So..... if you changed to a shorter torque arm (like, say, the Spohn unit) but didn't change the angle between the front and rear attachement points vs. the stock arm angle you wouldn't change the IC at all??

Obviously, as the suspension moved up and down the IC would move around differently than stock but....... at stock ride height, as described above, the IC wouldn't move at all vs. stock?

I'm only about 80% satisfied that I understand this.

 

I don’t know how you got there. If you shorten a TA the instant center HAS TO MOVE. It will always be in line with the front joint of the TA, so if you shorten it moves back.

Do us a favor, before one of us goes through trying to explain this through another angle go back and read what I wrote and look at the pictures that I posted. It’s all there…

 

Damon: The rear end will rotate, through the LCA around the LCA body mount - if the rear end were always pointing towards the lca, (or in the same direction with respect to the lca) then the lca front mount would be the IC. This is how those "Pete-Z" bars seem to work, etc.

But with a torque arm not only does the rear end rotate around the lca front mount, it also rotates around the LCA rear mount (where the rearend attaches to the LCA). This is because the torque arm mount is definining the point that the rear pinion has to "point towards" and to maintain that the rear has to rotate independently of the lca (lca rear mount) as the lca itself rotates.

So if you actually diagrammed the path of the rear end through a compression cycle it wouln't follow a circular arc - (so the instant center will depend on the current rebound/compression of the suspension - is that right?) - but as it is "instant" center it is really only applicable for one infintenssimally small degree - d0 - so basically only one single point.

Or I could be off, but that is how it currently seems to me.


Chris

 

Where can I see a pic of the aftermarket pivoting and sliding

torque arm?Who makes each?

My torque arm mounts to the sfc connection plates and has a

poly bushing w/3 holes for up and down(1 inch each).

I can't see how my tq.arm can rotate at all.

John

 

WS6- I THINK we're in agreement. What threw me was when you posted this:

Since the factory setup IS a sliding link that would imply that the LCAs DO have an effect on the IC. i.e. draw lines through the TA and LCA and where they intersect is the IC. In which case, a shorter TA (but at the same angle) would have no effect on the IC since the lines would still intersect at the same point.

I'm not trying to be a pain, and I have read everyting posted which is why I'm confused.

 

Please look again at this diagram from WS6 TA:

http://users.erols.com/mpikas/temp/I...erTA-slide.jpg

The IC is located where a vertical line drawn thru the sliding front TA pivot meets a line drawn thru the LCA mounting points.

The IC location you are describing is for a 4-link (or a 3-link). With a short upper arm 4-link, the IC changes a lot more with suspension movement than it does with the sliding pivot TA and LCA suspension.

 

The torque arm pivots around the front mount as the axle rises and falls. It is the axle/TA assembly that is rotating around the front pivot.

Photos:

Link type TA:

http://cjcfo.fbody.com/members/injun.../DCP03248a.jpg

http://cjcfo.fbody.com/members/injun.../DCP03250a.jpg

Slider type:

http://cjcfo.fbody.com/members/injun.../DCP03133a.jpg

http://cjcfo.fbody.com/members/injun.../DCP03132a.jpg

http://cjcfo.fbody.com/members/injun.../DCP03131a.jpg

 

Interesting pictures Injuneer…, FWIW, assuming that the booted part of the second TA is a sliding joint, that TA will work like something half way in between. The sliding joint will impart some extra force on the LCA’s like with a sliding joint TA, but the IC should stay at the pivot since it has to rotate around that point. I don’t know if it’s still the case, but a bunch of people had issues with that style sliding joint on spohn’s early TA’s.

Damon, what OldSStroker said…

 

Yes, the bellows covers a sliding joint, with the front/mount tube sliding over the rear/lower tube. There were several failures of the rear/lower tube in front of the point where the two tubes meet. The failure was due to bending. The "fix" was simply to strengthen the rear/lower tube with a chrome moly insert. Mine was a very early arm without the insert, but I had the insert added. The arm was still intact, but it was not given a very demanding test, since at that point the best the car had done was 11.1@128mph with a 1.6 60-ft.

Spohn no longer makes the "slider", only the "link" type. Both arms in the pictures are Spohn products.

