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

[bjkadron]

So I've never heard of people saying that the weight would change. Like Dion said, I think the concern has always been more of where the COC shifts the spring forces to the framerails from the TB crossmember. this would be the equal and opposite force of whatever weight transfer is happening. So worst case for a 3600 lb car with a 21 in CG and and a 58" track at 1.5 g would be ~500 Lbs plus any addition shock, bump or braking loading. Then you would need to figure out the moment arm for the change in force transfer to the framerails. So the actual change is a lot more than just a basic 200# number. That being said, most performance cars have some combination of frame connectors, torque boxes, inner fender supports, or core support bracing that would offset this. not to mention all cars are overbuilt by some factor of safety to begin with.

short answer: Is it gonna snap in half? Probably not but it is is going to add some extra stress so I would recommend some chassis stiffening improvements anyway.

 

[72bluNblu]

I was just reading something on the book of faces regarding coil over swaps and the negative affects of additional weight on the front frame rails. I know everyone has heard it before. "The front of these cars weren't meant to handle all the weight. The torsion bar cross member is under the car to hold the weight." This is actually something I've thought about and at the time of my swap I really wished I had a set of scales so I could do before an after corner weights. But I don't have scales and life went on. Prior to the light bulb just going off as I read on FB, my stance was my car has thousands of street miles and hundreds of autocross runs on the HDK and not a single panel gap has changed and my car hasn't folded in half like all the armchair quarterbacks said it would. However, I have new information now as it relates to this. When I was at the CAM Challenge in St. Louis, @JBrian was there with his Gen 3 hemi swapped 71 Duster. He's running pretty much the same exact T-Bar setup I had prior to the swap. Part of the requirements at these national events is to weigh the cars to make sure they fit the rules. Guess what? Our front corners were damn near the same. In fact, our front right was exactly the same weight. So if the beloved torsion bar system transferred weight to the center of the car, how is it that our front weight was damn near identical? Let's get a little more finite. His car is 200lbs more than me (drivers in the cars) and all that weight showed up on the back of his car. However, he's got some stereo equipment and some other things that will generally add up to more weight. (I am still wondering where that 200lbs is though.) So I'll go ahead and give these people the benefit of the doubt and say the torsion bars are moving that weight to the rear tires. That equals 5% of the total weight of the car being moved to the back because of a t-bar. Perhaps, but I'm not convinced. 200lbs isn't going to fold the car in half because I don't have J bars.

That’s actually not an argument I’ve heard before. I know after doing the math on the HDK components vs stock and even stock with aftermarket stuff the coil over conversions were typically a little bit lighter as far as actual weight goes. It wasn’t a lot comparing like to like, especially with a manual set up on both it was only like 30 lbs or so.

Now what @bjkadron was saying about where the suspension loads are carried in the chassis is a different story. But that’s not something you’d see on corner weights. The torsion bar suspension transmits the torsion bar loads radially into the K frame and torsion bar crossmember, where obviously the coil overs transmit their load vertically into the frame rails. The original chassis isn’t very stiff there- hood shake is an issue even on stock cars. So the concern would be more vertical flex between the frame rails and the firewall/rest of the chassis.

That said, if you’re doing any kind of racing the chassis gets reinforced anyway. With a coil over conversion adding j-bars or further triangulating the rails and firewall would help carry those loads.

The other thing I’ve heard would be the center of gravity, because of how the coil overs are arranged vertically vs the torsion bars which are down low. But that’s not the easiest thing to actually measure, not the height anyway with that kind of accuracy. And with the COG I wouldn’t suspect it would change much front to rear, as your corner weights show. The more interesting part would be if the height changed, if the coil overs raised the COG any compared to the tb’s.

But even with that, because the coil over system tends to be a bit lighter, I wouldn’t expect it to be a really significant change. The weight of the engine and transmission dominate that calculation. The hemi swap probably changes it more than any of the suspension changes.

