How much front torque arm height is good and how too much or too little is bad.

I made a bracket to add to my front mount and added 1 inch of height. Waddaya think?

TIA,

 

Instant Center is the point about which the rear axle (in this case) rotates when it either moves up or down with respect to the chassis.

A torque arm suspension like the f-body is the easiest to understand: the IC is effectively at the front torque arm mount. It gets a lot more complicated for 4-link, IRS, leaf spring, etc. suspensions. See update by WS6 TA below!

I believe you are referring to rear anti-squat in your question about torque arm front mounting height. Anti-squat means that torque reaction during acceleration (RWD) tries to force the rear of the body away from the axle, or lift it. To do that it puts more force down on the rear tires, and you get more traction.

You can visualize your IC, and anti-squat on your car with some tape, string and a tape measure. Here's how:

Locate the center of gravity height of the vehicle. On an F-body, the height is about 21 inches, which is just about the top of the wheel rim.

Tape (or nail!) one end of a piece of string on the road near the bottom center of the rear wheel at the fore-aft center of the tire patch. Stretch the string tight and tape it to the top of the front wheel rim on the vertical centerline of the front tire. Call this the 100% anti-squat force line.

Now crawl under the car and measure the height from the road to the stock center of the front torque arm mount, and the fore-aft location with respect to the rear or front wheel. On the door, put a piece of tape at that height and fore-aft location. An x or small dot on the tape is good.

Now, if that dot is on the string line, you have 100% anti-squat, and the rear of your car doesn't drop during acceleration. If the dot is above the line, you have more than 100% anti-squat.

If it's below the line, you have squat (or 'pro-squat") and the rear end will drop during acceleration. Some squat is the normal case for a passenger car.

You can now easily visualize how much you've changed things by moving your mount up 1 inch.

Now visualize a shorter torque arm, say 40 inches from the center of the rear wheels, mounted at the same height as the stock arm. Notice that the mounting point, the Instant Center (IC) gets closer to or above the string, so you have more anti-squat.
That's why shorter torque arms help forward bite.

The bad news is that shorter torque arms and very high percentages of anti-squat have their own problems, which is more complex.

Here's a reference if your want to read more about this.

http://www.raceglides.com.au/TechInfo.htm

 

No, actually it isn’t. That is only the case for a conventional TA setup with a pivot at the front of the TA and then it would have to have LCA’s that were in the same plane as the axle/TA (in other words, they would bolt to brackets at the front of the axle tubes, not below them and would do nothing but locate the axle ends). Also, if that was the case, then LCA relocation brackets wouldn’t do anything except induce bump steer in the rear axle.

On f-bodies, with their sliding link TA the IC is computed by drawing a line perpendicular through the sliding joint at the end of the TA and then a second line through both pivots on the LCA’s, and the intersection of those 2 lines is the IC. Then you compute the % antisquat just like OldSStroker described.

Essentially, what this does in the long run is that changes in TA angle make almost no change in % anti squat, changes in TA length do, but not as much as you’d expect (by the time you get it in a real happy place you’ve pretty much ruined your braking unless you’ve decoupled your TA in the process), but messing with the LCA angle makes a big difference (in my opinion, too much of one unless you’re running on real, wrinklewall slicks/dot slicks)

I do know that the claim is that Madman (and others now) are moving the TA pivot up to get their setup to work, but I suspect that there is really something else that’s making it work. I haven’t actually seen pics of their setup so that I could make a guess what it is.

 

Oops! You're right, WS 6TA. Thanks for catching that. I agree with all you've said here.

To quote Terry Satchell (who may well have designed this suspension):

"This suspension has a fixed length side view swing arm that is borderline acceptable. Power hop and/or brake hop can occur with this type of suspension. The amount of anti-squat obtainable is limited because the height of the side view instant center can never realistically be raised even as high as the crank centerline. Thirty percent is about the maximum obtainable. To assure roll understeer, the lower control arms must angle down towards the front. This also means that the side view swing arm instant center height will be low."

Redbird, forget what I said about locating your IC. Yeah, raising the front torque arm mount on a f-body doesn't do much other than change the pinion angle a little.

