924Board.org

[gegge]

Running stock ride height with A-arms level with ground seems too high.

I would like to lower the car minimum 2" but I am concerned about camber compensation and bumpsteer. And what about the rollcenter, it will drop faster than center of gravity making the car less stiff? I got 400lbs springs and don´t want the car stiffer than that.

On 944 cars they recommend a ride height of 25" from ground to the edge/rim of the fender/wing, thats is approx the height of the wheel.

Looking at the British 924 racing series for example, and they are running there cars really low and small 15" as well.
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Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[924RACR]

I measure mine the way our rules are set, which is height to the bottom of the rocker, not including the pinch weld. The limit is 5", and we run about 5.25" in the front, 5.5" in the rear. A-arms are definitely past horizontal - but it's fast. I either haven't noticed or am used to the bump steer problem...
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[emoore924]

Just curious, why do you think the rules measure from the rocker? Is there something else in there the says you can't change your suspension mounting points so the CG can't go lower?

hmmm

[Fifty50Plus]

Some cars have pinch welds at the bottom of the rocker, others don't. The 5 inch rule is most fair when ignoring the pinch weld. In SCCA Improved Touring classes you are not allowed to move suspension mounting points. In GT and production you can.
Chuck
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1979 924 NA race car H-Prod SCCA
1982 924 NA race car - Sold
1981 924 Turbo sold
1982 924 Turbo sold
1972 911 E race car - traded for Cayenne Diesel
1975 914 1.8 Building for H-Prod SCCA

[924RACR]

Indeed, this is comparing many different types of cars in the same classes - easiest to check, and most consistent.
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[Rocco R16V]

having had a car that was lowered too low, then going to a proper susp setup i can say the difference in handling is enough to want to keep it at a proper height.

unless you do proper lowering with droped ball joints and steering rod fix i think you will be happier with how it drives at the higher ride height.

my 2 cents
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"Government exists to protect us from each other. Where government has gone beyond its limits is in deciding to protect us from ourselves. "
Ronald Reagan

[ic932]

At the moment I'm using a Sachs/boge front spring lowering kit. It is purpose made for the 944 but is now NLA. I don't know the spring rate but it's a lot firmer then the white dot marked springs found on the later 944's. The wishbones are tilted up quite a bit. I trust Sachs/boge to know what they are doing...but then again why was the kit dis-continued lol.

[bass gt]

924's have inheret bump steer issues once you lower the car to the ponit the wishbones are pointing up! Make a simple bump gauge and you'll see how bad it gets.
Both myself & Simon (Simsport) fitted a bump steer kit which he got from the US. On my car, it totally transformed the handling. No more sawing at ther wheel through fast corners!!
Search the web, these kits are readily available

Steve
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Front Wheel Drive is the Devil's work.

[gegge]

Quote from www.modified.com/tech Making It Stick Part 3&4

"The distance between the roll center and the center of gravity is called the roll couple. The roll couple is the lever arm that centrifugal force working on the CG uses to make a car lean over in a turn around the roll center. In a rear- or mid-engine car, these approximations apply to the opposite end of the car.

The longer the roll couple, the more weight is transferred to the outside wheels during cornering and the more the car will want to roll in a turn. A longer roll couple makes cars slower to respond to steering input. The resulting weight transfer from a long roll couple also uses the inside tires less effectively during cornering, thereby reducing the available grip.

The often-overlooked disadvantage to lowering is that roll center drops more radically than the center of gravity on most cars. This increases the roll couple and can cancel any weight transfer advantage. The huge roll couple created by overlowering will require an overly stiff suspension to control body movement.

And when your suspension is too stiff, it won't absorb road irregularities effectively, which will make it harder to keep the tires in contact with the ground. You can't drive fast if your tires aren't on the ground.

On most cars, the ideal location for the roll center is 2 to 5 inches above the ground for the front suspension and 4 to 10 inches above ground for the rear suspension. With the rear roll center higher than the front, the car will transfer more weight to the front, making it more likely to understeer. Most purpose-built racecars utilize this design because it allows them to be tuned for slight understeer at high speed and more oversteer at lower speeds.

On MacPherson strut-equipped cars, the wheel will gain negative camber under roll as long as the lower control arm is positioned less than 90 degrees relative to the strut axis. Unfortunately, many enthusiasts with MacPherson strut-equipped cars lower their cars too much and make this angle greater than 90 degrees. Beyond 90 degrees, the suspension will gain positive camber instead of negative as it compresses, significantly compromising grip

Bump ster - In order for a MacPherson strut suspension to have no bump steer, the tie rods must lie in line with the lower contol arm with the inner tie rod end in plane with the inner pivot of the control arm.

Toe steer - At stock ride height, the trailing arms are usually parallel to the ground. When the car rolls, the outboard and inboard arms swing in different directions. The resulting arc-shaped axle path shortens the car's wheelbase during compression and droop. Since each arm swings equally in different directions, the axle's toe doesn't change because each end of the beam axle is pulled the same distance forward. If the car is lowered too much, both trailing arms point downward toward the front of the car. At this angle, the arms don't move equally in opposing directions. Under roll, the inside arm will push its side of the axle rearward while the outer arm will pull its side of the axle forward, causing understeer."
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[gegge]

And this is why you shouldn´t lower your car with soft springs and using r-compond or slicks:

http://zzyzxmotorsports.com/library/strut-tech-20070918.pdf

The only way to do it right is to get smaller wheels I guess.
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[flosho]

gegge, what does all that pdf mean..
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[This Space For Rent]

[!tom]

It's a demonstration of how adjusting one aspect of the suspension without taking the entire system into account can have undesired effects.

Lowering the car without making other adjustments can have undesired effects.

It is illustrating general principles -- it is not particular to any specific car, other than MacPherson strut systems.
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78 924 NA
5-lug

[flosho]

But where is our goal "instant centers" and roll center height?

What does it mean when we lower our cars 2 inches and it changes the roll center from 3.432" to -4.02" below ground?

Or changes the "instant center" from 98" to 145"? What is the 'real world effect' of these figures? Hypothetically of course.

Ok from a quick read of wikipedia, if you are increasing the instant center, you induce understeer in the car, as it is turning in a larger arc? Possibly more suspension movement and weigh transfer?

*edit, ok again reading it, its basically saying using camber plates can offset the fact that our control arms are past parallel. With longer ball joint pins it would help this and camber plates would help even further.
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[This Space For Rent]

[gegge]

Flosho, if you lower 2" the car and the result is that the roll center drops from +3,4" to -0,4" then the roll couple will increas by 1,8". To compensate that extra level arm you have to increase the stiffness of your springs by 10-20% (guessing). So what have you accomplished? A lower and hard sprung car with the similar initial roll stiffness BUT worse grip due to the harder springs. No big deal with no camber or full camber.

But that is not the end of the story. When the car rolls, for example by 4° like in the PDF - the roll center drops from +0,3" to an even worse -15,3" (fig8) with full camber. The car will handle just like the center of gravity is not fixed - imadgine travelling with the car with an 400lbs animal running around inside the car. Not exactly the idea of balance...

Please note that if you lower the car with NO camber, roll center actually improved roll stiffness going from +2,8" to +6,4". Can be a good thing (but I am not sure how to interpret the 100" to the far left... )

This is the behaviour of the MacPherson and not Porsche specific.
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[gegge]

As I understand "instant center" - a geometrical term in the suspension setup. Rollcenter is more important, it is the pivot point of the roll. You want center of gravity as close to rollcenter as possible. In real terms; a high roll center and a low center of mass/gravity in the middle of the car.
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs