killer305

I posted this in drivetrain but with no response so im coming here. im trying to figure out how to come up with the instant center for the rear end. would it be where the torque arm mounts? I would think that it would be the intersection point of the LCA's and torque arm, but usually the LCA's and torque arm are almost parallel, putting the instant center way in front of the car soi know that cant be right.

Z28barnett

Go to Frrax.com and search. Those guys know suspension.

Unbalanced Engineering has a site that explains the instant center created by their uncoupled torque arm.

Their is also a roll center, that is defined by where the panhard rod crosses the centerline of the rear axle.

Z28

OldSStroker

Assuming you mean a Camaro rear suspension with a fore/sft sliding front mount of the torque arm, the side view IC is found as follows:

Extend a line from the rear pivot of the lower control arm (LCA)thru the front pivot forward past the transmission mount. Next a line is drawn perpendicular to the front end of the torque beam where it is attached at its rubber slider joint. This is basically a vertical line with the car at normal ride height. Where this vertical line crosses the line of the lower control arms is the side view Instant Center.

This is quoted from Terry Satchell, a GM suspension engineer very familiar with this subject.

Note that you can easily raise the IC by dropping the rear pivot height of the LCA. Note also that different ride heights (lowered or raised car) change the IC height. Note that lowering the rear drops the IC, and therefore reduces the anti-squat unless LCA pivots are relocated.

Jon A

http://www.jonaadland.com/Ta.jpg

OldSStroker

We read the same textbooks, Jon. Thanks for posting the picture.

Chapter 17 is excellent. All was written by Satchell.

killer305

thanks alot that explanation helps alot so the instant center can be raised and lowered, not moved back and forth unless you get a longer or shorter TA. thanks again

Damon

That's correct.

I used to think the instant center was exactly where the torque arm attaches to it's forward mount near the trans, but was shown that the angle of the LCAs affects it's location up and down. It's front-to-back location is not movable, however. At least not with a stock setup.

FASTFATBOY

Everything I have read is the instant center is where the two angles intersect. Which is the reason for dropping the LCA rear mount point. Anyone?



David

OldSStroker

The simple answer is to increase anti-squat. Look at this diagram originally posted by Jon A:

http://www.jonaadland.com/Ta.jpg

From my previous post, the Camaro rear IC isn't where two angles intersect.

Draw a line from the center of the contact point of the rear tire with the ground (where the dashed line meets the ground) through the IC (Instant Center). As you raise the IC by dropping the LCA REAR mount without moving the front mount, the angle between this new line and the ground increases. Antisquat is figured from the tangent of this angle. More angle = larger tangent and therefore more antisquat.

You need more information to actually calculate % anti-squat (AS), but remember that a higher IC location means more AS and more tire load.

Anti-Squat could be another discussion entirely.

Jon A

Anti-Squat? http://www.jonaadland.com/Antisquat.JPG (The middle one for a solid axle rear suspension.)

As usuall, "The Other Jon" did the typing for me quite well. And he's quite correct--the value of/how much Anti-Squat is optimum for what application, etc, is a subject that will fill pages of debate for years to come.

Another point--as I can see the question appearing soon--why didn't GM just angle the LCA's up in the first place? Well, a side-effect: The inclination of the rear roll axis which is just fine at low speeds and even high speeds in a straight line is a detriment to stability and controllability while high-speed corner-carving.

OldSStroker

Rear LCA angle is important for rear roll steer, or "bump steer" as the car rolls in a corner. Slight roll understeer is desirable, but that requires the LCA to angle down at the front, not up. Then there is power hop and brake hop and the beat goes on. In suspensions, almost everything is a compromise. Well, there is the decoupled rear suspension, but that becomes very complex.

SStrokerAce

How about a comment on that from the other Jon, I think he has a idea about that too.

