by three corners.

In driving the '03 car after driving the '04 car, I can easily say that I think the floating undertray wasn't optimum design. Or at least the way we had it. We comprimised too many things to make it work.

As for the differential loading, you wouldn't notice it. The one thing I did notice in the driving comparison, the '03 car gave this sensation of "delayed" turn in. This is in part, in my opinion, due to the lateral "slop" designed in the system.

As for windtunnel tests, we have results that confirm CFD. We haven't tested cross flows, or cases where the car is in roll. But, the idea of the floating undertray is that no matter what the car did, the undertray and wings remained at the same height and angle with the ground. Something that aerodynamicists love apparently.

But, if you look at the bottom line, the '04 package is lighter, and the car is significantly faster for it.

[Joel Miller]

Thanks Erick.

I gather UTA did a chassis-mounted system in 2001. Any chance there could be a return to that system to save further weight? Or are the benefits of unsprung downforce (such as not compromising spring rates, const ride height etc) too great to ignore?

We are using a different suspension that makes the car very stiff in roll, with independently tuneable pitch/bump characteristics. With this, it seems an unsprung system would have less benefit, so we are trialing a chassis mounted setup.

FSAE is one of the only formulas that allows unsprung wings. What are your thoughts on the benefits of unspung downforce? Is it worth exploiting? How does the '01 car drive?

Cheers,

[Denny Trimble]

UTA's aero packages have been:

2001: Sprung Everything (wings + undertray fixed to chassis)
2002, 2003: Unsprung Everything (wings + undertray connected to control arms)
2004: Sprung Undertray on Chassis, wings mounted to chassis but angle of attack coupled to each bellcrank (4x)

I must admit I'm thinking crazy thoughts about building an A-Mod car, so aero has been on my mind lately http://fsae.com/groupee_common/emoticons/icon_smile.gif Can't do moveable aero in A-Mod, except side skirts. Hmmm...

[UTA racer rikki]

Joel,

Just a slight modification to and to add to what Denny said.

F01 was just wings, no undertray. They were chassis mounted. In driving, the '01 car is bad at low speed because it's sprung like a go cart. It's so stiff that if you make any mistake in low speed corners you just slide out of the corner. At speed though, the thing is stuck so well that it's impossible to make it oversteer.

F02 and F03 has a heavy unsprung weight with the wings attached to a rigid undertray and the untray attached near the ball joints of the uprights. While '03 didn't perform to potential at competition, a similar package on the '02 car was 2.5 seconds faster than the 2nd place car in autocross.

F04 has the aero into bellcranks package with the undertray attached to the chassis. This package, in my opinion, is the easiest to drive fast. The car is still sprung softly enough that low speed corners suffer no detrement from the aero package.

In terms of the possible packages, I would never consider chassis mounted aero again. The unsprung system has major advantages over a chassis mounted system. If it's your first go at designing an aero system though, I would definitely consider a chassis mounted system first. Their are a lot of headaches and packaging nightmares in the unsprung undertray and aero into bellcranks.

As for unsprung systems, the aero into bellcranks wins for now. There are some things that could be done to improve the floating undertray. But all that comes from time and development. Unfortunately, with only a limited time on a team (haha) you can't do everything.

Denny, an A-mod car ehh??? Count me in! We can take 'em!

[Brodyj]

UTA dudes,

You should build a A-Mod car, BIG TIME. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif I have seen some of those A-Mod cars and they are absolutely horrid.

Why did you choose to suspend your undertray and run the wings unstrung and not the other way around? How much does your aero package and hardware weight? I really don't have a opinion I just want to hear your reasoning.

[Homemade WRX]

I am finding this thread to be quite helpful being we are just now looking heavily into aero after our last autocross and running into horrible aero issues...

[Storbeck]

"with the undertray attached to the chassis. This package, in my opinion, is the easiest to drive fast"

"I would never consider chassis mounted aero again"

Do you mean you'd never chassis mount the wings, but the undertray is okay sprung, or would you still do unsprung undertray if you had it to do again, despite the success of the 04 car with chassis mounted undertray?

