E46 Fanatics Forum banner

Concept for roll center correction. Fabricators and engineers please chime in.

31K views 89 replies 11 participants last post by  IrocThe325i 
#1 · (Edited)
Okay, so, as most of us know over lowering a car with a Macpherson strut suspension design causes poor roll center. Poor roll center means that regardless of how low you go you will not reduce body roll as expected. Lowering does lower center of gravity but that doesn't mean sh*t if the end of your control arms are pointing upwards at a 60* angle.

So heres an idea that seems pretty simple which makes me believe i'm over looking something substantial.

On the Knuckle (spindle/hub) Why can't you extend the little arm that protrudes downward where the ball joint bolts through? If you cut it and welded an inch and a half or so and gusseted the arm, leaving room for the balljoint bolt of course, would it crack off and cause you to flip and burn to death???

Im trying to find a diagram to modify to help provide a better explanation of what im talking about.

Im considering having my fabricator friend modify a set of knuckles and test it out. Although i rather use a beat up old e46 than mine.

All you people that are most likely smarter than me, throw some input my way and tell me why or why not.
 
See less See more
#4 ·
DO NOT MODIFY THE SPINDLE

If you want to roll sender correct your control arm, you have 2 choices:

1- Instead of having a ballpoint at the end of the control arm bolting to the spindle, have a spherical bearing installed. Fabricate a long bolt that has the same taper as the OEM ball joint to fight snugly in the tapered orifice. Using a conical spacer and tubular "pipe shaped" spacers until the desired angle of the control arm is achieved. The bolt must be long enough to go completely through the spherical bearing, pass through another conical spacer, to allow full movement of the bearing, then cap it off with a castle nut and cotter pin.

2- Fabricate a offset bar to bridge the 2 tapered holes of the ball joint and tie rod. The bar has to be the correct height to correct the angle of the control arm since it is not adjustable. 2 tapered holes have to be drilled in the bar, relocating the tie rod and ball joint lower towards the ground than before. Generally this setup is used in drifting where they try to achieve more steering angle by relocating the tie rod pickup closer to the center of the spindle, thus giving you reduced steering feedback but increasing steering angle (changed the fulcrum of the tie rod assy).

Hope this helps, but don't modify the spindle, it's only a matter of time before it cracks, regardless of how good the welder is.

If you want picture examples of what I'm talking about, visit Driftworks website and lookup their Geomaster knuckle kits for Nissan 240sx/Silvia.

Good luck.
 
#7 · (Edited)
Yessss perfectt!!!!
View attachment 471674

but how would this work for an aluminum control arm?



Not from my iphone
Um, seeing that we're talking funky angles, I wouldn't trust an aluminum control arm that's designed virtually flat. On my racecar, or on any car, I would only adopt something like that if I had a custom-built steel control arm.

In the photo you posted, look at the arm - it's still and look how much reinforcement was done to it. I wouldn't even consider aluminum.
 
#8 ·
Honestly there is no issue using an aluminum control arm. Problem would be the angle if the tip of the control arm (specifically the portion where the ball joint is). I doubt the original control arm would have the correct angle without modifying it. Only way to know would be to try, I'm game so if someone has a control arm they replaced hanging around, I'll through a spherical on it and have a look.

Realistically speaking though, to utilize the spherical roll center corrected ball joint, you would also need to use the roll center corrected tie rod (which requires 17" rims minimum). Now that you're getting into heavy mods, the aluminum probably won't keep up with the stronger steel or chromoly replacements. So you would fab up a tubular steel replacement. Since you're going through all this trouble, might as well convert the inner ball joint to a spherical a well. Since all that work has been done, might as well use the 3 point OEM design and integrate a fully adjustable caster portion into the tubular arm.

Change one thing, 10 more follow, especially when you're messing with something as touchy as roll center correction.
 
#12 ·
The car in my sig had the roll center changed. Just a machined piece to move the pivot point of the control arm lower. The same thing was done with the tie rods to fix bump steer. The catch was that 18" wheels were required to clear the newly relocated suspension pieces.
 
#14 ·
And yes 18" rims are required. The ken tie rod has a small bend in it so when you turn the wheel to full lock, the inside of the rim doesn't hit the tie rod. With 18" wheels and a roll center corrected heim joint tie rod, the wheel clears the straight tie rod.

Using spherical bearings as ball joints in the control arms instead of actual ball joint, you would never need replacements (if mil spec is used) and they would be 100% adjustable. So every time you do something on your suspension, a quick measurement at the alignment shop, a shim or 2, then bingo, perfect alignment for the best suspension behavior and handling.

Basically roll center adjustment is to retain OEM geometry when lowered, widened or raced when slammed.

The car behaves the best using OEM geometry, and slamming the car without roll center adjustment actually kills the OEM geometry causing bump steer, bushing binding and so on.
 
#29 · (Edited)
why cant they make something as simple as a ball joint spacer?
A ball joint spacer won't allow for the tapered section of the ball joint stud to sit properly in the knuckle.
I think it could be done.

Take a cylinder, machine a matching female conical seat, and a matching female thread to the control arm ball joint. Put wrench flats on the cylinder, and then you can tighten the "spacer" onto the ball joint. At the other end cylinder, give it an external thread and conical seat just like the ball joint. Mount everything like normal.

You would have a minimum length because the entire ball joint would need to fit within the spacer, but any ideas why that wouldn't work?
 
#37 ·
Sorry I had a house warming party yesterday and couldn't follow anymore.

As for machining basically a ball joint extension like mentioned above, the issue would be with the angle the control arm would now be at. Picture a 4" spacer between the ball joint port on the knuckle and the control arm, that is now your weakest link due to shear forces, the control arm would most likely interfere with the disk as well. The number one issue for companies making this kit is for the control arm portion surrounding the ball joint, hitting the disks. Just because it doesn't touch when static, bushing deflection, and general flex would distort the arm enough to hit the arm.

There are only 3 ways to go about this, when I get back in an hour or so, I'll draw something up and share.
 
#39 · (Edited)
is roll center effected by only the angle in which the control arm pushes against the inner bushings???

so if the control arm had a bend in it to counter the overall angle upward would that fix it?

orrr is it the point of force's height relative to the mounting locations of the control arm.

Not from my iphone
 
#43 · (Edited)
I'm surprised nobody has posted a diagram of a MacPherson strut roll center. I took this from my vehicle dynamics textbook. It will help you visualize what can be done to raise the roll center.

From the diagram you can see that you can raise the roll center in several ways after lowering:

1. raise inner joint
2. lower outer joint
3. Increase shock lean towards inside
4. increase track width (seems counter intuitive, can someone check that out)
 

Attachments

#50 ·
I'm surprised nobody has posted a diagram of a MacPherson strut roll center. I took this from my vehicle dynamics textbook. It will help you visualize what can be done to lower the roll center.

From the diagram you can see that you can lower the roll center in several ways:

1. Lower ride height
2. Lower innder joint
3. raise outer joint
4. Increase shock lean towards outisde (more vertical from picture I believe)
5. increase track width
The significance of the roll center can only be appreciated when the vehicle's center of mass is also considered. If there is a difference between the position of the center of mass and the roll center a moment arm is created. When the vehicle experiences angular acceleration due to cornering, the size of the moment arm, combined with the stiffness of the springs and anti-roll bars (anti-sway bars in some parts of the world), dictates how much the vehicle will roll. This has other effects too, such as dynamic load transfer.
Lowering the roll center relative to the center of mass increases body roll and suspension compliance. We are trying to raise the roll center to correct it after lowering it when we lower our cars.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top