911 rake front/rear

[jyl]

Paul, did you mean same size (225) on all four corners? I thought SCs took wider tires at the rear, like my Carrera?

[Schrup]

John, would it make any difference? If so what? I'm pretty limited on size selection for a 15" tire. I wanted 245/50 rear but can't find them. I'll lower the a$$ after I get tires, but I must say I really do admire David Starsky, can't wait for the movie.

[speeder]

Island, I understand what you are saying about rake affecting CG, but really it is the height of the car that changes that. Obviously one end has to go up or down to change rake, but one could change the CG infinitely w/o changing rake, just by raising the entire car w/ same rake.

And Bill, that deck lid that was left up must have had different struts on it than mine, to stay up @ 70mph, or whatever the equivalent wind factor is when a rear spoiler's effect becomes noticable, lift-wise. Not calling the story B.S., just a little far-fetched.

[Zeke]

Would there be some consideration on the belly pan being hiked up in the rear? A lower front and higher rear might cause a venturi under the car or ground effects? If true, then this could affect the CP as well.

You know I'm just wantin' to believe that there is this vacuum at the bottom rear of the car. Or at least neutral due to rake. Diffuser anyone?

[jyl]

I'm not sure exactly what difference equal-sized F and R tires would make. I thought that, as Porsche specified larger rears for the SCs, that was probably the way to go. I have also read here that 225s may not fit, or take some work to fit, in the F.

A search of Tire Rack shows one okay choice (Dunlop SP8000) in 225/50-15 and 245/50-15, lots of good choices in 205/50-15 and 225/50-15, and a few so-so choices in 205/60-15 and 225/60-15. I also found some choices using 215 and 235.

BTW I had SP8000s, and they were okay. I prefer my SO-3's, seem better in the wet.

[masraum]

The reason for the larger rear tires is to help neutralize the affects of all of that weight out back. By putting same size tires on you are more likely to end up going the wrong way. More traction at one end is like having less traction at the other end.

[Schrup]

John, the SP8000 245/50s are on back order & they don't know when the plant is going to make a new batch. Steve, thanks, that makes sense, I'm going to try to find 205/55 for the front, & 225/50 rear. Wish I could afford to switch to 17 or 18" Fikses right now, but my interior needs my money first. I've got a DE in Oct. & I need tires. I would imagine lowering the rear will help keep me going in the right direction. I think the angle of the picture make it look more jacked up than it is.

[island911]

Hey Paul, definitely try to find 205/55 for the front. Wider is not always better.
. . .And I agree, the angle of the picture make it look more jacked up than it is. You should have posted these:

[island911]

Originally posted by speeder
Island, I understand what you are saying about rake affecting CG, but really it is the height of the car that changes that. Obviously one end has to go up or down to change rake, but one could change the CG infinitely w/o changing rake, just by raising the entire car w/ same rake. . . ..

-sigh- you had previously said "it (rake) has no effect on CG." . . . i was clarifing that it is a rare instance that rake has no effect on CG. I was NOT saying, or implying, that CG has effect on rake. It's kinda one of those 'multiplicitive inverse of the negative reciprocal' things.

[ted]

I like a little rake.
On heavy braking (more likely on the track) a 911 with too much rake can make the rear nervous.

[350HP930]

Check out the current rake on my 930 . . .

[masraum]

looks just about right! Now all you need is some Flames (painted on the front) and you would have a ride almost as cool as the one in Batchelor Party!!

[RoninLB]

Originally posted by speeder

And Bill, that deck lid that was left up
. Not calling the story B.S., just a little far-fetched.

Naw.. I don't think so..
Bill wouldn't post anything close to BS.
Maybe the thread needs more involved details for a full story. But it's mucho plenty good enough for me. very cool.........Ron

[Eric Coffey]

Originally posted by Bill Verburg
Eric, please explain your statement.

The aero benefits are the reduction of air under the car and raises the rear . These 2 effects lower lift at the front and reduce lift at the rear. That is also the effect that the front and rear spoilers have. The effect of the rear spoiler was a serendipitoius discovery, the rear lid was left raised in a wind tunnel and the reduction of rear lift was noted. The attitude of the car just enhances the effects....

It is also true that the aero ctr. is moved forward which enhances stability in crosswinds.

The ctr. of gravity is lowered whenever the front or rear is lowered. This is not relevant once the car is at optimal ride height

Weight transfer is a function of suspension geometry and spring stiffness.

Yep, totally agree with the aero benefits to forward rake. I was just tying to convey that when discussing rake, there is more to consider than just it's effects on aerodynamics.
Rake effects mechanical grip as well. It is simply another handling element to fine-tune the balance of the car. For instance more forward rake gives you less longitudinal weight transfer under braking, decreases corner-entry under-steer, and effectively stiffens the front roll resistance. Of course the inverse is true with decreasing rake. And as stated earlier, I prefer a bit more forward rake on the 911 (25" fender height at all four corners for example).

[Bill Verburg]

And Bill, that deck lid that was left up must have had different struts on it than mine, to stay up @ 70mph, or whatever the equivalent wind factor is when a rear spoiler's effect becomes noticable, lift-wise. Not calling the story B.S., just a little far-fetched.

Oh ye of little faith.....


The above is part of the documentation.

There is a small effect on weight transfer
Here is where the center of balance of a 996 is, 911 is not far different


lowering the front or raising the rear will increase oversteer probably due to the cgbeing higher than the roll ctr of the front axle

[stlrj]

reduction of air under the car

For some reason this dosen't sound right. If you reduce the flow of air under the car with no reduction of air velocity above the car, then wouldn't you be increasing aerodynamic lift? Because, according to Bernoullian principles, " the decrease in pressure as the velocity of a fluid increases", when you decrease the velocity under the car relative to velocity above the car you are, in effect, increasing aerodynamic lift.

Any Aeronautical engineers out there who could verify this?

Could this be why the factory settings result in very little if any rake at all?

Cheers,

Joe Garcia
86 Carrera
Redwood PCA since 1976

[Tyson Schmidt]

Joe, stick your hand out the window at 60mph. Try different angles with your hand and get back to us on which provides downforce.

[makaio]

As usuall, Tyson's right!

[island911]

Originally posted by makaio
As usuall, Tyson's right!

Hmm
While I'm not an Aeronautical engineer I did take an Applied Aero class as part of my ME degree.

In a nut shell, the 'hand int the wind' is somewhat a wing analogy (thought a lot of what you feel, there, falls into the 'lifting-body' catagory)

Anyway, ground effects are about shearing the air, where the wing in free stream is about pressures created though differing stream velocities. (Bernoulli / "Circulation")

[Tyson Schmidt]

It's true that by reducing the air under the car, you increase the air going over the car. The air going over the car is accelerated by the upward angle of it's shape. This will exagerate whatever characteristics the back half of the car has.
A 911 has a deeply sloping rear which causes lift. By lowering the nose, or adding a front spoiler/splitter, you will be increasing the rear lift. (That's why a rear wing is needed to balance out a front spoiler) Increasing rake though, will give the underside of the rear an upward slant, which will yeild a slight venturi effect which will reduce lift in the rear, effectively offsetting this effect. Ever looked at the shape of your rear bumper gaurds? The bottoms are steeply angled to get a little of this effect. The engine undertray on the 964 and 993 also have the upward slope to achieve the lift reduction.

Put simply, the rake of a car is changing the angle of attack just like the whole car is a wing, and you are increasing the pressure on top, and reducing it on the bottom.