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Lowering and Roll Center change?

EngagRelyt

EngagRelyt

New member
Joined
Jun 25, 2007
Location
Dartmouth, Massachusetts
I have an 1988 240 DL, right now it sits on cut springs about three inches down from stock in the rear an 2 inches in the front. I got a set of extra springs. And I was think for the hell of it going extra low with my current springs (2" between crossmember and ground).

Right now my control arms paralell to the ground maybe a little less. I have heard going real low and causing the control arms to go being paralell or more to being less and this lowers the roll center.

So what is a specific disadvantage of having a low roll center, in what aspect does it hurt your handling?
 
The stock 240 roll center is barely above ground. 1" lower and it not only migrates above/below ground, it goes left/right in turns. It's really bad.
The lower the roll center, the easier it is for the body to act like a lever and induce roll in corner. So in some cases, lowering a car makes it lean more in turns, given the same roll rates.

The advantage of a roll center near the ground is it gives a quick roll transition compared to a high roll center.
 
Yet, in the real world, with Bilstein front struts, cut super soft front springs, 25mm front and 22mm rear bar(cut wagon springs), my car seems to corner flat according to outside observers. I'm sure with some 225 R compound tires that story would be different, but the 235 street tires weren't too bad either.

My roll center has to suck:
IMG_4494.sized.jpg
 
Well lowering your car lowers the center of gravity, which will improve transient handling and such. It's a tradeoff between roll center and cg in suspension design.
 
Its also important to keep in mind where the rear roll center is in relation to the front roll center. The slope of the imaginary line between to the imaginary roll centers is important and can affect transient behavior.

With a solid axle rear we are stuck with a higher roll center, its essentially in the center of the differential, so no matter what you do with the front. The rear is stuck high in the air. So ideally you want the 240 roll center to be a bit above ground, not much but ground level isn't really ideal.

Some downward angle from the rear to the front is ok but too much isn't soo good.

As we understand the roll center drops below the ground as we lower the front of the car without correcting the rear. Which is a problem for the front but also the vehicle as a working system because the rear is still up in the air.
 
blkaplan said:
With a solid axle rear we are stuck with a higher roll center, its essentially in the center of the differential, so no matter what you do with the front. The rear is stuck high in the air. So ideally you want the 240 roll center to be a bit above ground, not much but ground level isn't really ideal.
Click to expand...

So fab up a Mumford Link setup, get the Roll Center down closer to that of the front.
 
m4ff3w said:
So fab up a Mumford Link setup, get the Roll Center down closer to that of the front.
Click to expand...

for that much trouble it would be worth looking into fitting a multilink IRS... to get the weight of the rear diff off the suspension...
 
dbarton said:
Trying to grasp this stuff....
Lowering the rear of the car obviously lowers the body panhard mount. Are you saying extend the body mount lower as well as the axle mount?
Dave B.
Click to expand...
Yes. Lower both mounts to physically lower the panhard rod relative to the axle and make it parallel. I'm no expert on solid axle roll center height, but it's my understanding that the roll center is dictated by point where the panhard rod passes the differential.

Here's an example. Nevermind the rear mount turbo I was photographing at the time.

panhard1.jpg

panhard2.jpg
 
^that's sweet^
blkaplan said:
Its also important to keep in mind where the rear roll center is in relation to the front roll center. The slope of the imaginary line between to the imaginary roll centers is important and can affect transient behavior.
Click to expand...
This probably has something to do with why my car likes to lift its inside rear tire so much... I can't wait to get some stiffer springs and proper geometry on my car... :oogle: But that's only because I actually care about how my car reacts when it's driven near 10/10ths.
 
the amount the tire gets lifted is also directly affected by the angle of the panhard bar,

its called "torque wedge" if its angled past horizontal it will want to lift. if you angle it back the other way it will actually push down :nod:

about level gives more even loading to the wheels.
 
klr142 said:
Yet, in the real world, with Bilstein front struts, cut super soft front springs, 25mm front and 22mm rear bar(cut wagon springs), my car seems to corner flat according to outside observers.
Click to expand...

there you go. and it might even be on a bump stop in turns for all I know.
I read the books and did the math, and I did the experiments. It's a real effect.
 
blkaplan said:
the amount the tire gets lifted is also directly affected by the angle of the panhard bar,

its called "torque wedge" if its angled past horizontal it will want to lift. if you angle it back the other way it will actually push down :nod:

about level gives more even loading to the wheels.
Click to expand...
:grrr: For no adjustability.
mikep said:
there you go. and it might even be on a bump stop in turns for all I know.
I read the books and did the math, and I did the experiments. It's a real effect.
Click to expand...
I agree, but the car still corners pretty flat. It only has the internal stops in the Bilsteins now up front.
 
I have a book here in front of me that states the following, loosly translated to english:


Every wheel suspension system has a roll center that is based on it's geometrical dimensions. The centrifugal forces generated in cornering tries to sway the chassis around this roll center. Both the front and the rear suspension have their own roll centers and the imaginary line between these two can be regarded as the roll axis of the chassis.

The tilt angle of the chassis is determined by the magnitude of the forces acting upon the car and by the distance between the roll axis and the mass centre point of the car.

Centrifugal force can be thought to concentrate to the mass centre point. The higher the mass center point is, the larger will the tilt angle of the car be. In Theory in such a case where the roll axis of the car should pass through the mass center point of the car, the car would not tilt at all regardless of the speed used to drive through a corner. However in practice this can not be achieved because the suspension system is a dynamic environment and the atributes may change in operation.

The placing of the roll center and thus the placing of the roll axis gives a great possibility to alter the characteristics of the car. For example, if the front roll center is lower in relation to the rear roll center , and the mass center point is approximately in the middle of the axles horizontally, the front of the car tries to tilt to a greater angle than the rear because of a greater moment acting upon it.

However, as the car body is there to prevent the other end from tilting more than the other, the car tilts to an average tilt angle. In other words, the front end tilts to a smaller angle than what the forces acting upon it would require, and the rear end tilts to a larger angle. This leads to a greater weight transfer from the inner rear wheel to the outer and thus leads to a more oversteering car.

Respectively placing the front roll center higher the car will become more understeering.
 
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klr142 said:
Yet, in the real world, with Bilstein front struts, cut super soft front springs, 25mm front and 22mm rear bar(cut wagon springs), my car seems to corner flat according to outside observers. I'm sure with some 225 R compound tires that story would be different, but the 235 street tires weren't too bad either.

My roll center has to suck:
Click to expand...

that swaybar has got to help with the flatness for sure....by they way, your chassis is too clean, do you really drive that car???? :-P
 
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