Thanks again WS6 TA.

Jon

 

I'll have to really study this stuff. I got the idea to RAISE the front TA mounting point from Brett (BMR) and MADMAN.

Thanks again Gentlemen!

HInk

 

WS6- I'm not really much of a theory guy but I think that instant center only deals with the pivot point under the TWISTING force of the axle under torque and has nothing to do with the lateral or pushing force.

Handling the twisting or rotation force is ALL the job of the torque arm. The lateral force is ALL the job of the LCA's.

So if the intant center just deals with the the twisting forces of the axle the instant center would just be the front torque arm mount location.

However, the axle can obviously still generate downward force because of the LATERAL force put through the lower control arms. If they angle UP to the body they will force the axle down under lateral acceleration, but the force this generates has nothing to do with the twisting force on the axle or the instant center location.

An I all screwedup on this? I know from racing these things it's how it works, but I'm not sure if that means I fully understand the theory of an "instant center." Does the instant center deal with both twisting AND lateral forces?

 

Damon, how about you’re somewhat screwedup?

The problem is that you’re trying to oversimplify the situation. Both the TA and the LCA’s counter the rotating force of the axle. I’m not exactly sure what you mean by lateral force, but if you’re referring to the front to back location of the rear axle both are responsible there also. Because of the geometry of the of the rear suspension, without the TA the rear axle would be able to flop forward and backward a few inches.

You could divorce the 2 functions, and then you’d have the true TA rear suspension that I described in my first response. The LCA’s would be attached to the front of the axle in the plane of the axle, and even without a TA it would not be able to move forward and rearward (well, in reality, to truly achieve this the rear pivot of the LCA would have to be around the axle housing). In that case the TA would pivot around the same point all the time and would define the instant center, and also be the only factor in controlling rotating force.

Make sense?

Redbird, like I said earlier... Madman 'claims' (by that I mean that people talking on the lists are saying, I haven't heard this from madman, and I’m not sure that others have either) that moving the height of the front of the TA is what's working. I'll tell you positively, that it is not causing more traction, there is something else about their setup that is helping. For me to tell you what is happening I’d have to see what their changes are, and I’ve never even seen fuzzy, dark, or any kind of picture of their suspension.

 

I'm a little confused about how the TA can apply a force in the axial (front-to-rear centerline of the vehicle) direction. In the stock setup, there is a bushing that in effect is a "slip joint". Other than a small amount of friction in the bushing, I don't see an opposing front-to-rear force in the TA when I draw the free-body diagram. The slip joint is necessary, because if it was not there, the rear axle would be in "bind" as it swings in one arc from the body mount of the lower control arm, and would be trying too swing in a different arc from the front mount of the torque arm.

It would seem the TA keeps the rear axle assembly from rotating about the axle centerline, and about the rear LCA mounting bolt by pure "torque" reaction, and not through compressive axial loads to the TA itself.

All good TA designs incorporate the "slider" or a link to allow for the difference in front-to-rear displacement of the axle as it swings in the arc of the LCA's Spohn first use a true "slider" and then went to a "link" design.

The interesting thing about the Madman arm, at least the version I personally saw installled, is that it does NOT provide for this front-to-rear displacement. It bolts directly to a rigid tab on a new cross-member that essentially replaced the "g-load" brace, and attaches to subframe connectors at the ends. The rigid bolt tab does have an extra hole in it, that allows the TA pivot point to be moved up and down, and perhaps altering the inherent "bind' of the Madman design is what alters the net traction when the front mount point is raised.

When I saw the Madman arm for my buddy's car, I told him is wasn't going to work, that it was in bind. On the first hard launch (and at this point the car was only running around 750HP) the arm failed, ripping the welds in the axle end mount apart - in my analysis a combination of the net forces induced by the bind, poor layout of welds and poor weld quality.

The arm was reinstalled, and this time all the welding was beefed up, but it was still a "rigid" (fixed front pivot) design. The car never really 60-footed like it should. At this point he took the car to Steve Spohn, and Steve told him the TA was putting the rear suspension in bind and the suspension couldn't work at all. The situation was corrected and the car started pulling low 1.3X 60-foots, and pulling the front wheels 4 feet in air. .

