-nick

All that I can tell for sure is that flipping gains an extra few mm of compression travel before the control arms contact the frame. Interference happens at about rs-45mm otherwise, which is a wheel position that no one will ever see anyway. Hard to envision how it would effect the geometry.

edit- interference happens at about rs-45mm

prschmn

The change is angle is going to raise the instantaneous roll center height a bit. Might be useful
on a lowered car. I haven't had enough coffee yet to think it through further than that yet!

porsche mania

I've done this on my 964. The change brings the a arm closer to level. The more the a arm moves through its range the effective length of the control arm increases causing bump steel the mod returns the a arm closer to level where there is less change to control arm length through movement. Think that makes sense.....

porsche mania

You can see how moving it back would effect how level the a arm is. This is at rs - 25mm

budge96

To me it appears that mounting the ball joint above the AArm in this manner would create
difficulties in making easy castor adjustments..
I've see it done a few times mistakenly only to be reversed and mounted in the lower position....Bert

porsche mania

I wouldn't say it made it harder. You just have to use a shallow allen socket and a ratchet rather than a t bar. Certainly not difficult and fairly common tools as well!

prschmn

The one inherent thing I can see is that changing it also changes the relative angle between
the A frame and tie rod-hard to say if that's good or not without careful analysis.
It's possible that part of the original design was intended to provide more-or less
ackerman effect-though that would hardly affect track use .
It's important to keep in mind that any Macpherson arrangement is inherently compromised
as far as bump steer goes. When I was racing open wheel I took great pains to get bump steer
down to under .010-hardly doable with a 911 but in any car the more you have the less comfortable the drive. Though that doesn't affect theoretical lap times it sure does make it easier!

Spyerx

Most / all the race cars are setup this way.
It's the geometry advantage.

onevoice

Some here need to go have a first year engineering student explain free body diagrams. The lower arm is nothing more than a connection between the inner pivot, and the lower ball joint. It can be straight, it can be curved, it can U-shaped, it doesn't matter at all. The shape of the connection does nothing to impact the loads or pivots of the member. You could mount the ball joint on top of the arm with several inches of spacers between the arm and joint and it would likewise have NO IMPACT AT ALL. The joints in the arm will still be in exactly the same position, move in exactly the same paths, and transmit the exact same forces.

The only possible impact here is to put the arm in a more favorable position for clearance, or to put the joint in a less likely to bind position.

Vandit

The top mount doesn't move with suspension travel. The bottom mount (control arm) does. McPherson strut suspension. This relationship in movement affects the camber curve and, in turn, steering, as the car's camber changes through its suspension motion range.

The geometry's sweet spot is when the control arm is parallel with the ground. The further away from parallel with the ground the control arm sits, the more you're starting off on the wrong foot when it comes to geometry.

Given a lowered ride height, where the control arm ball joint side may potentially be pointing upward, this flip will help bring the arm closer to parallel.

There are also roll center implications. Go look at the link I posted earlier and will include here too.

http://www.meganracing.com/tech/faqs...ect=Suspension

onevoice

This is where the misconception lies. The front geometry has nothing at all to do with the CONTROL ARM being parallel to the ground. It has everything to do with the relative heights of the INNER PIVOT and BALL JOINT pivot. The arm connecting them doesn't in any way change those pivots. Just as mounting the ball joint on top or bottom doesn't change the pivots. The control arm could could be angled down until it touched the ground, then the ball joint spaced back up, and if the pivots were in the same locations, nothing has changed. The control arm is a link connecting two pivot locations, nothing more, it doesn't matter the angles it takes to connect the pivots. Yes, the heights of those pivots have great impact, and it may be convenient in some vehicle designs to reference the control arm being "level" when it works out that it actually is level when the pivots are in the most advantageous position, but it isn't the level of the control arm that is impacting the system.

Imagine if the control arm was designed to be much thicker on the inside, and taper to be much thinner towards the wheel, instead of flat as it is now. The top and bottom wouldn't be parallel, which side would you want to be parallel to the ground?

porsche mania

The thing is we are talking about a specific car. Yes what is important is the line from ball joint to a arm pivot and on this car that follows nearly the same line as the a arm thus making it way easier to just say the a arm wants to be as close to level as possible to reduce the effects of suspension travel on toe angles. The fact that the control arm follows nearly parallel to the a arm is also made it far easier to just say that the a arm wants to be level. As for it working at any angle as long as they follow the same trajectory is OK in regards toe but your forgetting all about roll centres and camber changes.

Bill Verburg

Top mounting the ball joint doesn't change the A-arm angle but it does change the virtual A-arm angle which is what you care about. The virtual angle determines where the instant centers and thus roll center are/is located as well as the scrub radius and also the virtual arc of the A-arm ball joint, on a lowered car you care about the concentricity of the arc of the A-arm and tie rod ball joints. Raising the A-arm ball joint raises the instant centers and thus the roll center and probably increases the concentricity.

-nick

Edit: No need for a bad example to confuse the discussion!

But this is probably still relevant- ha! Just checking, none of us do this for a living I hope?

ToSi

You have that backwards. Unless you move the ball joint (relative to the wheel center and/or tierod end), there is no impact to geometry. Draw a line through the center of the ball joint & inner bushing pivot axis. That's all that matters in this case.