steve_c

lons94z,

I still dont get it how you dont see how lca movement causes fore and aft movement. Someone needs to draw a picture, Jon A please help...

If both lca's have spherical ends, that means no fore/aft movement due to bushing deflection, then how will the torque arm move in/out of its mount. Explain this.

How do lca's move anyways? Remember axle steer? what happens... one arm's effective length shortens, while the other arm's lengthens.. simulating a toe in of the axle.

How does the length change?... the body of the car leans over and the arm swings at an arc.

Ok assume the lcas are parallel before a right turn, and let's look at the outside wheel or left rear corner of the car. In this case the body of the car leans over and the lca pickup point on the frame lowers vertically relative to the axle pickup point.

You can draw this as a right angled triangle, where the hypoteneuse is the real lca arm length, and the effective length is the bottom side of the triangle. This bottom side is now shorter.
This moves the outside wheel in, changes the wheelbase a bit creating toe in.

If the axle moves in now, the torque arm which is connected to the axle must move in now...

You have to see the traclink under the car during installation, If you were to disconnect the rear swaybar and let the axle drop, you would see that the arm slides out of the cone mounted to the main beam. I've had one, installed it myself, and know people with some, alot of those are autox'ers, road racers...
There is a sliding member, the arms works very well, it does what it says in writing (just like you read on the GW site). Yes the same writing that everyone else copies (spohn ,bmr, etc...)
It's won national championships, I have friends with 422ci strokers generating 500rwhp that use it. No torn sheet metal.

It's a serious piece, but its not going to give you a lexus smooth ride.

Steve

lons94z

I thought your point centered around hitting a bump with both wheels. The arc swung by the arms would then cause a very slight fore/aft movement. Again VERY SLIGHT. Re-read the PHR example!

When roll steer happens the arm will not move in or out because one side moves in, the other out. It does not actually toe. The whole axle moves.

Now picture this ridid TA one may have. An LG is PERFECT example as it does not have rod ends at the axle mount. When the axle rotates the arm HAS TO ROTATE WITH IT. There is no way around this. It is bolted to the axle. And since the front slides (swivels in the LG example) Then the arm will move in and out of its mount when this happens.

Now if we set the car up with a perfectly rigid arm it would not have axle rotation which would limit traction and cause brake hop.

Again all easy enough to understand without a picture.

steve_c

>When roll steer happens the arm will not move in or out because one side moves in, the other out. It does not actually toe. The whole axle moves.

What moves in, the other out? > I assume you mean the control arms?
control arms dont move in/out, they move in an arc like Jon said.
When this happens the effective length of the arm shortens... its a virtual length, it's not real.
This pulls in the axle, and makes the torque arm move..

Dude wether or not a torque has rod ends (which are used to adjust pinion angle) or not..., the arm is fixated on the pumpkin, nothing rotates. If it did you would have all kinds of pinion angle change, which would result in poor traction and wheel hop.

This is why Sam went to the Random Tech, because the stock arm flexes, and changes the effective pinion angle.

Engine torque causes the pumpkin to move in an arc, like the lcas, since it is fixed about the torque (assuming its nice and rigid). This causes the arm to slide in/out.

lons94z

First off if the only reason Sam went to a Random Tech arm was to eliminate flex why didnt he just weld a bunch of braces to the stock arm? There is more to it than just eliminating flex. Like weight, siffness, pinion adjustment, free movement, etc etc.

And the other point is that you finally agree with me. And this is the 3rd time you changed your mind. First the arm slid in its mount because of the LCA arc that is swung. Also you said that it was when both wheels hit a bump. Then it was caused by the LCA because of roll steer. And finally the right answer because of axle rotation.

So you finally know how the torque arm moves fore and aft in its front mount.

steve_c

I just wanted to emphasize the importance of the LCAs, I still don't think you get it, but I'll leave it be.

Sam could of reinforced the stock arm. Everything about the stock torque arm is good, the only issue with it is that it flexes alot.

Pinion angle is something you shouldn't screw around with, it robs horsepower and causes u-joints to wear. It essentially preloads the rear end. The stock arm angle is set at 0 from factory, and there's plenty of 10 sec guys using it still.

Stock arm has free movement, and weight isn't a big deal.

I've always known how the torque arm works. Your comments are usually devoid of logical statements and full of ambiguity, that everyone ends up confused.

Be a little more concise when you write your comments.

I CORNER1

Wow!! Lots of debate on this one!

As I have not had the opportunity to get the car up in the air (just got home from work), I still don't know the cause of the breakage of my HPM Megabite (fixed-front pin) Torque Arm. The cause is probably located to the adjustable coupler or the fixed poly mount. We will see. At any rate, I am going to go away from the fixed-pin front-mount design, due to its bind limitations. I am going to use a design which does not restrict fore-to-aft movement of the TA. I am still leaning towards the Spohn design for several reasons. Let me explain and maybe you guys can help me.

Let me take the liberty of "quantifying" the values of movement that everyone seems to be arguing about. It is really "much-ado-about nothing"!

