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I'm sitting here with a lousy head cold and my brain refuses to do any thinking beyond looking for part numbers in PET.

Anti-Dive is an area I've been interested in after becoming aware of how the front a-arms function and the process of eliminating stiction when replacing the bushings with harder material by essentially aligning the a-arms in a straight plane. Elephant Racing has an excellent write up on their site (Tech Topic - Suspension Binding).

I am interested in what changes (if any) occur as a result of using spherical bearings to achieve collinear alignment as shown in the image on proffighter's post. These, in effect, lower both end of the a-arm by approximately the same amount but is that sufficient to change the anti-dive.

Time for someone with more suspension theory or practical knowledge to add to the discussion.

Also notice from the images of the 930 anti-dive spacers that they are not symmetrical but create an angular displacement to the front of the a-arm which I believe may help to offset any natural stiction in the original factory rubber bushings. I notices this angle with the pieces off the car. Compare the 930 image with the SC.

I've eliminated stiction by both using the Elephant Racing kit and by using washers between the a-ram mount and chassis to align the a-arms. Due to the anti-dive question, I'm not certain which is preferred.

Good stuff. Continue.

Jim

Old 03-09-2010, 08:11 AM
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Max Sluiter
 
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The anti-dive effect comes about almost entirely from the change in axis angle. Moving the whole A-arm down in the same attitude will move the SVIC location only slightly so the anti-dive effect will be much less.
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911S
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/
Old 03-09-2010, 04:30 PM
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Quote:
Originally Posted by Flieger View Post
The anti-dive effect comes about almost entirely from the change in axis angle. Moving the whole A-arm down in the same attitude will move the SVIC location only slightly so the anti-dive effect will be much less.
That was my assumption. Thanks.

Your discussion of anti-dive is interesting and I intend to study it more. Anti-dive doesn't get a great deal of attention in most suspension tuning sources.

How much axis angle change is necessary to make a difference in the anti-dive characteristics that can be felt by the driver and what are the characteristics that the driver senses?

Jim
Old 03-10-2010, 08:38 AM
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*Superceded by following posts.*

Most of what I know comes from the Milliken book Race Car Vehicle Dynamics ISBN 1-56091-526-9 from the SAE.

The anti-dive is determined by the line of action of the braking force: contact patch to side view instant center. The instant center is determined by the intersection of two "imaginary" lines. The first is a line perpendicular to the strut centerline. The second is the axis of the A-arm. Where the two meet in space is the instant center- the pivot point for the wheel at that single moment in time. The instant center changes as the suspension moves throughout its range of travel. The strut inclination (caster) will get more severe as the suspension compresses. This will cause the IC to move down and forward- assuming positive caster and keeping the other variables constant. Angling the axis of the A-arm as in the 930 will tend to raise the IC and bring it forward.

Braking force is transmitted from the contact patch to the SVIC. The moment of the braking force * the height of the center of mass above ground level must be reacted by the changing vertical loads on the wheels. If the IC is on the ground, then all the vertical wheel load is taken by the springs since the load is perpendicular to the SVSA. Increasing the angle of the SVSA inclination will send some of that through the suspension member instead. The total longitudinal load transfer remains the same but more anti-dive causes the load to transfer faster since springs are not in the mix.

The anti-dive is further complicated by the need to consider the rear SVIC and the rear braking force (different from the front) with any anti-lift that it has.

To find the anti dive, one sums the forces and moments about the SVIC. The braking force is applied at the CG and causes a couple around the SVIC. This is mass*acceleration*%front braking.*SVIC height. Divide by SVIC length to get anti-dive force, which is the force pushing the tire into the ground due to applying a horizontal force through a slanted suspension member.

Anti-dive accelerates the rate of load transfer to the front wheels under braking. It feels like very stiff sway bars but in the longitudinal direction.

The amount of anti-dive wheich results from a given change in A-arm angle depends on CG location and the inclination of the strut.

