woodysjh

Does anybody have a DIY for the RX8 that is like the Mercedes SLR McLaren air brake on the trunk of the car?

Zoli007

Since when did the RX8 have 600 HP?


Until then you dont need an air brake, and the SLR doesnt need it either.

pcimino

Yes, there's a little button, lower left, next to the hood release. All you need are some big spings on the trunk hinge.

KYLiquid

hrmm it would be cool, but its really not needed. I also saw a review of the SLR where they did a brake test with and with out the air brake. I think The stoping distance was nearly the same +/- 10 feet from 120. Should help out at higher speeds.

pcimino

I don't know how big an air brake we're talking, but let's say 2 sq feet...

F = 1/2 ρ CD A v^2
ρ (Greek letter "rho") is the density of air = 0.002377 slugs/foot3
CD for a flat plate is 1.28

So F in lbf = 0.00304256 * (Velocity Squred in FPS)

So braking force is:
30 MPH ~ 44 fps 2.7 lbf
90 MPH ~ 132 fps 24 lbf
120MPH ~ 176 fps 94 lbf

You could design a plate to have more drag than a simple plate, such as vanes, create lift (downforce) adding induced drag on top of the parasitic drag. Or you could increase the surface area.

Sigma

A DIY for something that would require replacing the trunk with a custom-built one, creating a liftable panel for it, engineering hydraulics (or some sort of electrical actuating rods), engineering and creating a computer to tap into the existing brake functions to operate the thing, and then somehow making the whole thing work.

Yeah, that sounds like one hell of a DIY project. Involves a lot of custom work and would probably cost thousands when all was said and done.

KYLiquid

2sq of surface area sounds about right for air brake. But the drag you calculated for the air brake... is that the maximum? cause the wing doesnt do completly vertical so that acutal drag may be different.

EDIT : Mercedes says the air brake deploys at a 65* angle "to increase drag and provide downforce for faster braking.

pcimino

It was a rough and dirty estimate.

You could take my first tongue-in-cheek suggestion and actually use the trunk. This assumes that WOODYSJH wants a functional brake, not just a cool looking brake.

Add ribs to stiffen the trunk lid, add a hydraulic strut that deploys when the trunk is released and then add a switch to the brake light along with some logic that says "don't deploy unless above such and such a speed", and maybe "I don't have any loose laundry floating around in the trunk".

For a "cool" air brake, you're right, pricey. I'd put something on the rear fenders that deploy up and out, if you could find some structure there. Another option would be the roof or replace the rear vent windows.

Someone in some thread suggested openning the rear doors. THAT would give you some serious surface area. ;-)

He could always go with a drag chute, but then you either have to completely stop and roll it up or cut it loose.

KYLiquid

http://www.germancarfans.com/news.cf...es-benz/1.html

Information on the 'stoping systems' for the SLR.

I dont doubt that the spoiler adds stability during high-speed stoping as well as drag to help slow you down....but I think the car would stop almost equaly as well from most speed without the airbrake.

as pcimino did the math, i dont see it helping out much below 120mph.... but i bet it sure is nice when hauling that monster down from 208mph to 55mph on the interstate...lol...take that highway patrol!

EDIT: had to fix the link.

pcimino

Quick search for "speedbrake kit" came up with this baby. Didn't hunt for the specs, but should be able to install one or two of these babies in the trunk. Again, will need to be tied to something structural and/or beef up the trunk lid. There's a flash anim showing them deploy.

http://www.preciseflight.com/sb.html

KYLiquid

If you didnt have to open the front doors to get to the back doors, you could rig up an electric lock to open the suicide doors at speed. that should equal gobs of instant drag :p for about .2 secs then the doors fold completly backwards and become one with the rear fenders...lol

you seem to know your stuff about the math of this stuff. From the link I posted above....how do they calculate the '2000hp' of braking force? really I am just interested in the rx8's stoping power. Also 62.5mph>to>0mph in 114 feet doesnt sound that amazing, isnt the rx8's stopping distance (with sports brakes) around high 120's low 130's? I know its about as easy to lower the stoping distance in a car as it is to get HP with bolt-ons in the rx8 (other than major upgrades) but I know our cars have a very good breaking system...maby im thinking the numbers are lower than they are but 114 feet isnt THAT impresive given all the technology in the car...of course it is pretty heavy.

KYLiquid

very cool. I tink it would be possible, as you said, to mount these at the bottom of the body, just infront of the rear wheels, Im willing to bet you could tie into some of the cars frame down there. If these were 6" long or so, they would reach out into the airstream along the side of the car. Im thinking the area just infront of the rear wheel...where the little plastic paint protection is. Interesting. Some good aero testing to see how the air moves over the body would also help in findign the right placement/angle. very interesting stuff...good find!

