The bantering about factory vs aftermarket K frames made me wonder if there had ever been an instrumented test to compare the offerings.

The K frame obviously has to locate the lower control arms and contain the resulting forces but a big part that is frequently overlooked is the structural rigidity they add to the vehicle. The engine bounces around on that thing too.

I don't have a dog in this fight but the engineer side of me is kinda interested.

They should each send me a frame. I'll build a rig and do a test to compare rigidity. There's a non-welded B body K frame lurking around my place that I'd test.

I like that idea.

I'll talk to the maker of my aftermarket kit and see if he'll jump in on it.

I've been wondering for awhile now how much it would benefit rigidity when a K-member would actually be welded to the framerails of a A-B-E body Mopar.

While you're at it maybe you should test a C-body 'frame' as well, where the 'K' itself is welded and the attachment bolts are located at the body of the car, either with, or without rubberbushings in between.

It could take a fairly simple rig to test the compliance of the K, however accurately representing loads applied from the engine, steering and suspension could be fairly entertaining.

I have thought about adding some additional mounts from my K to frame normal to the factory mounts. Though I'm not sure I'll get that far..

I'd be most curious about suspension compliance, bump steer, roll center migration, camber change, etc...I haven't come across much information on that.

BBM, the C-body subframes changed drastically between the models. The Imperial is much heavier than the others and even has a rubber isolated K frame separate from the subframe.

Chrysler on the left. Imperial on the right.



Goody, some of that stuff can be changed to some degree without too much trouble.

I think Ideally you'd want to do something like a pothole deflection test more than anything. Something to actually load the k-frame as the suspension would in a real world condition.

The thing you find often in aftermarket K's is that they have the LCA mounting points pretty far behind the rear k-frame bolt so the load is not spread over the K-frame and especially the factory bolts as it is in the stock setup.

Leave the K a bolt on but weld in some bars, huge difference.

This is a great topic and one that I have also wondered about. All of the aftermarket replacement front suspension kits have their own version of a "K" frame, but few seem to offer the OEMs robust resistance to "Parallelogramming."
(Is that the right description ???)
The RMS has a cantilevered outer tie rod connection. THAT looks hokey to me.
Someone wrote elsewhere...."All of this just to run a REAR sump oil pan..."
Funny. They also get a steering rack. Some feel it is a more precise setup. I'm still not impressed enough to dump my well prepped, aftermarket fortified stock based setup.

Cool, a real comparison finally. Will be watching this for sure. On the 70 k that I'm going to use in my car, I counted up 35 factory spot welds to fasten the two main units together. To this I added 102. It would be interesting to see a comparison between a stock unit and a reinforced one. Keep at it Feets!!!

There are a lot of bright people here, nobody yet has all the answers, together however, a strong and solid conclusion might be possible.

I would like to hear from Andy F. I already respect the words of Rick Ehrenberg who prefers the stock based stuff.

There might have been something lost in translation here. With the original post in mind, referencing,



is where I was referring to a fairly simple rig could be required. Something like torsional rigidity with a static load applied across the frame mounts would not be complicated, in fact, simple. Long "infinitely stiff" moment arm and apply a weight while the other side is constrained. Constrained to something like the couple thousand pound lathe that resides in Feet's shop.

One could deduct K-A is X% more compliant than K-B, given this simple, very repeatable test with minimal variation. That may, or may not, be very useful data, but data nonetheless.

Where I was referencing testing real loads through the K as "entertaining" was a sarcastic twist for not easily done. Such as,

"it'd be entertaining to mow your lawn with a pair of scissors"

Possible, but likely more complicated than it was worth. In other words, I agree, it would not be easily done. Depending on the fidelity of the information that is expected from the test would determine the required testing rigs.


FSAE teams have been using very simple test rigs to determine their chassis deg/ft-lb torsional stiffness to compare against FEA results. Something similar was my basis for a "simple" k-member rig.

I would assume for our interests in a K, we would want the test for twist to be 90 Deg from what your rendering shows? And we don't want the arm to add its stiffness to the K?

Correct.


As with my experience in applying loads, the part should be tested as installed. In my opinion. As the frame would tie the two mounts together while installed in the vehicle, it should be tested as it was intended to be mounted during use. As long as the test was consistent between the mounts, the % difference would still be accounted for.

As an example, a common motorcycle chassis uses the engine as a structural member. Testing the stiffness of the chassis without the engine would not be representative to it's "as-intended" condition. For this instance, the frame members are going to be there, no matter what K you use.

Even though this doesn't answer exactly - XV did this in 2006 by testing overall unibody rigidity on a surface plate. Once the reinforcements were installed, the unibody stiffness ranked in line with modern performance cars. I think the K isn't too bad for a stamped piece. When bolted to rails that don't move, its a lot better.

Unfortunately all I've seen from that testing was a video where someone talks through those numbers. No datasets, graphs, etc that give some sort of reference to the testing. I'd love to take a look at results.

I'm not putting to much thought into this one, if I'm going to driving a 45 year old car that fast that it is twisting the k member , be it stock or aftermarket the car is going to have a full cage in it.
I'm really not a big believer in welding to much on the k member other than steering box support and the lower control arm mounts to keep the pins from ripping out, after that I don't think anyone could prove any noticeable difference in handing. Same with those lower control arm plates that people add, you are just adding weight to the suspension, at most I could see bridging under the sway bar mount.
I would still put money on it that if you had 2 cars with the same tires, horsepower, driver ect and you did one with stock style suspension and one with a good aftermarket coil setup that they would both go pretty much the same speed around any given track when you had them both set up good.

Any car with only a set of 245's on the front is gonna be slippery. We need threads on how to fit a 295 or 315 on the front of these things. Then with that kind of grip, you might start noticing some REAL advantages to suspension mods IMO.

Parallelogramming is the right word and if you look closely at the Gerst aftermarket K member it's LCA inner mounting points are also cantilevered. Being in double shear, not too sure how that'll play out. Without testing who knows and anecdotal track experience isn't testing per se.

Wonder about doing a wide body kit, something similar to what the chebbie and even the ricer crowd do? basically flare the front (and the rear) fenders a couple inches out to clear a 315 tire? Yeah, it'd upset the look of the car and really piss off the purist but would work.