DOES THE HDK SUSPENSION K-MEMBER HANDLE BETTER THAN A T-BAR SUSPENSION?

[DionR]

I was BSing with my QA1 right rep as they were getting ready to launch their GERST stuff. He said the higher ups were sarcastically saying how Mopar people are a "special kind of customer".......I didn't miss a beat and replied "Yes, we are"

I am convinced that many of the people in the industry think that Mopar people don't care about handling because they don't buy expensive coil over kits. The GM guys need them and spend the money, and so because the Mopar guys don't they must not care. But maybe instead it is because the Mopar guys are able to do more by updating the stock suspension and it flies under the radar of the industry guys. I think too many of those guys view the world through GM eyes and can't understand because it doesn't fit their paradigm.

Not to say Mopar guys aren't weird, because we are.

 

[DionR]

I have the USCT braces sitting here and they just plain aren't going to clear a 275 tire as far as I can tell so I can't use them to help.

Sticks in my head that @72bluNblu put those on his Duster. Maybe I am remembering wrong?

 

[DionR]

Is an automatic far behind?

Nope, already bought the T56 Magnum for it.

 

[J-c-c]

I am convinced that many of the people in the industry think that Mopar people don't care about handling because they don't buy expensive coil over kits. The GM guys need them and spend the money, and so because the Mopar guys don't they must not care. But maybe instead it is because the Mopar guys are able to do more by updating the stock suspension and it flies under the radar of the industry guys. I think too many of those guys view the world through GM eyes and can't understand because it doesn't fit their paradigm.

Not to say Mopar guys aren't weird, because we are.

Frankly I don't care much what others think about me. I am interested in how they think ofwhat they do, so I can learn from their thinking, but that many times that seems to be lacking.
Regarding mopar guys and handling, in 1968 in HS I went to the junk yard and retrieved a mopar front swaybar to fit my 62 Polara 500 that I could just bolt on, since it attached to the brake struts oddly back then. Before graduating, I found some 11" front brakes to replace the OEM 10" drums. I had G60's on the fronts, front and rear spoilers. Handling has always been my thing.
Seems normal to me, everyone else is weird?

 

[DionR]

Thanks for making the effort to test it in this way.

Because that design turns it more into a cantilever beam and assuming the force at the center of the bar is the same in all cases, simply moving it closer to the tube by mounting the bushing vertically as seen in the RMS Alter-K-Tion is an improvement (though the sway bar is very low hanging otherwise), it's just flat out deflecting less. Let's say that with weight transfer included the car might have 800 lbs by jacking up one corner in a static condition.
View attachment 1716197929

The issue with the parallelagraming is -> that is the most weak direction of the car and where its quite relevant for cornering, if, for example, there was no k-frame, the car in front of the windshield is kind of like a cardboard box with the top and the bottom removed. It would parallelagram or even splay very easily. The K-frame of any type helps resist it, and the k-shape would be harder to twist with a load applied at the LCA pivot point than having a simple crossmember at a right angle. The more the load is behind the rear most bolt, more lever arm, meaning more flex each time. The stock one the arm is behind in the same way, but the pins are directly behind the rear bolts for the k-frame. It's actively reducing the twisting directly into both the k-frame and the chassis without any cantilever lever arm. This is where adding something behind comes in because it would generally stop any of this twisting action relative to the body and spread out the load. You can see this with something like the MM mustang K-member. It's not possible with the OE type oil pans and the available headers to have the crossmember go directly through the LCAs which would be an alternate design.

I don't know the reason for the cross bar having a rod end. The left to right side can twist meaning if, for example, you went into a corner and the left side mount is attempting to move in an arc due to the twisting of the k-frame about the axis going front-rear in the car Since there is a ball there, it just keeps the straight line distance to the other side the same only in the cross car direction, it will do zero for any simultaneous front-back movement because it will just pivot on the rod end instead of having to bend the rod to deflect. With only one bolt on each side, it can also run an arc. The degrees of freedom are not adequately restricted to get the best possible effect. For example, my truck has a removable crossmember similar to the transmission crossmember on our cars in roughly the same area and it has two bolts on each side with what's essentially a box tube in the middle. It has no degrees of freedom other than literally twisting the steel.

The frame rails themselves are pretty weak and nothing like say a hydroformed pickup truck frame. They're just supported by the inner fenders. The cars flex all over the place which is why we have seen cars with high wheel rates cracking the spot welds or inner fenders. It just fatigues out with the higher loads.

