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Theory Question - Core Support Stiffener for Late B-bodies #2773499
05/11/20 12:45 PM
05/11/20 12:45 PM
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DynoDave Offline OP
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I'd like to get this groups thoughts on the value of modifications for increased stiffness in the core support of a late ('73-'79) B-body.

A quick summary of what the car is, where it's at, and where I might like to go with it.

The car is a 1977 Cordoba, 400 4bbl. car. 38k miles, all stock (and worn). A full front end rebuild will be performed - ball joints, control arm bushings (OE rubber), maybe beefier tie rods, a quicker ratio pitman if it will fit, larger torsion bars, probably staying with factory anti-sway bars with urethane end links until I see the effect of the heavier torsion bars. I will run aluminum pucks/bushings at the K member, and replacing the iso-clamps on the rear axle with an earlier solid design.

Goal: This is my second big block Cordoba, the prior being a '78 that I actually did a little autocrossing in. I also taught advanced driver education (for police, fire/EMS, executive chauffeur) in a St. Regis R-body. So I know what it can do, and what it's shortcomings are, in my opinion. Generically, I want to improve this cars handling, but have no true pro-touring, Ultimate Street Car Challenge. or regular/weekly autocross or road course work plans. But if I decided to make a run through the cones, or spend a day at a local track, it would be nice not to be cornering on the rims. A roughly 300 horse all iron bigblock, 727, 3.23 geared 8.75 will power it. Sub-frame connectors and torque boxes of a home made variety. Hoping to shave a few pounds where practical (battery, bumper reinforcements and bumper shock absorber system), but no exotic materials. Probably two sets of rims...an '80s vintage set of forged aluminum factory Cordoba rims with some T/As or something similar for the street, and perhaps some 17s with a little better tire for play. Effort, but not max effort by any means.

This car has been in our family since new, and has been sitting idle for about 20 years while the kids grew up. Now I have a chance to dig into it. So I started with work to replace/improve the battery situation (losing 50 pounds off the extreme left front corner of the car), and having the stock radiator rebuilt (going from single core to double). Doing some clean-up while I'm in the engine compartment. With the fan, clutch, shroud and radiator out, visibility of the current core support lower is very good.

Looking cross-car in section (profile), it looks approximately like this.

[Linked Image]

Front of car is to the left. Red is a more conventional looking right angle style support running between the body frame rails. The black is a second layer, spot welded together to form a box of sorts, then shaped to form a bit of a skid plate that totally protects the lower tank. That black layer measures roughly 0.1" (3/16th - ish) with my caliper in thickness. Sorry, my hand drawing is an approximation. Green would be the condenser, and behind that the radiator.

So, given the front suspension that is fully mounted on the K-member, (see Magnum picture below), with the elements listed above: Is it worth the effort to crate some additional reinforcement of the lower core support while I have good access? Like getting a second lower "skid plate" from a salvage yard, and welding it over the top for increased material thickness?

I appreciate any input you might have on this topic.

[Linked Image]


Last edited by DynoDave; 05/12/20 12:02 AM.

DynoDave
Walter P. Chrysler Club - Great Lakes Region
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1977 Chrysler Cordoba
Re: Theory Question - Core Support Stiffener for Late B-bodies [Re: DynoDave] #2773619
05/11/20 05:46 PM
05/11/20 05:46 PM
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Do a walk thru of the forces involved here in your problem, All forces start at the interface between the tire and the road surface. They next go thru the rim, the hub, the spindle and spread out thru the ball joints, to the UCA that attaches to the a bracket on the unibody frame rail, and the LCA goes straight into the K frame. Also keep in mind, the forces are acting on the mass of the vehicle and cornering, turning, inertia forces that mass resists, The largest single most dense mass (engine) on the entire car is located/attached directly to the K frame. Of course other masses, the unibody, interior driver are located away from the K frame and have the advantage(?) of leverage in transmitting their. maybe smaller(?) forces (moment) by simply having less mass. Also don't overlook the fact the mopar OEM K member is the single most robust/rigid single component on the car. So the "weak" link in this unibody system of transmitting the UCA portion of forces into the chasss is the connection noted above. Also understand, these tire/road surface generated forces are divided into the the spindles vertically by the ratio the spindle is located between the vertical height of the ball joints, or about 1/3 UCA and 2/3 LCA. We are disregarding sprung weight which goes 100% into the K Member. The shock loads goes into sheet metal, and only need to resist vertical motion during transients. If the sheet metal flexes with shock loading, it then becomes relatively another "stiff" spring, and can be accounted for by TB choice and shock rates. However this adds complexity in race set-ups, and is not a chosen goal. For that matter, since NOTHING is perfectly stiff on a car, everything becomes a spring, and tuning everything together achieves goals sought. Brake forces are a component in all this, but not a huge concern if everything else is working as needed.

