911 rake front/rear

ohecht · 07-18-2003, 09:08 AM

I saw Ray Scruggs' recommendation that the rear of a 911 me 3/4 to 1 inches higher than the front.

Does this sound right to the experts? My car is an 82 SC.

Thanks,

Olivier

Schrup · Schrup's Garage

I'm no expert, but with 60 series tires I think a slight rake looks cool on a SC. I think mines about 3/4"

IROC · IROC's Garage

Most 911s are set up with some amount of rake (with the rear a little higher). I haven't seen any definitive reasoning why. I had mine set up pretty much level until I put larger tires on the rear. The "no rake" set-up works well, too. I even know a knowledgeable 911 mechanic that recommends no rake.

Mike

masraum · 07-18-2003, 09:36 AM

According to Bruce Anderson a 911 handles best with about a 1 deg rake.

Paul, why the giant tires in the back. Aren't those 16" wheels? The stock sizes (arguably the best sizes) for those are 205 55 16 and 225 50 16.

Generally the US spec cars have had their rear ends jacked way up in the air (by at least an inch) due to the bumper height laws here.

ohecht · 07-18-2003, 10:25 AM

Steve,

What does 1 degree of rake translate into in terms of how high the rear is compared to the front at the fender? Did Bruce give his recommendation in those terms as well?

I am sure I could calulate it with the right measurements, but that would just add variables.

Olivier

Neilk · 07-18-2003, 10:38 AM

It's about 1/2" measured at the top of the fender wells. The front being higher.

RoninLB · 07-18-2003, 11:03 AM

Quote:

Originally posted by masraum

According to Bruce Anderson a 911 handles best with about a 1 deg rake.

I couldn't find the hard info..

but I believe the 1deg./1in. of rake puts the center of gravity under the dash...........Ron

Eric Coffey · 07-18-2003, 11:05 AM

Yep, kinda confusing, but .5" higher in front, from the fender lip to the ground, equals roughly 1 degree forward rake. However it is Ok (and desireable IMO) to run a little more forward rake. I simply set the same height on all four fender-to-ground measurments. This will give you a bit more agressive rake, and make ride-height set up easier.

IROC · IROC's Garage

Changing the rake of the car won't affect CG - the CG is "where it is" regardless of the orientation of the car.

I always suspected the rake thing was aerodynamically driven. Maybe it's for looks. :>)

Mike

speeder · 07-18-2003, 11:30 AM

The slight rake is for aerodynamics/weight transfer. IROC is correct, it has no effect on CG.

pschrup, why would you put taller-than-stock tires on a 911? That's a new one. That's a good looking car you've got there, but if you got it set up right it would look a lot better. Looks like Starsky or Hutch owned it.

RoninLB · 07-18-2003, 11:35 AM

Quote:

Originally posted by IROC
Changing the rake of the car won't affect CG - the CG is "where it is" regardless of the orientation of the car.

I always suspected the rake thing was aerodynamically driven. Maybe it's for looks. :>)

Mike

excuse my wording..
the hard info said something about air movement /lower psi maxed out under the dash..........Ron

Hockey fan · 07-18-2003, 11:43 AM

Zebra Three ready to roll!

ohecht · 07-18-2003, 01:15 PM

I thought I was sure that the rear was "supposed" to be higher, but am I wrong?

Olivier

nostatic · nostatic's Garage

Quote:

Originally posted by IROC
Maybe it's for looks. :>)

Does any of the other gobbledeegook really matter?

Just trying to emulate a jacked-up hemicuda...

Bill Verburg · Bill Verburg's Garage

When there is a bit of forward rake the measurement from the front lip to flat horizontal surface will be ~.5" higher than at the rear lip due to their diffing shapes.

The purpose of the rake is aero.

This is 25.7" f & 25.3" r

masraum · 07-18-2003, 02:17 PM

I think what we are looking for here is center of pressure, but I'm not certain.

More rake will create more forward weight transfer which can be used to tune handling.

Eric Coffey · 07-18-2003, 02:58 PM

I think Steve is correct. Rake is not just an aero consideration.

Bill Verburg · Bill Verburg's Garage

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.

Schrup · Schrup's Garage

Steve, my tires are 205/65/15 fr & 225/60/15 rear. The PO put those on & I have a reciept for the speedo calibration. My wife said I couldn't get new tires till those are worn out. Mission Complete

I'm buying 225/50s SO3s for all 4 corners. After I wear them out I hope for some 18" Fikses.

island911 · 07-18-2003, 03:39 PM

hmm.. . on Pauls 911, he clearly lists that it is a 1980 911SC Weissach Edition. This means 7&8's x 15"

I also apears to have been lowered in front, with stock (US) height in rear. (Lowering the rear would be good for various reasons.)

Without a doubt, rake will infulence CG & CP.

CG, center of gravity, being referenced from the ground, means the rake can have zero impact on CG only if the rake is changed by rotating about the CG (good luck with that.) For practical purposes, putting the rear in the air (like the weissach above) the rake is brought up along with the CG.

Then there is CP, center of pressure, an aerodynamic center of "-gravity" . . .though here the "-gravity" is the culmination of aerodynamic lifts and drags.

Rake will certainly effect this as well.