I've got some nasty looking bushing, rubber is cracked.
Does the Reinforcing kit negate them or do I still need to replace them with new ones?
I have the cross member out already as I'm installing the dual mount rear spring.

 

If you mean the aluminum disks, the are just supports under the bushings. You still need to have good bushings in place.

 

I've got them in stock if you're in a hurry.

Andrew
800-442-0335

 

But the stock bushings rarely need replacement. Surface cracks are pretty well meaningless.

 

These are pretty deep cracks

 

Yes they are.

If you analyse what these bushings actually do, the direction of the load path, what effect they have on suspension geometry etc, you'll not only forget about changing them, you'll not bother installing the zoomy swish aluminum disks.

I bought first, thought later. Bad, bad me.

 

Not being an engineer, I'd have to guess
I'd guess their main job is to isolate the diff from the frame and to resist the twisting motion applied by the drive axle.
I would assume that cracks mean the rubber has dried out and therefore less effective at isolating noise and/or resisting the twist.

Am I even close?

 

Since you are there and if the bushings are in as bad of shape as you say I would replace them. I would also get a new snuber bushing as well. And contact VBP or Van Steel for there Reinforcing kit. And I think they also carry the Bushings as well. That way you can get everything from one place with only one shipping cost.

Gullstrand ( I hope the Spelling is right lol. ) used to carry a kit to get rid of the bushings and put in steel plates to stop all movement. But they no longer have the kit and say that with the locating discs and a Solid disc to replace the front snubber bushing it is no longer needed.

 

The ones made by Dick Guldstrand are aluminum, not steel. I have them on my '64.

 

Yes the Locaters are Aluminum. But the old kit did not use the bushings at all. And replaced them with Steel Discs. The ones I installed were steel but that was in the early to mid 70's.

But all that is moot now.

 

Yup. The primary load under most conditions is in the vertical plane, essentially the weight of the rear half of the car pushing downwards via the diff and spring etc., and varies only as the car rises and falls on it's suspension. The cross member is being forced towards the frame (away from the pucks). The only time it achieves zero and goes in the opposite direction is with the weight off the wheels.

The torque applied by the driveshaft to the diff (not half shaft torque) is resisted by one bushing going into compression and the other going into extension, but given the length of the cross member, the effect (lever arm) is minimal. The snubber bushing at the front of the diff resists the wheel torque which 3-4 times high than diff input tq.

The whole reason for using rubber and not poly or solid is to dampen vibration as you mentioned.

I used the aluminum pucks for 5-6 years I guess. During the frame off I had a look at them and noticed no wear marks whatsoever, meaning that the old, ugly, cracked bushings had never deflected sufficiently to allow any contact. I re-installed the crossmember with fresh paint and left the original bushings, warts and all. The pucks now make good drink coasters in the garage.

 

Now I'm tempted to remove them, just so I can cut then in half and see how deep the surface cracks go

 

You must have only straight roads in Canada? My diff crossmember gets lots of lateral forces when I go around turns.

The rotational force twisted my crossmember due to a week bushing on one side since it was able to flex the crossmember as the bushings moved

 

True, but the bushings go into a compressive mode across the widest part of their cross section. Area vs. load in this plane means the bushing barely know anything is going on. The magic pucks don't come into play here either.

Don't understand how your crossmember would become twisted by a weak bushing. (?)

 

Try running autocross with sticky tires.

 

OK, I'm a dummy. How can the lateral forces induced in high G cornering induce a twist due to bad crossmember bushings?

 

These are the Guldstrand parts I use to locate the crossmember and the differential. They are still offered:


The two large aluminum disks locate the crossmember (above the rounded areas at the ends of the photo). The crossmember locates the differential. If there is slop, the differential can torque out of ideal alignment and induce wheel hop, etc.


The small aluminum disk replaces the front cushion and more securely mounts the differential, again to avoid twisting, wheel hop, etc.


Seems pretty straight forward to me (and Dick Guldstrand)

 

The crossmember bowed from the differential trying to rotate. The bolt in the bushing on the passenger side worked as a pivot point and as the force pulled the inner side of the crossmember downward the outer side was able to move upward since it was not stabilized by the bushing

 

Yeppers here is the Aluminum disk that replaces the front cushion.

 

Yes, except the OP is not asking about the front (snubber) bushing, only the crossmember units. The snubber bushing resists the torque reaction of the differential housing, whether it's of benefit to install a puck instead of a rubber isolator I'm not so sure.

Rotate in which direction from what force? The weight of the car is pushing the c/member bushings into the compressed position. I cannot imagine any driveline force or driving manoeuvre that would reduce this 'preload' to zero and continue to reverse the load sufficiently to allow one end of the c/member to move away from the frame far enough and with enough force to bend the opposite end of the c/member. Are you sure it wasn't a bad snubber bushing?