Nobody has made the case to me that lower radiator gusseting solves many real problems, I have read comments about multi post shaking videos, but not sure what the problem is except for marketing. On this topic, I have little concern if my radio antenna shakes on a multi post shake rig, and why I don't make it out of 1.625" CM tubing.

Since all the forces acting on K member come from tires or to the rear of the K member, reinforcing anything in front of the K member to achieve added stiffness, is inefficient engineering design.

Meaning ,solving other problems might be better use of your time, efforts, and money, like LCA 1/2" steel gussets eyes. Don't get me started. laugh2


"Respect and Contempt are best served when justly earned"
Re: Theory Question - Core Support Stiffener for Late B-bodies [Re: jcc] #2773636
05/11/20 06:43 PM
05/11/20 06:43 PM
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DynoDave Offline OP
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Lots to think about jcc, and since we will never have a late B on a shaker or any other scientific measuring device, this whole exercise is "best guess", and that's why I came here to ask folks for their thinking on this. So thank you for yours.

The late B, starting in '73, is sort of an odd duck because the upper control arm mount moves to an extension of the K-member, not a welded part of the inner fender and uni-body like on my '72 Charger or '70 Duster. Like the earlier cars, that K is bolted solidly (or will be) to the body. Is the K helping the body not flex, or is the body helping the K not flex, or a little bot of both?

If the K tries to flex, then it will have to flex the body it's bolted to. If that's the case, and if you picture the body with the K removed, then that core support becomes the only substantial cross-member in the body structure between the the torsion bar cross-member and the bumper. And I would "guess" therefore that it provides a meaningful function on controlling body flex. It's an interesting argument, as WITH the K in place, the K becomes by far the most substantial body cross-member, being dual purpose in that regard.

I think in a more max effort car, with bigger, stickier rubber, it might be worth the effort. But my guess is that for my application, it's not worth the weight or the trouble. But that's why I wanted to hear what other thought.


DynoDave
Walter P. Chrysler Club - Great Lakes Region
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1970 Plymouth Duster
1972 Dodge Charger Rallye
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1977 Chrysler Cordoba
Re: Theory Question - Core Support Stiffener for Late B-bodies [Re: DynoDave] #2773720
05/11/20 11:46 PM
05/11/20 11:46 PM
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I apologize, yes a "late B" in my mind is anything after the 62-65 B's, and that is a wrong assumption on my part. My knowledge of anything after 72 is rather limited, and I might have steered you in a wrong direction because of that. So reader beware.
But my "theory" (?) still stands. grin

Regarding your thinking about being the only real crossmember in front of the K, yes, that's correct, but other then maybe aero loads, nothing is inputting forces into the chassis forward of the K, except the mass of the items in that area, bumper, brackets, headlights, etc.

Maybe an analogy, like trying to hold a muffler's weight and position by using a pipe hanger at the end of the tailpipe, rather then at the muffler?


"Respect and Contempt are best served when justly earned"
Re: Theory Question - Core Support Stiffener for Late B-bodies [Re: jcc] #2775393
05/17/20 12:20 PM
05/17/20 12:20 PM
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I would think a solid mounted K frame will perform the same function. If you were leaving the K rubber biscuits in place then I could see it of being a benefit. Besides, the bumper also helps with this as well.

Pull the K frame completely out and weld the entire thing up instead.

Re: Theory Question - Core Support Stiffener for Late B-bodies [Re: MarkM] #2775435
05/17/20 02:11 PM
05/17/20 02:11 PM
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Having been into a 73 rubber B I'd definitely solid mount the K like planned but would like to throw this out there.

What about bracing the upper control arm mounts to the inner fender? Similar to earlier B and A bodies? Looks doable on the 73 74 cars, not sure on 75 up.

A monte carlo bar arrangement might help too. Id do either before working on the core support.

Re: Theory Question - Core Support Stiffener for Late B-bodies [Re: ruderunner] #2775569
05/17/20 08:06 PM
05/17/20 08:06 PM
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DynoDave Offline OP
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Thanks guys. I appreciate your input.

While the inner fenders are steel, I don't know that they have the strength to help much. And then you wouldn't be able to drop the K either.

That Monte Carlo bar idea is an interesting one. Would be quite the arch, and with the low hood line of the Cordoba, I don't know that there's room for it.


DynoDave
Walter P. Chrysler Club - Great Lakes Region
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1970 Plymouth Duster
1972 Dodge Charger Rallye
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1977 Chrysler Cordoba
Re: Theory Question - Core Support Stiffener for Late B-bodies [Re: DynoDave] #2775698
05/18/20 12:31 PM
05/18/20 12:31 PM
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Yes, the Monte carlo bar for one is much more efficient design and IMO effective, but nearly impossible to fit in our cars. Being an arch is mostly effective at bending and storing energy in this situation, the opposite of your primary goal I suspect. One must accept a number of compromises in this endeavor, and best to know what they are before deciding.


