Torsion Bar Tango
|G-machine suspension tech, Mopar flavored
By Andy Finkbeiner Photography by Ron ValeraTorsion bar suspension illustrated by a 1965 Coronet. Chrysler used this same basic design from 1962 until the late 1970’s. Look at all the room! This same basic design has reappeared on the 2002 Dodge Ram trucks so you know it is a proven design.Torsion bar suspensions have been used in everything from sprint cars to tanks over the years, but their use under Chrysler passenger cars during the 1960’s and 1970’s is what interests us g-machine guys. Rather than copying the coil spring design that Ford and GM were using on their passenger cars, Chrysler went with a torsion bar design. With the torsion bars tucked down between the frame and engine, the Mopar engine compartment was roomy enough for a Hemi. And as anyone who has ever busted their knuckles changing plugs on a big block Ford Mustang knows, getting rid of the spring towers in a tight engine bay is a great idea!
Even though the Mopar torsion bar front suspension has been around for over 40 years, most people don’t understand it as well as they do coil spring suspensions. But it is a fairly simple design, and once you get familiar with it, you’ll be impressed with how easy it is to work on. Now that Mopar Performance is selling a complete selection of high rate torsion bars, there is no reason for the Mopar g-machine to be left behind when the road gets twisty.
Chrysler passenger car torsion bars came in several different lengths but only the A-body and B-body bars are still available new from Mopar Performance. The A and B-body bars share 1.250 diameter hex shaped end dimensions, but they have different lengths. The A body bars are 35.8 inches long, while the B body bars are 41 inches long. E body cars also used the 41 inch B body bar.
Pictured are torsion bars from C,B and A body cars. Lengths in this picture are 47, 41 and 35.8 inches. Several other lengths exist but these are the most common. All bars use a hex shaped forged into the ends as a mounting location for the suspension and frame. Notice the smooth radius from the hex end to the bar diameter.
Factory installed torsion bars can be identified in several different ways. Different colored paint splotches were used on the production line to identify the bar both for size and location. The left side bar had two splotches, the right hand side just one. These splotches are located about 12 inches forward of the anchor end of the bar. Orange splotches on this set of 41 inch bars identify them as being from a B body with a slant six engine.
The end of the bar contains several different marks including the part number, the manufacturing date and a mounting location code.
Here is an illustration of the ID marks on a typical torsion bar. The 890 is the last three digits of the part number. In this case, the full part number is 2535890 which decodes as a 0.850 diameter passenger side torsion bar for an A body car. The R means right hand, or passenger side application. You’ll notice that the part number is even and the bar is right hand application as is typical practice for Chrysler engineering. Parts which mount on the driver’s side of the car typically have an odd part number. The 165 is the date code. In this case, it stands for the 16th week of 1975 although it could also represent the 16th week of 1965.
Mopar torsion bars are preset at the factory in the direction of use. You can observe this preset by laying the bars down on a flat surface and observing how the bar is twisted. The driver’s side bars will have a 30 degree right hand twist. A passenger side bar will have a 30 degree left handed twist. Pre-setting the bars in this manner improves the load carrying capacity in the pre-set direction. But, the load carrying capacity is reduced in the other direction of twist. This is why it is so important to get the bars installed on the correct side of the car. This twist is put into the bar during fabrication and is done to strengthen the bar. If the bars are put on the wrong side, they will be loaded against the pre-set and will not be able to carry their rated load.
Theory of Operation
A torsion bar is one of the simplest shapes for a spring. The bar is subjected to a twisting motion by the car’s suspension, and resistance of the bar to that twisting force is what supports the weight of the vehicle. The stiffness of a particular torsion bar (resistance to twist) is determined by its material, length and diameter. Since the length, material and mounting configuration are all out of the control of the typical g-machine enthusiast, we only need to concern ourselves with diameter.
What many people fail to appreciate is how important diameter is to the stiffness of a torsion bar. For instance, if you double the diameter of a torsion bar while holding all other variables the same, the stiffness will increase by 16 times. So a bar that has a spring rate of 100 pounds per inch would go to 1600 pounds per inch if the bar diameter was doubled. This is why the factory torsion bars are produced with diameter increments of only 0.020. A small increase in size makes a big increase in spring rate.
Here we see a torsion bar mounted into a lower control arm. As the lower control arm is moved up and down by the wheel, the torsion bar is twisted. The ability of the torsion bar to resist this twisting motion is calculated in pounds of force per inch of travel. For a 3500 lb g-machine we want about 180 lbs per inch of wheel travel.
Mopar Performance currently offers 6 non-production bar diameters to fit the 1962 to 1972 B body chassis. When you combine these 6 aftermarket bars with the 4 production diameters, the B-body based Mopar g-machine has a total of 10 bar sizes to choose from. As the chart shows, these bars range from a little less than 100 lbs. per inch to over 400 lbs. per inch.
Spring rates for torsion bars are measured at the lower ball joint so they are representative of what the wheel sees. On cars with coil springs, the wheel rate is much less than the spring rate due to the geometry of the suspension. So don’t try to compare a 200 lb/inch torsion bar with a 200 lb/inch coil spring unless you know you are comparing wheel rates.
The stiffest torsion bar available on production B and E-bodies was the 0.920 diameter “Hemi” bar. Today, the 0.920 bar is the limpest bar that Mopar Performance offers outside of the super lightweight drag race bar. Evidently, Mopar Performance now recommends much higher wheel rates than the factory did 30 years ago.