% anti-dive front= [W(a/g)(% front brake balance)(SVSA height/SVSA length)]/[W*(a/g)*(h/l)]

% front anti-dive = (%brake balance front)(tan p)(l/h)

where p= angle on the line from contact patch to IC, h= height of center of mass, l= wheelbase

That is all for now. I am going to do some more studying on this topic to better understand the dynamics myself.
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911S
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/

Last edited by Flieger; 03-13-2010 at 05:23 PM..
Old 03-10-2010, 06:00 PM
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Here is another way to look at things, just as with roll centers and lateral load transfer;

The line of action of the braking force is a slanted line along the tire contact patch to SVIC location. There is a force component through the suspension arm (radial direction) and a force component perpendicular to the suspension arm (tangental). Using some trigonometry, one can convert the braking force in the horizontal direction into radial and tangental force components. This gives rise to a vertical component in order to complete the vector addition triangle. This vertical component pushes up on the sprung mass.

The radial component is not reacted by the springs since it is through the SVSA. The tangential component is in the rebound direction of spring displacement, therefore, it subtracts from the load on the springs and therefore reduces the pitch angle.

SVIC= side view instant center
SVSA= side view swing arm
Force vector = from tire contact patch to SVIC
Θ = angle between force vector and horizontal
d = horizontal distance from tire contact patch to SVIC

Fradial × cosine(Θ) + Ftangental × sine(Θ) = Fbraking + Fanti-dive

Ftangental = Fanti-dive × cosine-1(Θ) ,alternatively, Ftangental = Fbraking × sine-1(Θ)
Fradial = Fanti-dive × sine-1(Θ) ,alternatively, Fradial = Fbraking × cosine-1(Θ)

Fbraking × cosine-1(Θ) × cosine(Θ) + Fbraking × sine-1(Θ) × sine(Θ) = Fbraking + Fanti-dive

sine(Θ) ÷ cosine(Θ) = tangent(Θ)
cosine(Θ) × cosine-1(Θ)= 1
sine(Θ) × sine-1(Θ)= 1

Fbraking + Fbraking × tangent(Θ) = Fbraking + Fanti-dive

Fanti-dive = Fbraking × tangent(Θ)

As stated above, Fanti-dive is the vertical component pushing up on the sprung mass.

Remember that the total longitudinal load transfer in steady-state braking will be independent of anti-dive %. Here are the equations.

Wcar = weight of car
Wfront = weight on front wheels
Wrear = weight on rear wheels
mcar = Wcar ÷ gravity
Fbraking = mcar × abraking
h = height of center of vehicle mass
ℓf = length horizontally from front wheel to center of mass
ℓr = length horizontally from rear wheel to center of mass
ℓ = wheelbase

sum of moments about the center of mass:

I is the moment of inertia, hard to figure.

ΣMG = Fbraking × h –Wfront × ℓf + Wrear × ℓr = I × 
Wcar = Wfront + Wrear

ΣMG = Fbraking× h –Wfront × ℓf +(Wcar - Wfront) × ℓr

Fbraking × h + Wcar × ℓr = Wfront × (ℓf + ℓr) + I × 

Wfront = ((Fbraking × h + Wcar × ℓr) - I × ) ÷ ℓ

Because anti-dive arises from the application of braking force through an angled suspension member, it acts as quickly as the braking force is applied. The acceleration is linked to the braking deceleration. It reacts much quicker than springs since spring force depends on displacement which means integrating twice with respect to time the pitching angular acceleration of the center of mass, then multiply by ℓf. The angular acceleration would be

k = spring rate
x = spring displacement from static ride height

((Fanti-dive+ kx) × ℓf )– (Fbraking × h) – (Wrear × ℓr) = I × 

For maximum braking/weight transfer,

(((Fbraking × tangent(Θ) + kx) × ℓf) - (Fbraking × h)) ÷ I = 

We want  to be small. Negative is pitching forward.

From the form of the equation, we can see that having the center of mass low and rearwards is good. Having a high Θ is good.

x depends on 

For constant , we could use:

x ÷ ℓf =((t2) ÷ 2) + Ct + C where C is the integration constant. Ct is initial angular velocity, 0. C is initial displacement, 0.

However,  is not constant. As the springs compress,  drops.

If anti-dive were zero, all of the pitch reaction would come from the springs so for quite some time, kx would be too low and  would be negative.