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Re: Theory Question - Core Support Stiffener for Late B-bodies [Re: DynoDave] #2777718
05/24/20 03:58 PM
05/24/20 03:58 PM
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Originally Posted by DynoDave
Thanks guys. I appreciate your input.

While the inner fenders are steel, I don't know that they have the strength to help much. And then you wouldn't be able to drop the K either.

That Monte Carlo bar idea is an interesting one. Would be quite the arch, and with the low hood line of the Cordoba, I don't know that there's room for it.


Been awhile since I've been up close and personal with the later Bs. I thought the inner fender was more of a liner than a shear panel like the earlier cars.

Re: Theory Question - Core Support Stiffener for Late B-bodies [Re: DynoDave] #2778640
05/27/20 10:17 AM
05/27/20 10:17 AM
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The frame rails on the late b are essentially flapping in the breeze. I think your boxing the radiator support is a good idea. I'd also suggest doing a pipe from the firewall down to the end of the frame rails to triangulate them and to stiffen them up. I wouldn't make them straight. Not only would it interfere with the bolt on inner fenders, but a kink would also give it a crush zone in case, God forbid, a front end collision. Solid iso biscuits are best, but because of their thickness, the K-frame's loads will have a larger torque vector on the frame rail rather than a non-iso mopar, so it won't be doing as good a job as the other Mopars. A late B K frame is mondo heavy compared to the earlier ones, too. There's ways to lighten that up without sacrificing stiffness (think installing windows, but welding up the trim) but now we're in mission creep regarding not going max-effort. A Monte Carlo bar from the firewall to the top of the fenders is almost worthless as there is no strength up there on a late B.


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Re: Theory Question - Core Support Stiffener for Late B-bodies [Re: cudazappa] #2778717
05/27/20 12:40 PM
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Originally Posted by cudazappa
The frame rails on the late b are essentially flapping in the breeze. I think your boxing the radiator support is a good idea. I'd also suggest doing a pipe from the firewall down to the end of the frame rails to triangulate them and to stiffen them up. I wouldn't make them straight. Not only would it interfere with the bolt on inner fenders, but a kink would also give it a crush zone in case, God forbid, a front end collision. Solid iso biscuits are best, but because of their thickness, the K-frame's loads will have a larger torque vector on the frame rail rather than a non-iso mopar, so it won't be doing as good a job as the other Mopars. A late B K frame is mondo heavy compared to the earlier ones, too. There's ways to lighten that up without sacrificing stiffness (think installing windows, but welding up the trim) but now we're in mission creep regarding not going max-effort. A Monte Carlo bar from the firewall to the top of the fenders is almost worthless as there is no strength up there on a late B.


I wish I was more knowledgeable about "late" B's. A lot here to digest. But I will mention a minor detail, adding a straight "pipe" from firewall to frame rails, only becomes a "spear" IF a load is applied on axis to the center of the tube, nearly impossible, and adding any bend to allow it to collapse in unforeseen circumstances, obviously significantly weakens its stiffness, which is then overcome by increasing its dia/wall thickness, in a vicious design circle. I solved this problem by using CF tube, it just shatters. IMO, most race cars put a bend in the forward down bars mainly for fit. The orientation, again IMO, becomes more critical for safety issues the more horizontal the bars(s)(any) become.


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Re: Theory Question - Core Support Stiffener for Late B-bodies [Re: jcc] #2778862
05/27/20 07:23 PM
05/27/20 07:23 PM
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FWIW, when the 3rd-Gen GM F-bodies were designed, they found that a rad support reinforcement helped handling.
IIRC, Z28s & TransAms got that. Those were the first truly unibody F cars.
I think the point was to help fight flexing frame rails.
If you want that structure stiff, so the suspension action isn't masked by structure flex, look at triangulating the rails into surrounding structure.
Doesn't have to be in the engine compartment either, look at the wheelhouse area.

Re: Theory Question - Core Support Stiffener for Late B-bodies [Re: topside] #2779181
05/28/20 05:53 PM
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I can't begin to relate to GM bodies, so I little idea how they might or might not relate here.

But I will add "everything effects everything else" on a performance car, meaning the littlest things can change the results significantly, and why I can't participate in comparing a TB car to a coil car regarding frame stiffness concepts..


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Re: Theory Question - Core Support Stiffener for Late B-bodies [Re: jcc] #2779321
05/29/20 12:45 AM
05/29/20 12:45 AM
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JCC, I do like it when you post about Tech stuff.
Thank you.
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