I'm no mechanical engineer, but I did stay at a Holiday Inn Express one time.



That said, I for one have always been leery of mounts that cantilever. Not that it is always possible to do, but I just like things triangulated and supported on both ends when possible.

 

[DionR]

Regarding mopar guys and handling, in 1968 in HS I went to the junk yard and retrieved a mopar front swaybar to fit my 62 Polara 500 that I could just bolt on, since it attached to the brake struts oddly back then. Before graduating, I found some 11" front brakes to replace the OEM 10" drums. I had G60's on the fronts, front and rear spoilers. Handling has always been my thing.
Seems normal to me, everyone else is weird?

A great example. Most of the guys at Hot Rod would probably have pegged you as someone only interested in restoration of your car, because in their mind the only way a car can be made to handle is to buy aftermarket parts. And thus (to them) you don't care about handling.

 

[DionR]

If you're going to go to all that work why not just cut the existing front stub off and build a new one that does what you want?

At what point is a full chassis a better option?

At the same time, Joe ran against an E-Body with a full chassis at Moparty this year and was much faster. Super nice car, I got to see it at the final Ultimate Street Car Challenge road course day after SEMA. But not as fast on the autocross as Joe was.

 

[72bluNblu]

I am convinced that many of the people in the industry think that Mopar people don't care about handling because they don't buy expensive coil over kits. The GM guys need them and spend the money, and so because the Mopar guys don't they must not care. But maybe instead it is because the Mopar guys are able to do more by updating the stock suspension and it flies under the radar of the industry guys. I think too many of those guys view the world through GM eyes and can't understand because it doesn't fit their paradigm.

Not to say Mopar guys aren't weird, because we are.

The GM suspension design definitely has some inherent issues. Like on the Camaro's the motion ratio is terrible. So that's why guys like Dick Gulstrand ran 740 lb/in springs on those cars to set up for racing/handling. You're running a spring rate of 700 something to get a wheel rate in the mid 300's.

Sticks in my head that @72bluNblu put those on his Duster. Maybe I am remembering wrong?

I have them on my Dart GT. The Duster has J-bars in the engine compartment. I haven't tried putting the Duster's 275's on the Dart but my plan for that car was less aggressive, I was only going to run 245's or maybe 255's on it.

My understanding is that the USCT inner fender braces would need to be notched for 275's, although like everything that would depend on the ride height.

 

[DionR]

The GM suspension design definitely has some inherent issues. Like on the Camaro's the motion ratio is terrible. So that's why guys like Dick Gulstrand ran 740 lb/in springs on those cars to set up for racing/handling. You're running a spring rate of 700 something to get a wheel rate in the mid 300's.

Exactly. Plus the same sucky motion ratio also made the shocks pretty unresponsive to tuning and cost a fortune to get to work.

And this is what the magazine guys saw and assumed was the case with Mopars as well, because the mighty Camaro couldn’t be inferior to a Mopar. This is what the experts and internet tough guys propagated far and wide. Result being, most uninformed people want to throw it all away regardless of the make, age or condition.

At least that is my take on it.

Edit - for what it’s worth, in the link I posted in #97 the guy was talking about 1500# springs. Insane.

 

[HemiDenny]

I am convinced that many of the people in the industry think that Mopar people don't care about handling because they don't buy expensive coil over kits. The GM guys need them and spend the money, and so because the Mopar guys don't they must not care. But maybe instead it is because the Mopar guys are able to do more by updating the stock suspension and it flies under the radar of the industry guys. I think too many of those guys view the world through GM eyes and can't understand because it doesn't fit their paradigm.

Not to say Mopar guys aren't weird, because we are.

IMO, their remarks were that for the most part, Mopar people are not a bunch of sheep, not afraid to voice their opinion and ask questions.....case in point, this thread.

I do not think I EVER had a customer just buy....99% of the new calls I get start with "I have a few questions" and most of those calls last a minimum of 30 minutes. I love it... because I am one of you.

 

[mopowers]

I do not think I EVER had a customer just buy....99% of the new calls I get start with "I have a few questions" and most of those calls last a minimum of 30 minutes. I love it... because I am one of you.

Glad to hear it's not just me.

 

[J-c-c]

A great example. Most of the guys at Hot Rod would probably have pegged you as someone only interested in restoration of your car, because in their mind the only way a car can be made to handle is to buy aftermarket parts. And thus (to them) you don't care about handling.