If anti-dive were too high, the sprung mass would actually pitch rearward under braking.
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911S
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/

Last edited by Flieger; 03-11-2010 at 05:21 PM..
Old 03-10-2010, 09:36 PM
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Rats, the subscripts did not come over from the word document.
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911S
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/
Old 03-11-2010, 05:21 PM
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Flieger,
what you say is correct. But anti-dive/anti-squat has to be equall (not only dealing with the front geometry) in a Porsche. I believe your formula's are for other racecars. Any time you try to calc. dive you have to deal with squat. Also with anti-dive comes a change in bump steer.
Bernie
Old 03-12-2010, 12:30 AM
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Yes, to simplify the calculations, I modeled a maximum braking event- full weight transfer to the front wheels. However, the braking force from the rear wheels still has the same vertical moment arm so the results should not be too different from a fast sports car going maybe 8/10ths on a track. The conclusions/trends of what increased anti-dive does and about how to get more anti-dive are still valid. To actually come up with real numbers would require some very involved and accurate measurements of suspension geometry and vehicle mass locations- I, SVSA inclination, CG height, weight distribution, braking force, brake bias,...

The rear does have anti-lift due to the semi-trailing arm geometry, however, the vertical anti-lift/anti-dive force is a result of the horizontal braking force. The rear usually does a lot less of the braking than the front so the front anti-dive dominates. The total braking force and moment arm are the same, so using all of the braking at the front I feel is a reasonable simplification. To calculate rear anti-dive just use the brake bias times the total braking force and use the rear SVSA IC, then sum that with the front.

The anti forces are only due to the horizontal force, it is why there can be no anti-squat on the front wheels of a rear-wheel drive car.

How do you define bump steer? Changes in toe due to suspension deflection up/down? A change in toe is not a big issue if it is symmetrical, which is the case when the wheels are somewhat equally loaded under straight-line braking. Also, if the change in toe is a small change in the value od toe-in or toe-out, that is not that bad. The worst is when the toe goes from toe-out to toe-in , as when the front strut suspension geometry is excessively lowered. Compounding this is that in a turn the outer wheel could be over the hump and toeed-out while the inside wheel is still toed-in due to the opposing suspension deflections.

Anti-dive reduces suspension deflection under braking, though, so there should be less toe-change and more stable steering during steady-state trail braking, if the initially faster load transfer does not swing the tail around.
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911S
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/

Last edited by Flieger; 03-12-2010 at 01:25 AM..
Old 03-12-2010, 01:16 AM
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This does not really have anything about anti-dive but here is some more interesting suspension geometry.

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1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/
Old 03-12-2010, 01:23 AM
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Flieger,

Maybe to much neg camber on that one! Not sure I understand what they were working toward with the severely canted wheels.


Is Anti Dive geometry (AD) like Roll Center geometry in that it effects the 'rate' of suspension compression?

That is, a more favorable AD means that with a given spring rate the suspension will deflect more or less depending on the AD?

With your model, could you come up with a rough estimate as to how much as a 930 v 911 front set up might reduce deflection, probably as a percentage?

I am sure front toe might benefit some but a you noted if it is balanced its effect might be modest. It would seem it would seem reducing camber under braking would help the front to keep more tire on the road under braking.

That is, if we can reduce front dive from say 3" to 2", neg camber under braking might go from say -2.5 v -2 under braking.

If so, and we double our spring rate the front would deflect say 1.5" v 1" with a more favorable AD and neg camber might move from say -2.5 to -2.25.

I guess to me the big question is if we go to the effort to pull off a full 930 front change over, how much as an approximate percentage, might it reduce front deflection.

Thanks for the topic and info!
Old 03-12-2010, 08:03 AM
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The Cal Poly guys were compensating for the lack of cornering force that the thin scooter tires deliver compared to the conventional wider slicks used by all the other competitors. I don't recall why the (Zero motorcycle, I think) electric motors were not compatable with the wide slicks. Anyway, camber is a more efficient way to generate lateral force from a tire than is steering slip angle. This is because the lateral deflection of the tire contact patch is midway along the length of the patch. With steering angle, the lateral deflection is mainly at the extremities. The pressure distribution on the patch is greatest in the center, so the camber has more effective lateral force rate than does steering. The Cal Poly guys actually had Miliken here on campus to check out the car, which he thought was cool. I wish I could have met him.


I do not have enough measurements to make actual numbers for you with regards to anti-dive force and SVIC. It will vary between each car and each suspension setting and each point in the suspension travel. That is why it is an instant center. It is only good for that instant. I wish I could give specifics, though.

All I can do is explain trends. A 930 will have more anti-dive than a 911 because a 911's A-arm is basically flat so the SVIC will have to be at the height of the A-arm axis. It is just a matter of how far back, which is determined by caster.

