Tom Suddard
Tom Suddard Digital Experience Director
2/23/18 8:22 a.m.

This article presented by RHR Performance

We talked to Olaaf Rossi, co-owner of RHR Performance, a company that has a simple goal: Make a 21st-century approach to downforce accessible to the average racer.

Read the rest of the story

sleepyhead
sleepyhead HalfDork
2/23/18 9:37 a.m.

"You'll need more rear brake bias, because the car will have a higher percentage of weight over the rear wheels"

I think the more correct statement is: "you'll need more rear brake bias, because the downforce will mitigate the braking-based forward inertial weight shift, making the rear brakes more effective.

There isn't a higher percentage of weight over the rear wheels, otherwise you've done what they suggest you not do:
"One example of this is the car's center of pressure, where the center of an aerodynamic package's downforce is located. This needs to match the car's center of gravity; otherwise, the car's weight distribution will change with speed, making it unpredictable and harder to drive"

There is a higher percentage of force over the rear wheels, thanks to the aerodynamic downforce;  but "weight" is usually reserved to describe the downforce contribution of the vehicle's mass.

sorry to be pedantic, ymmv

thanks for the article, I look forward to picking apart... I mean... reading future installments cheeky

Tom Suddard
Tom Suddard Digital Experience Director
2/23/18 9:58 a.m.

I'll admit–that was a bit of a linguistic shortcut to keep this article accessible to a broad audience. 

But, if we're truly being pedantic, I am technically correct. Why? Because a wing, uprights, bracing, and diffuser weigh more than a splitter. Therefore, a car with a downforce-adding aerodynamic package will have more weight over the rear wheels than it did before adding the aero goodies. Sure, only a few pounds, but that's what being pedantic is all about. cheeky

sleepyhead
sleepyhead HalfDork
2/23/18 10:04 a.m.

I'll post the GIF I go to whenever I realize I don't want to get into a pedantic spiral....

Tom Suddard
Tom Suddard Digital Experience Director
2/23/18 10:08 a.m.

Did somebody say spiral?

maschinenbau
maschinenbau Dork
2/23/18 10:12 a.m.

I'll admit, I clicked for GMT360's with aero kits. I never know what to expect on this site.

Tom Suddard
Tom Suddard Digital Experience Director
2/23/18 10:14 a.m.

Now THAT is an article I'd write. 

nderwater
nderwater UltimaDork
2/23/18 10:53 a.m.
maschinenbau said:

I'll admit, I clicked for GMT360's with aero kits. I never know what to expect on this site.

759NRNG
759NRNG SuperDork
2/23/18 1:20 p.m.

I own the orphan of them all 2004 Bravada.....yeah where's the dang article ......I seem to recall that the youngest unser brother actually piloted an early trailblazer(gmt360) up Pikes Peak a while ago with some success.... 

sleepyhead
sleepyhead HalfDork
2/23/18 1:32 p.m.
Tom Suddard said:

Did somebody say spiral?

dude, everyone knows it's pronounced "zhaif"

 

Stealthtercel
Stealthtercel Dork
2/23/18 2:06 p.m.

Another guy here who thought "Cool! an aerodynamic Trailblazer!" and immediately clicked the link.  They're clever, these Suddards.

sleepyhead
sleepyhead HalfDork
2/23/18 2:17 p.m.
Stealthtercel said:

Another guy here who thought "Cool! an aerodynamic Trailblazer!" and immediately clicked the link.  They're clever, these Suddards.

honestly, this photo:

has all the elements for how to make such a beast... at least theoretically... I don't think anyone's tried it practically, yet

te72
te72 New Reader
2/23/18 11:25 p.m.

I've stared at my Supra for many an hour, scheming on how to plant it into the ground even more, how to make it as slippery as possible while doing so too... and yet... I'm still not sure that's a rabbit hole I wanna go down.

 

I'll probably focus on just the underside of the vehicle first. One thing at a time...

G_Jay
G_Jay New Reader
2/24/18 7:54 a.m.

In reply to Tom Suddard :

Regarding your reply to the rear brake bias from aero comment, a couple pounds aren't significant and if the aero downforce is balanced it shouldn't matter either.  What you didn't mention but will also shift the balance is the center of drag being raised due to a wing mounted up high.  Even with no downforce the resulting pitch moment will shift the tire normal forces rearward.

Knurled.
Knurled. MegaDork
2/24/18 8:38 a.m.

In reply to G_Jay :

Either way, the effect is that of having proportionally higher vertical load on the rear tires.  Assuming that you don't translate this greater load into harder braking...

 

Brake hard enough and you don't need rear brakes at all.  You typically only see this on short wheelbase/high CG devices like bicycles though, where max braking involves the back tire hovering over the ground.