Bret

Jon A

Yup, that's what I'm running now with the Unbalanced Engineering Decoupled Torque arm: http://www.jonaadland.com/Z28/Mods/UETA/PICT0049.JPG

It's about the best attempt to "have your cake and eat it too" one can do with a solid axle suspension IMHO. But even then there are compromises--it rarely comes together as nicely as it looks on paper. It does become quite complex to get it all set up correctly.

Anyway, this acts like a very short torque arm during acceleration, which moves the Side View Instant Center rearward giving you a huge increase in anti-squat. So much so, that I was able to angle my LCA's downward toward the front slightly (which lowered the IC and reduced anti-squat) to give me some roll understeer and gain high speed stability vs their being level as I had them set before (and I'm going to need all of the high speed stability that I can get with the new motor Bret's cooking me up!) and still get a very big net increase in anti-squat compared with level LCA's and a stock length torque arm.

What makes it work is the fact the torque arm is only active during acceleration. Such a short torque arm would cause unbearable axle hop during braking if it was simply a short torque arm. But this arm can only apply a force to the chassis during acceleration, during braking it "decouples" and the sliding link above it becomes active. This leaves you with a 3-Link suspension and a new instant center found like so: http://www.jonaadland.com/3-link.JPG .

So you angle the upper link in a way that gives you an instant center much farther forward during braking, reducing anti-squat (properly anti-lift during braking) and gives you a nice long SVSA so you don't get axle hop during braking.

That's the route I've taken while chasing the holy grail of suspension systems. But there are plenty of other good ones and in the end everything becomes a compromise no matter what you choose.

The stock F-Body suspension is arguably the best solid axle suspension ever put on a production car for handling as is and only gets better with some proper polishing. But if I was building a car from scratch I'd very likely do a 3-Link--they can be quite good and are very simple and light. Something often done with them is giving the lower arms an angle of convergence from the plan view. This creates another lateral restraint point and helps define the rear roll axis. If one works out all the angles correctly, this can allow you to angle the LCA's up toward the front to increase anti-squat and still have roll understeer as can be seen in the pic above. But that has its own compromises--you can't adjust your RCH without also changing the roll axis and you're limited how far you can lower it--I don't believe Trans Am cars these days generally do that because they mount the Watts so low in the rear they'd still have roll oversteer.

It's all about compromise....

1racerdude

If ya angle the LCA enough or drop the front of the TA to level with the ground where the two lines cross behind the TA front mount then where is the IC?
IMO it is at the intersect point and the front mounting point is the pivot point. Meaning the intersect point is where ya get your push upward and the front mount is where it lift's from.The further back it is the more rear lift or separation ya have in the rear under acceleration. All of this is in relation to the anti squat line and the CG.

OldSStroker

I might argue about the Camaro suspension being that good, but compared to most others it is.

Jon, have you ever looked at a "Satchell Link" 4-bar rear suspension. You can achieve a reasonably low RCH, good roll steer, a-squat, etc. It can package well, and all the links attach to the axle far outboard so there is virtually no bending load on the axle tubes. I've thought about a lightweight 9 inch housing assy with an aluminum pumpkin. I can see a setup like this with 50 lbs. LESS rear unsprung mass that a stock Camaro 7.5 in. Actually I'd like to do a very lightweight deDion "solid" with the Satchell Link. Might get the unsprung down to maybe 60% of stock. Camber and toe would also be easily adjustable.

The decoupled car we built was for roundy round and looked very much like the diagram. We weren't allowed coilovers so we had coil sliders and separate shocks, even on the 5th coil on the torque arm. Something like 36 rod ends of all sizes! We had the braking links set up to load the inside rear tire so it turned the car in under hard braking. The drive links were set up to keep power-on oversteer to a minimum. The driver was down on power (legal engine) but could out brake and put more down coming out and still stay inside when everbody else pushed out to the wall. Nobody that looked at it understood it. Those left-turn only adjustments wouldn't help you, but the decoupling does work, doesn't it?

I do like how you think...not just how you spell your name.

Jon B