Honestly, I think BOTH systems come down to tuning and set up. You can get good geometry with either system, if you tune the set up to your needs. My beef has always been the folks that claimed the coil over set up was somehow inherently better, which it isn’t. Both have pros and cons, but you don’t NEED coil overs to handle well. But that doesn’t mean they don’t handle well! It just means you can tune either suspension to handle well, and honestly getting either system to the same point is probably a similar amount of work.

 

[MMissile]

My cars require stock-type suspension for my racing classes, but if I were to do a coil-over front....HDK, without a doubt!

 

[racerjoe]

OK, so maybe I exaggerated a little. The car won't bend in half, but people are concerned about the weight on the front frame rails. Of course J bars will help. They will help any car regardless of suspension. I'm just stating there has been zero adverse affects that so may naysayers claim will happen. In fact, the left front corner of my hood used to contact the fender and chip the paint when I had torsion bars. I've touched it up since the HDK swap and it hasn't chipped off again.

I probably need to let someone drive my car that has a well setup T-bar car and get their opinion. That would be a real A-B test.

 

[bjkadron]

OK, so maybe I exaggerated a little. The car won't bend in half, but people are concerned about the weight on the front frame rails. Of course J bars will help. They will help any car regardless of suspension. I'm just stating there has been zero adverse affects that so may naysayers claim will happen. In fact, the left front corner of my hood used to contact the fender and chip the paint when I had torsion bars. I've touched it up since the HDK swap and it hasn't chipped off again.

I probably need to let someone drive my car that has a well setup T-bar car and get their opinion. That would be a real A-B test.

Maybe by next year mine will be setup and I can bring it down with me and we can run them back to back.

But I really think this is one of those situations that there is just more than one way to do the same lap time. There is pros and cons to everything.

 

[72bluNblu]

OK, so maybe I exaggerated a little. The car won't bend in half, but people are concerned about the weight on the front frame rails. Of course J bars will help. They will help any car regardless of suspension. I'm just stating there has been zero adverse affects that so may naysayers claim will happen. In fact, the left front corner of my hood used to contact the fender and chip the paint when I had torsion bars. I've touched it up since the HDK swap and it hasn't chipped off again.

I probably need to let someone drive my car that has a well setup T-bar car and get their opinion. That would be a real A-B test.

Again, it’s not the weight, it’s where the suspension forces are being transmitted. That’s an important distinction.

And it’s good that you haven’t seen any direct negative chassis effects. But you’ve been running the HDK for about 2 years now? And how many miles?

The chassis wasn’t designed to carry the magnitude of the suspension loads in the places where the coil over swaps place them compared to the torsion bars. Not that those areas aren’t designed to carry loads, just not the same amount. So it’s not something that you’re going to see in a short period of time. If you were blowing out spot welds or seeing cracking after a couple years that would represent a massive problem.

I get what you’re saying, and it’s definitely good that you haven’t seen any direct negative effects. But realistically I don’t think this is something where you’re going to see cracking or blown spot welds, especially in the timeline you’re at. But that doesn’t mean that the chassis isn’t being loaded differently.

But I really think this is one of those situations that there is just more than one way to do the same lap time. There is pros and cons to everything.

Exactly.

I still think the only real noticeable difference is going to be the feel of the rack vs the traditional steering box set up. And that’s not something that makes you faster. Otherwise any differences really come down to tuning. Like my car may feel different than yours, and that’s just tuning and set up between two torsion bar cars.

 

[mopowers]

OK, so maybe I exaggerated a little. The car won't bend in half, but people are concerned about the weight on the front frame rails. Of course J bars will help. They will help any car regardless of suspension. I'm just stating there has been zero adverse affects that so may naysayers claim will happen. In fact, the left front corner of my hood used to contact the fender and chip the paint when I had torsion bars. I've touched it up since the HDK swap and it hasn't chipped off again.

I probably need to let someone drive my car that has a well setup T-bar car and get their opinion. That would be a real A-B test.