Your point is not clear to me. My point here was at a time when I was still in high school over 50 years ago at the beginning of disc brakes I was adding a sway bar and bigger brakes, a square tire set-up and aero tricks which was so beyond the normal hot rod path at that time. Restoration really didn't even really exist yet except for maybe Duesenberg or a Tucker.

 

[J-c-c]

I didn't read anybody mention this K member solution, and I have toyed with it, because it has some really tough hurdles, but it ought to doable especially with a non TB IFS, is an OEM style carbon fiber mopar K member. It would check a lot of boxes.

 

[Phreakish]

I didn't read anybody mention this K member solution, and I have toyed with it, because it has some really tough hurdles, but it ought to doable especially with a non TB IFS, is an OEM style carbon fiber mopar K member. It would check a lot of boxes.

I'd sooner use die cast aluminum like many contemporary cars appear to. There are several processes which can mimic die cast results without the expensive tooling.

CF comes with many challenges. For a dedicated racer it might make sense, but for the typical cruiser or drag car most folks build it would be wasted effort IMO. Adding hardware attachment points to composites is an engineering exercise in itself, let alone preventing corrosion where CF and other metals meet. Designing for the proper load paths and developing a reasonable NDT program would probably require building more test parts than the number of retail units that would sell. Using chopped CF or trying to get a homogeneous CF mix winds up giving up a ton of the benefits but doesn't typically cut down much on cost. Heat resistance is limited by the resin/epoxy used as a binder too. Metals don't have that issue near as much.

Seam welding and lightening up a stock K is something most guys could do and doesn't take a doctorate in composite analysis to do well. Besides, the main benefit of CF is the higher modulus than steel, not necessarily it's strength. The higher modulus means the part can be lighter for the same stiffness, if done right. But a stock k is not optimized to begin with, though welding can quickly maximize what's available. Going aluminum could save some weight, but the modulus is lower and so stiffness gains would rely on increased section thickness. The end result does tend to still be lighter, but design choices still need to be made.

In the end, a CF k member might be a bit lighter and maybe stiffer but it would have several significant drawbacks and cost would be quite high. The real question though is whether the K is even a significant contributor to chassis flex. A super stiff K is kind of a waste if the front frame horns are still squirming about..

 

[J-c-c]

J-c-c said:
I didn't read anybody mention this K member solution, and I have toyed with it, because it has some really tough hurdles, but it ought to doable especially with a non TB IFS, is an OEM style carbon fiber mopar K member. It would check a lot of boxes.

I'd sooner use die cast aluminum like many contemporary cars appear to. There are several processes which can mimic die cast results without the expensive tooling.

CF comes with many challenges. For a dedicated racer it might make sense, but for the typical cruiser or drag car most folks build it would be wasted effort IMO. Adding hardware attachment points to composites is an engineering exercise in itself, let alone preventing corrosion where CF and other metals meet. Designing for the proper load paths and developing a reasonable NDT program would probably require building more test parts than the number of retail units that would sell. Using chopped CF or trying to get a homogeneous CF mix winds up giving up a ton of the benefits but doesn't typically cut down much on cost. Heat resistance is limited by the resin/epoxy used as a binder too. Metals don't have that issue near as much.

Seam welding and lightening up a stock K is something most guys could do and doesn't take a doctorate in composite analysis to do well. Besides, the main benefit of CF is the higher modulus than steel, not necessarily it's strength. The higher modulus means the part can be lighter for the same stiffness, if done right. But a stock k is not optimized to begin with, though welding can quickly maximize what's available. Going aluminum could save some weight, but the modulus is lower and so stiffness gains would rely on increased section thickness. The end result does tend to still be lighter, but design choices still need to be made.

In the end, a CF k member might be a bit lighter and maybe stiffer but it would have several significant drawbacks and cost would be quite high. The real question though is whether the K is even a significant contributor to chassis flex. A super stiff K is kind of a waste if the front frame horns are still squirming about..

Key words above "tough hurdles, but it ought to be doable..."

I believe "still squirming around" maybe be slightly backwards, in that the OEM K is what keeps the "frame horns" from squirming, and what the existing aftermarket crowd fails to understand with their offerings.

 

[Phreakish]

J-c-c said:
I didn't read anybody mention this K member solution, and I have toyed with it, because it has some really tough hurdles, but it ought to doable especially with a non TB IFS, is an OEM style carbon fiber mopar K member. It would check a lot of boxes.