 

The problem is that you’re looking at the TA as an entity by itself. If you neglect the other suspension links that would be a valid conclusion. OTOH, think of a 4 link, if you removed the top or bottom links what direction would it still be stable in? None.

With the LCA’s installed, the TA’s ‘preventing it from rotating’ also becomes ‘preventing it from pivoting on the rear LCA pivot and moving forward and backward.’

that depends on the geometry of the rest of the suspension. If the LCA’s are significantly shorter then the TA then in most cases you will need a sliding front link. I can think of at least 1 3rd gen setup for landspeed racing that has had the LCA pivots moved and longer LCA’s installed that essentially works like a true TA with the instant center at the front pivot. GNX’s used a very short TA with a bushing’ed pivot at the front that was almost the length of the LCA’s, it also hung down low enough that it was essentially in the same plane as the LCA’s making the whole assembly work as a true (by that I mean non sliding front link) TA.

Sounds like basically a screwed up copy of the GNX design. I’d still like to see a picture of it…

From what you’re describing, the higher you move the front TA pivot, the more bind you would get. It sorta moves the instant center upward, but only so much as the suspension can still move without binding, which probably is not much. I wonder if the’re basically shooting for turning the whole thing into a static linkage under load, sorta like a rail with it’s rear axle pretty much welded to the chassis. Might work on a race car but would be harsh on the street and undriveable on a road course.

You’ll find that I don’t have much respect for 99% of the companies making aftermarket chassis and suspension goodies for f-bodies. It seems to me that most of them don’t really have much of an understanding on how the f-body suspension was designed to work and they always make some very lame decisions. Spohn and BMR are slightly more successful in producing usable products only in the context that they really seem to try to avoid messing with the factory geometry and come close to reproducing it in a more rigid piece. OTOH, I thought that Spohn’s original sliding link used on their TA’s was a disaster waiting to happen (and it did, first few out there fell apart almost instantly).

 

Yep. You are sort of describing a "decoupled" suspension. By using 'birdcages' floating on the axle, you can achieve pivot on axle centerline.

We built a completely decoupled rear suspension for a circle track car a while ago. It could be adjusted for IC location on each wheel individually, and brake torque reaction on each wheel individually. The torque arm had a '5th' coil over to control the bite coming out of a turn. I think it had 36 rod ends, but boy did it work! The driver especially liked that we could get more braking downforce on the left (inside) rear to help turn-in, and still make it either free or tight coming out.

I have to agree that some of the aftermarket suspension stuff is scary. One of the suppliers of some nice stuff sells polyurethane control arm bushings with the caveat that they don't recommend them, and in some applications they will bind up a suspension, but they have a customer demand for them vs. the higher priced "multi-material" bushings and spherical joints. Anything for a $, I guess.

 

Heh, it’s surprising even to someone that really knows what they’re doing with suspensions what you can get a suspension to do if you really tinker with it. (I’d put myself at somewhat knowing, but not like a few people out there, unfortunately, I was a liberal arts major well, government/politics and law)

FWIW, I think you’re blending “decoupled” with “floating.” Floating sounds mostly like what you’re describing where some of the links are not able to exert any force in a direction that they are not supposed to through “floaters” which are bearing assemblies that hold the part in a cage that will not allow it to move at all in certain directions and freely in others. The most common (blatant?) example of this that I can think of is when using ladder bars with leaf springs you attach the axle to the springs with a floaters to allow the ladder bar complete control over the axle’s attitude, and the spring only acts as a spring, divorcing the to actions to specific parts designed to do that.

Decoupled is similar but it refers to having a separate part that is specifically designed to deal with one specific load (does not necessarily have to be floating, could be completely disconnected as in not touching from the other loads). A common example here is a decoupled TA, which really is 2 separate TA’s, one that acts like a shorter link that only comes in contact with it’s front pivot under acceleration, and a second that acts like a much longer link that only comes in contact under deceleration (most designs actually reposition this so that instead converting the axle rotation into a downward force at the ‘pivot’ of the TA, it becomes a forward force which acts as an infinitely long link). In this case the 2 separate actions work entirely divorced from each other and act with completely different geometry, where in the floating example they work together to perform different actions.

