C4 suspension LCA mounting method change?

69427 · #1 02-14-2021, 11:26 PM

I've had my (lighter replacement) aluminum LCA crossmember 90% finished for quite some time, but I'm not comfortable with the regular method of attaching the lower control arms (clamping/bending the crossmember tabs/flanges to prevent the steel bushing from rotating around the attachment bolts). I'm considering trying to fabricate a shaft piece that looks like what the C3 LCAs use, that then bolts to the front crossmember (or modify a C3 shaft, as the dimensions are somewhat in the same ballpark). This would allow me to just bolt the LCA shafts to the crossmember, with no tabs or bending stress involved. (I could also shim the shaft mounts outward if I wanted to change the camber or trackwidth.)

Just kicking around ideas at the moment. Please feel free to comment about this.

BBShark · #2 02-15-2021, 08:35 PM

Had to refresh my memory of the C4 rear suspension. I think this is the stock LCA setup right?

69427 · #3 02-15-2021, 10:33 PM

Quote:

Originally Posted by BBShark

Had to refresh my memory of the C4 rear suspension. I think this is the stock LCA setup right?
...................

Yeah, that's a C4 rear suspension, but I'm working on the front suspension LCA (A-arms). I'm trying to come up with an alternate method of bolting in the lower control arms without having to squish any parts of the aluminum crossmember I welded up.

BBShark · #4 02-15-2021, 10:57 PM

I don't know where I came up with rear suspension

Now I get it. Could you shim/spacer the bolts so they don't bend the aluminum tabs? The C3 part is a chunk.

phantomjock · #5 02-15-2021, 11:08 PM

There is a lot of stuff in that image - but I'm looking at the lower wishbone. Note how it mounts to the C3 front cross-member (K-bar/A-bar- I forget). That cross-member attaches (bolted) to 2 tabs on the C3 member.
Could something like that be in the works? It's the heavy - dog bones that bolt up.
Seems simple enough with your welding skill to fab that piece up. (No challenge intended.)
You could use heim/rod ends on either the outboard and/or inboard ends to give heaps of adjustment.

Cheers - Jim

phantomjock · #6 02-15-2021, 11:35 PM

I'll add; It looks like SuperBuickGuy as I recall has C4 IFS wishbones and uprights.

Pretty good shot over on a Mud-forum:
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Cheers - Jim

vette427sbc · #7 02-16-2021, 02:27 AM

Quote:

Originally Posted by phantomjock

..... It's the heavy - dog bones that bolt up......

Cheers - Jim

A cross shaft is going to be more than a few OZ’s... I vote some poly bushings to limit the squish. Can I assume you don’t want the tabs to squish for metal fatigue reasons?

69427 · #8 02-16-2021, 11:51 PM

Quote:

Originally Posted by vette427sbc

Quote:

Originally Posted by phantomjock

..... It's the heavy - dog bones that bolt up......

Cheers - Jim

A cross shaft is going to be more than a few OZ’s... I vote some poly bushings to limit the squish. Can I assume you don’t want the tabs to squish for metal fatigue reasons?

Correct. The C5-C7 LCA tabs on the cradle are part of the crossmember casting. Any tabs I put in my crossmember would have to be welded in (most likely). I'm a double E, not an ME, and I occasionally run into scenarios that are out of my comfort zone.

I can't disagree that the cross shaft would be an inefficient use of additional weight.

69427 · #9 02-17-2021, 12:07 AM

Quote:

Originally Posted by BBShark

I don't know where I came up with rear suspension

Now I get it. Could you shim/spacer the bolts so they don't bend the aluminum tabs? The C3 part is a chunk.

I've entertained that. The issue, as I understand it, is that the inner bushing steel tube needs to be compressed lengthwise to lock it to the tabs, so that the tube doesn't rotate relative to the through bolt during suspension travel. I keep envisioning these shim spacers in the same light/function as the rear axle preload shims on each side of the differential carrier side bearings. Perhaps if I collect a pile of assorted thickness 1/2" washers, I could use one thickness when welding the tabs in solidly, and then use (ie: pound in) a couple slightly thicker washers between the tabs and the bushing tubes. I think I'll measure a bunch of washers tomorrow to see if I can find a range of thicknesses.

I suspect you're correct that the C3 cross shaft is a chunk of weight.

phantomjock · #10 02-17-2021, 02:41 AM

Been a while so I won't run any numbers - but what about 2 separate sections with a web between? Think "I-Beam". Less weight - all those spars in aircraft etc are basically similar "displaced" members.

Cheers - Jim
Rather than add a post - I'll edit...
This looks like an interesting approach:

and:

Replicate for the LCA - You wouldn't need a full dogbone - just some bearing points. Remember that the LCA takes at least 60% of the load and the UCA 40 or less!
For more images:
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69427 · #11 02-17-2021, 09:14 PM

Quote:

Originally Posted by phantomjock

Been a while so I won't run any numbers - but what about 2 separate sections with a web between? Think "I-Beam". Less weight - all those spars in aircraft etc are basically similar "displaced" members.

Cheers - Jim
Rather than add a post - I'll edit...
This looks like an interesting approach:

and:
..............
Replicate for the LCA - You wouldn't need a full dogbone - just some bearing points. Remember that the LCA takes at least 60% of the load and the UCA 40 or less!
.....................

Yeah, I have been wanting to copy the UCA attachment method above for the LCAs, but I have never seen it done on any cars. Corvettes don't do it so that they can use the LCA eccentrics to adjust the camber. I adjust camber via the UCAs, so I'm not constrained there. The thought/possibility that I'm neglecting a key issue that makes this (UCA) mount design unwise for use in the LCAs was why I was entertaining the dogbone (C3 design) mount for my setup. This worry may be valid, or nonsense. I don't know which.

I don't have a C5-C7 UCA to look at here (all my stuff is C3 and C4). Is the UCA rubber portion of the bushing "glued/cemented/splined" to the anchor pin so that there's no relative rotation between the materials? From any pictures I've seen, there's not an obvious steel tube (that needs compressing) between the rubber and the anchor pin like on my C4 control arm bushings.

BBShark · #12 02-17-2021, 10:30 PM

I'm thinking that the forces acting on the A-arm are greatest under braking (which produces a rotational force) that tries to push the rear mount towards the center of the car and the front mount away. If you were to bolt simple plates onto the outside of the arm mounts, you could sim those plates through the bolted connection and shear forces axial to the plates would be countered by the bolts.

In between the A-arm pivots would be U bent bracket that would be bolted (or welded) but would not be a welded "tab" with a stress concentration at the bend.

This is all fabricated stuff with bolted connections which may be easier to make and with some adjustability.

69427 · #13 02-17-2021, 11:05 PM

Quote:

Originally Posted by BBShark

I'm thinking that the forces acting on the A-arm are greatest under braking (which produces a rotational force) that tries to push the rear mount towards the center of the car and the front mount away. If you were to bolt simple plates onto the outside of the arm mounts, you could sim those plates through the bolted connection and shear forces axial to the plates would be countered by the bolts.

In between the A-arm pivots would be U bent bracket that would be bolted (or welded) but would not be a welded "tab" with a stress concentration at the bend.

This is all fabricated stuff with bolted connections which may be easier to make and with some adjustability.

Yeah, I like that. I could probably do that with only minor modifications to the present configuration of the crossmember.