To get more anti-dive, one raises the SVIC. To raise the SVIC for the strut front suspension, one tilts the A-arm axis nose-down and reduces the caster of the strut. In the rear, one raises the pick-up point for the trailing arm. This is exactly what Porsche did for the 930 and some RSRs.

Continued in next post.
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911S
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/
Old 03-12-2010, 03:50 PM
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r = vertical distance between torsion bar center and top strut mount
h = SVIC height

ℓ = r ÷ (tangent Θcaster + tangent Θarm)

h = r - ℓ × tangent Θcaster

h = r – ((r ×tangent Θcaster) ÷ (tangent Θcaster + tangent Θarm))


Fanti-dive = Fbraking × (h + ½d) ÷ ℓ
d = tire rolling diameter
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911S
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/
Old 03-12-2010, 03:57 PM
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Red is the inertial reaction forces and the green is the actual force from the wheel, divided into the separate components in black.

At the instant of brake application, the spring force is zero since the displacment is zero (we are talking dynamics here, not steady-state).

The Fanti-dive is the instantaneous front load transfer (added to static front weight).

Steady state load transfer (once the springs are fully deflected and the pitch has stablized) is (W/g)×H/L where H is center of gravity height, L is length from tire contact patch to the center of gravity, W is weight and, g is gravity= 32.2 ft/s^2 for pounds.

vertical Fspring = ky = ((W/g) × H ÷ L) - (Fbraking × (h + ½d) ÷ ℓ)

Spring deflection = y = b × sine(Θ)
b is the length of from the torsion bar to ball joint
Θ is the angle of the A-arm viewed from the front.

vertical Ftorsion = (k × Θ) ÷ (b × cosine(Θ))

(k × Θ) ÷ (b × cosine(Θ)) = ((W/g) × H ÷ L) - (Fbraking × (h + ½d) ÷ ℓ)

solve for Θ to find vertical suspension deflection y using y = b × sine(Θ)


Add the spring deflection to the tire deflection to get the full pitch angle. This is assuming a race car with a rigid chassis and monoball bearings with negligable compliance.

The anti-dive force is reacted through the tire "spring" because it is a part of the SVSA. The anti-pitch reaction will therefore be delayed by the amount of time it takes to squish the tires, at which point the main springs will come into action to stop the roll.
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911S
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/

Last edited by Flieger; 03-23-2010 at 02:41 PM..
Old 03-12-2010, 04:56 PM
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What if we make some basic assumptions just for the example.

We know the wheel base, we know the weight distribution is about 40/60 so we might assume the balance point is placed 60% of the wheel base toward the rear. If we assume the roll center at that point is about 12" above the CL of the chassis I am guessing that gets us close to COG point. We know how much the the A arm attachment is tilted on a 930 front and back so if we measure its length we should be able to get the angle of additional tilt. Then assume the center-line of the chassis suspension attachment is say about an inch above the wheel center line in front and a bit more in back (1 deg rake). Or something like that.

Mostly just interested in if we can put the sensitivity into a range. Like dose it reduce squat say up to 10% or more like 50%?

This might tell us if this is even worth chasing on a 911.

Could be very exciting even if based on assumptions.

Just a thought.

Last edited by 911st; 03-12-2010 at 05:14 PM..
Old 03-12-2010, 05:11 PM
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This might also allow for playing with some of the assumptions and testing how much lowering the car or adding rake might effect dive or squat sensitivity.

Last edited by 911st; 03-12-2010 at 05:18 PM..
Old 03-12-2010, 05:16 PM
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Remember, the lengths are from an "imaginary swing arm". That is why the trig I went through.

Anti-dive will follow the graph of the tangent function.



with the angle determined by h and l lengths for the SVIC and the tire rolling diameter d.

tangent(angle) = (h+(d/2))/l
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911S
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/

Last edited by Flieger; 03-12-2010 at 06:05 PM..
Old 03-12-2010, 05:29 PM
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I made a clarification to the above post regarding spring deflection.

