Ahh, the age-old debate of which suspension is better for our beloved Mopars. There will always be opposition when this topic is discussed, but first, what exactly is “better”? Are the things that one person considers “better” actually relevant to everyone? Probably not. Chances are the die hard “coil over suspension is better” guys haven’t driven a properly setup torsion bar car. As the title suggests, I’m going to discuss the HDK Suspension K-Member as it pertains to handling. And by handling, I mean going fast, turning, and keeping up with, if not beating modern cars on an autocross. We all know aftermarket K-members offer lighter weight and extra room for big engines and headers so those topics won’t be discussed.
For reference, I have a 1970 Plymouth Duster that I use for cruising and autocross. I’ve only been autocrossing for a 3.5 years and have approximately 175 autocross runs on the car. I’ve never driven anything else on an autocross course, only this car. Until the summer of 2023, the car was setup with a stock style torsion bar suspension with parts designed specifically for improved handing. My t-bar setup consisted of the following parts.
- Sway a way 1.08 bars
- Hotchkis front and rear sway bar, non-adjustable shocks, and leaf springs
- SPC gen 1 upper control arms
- FMJ Spindles
- Aluminum tie rod sleeves
- QA1 adjustable strut rods
- Fully welded biscuit type K-member and LCAs
- Borgeson steering box
- 14” front discs and 12” rear discs
- Falken Azenis 200TW 235/40-18 front tires, 275/35-18 rear tires
- Alignment- 6.25 degrees caster, 1.5 degrees camber
This combination of parts resulted in a very good handling car. Unlike drag racing where the car with the most HP usually has the fastest car, the same doesn’t hold true on the autocross. Having a properly set up car is probably 75% of the equation. Driver skill is the last piece of the puzzle and is the hardest to make up. There is no replacement for seat time! I’ve never won a big event, nor am I the fastest at any given event. If I were to describe my skill, I’d say I am slightly better than mediocre. My car is always the oldest at my local events and I’m typically within the top 25% of the cars there. Most of which are modern compact cars.
In June 2023, my journey with stock style K-member and suspension ended with the installation of an HDK Suspension K-Member with coil overs and rack and pinion. I honestly didn’t know what to expect. Was I going backwards in my quest to have a 50+ year old car that can hang with modern cars around turns? Was it going to be a night and day difference? There was only one way to find out.
If you have been around the automotive market long enough, chances are you have heard someone say coil overs will be exponentially “better” for your car. But why exactly will this one part make your car better? After all, it’s just a shock with a spring around it. Sure, it can make ride height adjustments easy, but nothing is easier than adjusting the tension on a torsion bar. Fortunately, modern aftermarket suppliers are now manufacturing many options when it comes to larger diameter t-bars and there are plenty of non-adjustable, and adjustable shock options. Therefore, I’d say either option is good from an aftermarket support standpoint. From a assembly/disassembly standpoint, torsion bars can require slightly more work.
So, if we have determined the shock and the spring are the same as a shock and torsion bar, what exactly makes one configuration better or worse than the other? The uninformed won’t know where to go from here, but the big difference is the geometry of the moving components. Chrysler did a good job designing the front suspension. With a FMJ spindle, and a lower ride height, the geometry is pretty darn good. The camber gain with the FMJ spindle has been well documented and can be found online easily. With simply a set up adjustable control arms, you can dial in more caster and camber than you would ever want. Changing the upper ball joint height can further dail in the roll center and camber gain. However, the geometry adjustability pretty much ends with the upper control arm.
Nearly all aftermarket K-Members come with a rack and pinion. By design alone, you will get better steering response. While the Borgeson steering box is a huge improvement over the stock Mopar PS box, the rack is much faster. The Borgeson box is 3.5 turns lock to lock and the rack is 2.5 turns lock to lock. Besides the quicker response, the rack doesn’t have a dead spot in the center position like any steering box will have. There are products out there to quicken the steering box, but I’m not aware of a bolt in solution. This one could go either way based on driver preference because both are good.
Now let’s discuss the HDK suspension. Right away, the adjustability of the lower control arm alone gets a checkmark in the win column over stock components. Just having this one component being adjustable allows track width adjustments, easier caster adjustments, and wheel base adjustments when running aggressive caster. Upper and lower screw in ball joints allow options to use a longer balls joint for dialing roll center adjustments or even opportunity to use non-traditional spindles. More on that later.