Thanks for posting about your first hand experience. It's always good to hear from someone who's actually used one of these systems.

 

[Kern Dog]

I agree. The majority of guys that I've seen that change to these coil over systems never drive their cars hard enough to reveal any weaknesses.
Oh, it steers great, it handles so much better....
Yeah, a new 14 to 1 ratio steering rack will feel better than a worn out 24 to 1 manual or a sloppy stock power steering box.
Also, the term handling is a very broad term. To me, handling includes steering response, body roll, front to rear slip angles and oversteer/understeer.
To many people, "handling" seems to mean steering response alone because they don't push the car hard enough to know anything else.

 

[DionR]

Thanks for posting about your first hand experience. It's always good to hear from someone who's actually used one of these systems.

The thing I like about @racerjoe's feedback is he didn't just come from a worn out stock suspension and say "it handles like it is on rails". Much more of an A-B comparison since he was actively working on making his car handle with torsion bars and then swapped to the HDK kit and then continued to refine it from there.

So his input is much more of a matching use case too what I would want to know.

Not to say no one else has ever done similar, and not to ignore the others that have stated their results. But as far as I have seen no one else has put the work in beyond bolting a kit in and getting it aligned.

 

[72bluNblu]

The thing I like about @racerjoe's feedback is he didn't just come from a worn out stock suspension and say "it handles like it is on rails". Much more of an A-B comparison since he was actively working on making his car handle with torsion bars and then swapped to the HDK kit and then continued to refine it from there.

So his input is much more of a matching use case too what I would want to know.

Not to say no one else has ever done similar, and not to ignore the others that have stated their results. But as far as I have seen no one else has put the work in beyond bolting a kit in and getting it aligned.

Totally agree. He’s certainly the first that I’ve seen to actually look into the actual geometry of the coil over systems and make corrections, rather than just claiming it’s better and saying everyone needs one.

 

[Kern Dog]

I won't say that the OP convinced me to switch but I think he has helped us resist the idea a little less.

 

[racerjoe]

I won't say that the OP convinced me to switch but I think he has helped us resist the idea a little less.

View attachment 1716444896

Well I’ll let everyone know after 9/7 how it goes. That will be my first event with the latest iteration of the K and the sway bar I made. It will be at my local event so I’ll have some comparison to where I normally land in the finishing order.

 

[Kern Dog]

Some of my viewpoint is based on Rick Ehrenberg's assessment. In all fairness though, he is not infallible.
I agree though that the cantilevered tie rod end to steering arm looks hokey. I don't like the lower control arm since it lacks the triangulated shear that the stock design has. The issue of the load transfer to the frame rails will always be an issue unless they are somehow reinforced.
I don't see myself as a stubborn purist that thinks the factory way is the only way in all instances.
I hate that Ma Mopar put so much crap on the left side. Steering and shift linkage I understand but....Starter? ATF lines? KD linkage? This was all on the right on Chevrolets.
I understand the reasoning for rear steer but it complicates exhaust routing.
I'm not a fan of the ammeter or the ballast resistor.
LEFT hand threaded lug nuts wasn't all that smart either.

 

[TT5.9mag]

I agree. The majority of guys that I've seen that change to these coil over systems never drive their cars hard enough to reveal any weaknesses.
Oh, it steers great, it handles so much better....
Yeah, a new 14 to 1 ratio steering rack will feel better than a worn out 24 to 1 manual or a sloppy stock power steering box.
Also, the term handling is a very broad term. To me, handling includes steering response, body roll, front to rear slip angles and oversteer/understeer.
To many people, "handling" seems to mean steering response alone because they don't push the car hard enough to know anything else.

Not to mention most of the coil over set ups are replacing old worn out junk.