I just measured my Global West rear lower control arms (which use aircraft spherical bearings). They are pretty damn close to being exactly 19.0" long. I use the Global West anti-squat relocation brackets on my lowered car to optimize the instant center of the rear suspension. As my car sits on the level floor, the LCAs are "exactly" parallel to level ground.

Now I think that you all finally agreed that the arc of the LCAs is 99% of the cause of the fore-to-aft motion of the TA (ignoring movement due to sloppy rubber bushings or weak arm flex). Assuming that the TA does not flex, there is no fore-to aft motion added from the TA. Therefore, with my rear suspension in the steady-state mode, the LCAs hold the Torque arm as far back in the chassis, as it will ever be.

Now assume that I have vertical rear suspension travel of 2" of compression travel and 2" of droop travel from the steady-state ride height. Considering the use of a Right-Triangle, at 2" vertical suspension travel, the resultant horizontal travel of the TA will be 0.106" towards the front of the car [(19"-squared) = (d"-squared) + (2" squared)]. If the suspension travel were only 1" up and 1" down, then the distance would be reduced to only 0.0263". This isn't much movement, but is still important not to restrict to prevent binding (especially if no flex rod-ends are used). Also note, that under full power or braking, the TA itself helps limit the magnitude of the rear suspension vertical travel.

Now lets look at the Spohn design, that some love to hate (Herb Adams refers to this as the chassis mount or Reese Bar). The Spohn used to use a sliding rod (unpinned design), but have changed to the use of a "relocator" link (looks to be about 2.0" long) which is pinned both at the Chassis Tunnel Brace mount and at the TA. Assume that this link is oriented vertically when the car is in steady-state mode. Therefore, if the rear suspension compresses 2", the TA will be moved 0.106" forward. As the double-link will rotate forward about the Chassis Mount by 0.106" the front of the TA will be brought down vertically by 0.0028" [(2"-squared) = (d"-squared) + (0.106"-squared)]. Therefore, the vertical movement of the front of the TA can be considered to be negligible.

However, let's prove it. Spohn's TA is about 39" long (factory and LG arms are about 25% longer). Assume about another 3" distance to the centerline of the axle for a total effective length of 42". Again, for a 2" compression of the rear suspension, the front of the 42" TA mount moves down 0.0028". This results in a miniscule change in pinion angle of only 0.0038 degrees, using the Law of Sins [(Arc-Sin Theta) = (Arc-Sin of {(sin 90 degrees) x (0.0028") / (42")}]. This is nothing and essentially duplicates the sliding-rod design without the sliding noise or weakness.

Now correct me if I am wrong, but the 0-degree pinion angle is the reference from the centerline of the semi-horizontal differential pinion to the centerline of the semi-horizontal transmission tailshaft. Assuming the TA is set to position the differential at 0-degrees, the slight pulling down of the front of the TA will actually help maintain the pinion angle parallel to the transmission centerline.

In fact, I would argue that based on the above that the factory TA tailshaft mount is actually a "cost" compromise. If cost was not a factor, then F-Bodies would have been built with 4-link independent rear suspensions Think about it. The factory mount would actually change pinion angle when the suspension is compressed and unloaded, as the pumpkin is fixed to the rear of the TA. The pumpkin actually travels in an arc about the transmission TA mount, causing it to diverge from the transmission centerline.

Therefore, based on the above, the Spohn looks like the way to go. I get a shorter TA for greater rate of traction application. I get a strong tunnel brace to tie the Kenne Brown Double Diamond Subframe connectors to. I get 2 driveshaft safety loops and I get a very strong TA.

The theoretical downsides to short arms are some rear axle lift upon braking/deceleration. However, my very short HPM never exibited bad manners, during braking. Until someone makes a de-coupled TA that works, I guess there is no other choice.

As far as the Global West Traclink: It is also a short TA. It has an unusual method of attaching to the body sheetmetal. It does not appear to be as strong, as the Spohn. Finally, it is expensive. I love my Global West Control arms, but after my front adjustable height suspension problems (see other posts), it is a fact that all of their products are not perfect.

As for the mounting of the Spohn at the tunnel brace, I plan to weld in extra bracing, add extra welds to the KBDD subframe connectors and use larger ARP thru bolts, to keep it in place in both compression and tension.

Any comments/valid corrections guys?

Rick R

lons94z

I would leave it be to, if I had changed my story so many times that I had no "options" left.


Yea the stock arm is great

I guess competing on the National level is something someone who "open tracks for fun" would not understand.

Norm Peterson

That's correct for the case where the LCA's start out horizontal. If they're anything but, the initial TA plunge will be greater, since the initial arc motion at the axle would already have a horizontal component.

Also in such cases, the fore/aft motions at the left & right axle centerlines will not be equal and opposite, so there will be a little bit of TA plunge under pure roll. Not a whole lot, to be sure, but it does occur.

Running the same numbers with LCA's that are already 1" lower at the axle than at the chassis gives -0.0263" on the "bump" side and +0.106"on the "rebound" side (+ being forward). The next inch brings the "bump" side back to its original horizontal position (zero) while the "rebound" side moves to +0.238".

