coldfire

iTrader: (1)

i was browsing the awrracing.com site, under the RX-7 suspension section, and i came across something called "Roll Center Blocks: Eliminates roll center irregularities found when lowering vehicle".

anyone know what exactly these are?

there is also quite a few other neat items on the site...although i don't know if everything is for the FC or if FD and FB parts are also listed:
http://www.awrracing.com/pages/rx/rxsusp.html

Black91n/a

I have no experiance with these, nor have I ever seen one, a picture of one, or even seen a discription of one. Generally speaking such a thing will space the outer part of the control arm downwards relative to the hub. This corrects some nasty geometries that can exist on seriously lowered cars (you can get a positive camber gain on compression).

My guess with those is that it's a trapezoidal block of aluminum with holes in it so that you can bolt it between an S4 ball joint to an S4 A-arm, effectively moving the A-arm downwards while maintaining the rest of the suspension the same as before. That's what I would do if I was designing it. It'd be simple and easy to do.

coldfire

iTrader: (1)

hmmm, interesting. first time i have heard about this. doesn't sound like it is really needed unless you have significantly lowered the height, and you have the chassis setup so that it is really sensitive to minor changes (i.e. race prepped car).

some other cool stuff they sell though, like their rear toe adjusters, needle-bearing control arms, their engine mounts, and their rear camber adjusters (similar to the mazdatrix ones).

RETed

I would like to see pics of these things, cause they are nothing like the AE86 ones.
In fact, it's next to impossible to add roll center blocks to the bottom of the front struts...
They must be just shimming the steering rod ends?

All these things do is minimize bump steer by moving the steering rod connectors back to horizontal.


-Ted

Black91n/a

Actually Ted you're wrong. The steering has nothing to do with roll center and that wasn't mentionned in the breif description of the part.

Roll center is calculated (on struts) by projecting lines inwards along the control arm, and perpendicular to the stut tops. These lines will intersect somewhere on or beyond the other side of the car. Then if you project a line between the center of the tire's contact patch to the intersection point on both sides, then where those lines intersect (in the middle of the car when the car's level) that will be your roll center height. This is the point about which the car will roll when cornering. The moment (torque) that causes body roll comes from the centripital acceleration of the car's center of gravity and it acts on a moment arm that is the length equal to the vertical distance between the CG and the roll center. This is the roll couple. The roll center is also a dynamic point that moves about when the car leans into a corner, squats under acceleration and dives under breaking.

When lowering a strut suspension the roll center will drop faster than the CG, causing an increased rolling moment, and more roll for a given wheel rate. Roll center blocks will move the roll center up to compensate by moving the outer part of the control arm downwards in relation to the hub. This will leave ride height unaffected, but corrects the geometry. You would not want to add spacers to the strut, as that would raise the ride height, and it would be useless, you may as well just adjust the suspension upwards at that point.

There's also a nasty situation that can arise with a strut suspension. If the angle made between the strut and the control arm exceeds 90 degrees under static conditions the tire will loose negative camber, or gain positive camber under compression, causing the outside wheel to be canted the wrong way, seriously reducing grip. Roll center blocks can also cure that problem, returning the suspension to having a negative camber gain under compression.

Now when you do this there may be some bump steer as a result, which you can then tune out with different tie rod ends that space the tie rods downwards.

It's probably not a big concern unless you've lowered your car more than a few inches, to ITS levels or beyond let's say. It's usually better not to lower the car too much unless you're prepared to do a buch of work to fix all the handling problems that that may cause, but to know if it will cause problems you'll need to do a bunch of work.

RETed

Yack.
That made my head hurt, but I think I get what you're trying to say.
Pictures would've been worth a 1,000 words!


I would think this is something you would try to avoid at all times, and I cannot figure if this is even possible with the stock suspension unless we're talking massive droop?


Yeah, this was what I was talking about.
If you take care of the roll center but do nothing about the steering, then you gotta worry about bumpsteer...


The FC stock suspension cannot be dropped very low, or else you get headaches in the rear suspension.
It takes major redesign of the rear to get the camber to get within tolerable limits back there, and that would make the car very illegal for IT-S racing.

So with minimal lowering, I don't see what advantage having these roll center blocks on an FC???


-Ted

DamonB

Roll center for double wishbone or multilink:



Lines are projected from the upper and lower arms to the point where they intersect. A third line is projected from that intersection back to the contact patch of the tire. This is done for both sides of the car. Where the third lines intersect eachother is the roll center. Make note that often when the suspension is moving the roll center migrates. It moves up and down and even side to side; it's rarely in the exact middle of the chassis. To find the roll center you must always calculate both sides as the true roll center is the intersection of the third line from both sides, NOT where that line intersects with the chassis centerline. If the chassis is experiencing any amount of roll the roll center is rarely in the middle of the car.