Key words above "tough hurdles, but it ought to be doable..."

I believe "still squirming around" maybe be slightly backwards, in that the OEM K is what keeps the "frame horns" from squirming, and what the existing aftermarket crowd fails to understand with their offerings.

The k is dangling off of the firewall attached through the horns. Even if the K is stiff, the horns will still move about. The K will stiffen the horns some, but only right around where the attach bolts are. The k won't have any impact on how much the firewall flexes and won't contribute much to torsional stiffness which is what most cornering forces would impart. It would take some triangulating members from the A pillars down to the k to improve the moment of inertia enough to matter, and even then the torsion would just be pushed into the roof which isn't exactly stiff.
Point being, the k frame could be unobtanium with an infinite modulus and the mounting flanges on the front horns and front horns themselves would just wiggle about because there's not enough vertical cross section to add much to the moment and the torque at the mounting flanges of the horns will also wiggle about since they're not even as thick as the stock K steel. All a super stiff K would do is cause cracks to appear faster on the horns and at the inner fenders and firewall.

 

[DionR]

Your point is not clear to me. My point here was at a time when I was still in high school over 50 years ago at the beginning of disc brakes I was adding a sway bar and bigger brakes, a square tire set-up and aero tricks which was so beyond the normal hot rod path at that time. Restoration really didn't even really exist yet except for maybe Duesenberg or a Tucker.

My point was that from viewpoint of a GM journalist, you must not have cared about handling. If you did, you would have bought aftermarket parts, not stock parts.

 

[J-c-c]

I can't imagine anybody using a relatively expensive bespoke CF K with a stock chassis, but I can imagine a properly sorted out CF K member being far superior in a high-performance application than almost every aftermarket K currently offered.
I didn't address a few items you noted earlier, one being I believe elevated heat that can be accommodated with HT epoxies, I don't see chopped CF as ideal reinforcement, and without a TB suspension, metal attachment points are limited in number and see fairly predictable loads, (Rack & SB & 4 large mounting points). Another point that has to be considered is the largest single/ densest mass of the vehicle sits mere inches above the K, and the resultant inertia load paths it induces are rather short and simple. I see all these issues solvable with the needed will, and of course time, money, and testing.

 

[J-c-c]

My point was that from viewpoint of a GM journalist, you must not have cared about handling. If you did, you would have bought aftermarket parts, not stock parts.

Fair enough, but in 1968, none existed other than maybe Monroe air shocks.

 

[Phreakish]

I can't imagine anybody using a relatively expensive bespoke CF K with a stock chassis, but I can imagine a properly sorted out CF K member being far superior in a high-performance application than almost every aftermarket K currently offered.
I didn't address a few items you noted earlier, one being I believe elevated heat that can be accommodated with HT epoxies, I don't see chopped CF as ideal reinforcement, and without a TB suspension, metal attachment points are limited in number and see fairly predictable loads, (Rack & SB & 4 large mounting points). Another point that has to be considered is the largest single/ densest mass of the vehicle sits mere inches above the K, and the resultant inertia load paths it induces are rather short and simple. I see all these issues solvable with the needed will, and of course time, money, and testing.

If someone could make use of that bespoke cf K member, the non stock chassis would probably benefit from altered locating geometry, negating the benefit of a bespoke K member.

I would also posit that if one optimized the chassis and unibody, the stiffness of the k member would have a diminishing effect on chassis stiffness and that a decent motor plate would take care of most of the remaining forces needing resolved up front, which will tend to be mostly lateral.

One of the biggest chunks of mass may not have a huge moment arm to the K member, but plenty of the rest of the car does, including a hefty fuel tank and rear end. The rear end that is also putting all the motive force into the chassis. A chassis which also comprises 1/2-2/3 of the total weight, with its own cg pretty far aft of the front wheels and which has to take all the braking and rolling forces.

 

[J-c-c]

If someone could make use of that bespoke cf K member, the non stock chassis would probably benefit from altered locating geometry, negating the benefit of a bespoke K member. So then alter the CF K?