This is a very interesting discussion.

[UTA racer rikki]

Brodyj and Storbeck,

Hopefully I can clarify a few statements I made previously.

The unsuspended undertray was just too heavy. The undertray on the '02 and '03 car is in the order of 30 lbs. The new undertray is only 6 lbs with all the hardware to attach it.

When I said I would never consider chassis mounted aero, I was referring to the wings. I also think that getting the undertray unsprung would be worthwhile if you could keep the weight down. I think most of '04's success is that it is quite light with the full aero package.

Some of the A-mods are several years old. But the Bowland's cars are amazing. I would like the challenge of taking on the fastest cars in SCCA, but I know the top cars are really well thought out.

[Team MEM05 Drexel U]

Hey All,

Part of our Aero Team this year is focusing on an undertray design for the 2007 and/or 2008 year. Since we've never had enough participation until this year to consider such a design, we've spent a lot of time reading material as well as scanning the forums here for information from teams who have prior experience.

One of the biggest questions I have at the moment is how those of you with existing undertray designs formulated your coefficients for drag and lift calculations. I noticed UTA claimed to have spent time in Texas A&M's facilities which is awesome. The only wind tunnel we have in the area is owned by Boeing here in Philadelphia whom we're waiting to hear back from. This is important to us for calculating current down force w/o a tray as well as down force with a tray to see if the ends will justify the means. As we all know, a tray sounds beneficial, but our board will want hard proof if we expect to get any funding. Also, are there any other methods for determining the coefficients for an FSAE car other than a wind tunnel or would anyone have their own coefficients handy?

CFD software and calculations are obviously a huge benefit as well, but not only are license to such software a necessity but so is finding someone experienced in the program. I guess there's no chance anyone would be willing to share their CFD analysis, is there? We have received permission to use COMSOL but we've yet to find anyone with knowledge in the program.

Right now, any help regarding the analysis of an undertray would be extremely valuable. We considered various mounting points, including mounting to the control arms in order to place emphasis on the achieved down force focusing on the wheels. However, I believe we've decided instead to mount directly to the chassis due to other complications.

We'd also be interested in some design opinions as far as channeling and diffusers go. Any help that could be offered is greatly appreciated. Thanks for the time guys.

Mike
06/07 Aero Team

Drexel FSAE Home (http://www.pages.drexel.edu/fsae)

[Magnarama]

G'Day Guys,
I'm looking into the benefits of a undertray/diffuser for Adelaide's '06 car and I have similar questions to Mike. I've read the few books on automotive aero that I can borrow from my library and read previous aero reports from my team. In all the books that I've read they've given general pointers on how to design it but no real info on what shape the profile should be, ie looking at it from the side. If someone could point me to some literature I'd greatly appreciate it.

Also we have a wind tunnel but it is only about 800x800mm in cross section so the models have to be fairly small. I'm not too sure that models this size will give results comparable to the actual car. We were planning to do a lot of aero work in Fluent, which we have access to at Uni. Has anybody used this before and are their CFD results comparable to their actual results?

Oh and here's one last question for someone from UTA, if there's even anyone that can answer it I know people bugger off at the end of the year. I have been looking at pictures of the UTA car and how the floating undertray was mounted and was wondering if there were any issues with the undertray effecting the suspension travel? I would prefer to go with the floating undertray as opposed to chassis mounted due to the benefits of the relatively static height.

Thanks,
Tom

[andyman61]

Something to keep in mind when looking at ground effects in a wind tunnel (either front wing or tunnels) is the need for a moving ground plane for truly accurate results. Granted, at our speeds this is less of a deal than other series and can be argued as negligible, but your definition of negligible may be different than a design judge's. If you're doing CFD analysis, a moving ground plane is really easy to set up. It's only like 5 or 6 clicks in Fluent.

As for comaring CFD to empirical, yes it can be done. One of Cornell's aero papers (the first one I'm pretty sure) says that they got within 5% of lift, but drag was a lot harder to predict.