Roll center for strut suspensions:



Computing the roll center for a strut suspension is the same with one exception. Rather than projecting a line from the upper arm (you don't have one) you project a line from the top mount of the strut that is perpendicular to the strut axis.

Given the diagram you can see that if any suspension is lowered too much crazy things begin happening as the control arm angles become unfavorable. Using spacers etc at the upright moves the arm closer to its original angle before the car was lowered.

All reactions of the chassis occur at the CG. If the roll center is not in line with the CG a torque is generated which makes the chassis roll. Keep in mind that this torque is in addition to the one generated between the difference in CG height and the contact patch (ground).

RacerXtreme7

iTrader: (2)

This wouldn't work at all. You would need to move the Ball joint lower. Your method doesn't change a single thing from your discription (other then needing to lengthen your sway bar end links now). It sound like your placing a triangle shim (trapezoidal block ) between the ball joint and the arm. The points for institanious center follow the pivot points of your lower control arm, not the shape of them. In other words, its measured from the outer/lower ball joint through the lower control arm pivot mounted to the chassis. Your arms could be gull winged shaped or for that matter do figured eights, the line through the pivots doesn't change.

To lower your your roll center you'll need to lower your outer/lower ball joint. This will not effect spring rate or ride height either. I planned on doing this by using a longer joint pressed into the s5 lowers. I havent actually got any replacement ball joints yet or even seriously set a design aside yet, just some random thinking on the matter. I'm not only doing this to raise my roll center, but also trying to gain negative camber curve in the suspension sweep since struts tend to be lazy in this department. I will not be lowering the car much (perhaps 1"~1.25"). The rear I havent put much thought into yet either except maybe adjusting the ecentric toe bolts to the full up position and dialing toe back into spec via adjustable tie rods. This will raise the trailing links pivot point. Like I said, I havent thought all this through just yet just throwing out ideas. Finding roll center in a semi trailing suspension is a little more involved then struts.


On another side note, when strut cars are lowered a lot, you can get back some negative camber in the suspension sweep by moving your top strut mounts in toward one another. This of corse gives you more negative static camber which most would do in a heavily lowered race car anyway.

~Mike...........

Black91n/a

Damn, I was wrong about the method of acheiving the modified roll center. RacerXtreme7's right, you need to move the ball joint down, not the arm.

Now that that's said I'm not really sure how they'd accomplish that. Just having taller ball joints would put an excessive amount of force on them, leading to a vastly increased risk of failure, unless they weren't much taller at all, leaving you with very little improvement. The "best" way would be to have modified hubs to move the lower ball joint downwards, but that's not very practical.

Having these on a car that's only been lowered a bit would help give more camber gain on compression.

Wouldn't the roll center on a semi-trailing link suspension just be the intersection point of the axis of rotation of the semi-trailing links? If that's the case when you use the individual style camber adjusters you'd also be adjusting the roll center. Even if it's not true it should have an effect on the roll center.

KevinK2

"The moment (torque) that causes body roll comes from the centripital acceleration of the car's center of gravity and it acts on a moment arm that is the length equal to the vertical distance between the CG and the roll center"

A bit misleading, as this follows a descrption of the roll center of the front (or rear) suspension. To determine the roll couple, you find the front and rear roll centers, which define the roll axis. The roll couple is then the lateral g force of the sprung weight, times the vertical distance between the singular cg of the total sprung weight, and the roll axis. Just keeping everyone on the same Puhn page ....

jgrewe

iTrader: (1)

The FB can use 'roll center blocks' because of the design of the strut/ball joint area. The steering arm and ball joint can be lowered in relation to the spindle.
On the FC a monoball stud can be used as a long ball joint and washers are used to lower the actual pivot point of the lower ball joint. These things are used all the time on stock cars.

coldfire

iTrader: (1)

wow, tons of information in this thread!
i didn't expect this, but it's all been a good read.

DamonB

Yep.

Yep.

Alternatively you can leave the ball joints where they are but raise the spindle higher on the hub. Both accomplish the goal of lowering the car without forcing the geometry to go nuts but both require a different upright.

Nothing ever is when you're trying to do something the original designers weren't concerned with

Fortunately Bimmers have the same suspension type and there's lots of info:



http://e30m3performance.com/tech_art...nter/index.htm

DamonB

Why lowering a suspensions (especially a strut type) too much can be bad.

Black91n/a

Thanks for the information on calculating the semi-trailing arm roll center, I've wondered about that before. I guess my thoughts on the roll center before weren't very well formulated, especially seeing as it had no mention of the tires contact patches. I did get it right that it had something to do with the pivot axis of the arms.

By the pictures it would seem that the roll center wouldn't be affected by camber changes to the rear wheels via individual adjusters, is that correct?