Both the anti-dive force and the braking force are transmitted through the SVSA, which includes the tire "spring". The tire will be deformed by these horizontal and vertical forces in a somewhat elastic/linear way assuming no sliding. Therefore, the braking force is delayed by the F=k1x; x = ∫ax dt lag time. The anti-dive is linked to braking force so once the anti-dive starts, there will be further vertical deformation of the tire to generate the vertical component. Fanti-dive = (k1∫ax dt) × ((h + ½d) ÷ ℓ) so vertical deformation y = (k1 × (h + ½d))/(k2 × ℓ) × ∫ax dt

Since the acceleration is not consant, one uses integrals. Since the tires are very hard to quantify, being a "black art" of mysterious non-linearity, the k1 and k2 spring rates in the horizontal and vertical directions. are not known, nor are they likely the same.

Anyway, that math right there is just to show that anti-dive and the anti-pitch reactions will be affected by tire pressures and stuff like that.

Perhaps we should call the main forum's attention to this thread to get some fresh minds working at it. I have a few people in mind who might be able to contribute some good information. Also, I want to have some more eyes to check my work.
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911S
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/
Old 03-13-2010, 03:32 PM
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Would there be any interest in making some custom A-arm bearings that provide anti-dive geometry? They would be Teflon-lined steel sleeves and Aluminum mounting blocks. No rubber, no lubrication. The idea would be as a bolt-on for a 911 crossmember to give anti-dive similiar to, maybe more than the 930.

If there were enough interest, there is a small chance I could get some made. Do not hold me to any deadlines, though.



Not To Scale
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911S
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/
Old 03-23-2010, 01:40 PM
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Since there is not enough response for the custom bearings, I am going to make some spacers.

The front seems pretty straightforward but I want to know how the rear bearings of a Turbo are different. How was the 13mm lift achieved? Is the actual floorpan sheetmetal higher there? Is there a different crossmember? Are there spacers on the bearing mounts?

Any other photos people might have of this area- as the car sits or just a suspension part on the ground (for an upgrade/maintainance job) or a chassis without suspension (for painting) would be appreciated.

I also need to figure out if the spherical washers can account for all the angularity or if I will need to resolve the bolt holes to make them normal to the mounting threaded holes on the chassis.

Thanks.
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911S
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/
Old 03-30-2010, 12:37 AM
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Flieger

The 930's reinforcing crossmember is raised up relative to the chassis. I could look up how much, but it is in standard references like Anderson or Frere. And the front mounts are spacered down.

How is the crossmember raised up? Well, they took the threaded insert which goes through two layers of sheet metal, and into which the 12mm bolt that goes through the crossmember and the bushing cups, and raised it up. Maybe they made a different piece.

This fitting has a sort of flanged bottom, so that the cross member fits into the fitting and all the forces are not shear on the bolt.

They (and we, for practical purposes) are limited in how much the crossmember can be raised because you will soon hit the sheet metal. Plus the steering rack starts hitting the sheet metal.

I know about this because I modified my track car to achieve these anti-dive angles. I just cut off the rear chassis mount bases, and then fashioned a flange to mate with the cross member once I realized how all that worked. After a racing wreck I found my welds had broken for this flange piece, so I had to repair it.

I did not have to worry about suspension bushings/bind/whatnot, because I run coilovers, and the front and rear of the A arm are mounted in spherical bearings. So I gave no thought to what, if anything, Porsche did about this on the 930. I don't recall hearing about any suspension parts differences for those up front other than the hubs, but I've never owned or had apart a 930. My suspicion is that they just let the rubber do its job in the rear. It would be easy enough to angle the front mount, though I don't know if they did that either. It is not all that much of an angle change.

We all know about the different bananna arms and inner mount location in the back, which aided in anti-squat.

I can't say I noticed that my car worked better on the track after doing all this. But just doing it was its own challenge and reward, and it didn't hurt.

I'm a Milliken fan, too, though not as handy as you are with the math.

I keep hoping someone will have made detailed measurements (I tried once, but my garage floor isn't conducive and I didn't trust them and they were only in 2D plan vies) of a basic 911 suspension and will share them so we can plug them into the various computer models. As it is no one has ever shared what the CG of our cars is. That may be because no one has measured it (I tried once with corner balance scales, but got garbage so I was doing something wrong), or because no one who has taken the trouble to measure wants to share. Yes, I'd expect my track car (under 2,000 lbs and still slowly loosing weight, plus no roof/windscreen) to have a somewhat different CG height than my SC, which is still basically a stock car, though with a full roll cage). And so on with everyone else's cars. But it would still be useful to know beyond various guesses, like the CG is at the height of the top of the engine.

Walt

Old 06-26-2010, 11:54 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #40 (permalink)
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