I wonder if you could tie into the cars braking system to operate them off the brake pressure or if they could be run off the cars vacume under off throtle

staticlag

Ricey.

But:

Downforce on the rear of the car will do nothing unless you upgrade the rear brakes to 12+ inches. Even then, you would need a hell of a lot of downforce to actually make the braking power balanced (front vs back). Which means you would need a great coilover setup as well.

Rice = worthless mod.

devoid

Does anybody else think Knight Rider, and KITT in Super Pursuit mode when we discuss this? Or maybe its just me! :D

pcimino

2000 HP converts to 1,100,000 foot lbf/sec

That doesn't sound right AT ALL. Even if they inverted the calculation it doesn't make sense (1 HP = 550 ft lbf/sec).

Maybe they meant 20 HP ? Even then it's instantaneous. As shown above the force drops off dramatically with velocity, since its a function of Velocity squared

pcimino

If he's willing to weld the rear doors shut, the "Virtual B Pillar" becomes fixed, and usable for something like this. Don't know if I'd want to attach these to the rear doors without welding it. Of course if you're going to go this nuts, you need something to really exercide the speedbrakes...like a 3 rotor?

KYLiquid

yeah that is a hell of a lot of force. Do you know a way to calculate that much force over a specific distance to figure out how mang g's it would create? Would you need the weight of the vehicle?

Im just thinking that might be a way to double check the math. If the Car stops in 114 feet from 62.5mph and pulls 1.3g's how much actual braking force is there?

pcimino

Dist = 1/2 Accel T^2, solve for Time
114 = 1/2 ( 1.3 g * 32.2 fps^2) * T^2 solves to 2.3 sec

F = ma : 3000 * (1.3 * 32.2) / (32.2 slug conv) = 3900 lbf

Glyphon

edmunds has the 60-0 braking distance listed as 107.93 ft. various other sources list it at 114ft.

KYLiquid

I think 60-0 is 108 (107.93) and 62.5-0 is 114.

So going by edmunds site the SLR will do 60-0 in 108 FT the RX8 will do 60-0 in 114ft. Thats 6 feet....Obviously the RX8 weighs a lot LESS than the SLR, but that makes me pretty happy that the gap is so small. We dont have the fancy carbon/ceramic brakes, or the 8 piston calipers up front or an air brake, and we stop nearly the same. Thats pretty cool.

Curb Weight
----------------
SLR - 3734
RX8 - 3029

Glyphon

wait, you're contradicting what you just said...the 60-0 distances would be the same...

KYLiquid

lol now im confused.

I posted that the SLR did 62.5mph to 0mph in 114 ft. You quoted that and posted "edmunds has the 60-0 braking distance listed as 107.93 ft. various other sources list it at 114ft." I think you were talking about the SLR.

In my post I then said, 60-0 for the SLR is 108 ft (rounded from 107.93) so the 114 ft (from various sources, including M-Benz) is for 62.5mhp (100kph) so the extra 2.5mph equals 6 feet more.

I then posted the 60-0 times for the RX8 and SLR (as found on Edmunds, since you would hope that the 2 times are comparable and were tested in the saw way)

It just so happens that the RX8 is 60-0 in 114ft. So the SLR will stop 6 feet shorter from 60mph and the same distance from 62.5mph

I was just showing that with the help of the aluminium, and rotary engine these cars are very close in braking (at least from low speeds, say under 100, 100+...esp upwards of 150 would be a VERY differnt story) although clearly the SLR has the advantage in moving away from a stop.

I hope that clears it up....otherwise im not sure what your confused about.

Glyphon

AH. that does clear it up. the edmunds 60-0 distances were for the rx-8 (they didn't have number for the mclaren that i could find).

so, to clear things up, according to edmunds, the rx-8 does 60-0 in 108 ft, or equal that of the mclaren :D

gr8jab

These probably would not make very good speed brakes for a car. Contrary to their name, they are really 'spoilers'. As in "they spoil the lift" created by the wings and allow the pilot to decend at a faster vertical speed, while keeping his forward speed (thought the airmass) the same.

Some airplanes do have speed brakes that are designed to slow the aircraft, but they are much larger and are usually on the top and bottom surfaces. An example is the Russian Blanik sailplane. It has terminal velocity speed brakes on the top and bottom of the wing, probably totalling about 20 sq feet. While open, the plane's terminal velocity is less than Vne (maximum structural speed).

I know, I ramble too much...

cleoent

yeah here you go

http://www.rx8.com/airbrakelikemclarensimplediy.html