Not sure I follow what I'm reading as a TA being able to directly resist vertical suspension motion. Indirectly though SVIC and virtual swingarm considerations, yes . . .

Norm

ZBLKHELLRZR

Geez you guys really got in depth w/ this! I think I've read every post twice getting the jist of it. Only thing for sure now is I'm more undecided than ever. Both serve the purpose in different ways. I'm still gonna wait this out awhile longer no matter which one I get it'll be better than stock. LOL.

I CORNER

Clarification to the following"

"Not sure I follow what I'm reading as a TA being able to directly resist vertical suspension motion. Indirectly though SVIC and virtual swingarm considerations, yes . . . "

What I meant here was that under hard acceleration the TAs resistance to axle wind-up is to plant the tires harder (anti-squat) and the response to hard braking/deceleration is to keep the suspension from extending. In this way the TA actually limits the rear suspension movement under power/braking motions.

Rick R

LPEdave

I'll bow to all the math and just confirm that in practice if I take out my rear springs (LCA's and TA still in), and run the axle up as high as it'll go, and down as low as is reasonable (ie way more than 2" up and down), the driveshaft slides at most 1/4" back and forth in the tranny. I think that corresponds to some of the figures above. Not sure if it's relevant or useful to the discussion, though

Dave

I CORNER1

I had a short opportunity (with work obligations) to get under the car this weekend to see what the problem with my HPM Torque Arm breakage could be. No major member breakage, but the arm is scraping with suspension movement. Based on the origin of the sound, I think that the adjustable coupler is damaged. At any rate, I planned on replacing it for a higher-horsepower rated unit and to eliminate the front "pinned" mount.

I did take some measurements. I was wrong about the length from the TA bolt-holes on the differential to the centerline of the axle. It is longer - a full 8 inches. Therefore, the impact in movement fore-to-aft would be even less. So as long as the TA permits fore-to-aft movement the decision comes down to the optimum TA arm length.

So here are the relative lengths of the effective TA lengths.

HPM Megabite - 37" (29" + 8")
Spohn - 47" (39" + 8")
Stock or LG - 67" (59" + 8")

Therefore,
HPM was 55% length of stock length (about 82% increase in initial leverage to plant the tires).

AND
Spohn is 70% of stock length (about 43% increase in initial leverage to plant the tires).

From looking at the Global West Traclink photo, that TA looks about the same length as my HPM is.

As I said, the HPM had great initial traction, but then seemed to quickly run out of travel, as it raised the rear of the car and then did not grip as well. I think backing down to a slightly longer arm, will be a good compromise.

Hmmn?

Rick R

I CORNER

I just finished installing the Spohn LT1 Torque Arm and am very impressed. Let me start by saying that this is a very well made product, with tremendous strength.

I purchased the standard LT1 header version with Chrome Moly TA tubing, front chrome-moly spherical rod-end and 2 driveshaft safety loops.

I have 2 cautions for us lowered road-racers/autocrossers:
- The rear driveshaft safety loop will not fit most lowered cars. It contacts the passenger side of the driveshaft tunnel. Only the single loop TA will fit.
- The upper driver's side of the differential TA bracket protrudes out and contacts the driver's side of the driveshaft tunnel. You have to pound in the floor pan about 1" (still invisible under the carpet and seats). I also ground the edge off of the bracket itself for added clearance. Neither of these clearance tasks is a big deal, but you need to know that it needs to be done.

I adjusted the front rod end so the rotating link is positioned vertically when the car is at ride height. This will minimize any pinion angle variance, as the TA moves fore to aft.

With my 3.5" aluminum driveshaft I had adequate safety loop clearance, even after I shimmed the engine-trans up by 1/4" at the trans tailshaft for better driveline angle. I set pinion angle to - 1 deg from gravity with driveshaft at 0 deg and drivetrain at + 2 deg. I had to install Spohn tunnel brace over my KBDD subframe connectors.

Rod-End transmits more road noise (still acceptable to me) and seems more instantly responsive.

The Spohn TA is much stronger than the chrome-moly HPM TA I removed and I think that it will work well for autocross/road-race applications. The HPM poly bushing had squashed (cold-flowed) causing clunking and 1 of the adjustable coupler threads were stripped.

auto-Xer

Hey I CORNER!
Congrats on your new TA! Did you get the bolts from Spohn for the diff side of the TA? Maybe it comes w/ them now. The original bolts were too short and you had to remove the washers and the threads didnt come all the way through the nut He has grade 8 bolts on his site that are longer than stock to fit if you did not get them w/ the arm. Piece of mind...

Also, w/ your road noise, do you hear your diff now? I can hear it spin down when slowing down to stop. I was wondering if this is normal or if mine is going/something was wrong...

Thanks!
--Kevin

I CORNER

I bought the longer bolts with washers from Spohn.

I hear noticably more diff noise on acceleration and slightly more noise on de-celaration. I have a HD Torsen with 3.73 gears.