 

[goldduster318]

After seeing this - Scott Gottleib's Road Runner...This was a Gerst kit. You will notice these have TWO tubular crossmembers. Maybe the welds aren't exactly right but the one that broke is the crossmember nearest to the steering rack and torsion bar. To me that shows the relative weakness in the design of these things for a handling application. You see the sway bar and the steering rack mount to this. There is a lot of left to right shear load and a pretty good moment arm on this.

I am aware the HDK has a lot more weld area and is supported in two axis against the logitudinal tubes but there is also only one crossmember with similar loads going into it.

The design of basically right angles with no triangulation or gussets of any kind puts huge stresses at this point in any of these. This looks more like it's a fatigue failure than it shearing.

 

[NC Engine Builder]

I bet that not one of these aftermarket replacement systems have ever been run 100k plus miles in actual real life road conditions, and that thin tubing and design that lacks any real engineering, I doubt has the oem durability.

 

[TT5.9mag]

View attachment 1716444943View attachment 1716444944

View attachment 1716444945
After seeing this - Scott Gottleib's Road Runner...This was a Gerst kit. You will notice these have TWO tubular crossmembers. Maybe the welds aren't exactly right but the one that broke is the crossmember nearest to the steering rack and torsion bar. To me that shows the relative weakness in the design of these things for a handling application. You see the sway bar and the steering rack mount to this. There is a lot of left to right shear load and a pretty good moment arm on this.

I am aware the HDK has a lot more weld area and is supported in two axis against the logitudinal tubes but there is also only one crossmember with similar loads going into it.

The design of basically right angles with no triangulation or gussets of any kind puts huge stresses at this point in any of these. This looks more like it's a fatigue failure than it shearing.

Tom Gottlieb?

 

[bjkadron]

Tom Gottlieb?

View attachment 1716444951

Yep.

 

[RustyRatRod]

I think a lot of guys are just showin off using aftermarket suspensions.

 

[Super-Cuda]

Here's A Body members responses to CAP Auto Parts.

CAP Auto Products

I remember reading about this junk years ago.

Supposedly quality has improved since being acquired by QA-1

I for one am not about to be their test pilot.

RMS or HDK . Nothing else.

 

[goldduster318]

Apologies about mixing up the name but yes it's Tom Gottlieb.

Its not a CAP that was in that car, its a Gerst, which was also acquired by QA1. It doesn't appear that they changed the design at all...

It's fair to say the engineering on any of these is "light" meaning that I doubt there is any DV strength testing and much if any calculations beyond doing geometry for the angles on any of them.

I actually have no problem with using thin tubing if it's done right, if you go back far enough in the thread you can see where I posted about the Maximum Motorsports Fox Mustang K-frame and what that looks like. That outfit actually did all the engineering work (and in general used well-established engineering principles) and those things are honestly quite well designed given the parameters. That company does a lot of road racing also.

I'm not sure that it would take *that* much to make a huge difference in both the long term durability and stiffness of these.

I think all of these are fine for a straight line race car that gets inspected all the time. I really wonder about anything else. I know a lot of the cars these end up with may never pull much more than 0.5g lateral anyway.

 

[72bluNblu]

Here's A Body members responses to CAP Auto Parts.

CAP Auto Products

I remember reading about this junk years ago.

Supposedly quality has improved since being acquired by QA-1

I for one am not about to be their test pilot.

RMS or HDK . Nothing else.

As someone that owned CAP products and had a set of their LCA’s fail on my Challenger, and is also a current owner of the QA1 LCA’s that use a similar design I can tell you there’s no “supposedly” about it.

QA1 redesigned all of that stuff and fixed the production issues. The QA1 LCA’s on my Duster aren’t something I’m concerned about.

Apologies about mixing up the name but yes it's Tom Gottlieb.

Its not a CAP that was in that car, its a Gerst, which was also acquired by QA1. It doesn't appear that they changed the design at all...

It's fair to say the engineering on any of these is "light" meaning that I doubt there is any DV strength testing and much if any calculations beyond doing geometry for the angles on any of them.

