Rod thrown, what now?

[1998_K1500_Sub]

At some point in time, auto manufacturers realised that the bell housing was crucial in reducing NVH as any vibration there is amplified. The rods as I understand them can't add much in the way of torsional rigidity (ie, restraining the torque at the transfer box output) but possibly helps damp the cyclical vibrations. All guess work re GMTs.

This is akin to my view of their function as well, I can't see how they would do much for torsional rigidity... not to say they don't help some in that respect.

I had always figured they were put in the 4x4s b/c a 4x4 is likely to get bounced around much more than a 2x4 (construction sites, farm fields, etc.) and NVH would be improved by the added support at that gaping hole at the bottom of the engine / transmission interface.

But my 1995 2x4 S10 4.3L 4L60E S10 came with them those bars. Maybe it's a red herring.

 

[Pinger]

There's also the possibility that the additional length and weight of the transfer box causes a bending vibration that occurs where the engine and gearbox meet and the rods help quell it. Or a clashing of resonant frequencies not present in the 2WD's lighter/shorter transmission.

 

[stutaeng]

The rods? They probably act like a "couple," one in tension and the other in compression, and reactions at the other end of those rods get resolved at the engine block.

At least that's the theory of torsion and how to deal with torsional stresses. Either use a closed cross-sectional area element, or provide restraint on each side on the element under torsional stress.

 

[1998_K1500_Sub]

Here's my simple view of the matter, illustrated FWIW (using a Ford engine because it's the only picture I found usable).

The rods simply fortify the end-to-end strength of the engine - transmission assy between the front engine mounts and rear transmission mount.

Maybe these rods were added on the 4x4s because the of additional distance b/t the engine mounts and the rear mount (which is on the tailshaft behind the transfer case, isn't it?).

You must be registered for see images attach

The rods? They probably act like a "couple," one in tension and the other in compression, and reactions at the other end of those rods get resolved at the engine block.

Looking at my mind's-eye picture of how those rods are installed on my 1998 K1500, and how they were installed on my 1995 S10, I can't see how a couple formed by differential forces on those rods would counter the torque applied by the transfer cases / driveshafts, e.g., in "low" mode.

 

[Schurkey]

I really had never seen those rods until I bought my 2000 K3500, but that one does have them. It has the sheet metal flexplate/torque converter cover.

How can you have those structural rods, if you've got a sheetmetal inspection cover?

The rods on my '88 bolt to a cast-aluminum cover, which is held to the trans with six heavier bolts. The sheetmetal covers were held to the trans with four, small-gauge bolts.

Or has that been re-engineered for the Y2K vehicles?

 

[Supercharged111]

When I did the 4L80 swap on my 1500, I used a 2wd transmission with stamped steel bell and no rods. I always had a vibration around 3000 RPM. Later I got the cast aluminum bell and rods from the boneyard and the vibration has been gone ever since. So whether or not it makes sense is irrelevant, in my experience they really do make a noticeable difference.

 

[stutaeng]

How can you have those structural rods, if you've got a sheetmetal inspection cover?

The rods on my '88 bolt to a cast-aluminum cover, which is held to the trans with six heavier bolts. The sheetmetal covers were held to the trans with four, small-gauge bolts.

Or has that been re-engineered for the Y2K vehicles?

Edit: I meant to say that you got the sheet metal cover on 2wd or the ribbed cast aluminum torque converter cover on 4x4. I've never understood those rods or why they are there... somehow I got confused thinking they were used on the sheet metal cover.

I tried to get a photo this morning but it was too wet to crawl underneath. It looks to me like I've got the sheet metal torque converter cover. And I did confirm I have the rods, which I remember I had. See picture. Maybe I'm wrong. I'll double check later today.

For reference, we are all talking about the same ribbed cast aluminum cover, right? I just happened to be washing an engine this last weekend and cleaned 2 of these I had laying around.

 

[1998_K1500_Sub]

For reference, we are all talking about the same ribbed cast aluminum cover, right? I just happened to be washing an engine this last weekend and cleaned 2 of these I had laying around.

That's the cover I've seen used with the rods. The rods bolt onto the bosses, pictured (see added arrows on the picture).

And you certainly have the rods, one is plainly visible in your other picture... although obscured by the exhaust pipe so I can't tell plainly where it's affixed on the transmission.

 

[stutaeng]

I edited my previous response @Schurkey

Ok, so I took some additional photos. It looks like I have the cast aluminum cover, it's just smooth and doesn't have the ribs like the other photo I posted. The rods or struts bolt to the bottom of cover and at the engine mounts at the other end. I accidentally deleted the photo at the mounts.

 

[Pinger]

Here's my simple view of the matter, illustrated FWIW (using a Ford engine because it's the only picture I found usable).

The rods simply fortify the end-to-end strength of the engine - transmission assy between the front engine mounts and rear transmission mount.

Maybe these rods were added on the 4x4s because the of additional distance b/t the engine mounts and the rear mount (which is on the tailshaft behind the transfer case, isn't it?).

You must be registered for see images attach

Looking at my mind's-eye picture of how those rods are installed on my 1998 K1500, and how they were installed on my 1995 S10, I can't see how a couple formed by differential forces on those rods would counter the torque applied by the transfer cases / driveshafts, e.g., in "low" mode.

Now I have a better idea of where the rods are positioned I agree - not to resist torque. That was just a guess based on 4WD having low range and 2WD not.
I think you have it spot on with the 'extra length' between mounts and 'fortification' of beam strength.