SS #670 Modifications: Adjustable torque arm

I believe there are two main reasons to use an aftermarket torque arm. First, a strong design will greatly reduce flex the OEM version exhibits. Second, you can reduce compliance by switching from rubber OEM bushings to urethane or better yet using spherical rod ends.

After exploring some other options (see the details further down on the current page), I settled on the LG Motorsports G2 heavy duty torque arm.

G2 torque arm	Installed

The front attachment is on the transmission and uses a modified OEM mount with a greaseable aircraft spherical bearing welded to it. The arm is mig welded using 0.095 wall heat treated square steel tubing. The arm weighs 15 lbs, about the same as stock.

Installation notes

• You will want to have the car on all four wheels when taking measurements or making adjustments. I had good results using two pair of RhinoRamps. I drove the front wheels on two a pair and then jacked up the rear end and set the rear wheels on a second pair.

• I think it's wise to check your pinion angle before removing the stock torque arm. With my car on four ramps, the driveshaft was 2° and the pinion was 1½° which is -½° of pinion angle.

  Negative pinion angle means the driveshaft and the pinion both point down at the rear U-joint. When the car accelerates, the pinion rotates up, moving towards a pinion angle of zero.

• After installation, my pinion angle was -1 ½°

• You will want to have the rear of the car supported by the frame with jack stands in order to have enough clearance to remove the differential bolts. You'll also need to support the differential to keep it from rotating when you remove the torque arm.

  With the torque arm removed and the differential unsupported, the pinion tends to rotate up. What seems to work best is to support the differential from the rear with a block of wood. Resting the back of the block against the sway bar gives you a position with enough leverage to jack up the differential and rotate the pinion down.

• I used a SLP front driveshaft safety loop.

Summary

The G2 heavy duty torque arm made a subtle but important change in the handling of SS #670. It's expensive and somewhat exotic but like subframe connectors, its effects are important but quickly taken for granted. I like it!

My Previous Setup

The first torque arm I tried was a Spohn. As expected, it transmitted more noise into the cockpit but it also had a slight clunking sound when making a transition from forward to reverse, reverse to forward or hard braking to launching. It was also the tube-and-rod slider design which broke after about two years of daily driver duty.

Next I tried Spohn's updated design featuring a spherical rod end and shackles front assembly. This one certainly didn't look fragile but unfortunately it also made clunking sounds, sometimes the result of getting out of the throttle. After about six months, I reinstalled the factory torque arm to confirm the Spohn was the cause of the noise.

The Spohn arm is about twice as heavy as the stock version (32 vs. 15 lbs) but it looks like it would be bulletproof in a drag strip car. It's also adjustable, a feature I don't need but is desirable for drag racing.

While the factory torque arm attaches to the transmission, Steve Spohn's is one of several aftermarket designs that attaches to the floorboard. A nice feature of the Spohn torque arm is the shackle assembly that connects the arm to the cross member assembly. It allows front to rear movement of the differential.

The one I used was black with the NHRA compliant front driveshaft safety loop and the chromoly spherical rod end. I also requested that my cross member be made straight for extra ground clearance. (The normal version is curved and only hangs low where it needs to; where the exhaust pipe is.)

The arm is stout, the welds are pretty and the powder coating is thick. I hear that it takes six hours to fabricate one! The shackles assembly and rod end have grease fittings.

The torque arm cross member did an excellent job of tying my of my Kenny Brown subframe connectors together.

Installation notes

• After installation, I set my pinion angle to -1°.

• Even after removing the washers (as per the instructions), I found that the two M16 X 2 X 180mm differential bolts (approximately 7" long) were too short; when tightened, the bolt ends were not quite flush with the top of the nuts. I'm told the optimum thread engagement is approximately 1½ times bolt diameter so this is definitely not kosher.

  I eventually upgraded to these trick Caterpillar 9/16-12 X 8" bolts:

  Caterpillar bolts	Caterpillar logo

  The nuts (Caterpillar 5P-0539), bolts (9H-9129) and washers (5P-8246) cost less than $10. You can find a local source for these using the dealer locator on the caterpillar.com website.

  The holes in the differential housing and the Spohn torque arm were already 9/16".

• My Kenny Brown subframe connectors caused a couple of minor problems. First, the M8 X 1.25 X 25mm bolts that came with the torque arm were too short to go through both the torque arm cross member and the SFC plates. I was able to find M8 X 1.25 X 30mm bolts at Bowlin Bolt in Berkeley, California. I got them in black with matching washers.

  Second, to get proper exhaust clearance, the torque arm cross member needs to be mounted directly to the floor board mounts with the SFC plates on top. This strategy worked with the first generation Spohn torque arm but not with the new design; there's not enough clearance with the way the tubes are welded to the bottom of the cross member plates.

  Sandwiching the SFC plates between the floor board mounts and the cross member gives a little extra clearance below the exhaust pipe (remember my torque arm cross member is straight). But it also moves the safety loop down so that it's touching the exhaust in three places (I have the SLP dual-dual exhaust with a stock Y-pipe). This meant I had to have the safety loop mount tube lengthened. But you can't go nuts here; if you raise the safety loop too far, the driveshaft will hit it when the rear suspension is hanging down on the shocks. And cleverly, the balance weight on the aluminum driveshaft is 6" from the front U-joint, right where the NHRA specifies the loop be located.

• The factory transmission torque arm mount isn't used with the Spohn arm so I removed it. You end up with only one bolt long enough to hold the exhaust bracket (M10 X 1.5 X 80mm). I bought another one from the dealer (GM 11504745).

• The grease fitting on the spherical rod end is somewhat fragile; I broke mine trying to disengage my grease gun. The fitting appears to be an Alemite 3019 (5/16" diameter, 15/32" overall length, "drive-in" style for a 1/8" drill, no ball check).

• There are a number of installation mistakes that will cause clunking sounds from the rear end when making the transition from forward to reverse, reverse to forward or hard braking to launching. As per the instruction sheet, all the bolts and jam nuts need to be tightened during installation.

  If you don't tighten the bolts that hold the solid ends to the differential housing mount, the torque arm will shift. With the car on ramps, I didn't have enough room to properly tighten these. What worked well was let the differential hang down all the way and then use a ½ impact gun.

  If you don't tighten the upper jam nut, the torque arm will shift. This one is not normally removed so I used Loctite 262 on it.

Fasteners

Description	Specification	Tool	Count
Floor pan bolts	M8 X 1.25 X 30mm	13mm	4
Lower shackle	3/4-10 X 3"	1-1/8"	1
Upper shackle	5/8-11 X 3"	15/16"	1
Rear U-Joint	?	11mm	4
Solid end	5/8-11 X 2-½"	15/16"	2
Differential housing bolts	9/16-12 X 8"	15/16"	2
Differential housing bolts (stock)	M16 X 2 X 180mm	21mm	2

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Craig Leres