Just like almost all coil over conversion for Mopars, the HDK uses a Mustang 2 style spindle. There are many options available when it comes to the M2 spindle. There are inexpensive versions, there’s a Wilwood version that is approximately ½” taller than standard, then there’s a relatively new CPP option that uses the corvette hub and brake assembly. The options for brakes on these spindles seem endless. I chose to use the corvette hub style, mostly to get the much larger sealed bearing and a Wilwood 6 piston caliper. Every car guy wants options, the M2 spindle offers that and it works great for drag racers and cruisers. The biggest problem with the M2 spindle is its very short. HDK remediates some of the height problem by supplying a 1” taller than stock upper ball joint. However, this isn’t enough if you are looking for maximum handling capability. With the shorter spindle, the upper control arm has a downward angle toward the tire. When the suspension compresses, the UCA will push the top of the spindle outward until it gets to the level point before pulling the spindle back in. This results in camber loss. Not a recipe for great handling. The second problem with the UCA having a greater angle than the LCA, is the roll center is in the ground. The short M2 spindle puts a check into the negative column for the coil over conversion, but the good news is, there’s a way to fix this.
The Wilwood spindle will help a little, but it still isn’t tall enough. The first option is to get an even taller upper ball joint. There are several aftermarket ball joint manufacturers that make up to 1” taller ball joints. When it comes time to go beyond that, I was only able to find 2 manufacturers. Howe Racing makes a 1.5” taller and Allstar Performance has a 1.5” and 2” longer option. The pivot point for the UCA and LCA on the inboard side is approximately 12.5” apart. This means the distance between the ball joint pivots needs to be greater than 12.5” to have any chance of getting the control arms into a position that will have a favorable roll center.
In trying to continue using the M2 CPP spindle, I opted for the 2” longer upper ball joint. I was initially concerned about sacrificing strength, but it’s a strong piece. The shank diameter and threads are larger than the 1” longer piece but still has the 7 degree taper. Using the combination of the CPP M2 replacement spindle along with the 2” longer upper ball joint, and the standard lower ball joint supplied by HDK, the ball joint pivots are approximately 13” apart. This gets my roll center to approximately 2.5” above ground.
As of this writing, HDK has been made aware of the 2” longer ball joint and will likely supply it with the kit if requested.
During my quest to refine the geometry, I found Ride Tech is now offering a taller spindle with the corvette hub designed to be used on their Chevelle platform. Based on scaling some pictures found on the internet, it appears the spindle is approximately 2” taller than the standard Mustang 2 spindle. Considering the HDK uses screw in ball joints, even if the Chevelle spindle has a different ball joint taper than the M2, ball joints of the correct taper could be sourced. Like with most GM spindles, the steering arm bolts on. There seems to be endless options on length for this part, so dialing in steering angle would be another adjustable feature. Having a 2” taller spindle will allow use of the ¼”,1/2”,3/4”, and longer ball joints to dial in suspension geometry more precisely. I’m limited now since I’m using the 2” just to get the UCA past parallel.
For the guys that are educated in suspension geometry that state coil over conversion systems don’t have proper camber gain, well, I fixed that. With just the taller ball joint change, the standard HDK camber gain went from .7 degrees at 2” of compression to 1.8 degrees at 2” of compression. The factory suspension with an FMJ spindle is around 1.4 degrees based on an old Mopar Muscle Magazine article. Of course, ride height affects all these measurements so use these values at your own risk.
Back to the question at hand, can the HDK handle better than a T-bar setup? As of this writing, I’m going to say my car is handling better than it ever has. I’ll admit, I never took the time to dial in camber gain and roll center on the factory suspension, so perhaps there are some improvements I left on the table. Let’s just say it’s just as good and move on. The aftermarket doesn’t support our Mopars like other brands, so let’s just embrace the fact that there are companies out there willing to contribute to our brand.
Coil over conversions will never be for everyone. If you’re a die-hard t-bar guy and think coil over conversions are bad, I’d like for you to come ride with me around the cones. I plan to be at every Moparty running Grand Champion as long as I’m vertical. Come introduce yourself and let’s go for a ride. I promise you will have a grin from ear to ear after the first corner.