(I’m not sure if I just made this clear or muddy)

 

I couldn't figure how to "decouple" without "floating".

We used four birdcages so that each link did not influence another link (decoupled?). Only driveshaft torque went into the axle housing, and it was resisted by the TA. Brakes were mounted on individual birdcages with individual links. Same for wishbone trailing arms: they took only fore-aft loads and put them into the chassis at individual points. Panhard rod was used for lateral loads and roll-center height.

Anyhow, in my mind, we "decoupled" all the loads and only used six basic links (if you consider the TA+5th c/o as a single link).

Should we call you Mississippi?

 

There is obviuosly a misconception on Instant Center. AND according to Injuneer Fred my arm doent work. First and foremost my arm has been 1.24 60fts times on my cars PLUS 1.26 on lower hp cars.

The misconception is the way youset the arm up. My arm has 5 holes in the front and 2 sets of holes on the rear. The lower the front is the longer the I/C is. This allows the rear to plant and carry the front further out. A bar that is mounted up hill from the rear allows a sudden shock to the rear tires which plants them but most cars dont have the momentum to carry the front out past the 60 ft.

On a torque arm car you can rmove the lower control arms at the launch and the car will still go straight(with an antiroll bar). Same for your coil springs an the rear BUT thats another discussion.

On low hp cars we ste the t/a level or 2 degrees up to allow the rear tires to smack the tires. On high hp cars we run the t/a 3-4 degrees down from the rear to allow the cars to hit the tires and carry them past the 60 ft.

 

I don’t see anything here that disagrees with anything that I said (for that matter, I’m not going to pic through the original posts, but it doesn’t disagree with Injuneer’s general sentiments, it will bind and it will not work ‘correctly’ as it was originally designed to work). I’d still like to see good pictures of the thing (I have seen the ones that are posted on your page under the camaro (black car with red floorpan).

From what I’ve seen there it looks like it basically works as we’ve described here… in a way that you really wouldn’t want a car that sees any street time working. To be honest, you’ve turned it into something that works differently then the stock suspension works even though it appears to be a roughly similar layout. I’d go as far as saying that it does not work as a factory TA suspension at all, but more like an aftermarket ladder bar suspension.

Um, only if the antiroll bar is mounted really rigidly, and only on setup like yours that replaces the front sliding joint with a pivot. For that matter, if you could keep things lined up well enough I’d be willing to bet that that setup would work better with an arm like yours. But, saying that is somewhat like saying that a drag car will launch and make a pass straight with the steering linkage disconnected. Yes, it will, but only if everything works out perfectly on that pass, and I don’t know that anyone would be nuts enough to try it.

as the ride height drops the instant center drops/moves forward. If you dialed in enough antisquat that it would really work well for a launch starting from sitting on the bump stops you’d have too much antisquat when it lifts and starts moving. To some extent you could make up for that with very soft tires or a lot of power, but very few cars that don’t fit that category work well with that much.

For that matter, unless you’re on soft slicks, I don’t really believe that more antisquat will help the average f-body.

You know, with that setup I’d be more interested in knowing what you do to keep from carrying the front tires too high…

Just to be clear, I’m not saying that your TA doesn’t work, I’m saying from what I’ve seen and you’ve described, it is no longer working as a TA suspension, but as a ladder bar setup…

 

On the anti squat theory when you launch your car the rear end separates from the chassis. The rear end actually goes down toward the track and the body rises up. You actually are using the shocks to control the raerend not the shocks.

The t/a suspension major flaw is that there isnt anyway to get a short i/c. Most stock car i/c is around 80" out and 17" up. On some of the t/a that have a new crossmember they run 60" and 12" up. I have found that these cars like 45"-48" out and 6" up. That was the reasoning for my design. Also with my arm I normally dont run but 1 degree of pinion angle. I also use alot of the front suspension to control wheelstands.