I would also posit that if one optimized the chassis and unibody, the stiffness of the k member would have a diminishing effect on chassis stiffness so that would reduce the loads on the CF K and it could be made less robust and lighter? and that a decent motor plate would take care of most of the remaining forces needing resolved up front, which will tend to be mostly lateral. Not sure any motor plates add any real usable stiffness

One of the biggest chunks of mass may not have a huge moment arm to the K member, but plenty of the rest of the car does, including a hefty fuel tank and rear end. The car rotates on the rear axle in full grip situations The rear end that is also putting all the motive force into the chassis. which propels the car nearly always forward in a single direction and maybe under acceleration reduces front tractive reaction forces and under barking reduces rear tractive reaction forces A chassis which also comprises 1/2-2/3 of the total weight, with its own cg pretty far aft of the front wheels and which has to take all the braking and rolling forces. Seems like the rolling forces you note are almost always shared, and almost never exclusively on one axle.

 

[Phreakish]

Take a look at stock car chassis or other chassis designs that deal with high cornering forces. Cross members are pretty much all much smaller than a stock K because the rest of the chassis is designed properly already.

The idea you initially mentioned seemed to suggest vendors could offer a cf k member. If it has to be altered for each chassis design, there's no chance of making it a standard product is there? I'm pointing out that at the point when it's no longer a stock style K that there's no reason to keep anything resembling a k member at all. A welded in square tube could do the job since the frame horns will be plenty stiff enough already and the only benefit any cross member would be affording is lateral resistance.

Motive force may only push in "one" direction but those loads from the rear end actually react in a number of ways, and not all are linear. Torque is applied in two axes (around the crankshaft axis and the rear axle axis) as well as the longitudinal force developed by the wheels, which also apply additional forces and resultant torques due to rear suspension geometry. This is often manipulated to aid traction by forcing the read end down or lifting the rear up. Front suspension is often designed to develop similar forces under braking or turning. There's a lot to consider and none of it can be isolated nor ignored because a significant portion of the mass happens to be close to some particular parts.

Point being: most folks use a close to stock chassis with some weld in reinforcement as needed and convenient or affordable. The next stage is to custom build to the intended application. Go take a look at chassis designed for drag racing compared to the stock cars mentioned above and the differences are quite obvious. Drag cars don't deal with the same magnitude of cornering forces and so the structure is much lighter. There's no one chassis design solution optimized for both and so the choices made by each builder will be shaped by myriad design constraints, including many defines by class rules which may even be contradictory in some cases. None of those constraints seem to favor a cf cross member. I might even argue that the fact that cf is weakest in compression makes it uniquely unsuited as a cross member since the primary force that will be applied to it will be compression trying to push the front frame mambers together in a corner. The shape and size of the cross member does not add torsional rigidity to the chassis either.

With an adequate frame design, an engine plate would accomplish what's needed for a front cross member because the frame members would be more than strong enough in all degrees of freedom, but "boxing" across the engine bay by stressing the front of the engine through the plate would increase lateral strength and stiffness a whole bunch, much like a typical core support bar but without adding additional frame members and their weight. Point being that a custom front chassis that is properly designed wouldn't need more than a motor plate to deal with the forces present, unless taking crash safety into account or a need for being able to push on the front of the car.

The stock K member exists mostly to support manufacturing. It aids assembly and allows the chassis itself a huge tolerance envelope while isolating the more critical chassis geometry to a smaller, cheaper, and more easily altered and repaired subcomponent. Retaining it in a custom chassis would be fruitless since ease of assembly line operations isn't a consideration for a bespoke chassis and a stronger design can be obtained by eliminating it and fabricating the suspension directly on a properly engineered front clip. One which is most easily fabricated with tube steel weldments. Even the highest performing vehicles outside of F1 still attach a welded sub frame for the front and rear chassis for this reason. The reaction forces can be reduced by resolving them to mostly axial forces over a large bulkhead minimizing the difficulties of integrating attachment points. The cabin and bodywork are typically where composites come into play because it's far less critical.

Establishing fatigue limits for composites is also a nightmare, and for bespoke parts is near impossible except by rough approximation which if sized appropriately often negate any weight savings, and I'd it's not lighter it might as well be steel anyway. After all, there's a billionaire at the bottom of the ocean who guessed a little wrong on his approximation. The consequences at the top end of the drag strip or the end of a long straight are similar, which makes a good case for predicable manufacturing methods like welded steel.

 

[J-c-c]

And yet the same was the long stated reason why CF wheels would never be an offering.

 

[HemiDenny]

and who thought reading these comments would not be educational ?

 

[DionR]

Sticks in head that XV found that a stiffer lower core support made a significant difference in how rigid the frame rails were. Don’t remember if they had a k-frame in it but kind of seems like it did?

Been a lot of years, I would have to dig out the magazine articles to verify. And can’t do that for a little while.