I actually have no problem with using thin tubing if it's done right, if you go back far enough in the thread you can see where I posted about the Maximum Motorsports Fox Mustang K-frame and what that looks like. That outfit actually did all the engineering work (and in general used well-established engineering principles) and those things are honestly quite well designed given the parameters. That company does a lot of road racing also.

I'm not sure that it would take *that* much to make a huge difference in both the long term durability and stiffness of these.

I think all of these are fine for a straight line race car that gets inspected all the time. I really wonder about anything else. I know a lot of the cars these end up with may never pull much more than 0.5g lateral anyway.

QA1 did the same thing with the Gerst K member as they did with the CAP stuff. I know for a fact that they changed some of the metal thicknesses and redesigned some of the components to move welds etc.

I agree though, none of the aftermarket systems have the same level of engineering and testing as the factory parts do. Although, I don’t think you could argue that the factory engineers planned on having anyone run 275/35/18’s on the factory suspension either.

 

[goldduster318]

As someone that owned CAP products and had a set of their LCA’s fail on my Challenger, and is also a current owner of the QA1 LCA’s that use a similar design I can tell you there’s no “supposedly” about it.

QA1 redesigned all of that stuff and fixed the production issues. The QA1 LCA’s on my Duster aren’t something I’m concerned about.



QA1 did the same thing with the Gerst K member as they did with the CAP stuff. I know for a fact that they changed some of the metal thicknesses and redesigned some of the components to move welds etc.

I agree though, none of the aftermarket systems have the same level of engineering and testing as the factory parts do. Although, I don’t think you could argue that the factory engineers planned on having anyone run 275/35/18’s on the factory suspension either.

The design as far as how its welded/supported is identical. So even if improvements were made, unless they are just that much better welds, this can happen again, easily. The design of this particular one is just weak.

The factory suspension loads exactly nothing in the way that it's loaded in the Gerst/QA1 k-member as well as any of the other ones.

Other than the steering box, the stock part pushes all the load directly into the main member, basically on the same axis with small moments at worst. If you fully welded one of these out, and braced the steering box mount, I honestly don't think you can really break one of these. Everything is a 3D shape and also naturally arched or triangulated. It's naturally quite stiff. The steering box mount honestly isn't though.

Of course my 55 year old one has been taking P275 tires for 10 years (it may only have been 12-14k miles in that time) and it had a lot of miles and questionable weld quality to begin with. For something designed to be mass produced and to a price it's a solid piece. The engineering of it is fundamentally solid, especially for when it was made.

 

[davescuda]

I bet that not one of these aftermarket replacement systems have ever been run 100k plus miles in actual real life road conditions, and that thin tubing and design that lacks any real engineering, I doubt has the oem durability.

The Red Hemi Duster has earlier HDK stuff in it. The last I read in an article it had 120k miles on it with no issues. Denny probably can way in more on this.

 

[Super-Cuda]

View attachment 1716444962
The design as far as how its welded/supported is identical. So even if improvements were made, unless they are just that much better welds, this can happen again, easily. The design of this particular one is just weak.

The factory suspension loads exactly nothing in the way that it's loaded in the Gerst/QA1 k-member as well as any of the other ones.
View attachment 1716444968
Other than the steering box, the stock part pushes all the load directly into the main member, basically on the same axis with small moments at worst. If you fully welded one of these out, and braced the steering box mount, I honestly don't think you can really break one of these. Everything is a 3D shape and also naturally arched or triangulated. It's naturally quite stiff. The steering box mount honestly isn't though.

Of course my 55 year old one has been taking P275 tires for 10 years (it may only have been 12-14k miles in that time) and it had a lot of miles and questionable weld quality to begin with. For something designed to be mass produced and to a price it's a solid piece. The engineering of it is fundamentally solid, especially for when it was made.

Looking at this pic, it looks like it was never welded properly in the first place.

Look at the rust around the circumference of the tubing.

Looks like water got in there.

Junk.

Run, don't walk away from this ****.