 

 

GTXVette
GTXVette Dork
2/24/18 9:07 a.m.

 When Talking Braking, they say the Front Controls 70%, and yes that is weight shift and front end dive and a couple other Forces Too.  Would making the forces from Wing Be Better Used Pushing down In FRONT of the Rear Tires .   Now saying that I see the problem is Height and we already have Outlaw Sprint cars, so Duh

olaaf
olaaf
2/25/18 5:12 p.m.

In reply to sleepyhead :

One thing to note is that the center of pressure should never be in front of the cg- it makes handling very twitchy- you want it close, but behind the cg. Also, during braking the aero balance will shift, which direction and how much depends on the package. However, by pitching fore, the rear wing AOA increases, and thus produces more downforce. In a few weeks we'll publish an aero map for the S550 project on the RHR website. No matter what, the balance will shift, but how much it shifts is part of the game. In our S550 project, we'll illustrate an example where we had designed a more effective front splitter iteration that was NOT chosen, because while it was better performing all around, it was highly sensitive to pitch.

grover
grover Reader
2/25/18 8:48 p.m.

Bummer, I was hoping to see a cool aero suv

sleepyhead
sleepyhead HalfDork
2/26/18 6:44 a.m.

In reply to olaaf :

first, welcome to the forum olaaf!  thanks for popping in here.

Thinking about it, I understand the imperative of keeping the aerodynamic downforce balanced to be slightly behind the c.g.; otherwise you'll end up with a high-speed oversteer, effectively making it difficult to trust the downforce.

The AoA increase through brake application pitching is interesting... and makes sense; although I wonder how much of that is a component the increased rake of the body providing better flow attachment over the rear window, vs. the AoA of the wing.  Although, this change in AoA is clearly an important one when setting the angle of the wing statically, so you don't exceed the maximum lift coefficient of your foil section.

more generally, I understand there's a great amount of nuance required when it comes to communicating about Aerodynamics, and the complication of doing that succinctly because of the technical nature of that nuance.  Since you might see me some in the comments on this series, I'll give you the heads up of: I've got a BS in Aero, more through force of shear will, compared to most of the other people around me that tended to soak this stuff up like a sponge.  I'll also note, that my degree is a decade and a half behind me, and I spent most of that time performing a discipline closer to controls than aero... so I'm a bit rusty on this stuff... and cars are different enough from airplanes to further calcify my apparent technical understanding?  appreciation?  whatever.

thanks for providing the community an insight to your process, and I look forward to adding to my toolkit of knowledge on the subject.

olaaf
olaaf New Reader
2/26/18 8:59 a.m.

In reply to sleepyhead :

Hi Sleepyhead- thanks for the welcome-

And, you are right on about flow attachment, I’ll post some velocity slices on our blog when I get to the Aero maps, you will see marked differences is velocity and pressure during pitch over the roof and rear window. One thing I’ve noticed in the real world is that since so few invest in a quality splitter- (our own testing shows a large flat blade splitter on our S550 model produces of Cl of .29 and a Clf of .32 at static ride height, vs. a Cl of .62 and Clf of .79 for a AI legal splitter that has tunnels and a 3D airfoil shape on the leading edge, and a Cl of 1.05 and Clf of 1.38 for our psychotic unlimited splitter), the result generally means that most racers are nowhere near rear wing stall (in my experience at the track) at no pitch.

One other thing you can check on our blog (new post about our new diffusor) is that during pitching, it’s contribution to DF increases significantly.

And, you have me there with a BS in aero- I’m software engineer- our Aero consultant has a PhD, and he does all the hard work.

Thanks for chiming in,

olaaf

DjGreggieP
DjGreggieP Reader
2/26/18 11:28 a.m.

In for this. I want to develop a better understanding of aerodynamics. Thank you both Sleepyhead and Olaaf

sleepyhead
sleepyhead HalfDork
2/26/18 12:04 p.m.
olaaf said:

In reply to sleepyhead :

And, you have me there with a BS in aero- I’m software engineer- our Aero consultant has a PhD, and he does all the hard work.

Thanks for chiming in,

olaaf

No, I wouldn't say that... in general I try to avoid the "one-up-manship" that tends to frustrate most aerodynamics debates.  I was trying to use that as a shorthand for "I understand some of this stuff, so feel free to speak to me on that level".  It's probably closer to say that we both have a "practical understanding" of aerodynamics... only I went through a "classical training regime, followed by atrophy and self study" to achieve it.  You just went straight to "practical understanding" cheeky... after all, you get to play with this stuff on a much more frequent basis than I do, and with cooler simulation codes.

And, I'm all for more people being able to obtain that proficiency.

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