 

Beings Im useing the stock T/A ,in the stock location, with the stock front mount and boxed LCA in the stock location with a weld in anti sway bar on 28 x 10.5 tires with All Stock Front Suspension in a 3200lb car running 1.39 60ft and running the 1/4 at 10.38 at 130 , I would say all this talk is over rated for the average bracket car. But if anyone is willing to donate one of the T/As and some LCA lowering brackets, I would be willing to test it out.
Videos and Pics

 

10.90, yea, that’s pretty much why I tell most people that they don’t need anything to launch hard with an f-body, they just need to make sure what’s there isn’t worn out and is still working right. The only thing that I’ve found that makes a significant difference without taking something too far is an aftermarket panhard rod, which helps keep things straight on the launch and at shifts (I found that it fixed the ‘jumping sideways on the 1-2 shift’ problem on my cars).

Looking at the pictures of your launches, it looks like you could use _a little_ more antisquat, but I wouldn’t mess with it till traction actually becomes an issue.

At some point I’m going to give this a really serious look, since I feel that for ‘handling’ applications you really want your LCA’s parallel to the ground or _slightly_ nose down, which gives you slightly less antisquat from the factory setting, where most cars would benefit from a little more antisquat at the dragstrip then the factory setup, but nowhere near what most aftermarket stuff gives you.... I’d like to find a slightly better compromise that will work on the street, road course and dragstrip without any changes, but till then, stiffened stock geometry is best

 

Yep, LCA angle helps determine rear roll steer characteristics.
There's no free lunch with this suspension....well maybe.

With so much understeer in Autocross-size corners, we have considered trying some rear roll oversteer just for AuotX, and using the heavy front a/r bar to keep the front camber changes to the minimum. This should help antisquat at the same time. Bret wins almost all the time when he AutoX's a VW GTI, but he hasn't tried yet with his stock-suspension SS.

 

If you’re suggesting what I think you are (lowering the rear LCA mounts to get some extra antisquat and roll over steer), don’t, it’s a nightmare.

What you end up with is a car that oversteers under braking (expected effect) and understeers under acceleration (unexpected by most, resulting from the antisquat working, both hooking the tires AND forcing the rear suspension down), and almost feels random in places in between. If you just gave it to someone and didn’t tell then what you did they won’t be able figure it out and will think that it feels most like someone put a massive rear bar on the car and left the stock front usually it’s hard/scary enough to drive that they never find out that when they think that they’re going to get power oversteer they really will get understeer and mow down a row of cones (better then learning that lesson on the street).

This ranks very high on my list of “lots of people do this and don’t notice anything wrong but it’s really a DAMNED SCARY ride” list.

 

Thanks to both of you guys for hanging in there with me through my questions. NOW I get it. I'm sorry I missed that before. I'm going up a pretty steep learning curve here and like the ancient Chineese proverb says: You have to tell most people something 3 times before they actually hear it. Consider me "most people."

 

Thanks for the warning. Some thoughts:

1. I'm not advocating massive amounts of Rear Roll O/S (RRO/S), more like reducing the designed-in RRU/S. Lowering the rear makes RRU/S worse even though it improves the CG height. With the basically parallel LCAs of the F-car, the rear roll axis is parallel to the LCAs in the side view, so "nose-down" roll axis is RRU/S and vice versa.

2. Braking: Straight line braking gives no steer, correct? Braking and then turning in gives RRO/S as you said, and that's increased with a rebounding rear suspension. So far so good for a very tight AX corner if we keep it moderate.

3. Acceleration: While still turning (and rolling) there should still be a little RRO/S right? The compression of the chassis due to the limited amount of A-squat available tends toward RRU/S. Also with more load on the outside tire (still turning), that helps negate the U/S. There's enough power to induce O/S also. With a sophisticated driver, who doesn't treat the accelerator as an on-off switch, that's probably very controllable.

4. We would also change springs to control roll, and tune roll couple distribution with the springs and A/R bars.

5. All of this would be just for A-X, not street driving. It might be that a 3-position rear mount for the LCAs could be developed:
bottom hole for dragging (max a-squat),
middle for A-X
top for street. That's probably dreaming.

6. Everything needs to work together, of course, like any system. There's no magic amount of roll steer that's best, just like there is no "magic cam" that by itself transforms a C. Kent engine into a Superman. (numerous threads on many forums to the contrary! ).

This is more of a "thought experiment" now; Bret hasn't decided to A-X the Camaro.

Your input is always welcome.