Creating downforce with underbody mods

[JMortensen]

What is the purpose of the car? If you really want to make downforce from an undertray/diffuser, you need to run it within a few inches of the ground, and you're going to need to run very stiff suspension to keep it very close, and very level to the ground. If you can't do that, you're either not going to make any appreciable amount of downforce, or you're going to make the car very hard to drive (center of pressure moving all over the place when the car pitches and rolls)

 

So, can it be done? yes. Is it worth it? only for a dedicated racecar, in my opinion

Good point. Similarly it's probably worth mentioning that splitters can be a bit touchy when it comes to pitch sensitivity. If you're using soft front springs and the nose dives under braking that can suddenly get the splitter closer to the ground and increase downforce in the front.

[3star2nr]

very good point about the stiffer springs.

 

Heres the thing though, some one commented that the flat belly wouldnt make much of a difference unless the car was very low, thats not 100% true. I think that statement is a bit too general.

 

My car for example is a 3000GT, It comes with plastic undertray covers, to give it a some what flat floor.

 

I took them off because they were getting old, the difference in stability was very very obvious.

 

From as low as 60mph the front end would wander about, and the car was very susceptible to cross winds. It also felt slower up top. I used to climb to 120 with ease, and cruise at that speed perfectly stable. With the undertrays gone the car felt noticeably more unstable, and felt like it wasn't pulling as hard as it used to.

 

Therefore I think the flat belly will make a difference, even with stock ride height. And will have an even greater effect once the car is lowered.

 

IMHO if you have the cash and time I say go for it.

[tonycharger72]

"Heres the thing though, some one commented that the flat belly wouldnt make much of a difference unless the car was very low, thats not 100% true. I think that statement is a bit too general."

 

I read through some articles from autospeed.com a while ago about installing flat underbelly pans, from memory they started out trying to increase downforce but ended up not really focusing on that goal and were more interested in Engine Cooling, Intercooler Cooler and Fuel Efficiency,

 

Think they used a Toyota Echo and on a highway, travelling at 80km's/hr reduced fuel consumption from 5.5-6L/100km's to in the mid 4's with the addition of a well designed under engine bell-pan that sloped down towards the front wheels - and the other car was some 7MG powered early 90's toyota - with front spoiler, under engine belly pan and "correctly" placed hood vents, all done with the aid of an Magnehelic Differential Pressure gauge, were able to get the radiator to work better and air to go in through the intercooler hood vent instead of being pushed out of the engine-bay through it, which is what it was doing before the underbelly pan from memory,

 

Even if you don't get more downforce, which you probably won't without dropping the car a bit, you could possibly end up with a radiator that works better and a car that is slightly better on fuel - if you design it correctly!!!

[JMortensen]

I think you guys are confusing drag and downforce, and also the effects of hood vents vs belly pans. Hood vents can vent under hood pressure, and under hood pressure can cause a large amount of lift. So comparing similar cars, one car with hood vents and a belly pan to a car with hood vents to a car with a belly pan to a car with neither would be a better way to clarify what is causing what.

 

If you look at wind tunnel testing like the Lotus test done in Racecar Engineering last year, you'll see that a car with a flat bottom and a diffuser didn't start to really work to produce downforce until the outer diffuser fences went down to about 2" off the ground. The flat bottom car is not the best design for downforce either. It's what most forms of motorsport were left with after ground effects cars were banned. In order to make it work to produce any significant amount of downforce the car is going to have to be LOW.

[3star2nr]

Interesting points j, what about the effect of reducing drag on the car though. The flat bottom car should produce less drag correct?

 

I think you gain alot more than just downforce by running a flat bottom

[tonycharger72]

Jon I am pretty sure they started off with a Cressida with a single bonnet bonnet scoop (little thing) mounted at the front LHS of the bonnet right above the intercooler, intially air from inside the engine bay was pushing out through this opening, not actually sucking it in,

 

After slapping on a belly-pan and additional bonnet vents air ended up being sucked through this intercooler bonnet scoop and the radiator worked a lot better,

 

However, the point I was getting at was the people in the autospeed articles started off trying to get more downforce for their street driven cars but instead, it appears, got caught up with things they didnt expect to happen, which was the radiator and intercooler working better with improved fuel efficiency via the installation of a belly-pan and appropriately place vents,

 

Another point from memory was it was pretty much impossible to create much downforce on a street driven car with stuff like front spoilers and splitters because you had to get it really low, impractically low just like your saying mate!

 

So I think there is a place for this sort of stuff on a street driven Zed, just not for the sole purpose of increasing downforce, or probably that purpose at all !!!

[260DET]

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Another point from memory was it was pretty much impossible to create much downforce on a street driven car with stuff like front spoilers and splitters because you had to get it really low, impractically low just like your saying mate!

 

................................

 

I don't think you meant to say that

 

On the under car stuff, for all the work involved the return would be awful small with a S30. By 'under car stuff' I mean anyhing from the gearbox back to the diff. Splitters, spoilers, airdams, wings, diffusers, undertrays......... They all can be used on the S30 as has been discussed many times.

[tonycharger72]

I don't think you meant to say that

 

Yes I did !

 

But it was a bit of a blanket statement - all cars, all spoilers I am sure is not exactly correct!!!

 

However, the point of the article was that the fellows doing the tests on their particular car's different front spoiler creations was that unless they reduced ground clearance to an unrealistically low amount (for a street driver car) they couldn't really increase downforce at the front of the car via a spoiler - not unless you could mechanical raise it when required

[260DET]

Just look at the racing development of the S30 and you will see why I made that statement, keeping in mind that the early spoilers were quite modest by modern standards yet they worked.

 

Autospeed does some good stuff but I'd be careful about any general conclusions, aero is very much about each individual cars body design and shape.

[Gollum]

Yea, be careful with blanket statements. They can get you in trouble, I know.

 

I think the S30 will benefit a lot more from limiting air under the car, than trying to keep the air that is there clean. I think a good front end that incorporates a splitter and blocking off most of the open area, combined with side skirts will do so much more for downforce than any belly mods/ diffuser could ever do.

 

It's also more practical for a street driven Z... I'm not saying a smooth belly is a bad idea, because I plan on smoothing out the entire bottom end of my current race car project (s130 though). I just think that the belly is that LAST key to aero to be focusing on. The front end is the biggest issue with the S30. I'd like to see more people designing front ends that fix the issues yet STILL look good.