Optimal wheel rate is going to be different for each car since it depends on factors such as overall vehicle weight, weight distribution, anti-roll bar diameter, wheel size, and driving style. But most g-machine B body cars are going to want to start with either the 0.920 bar or 0.960 bar and go up from there.
|A handy rule of thumb is to pick a bar that has a wheel rate that is 1/10thof the front-end weight. For instance, a 3600 pound car with 50% of the weight on the front end has a front end weight of 1800 pounds. A good starting point for the wheel rate would be 1800/10 or 180 lbs./inch bar. Closest bar available for the B body is the 1.00 inch bar with a wheel rate of 186 lbs./inch. For an A body, the 0.920 bar at 150 lbs./inch is probably the best choice. The next size up is the 0.990 bar at 200 lbs./inch which is probably a tad too stiff for most folks. Remember, the A body bars are shorter than the B body bars which makes them stiffer.This rule of thumb calculation is derived from the formula for natural frequency of sprung bodies. This 1/10 rule of thumb provides you with a frequency of about 1.40 cycles per second which is typical for high performance cars. For a more in depth discussion of this topic, check out “How to make your Car Handle”, by Fred Puhn. Be careful with this concept though, since more is not always better. A super stiff front suspension will have a very high natural frequency. If the natural frequency gets high enough, the car will actually be painful to drive.An interesting side note: The 1.00 bar in a 3600 pound B body provides a ride that is noticeably stiffer than the factory springs. Yet, this same wheel rate of 186 lbs/inch was the standard spring rate in the cushy riding Chrysler Imperial. How could that be? The difference is that since the Imperial weighed at least 5000 pounds, the natural frequency of the system is lowered back down to about 1.1 from 1.4. This just goes to show that matching the torsion bar rate to the car weight is the important factor. That is why the rule of thumb of spring rate being 10% of the front end weight works so well. By following that formula, you account for the car weight in your decision.
Drag Race Bars
A fairly common question people have is if they can use the drag race bar on street driven cars. Obviously, this low rate bar is going to have difficulty controlling a heavy car during cornering so a person wouldn’t want it for any type of daily driving. But the real reason to avoid such a bar on the street has to do with fact that this bar could be dangerously overloaded in such situations. Remember, the torsion bar is nothing more than a spring. And much like a valve spring that is overloaded, an overloaded torsion bar can fail. With a spring rate of only 92 pounds per inch of travel, the drag race bar has to be severely twisted to support the nose of a heavy car. Such a dramatic amount of twisting sends the internal stresses sky high. If the car hits a large bump and bottoms out, this overloaded bar could snap from the stress. Even if it doesn’t snap right away, the life of such an overload spring is going to be fairly short.The importance of matching the weight of the car to the bar size is shown by calculating twist as a function of force.Bar size Twist required to support 1000 lbs Stress in bar
.840 52 degrees 112,000 psi
.920 36 degrees 85,000 psi
1.00 26 degrees 66,000 psi
As you can see, the .840 diameter bar must be twisted twice as much as the 1.000 bar in order to support 1000 pounds of weight. This severe twist increases the stress in the bar by almost a factor of 2. Bottom line is that the drag race torsion bars are just too small for street use on the heavy B body. Minimum bar size should be the 0.920 bar in order to keep the stress level within reason.
One item to consider when using drag race bars is to use a heavier bar as the car gets faster. You’ll want to look at this as a way to control the front end of the car and prevent it from wheel standing or hanging up in the air at the fast end of the track. A heavy car probably needs a softer spring in the front to get some weight transfer to the rear tires but as your car gets faster and faster, you’ll want more control up front.
Removing and replacing the torsion bars is quite easy when compared to the work required on a coil spring car. The service manual contains complete instructions, but we can quickly summarize them here for those of you who haven’t picked up that important item yet. The front suspension needs to be unloaded in order to remove the torsion bars. Best way to unload the suspension is to get the car up on jackstands with the front wheels hanging free. Fully release the adjuster mechanism in the lower control arms so that the bars are unloaded. After removing the retaining clip at the rear of the bar, the bar can be driven out with the proper tool. Sometimes the rubber bumper supporting the upper control arm will need to be removed in order to allow the suspension to fully droop and unload the bars. The factory tool is the ideal one to use, but substitutes can be purchased or made. Be very careful not to damage the bar in any way during removal or storage. Any surface defect on these highly stressed springs can cause instant failure.At the bottom is a home made torsion bar removal tool. This aluminum block was quickly whittled out on a milling machine. By clamping this tool onto the bar and then striking the tool with a hammer, the torsion bar is safely driven out. The upper tool is the one that is recommended in the service manual. Miller Special tools carries the correct torsion bar removal tool under part number MLR-C-3728. Miller Special tools can be ordered from the parts counter at any friendly Dodge dealer, or contact Miller on the web at www.miller.spx.com if your local dealer pretends to not know who Miller is.
Here the Miller supplied tool is clamped in place on the car. A few quick blows from a small 5lb sledge hammer and the torsion bar will come right out. Providing of course, that you removed the retaining ring located at the rear of the bar!
Aftermarket Torsion Bars
There have been a lot of complaints about aftermarket torsion bars not fitting correctly. This is a real problem for a few of the vendors because they do not build their bars with the correct hex offset. The hex ends on the torsion bar are typically aligned with each other when the car is at a normal ride height. The hex in the torsion bar crossmember is arranged with the points in the vertical direction. At normal ride height, the points in the LCA (lower control arm) are also arranged in a vertical direction. But, the torsion bar has to twist in order to support the weight of the car so the hexes on the bars need to start off mis-aligned with each other. Ideally the bars are mis-aligned by just enough so that when they twist under load, the hexes are aligned in a verical fashion. For some unknown reason, several vendors sell their torsion bars with the hexes aligned. If the hexes are aligned without any load on them, they can’t possibly be aligned when loaded.
The amount that the hexes need to be “clocked” depends on the bar rate and the weight of the car. The stiffer the bar, the less they need to be clocked. Likewise, the lighter the car, the less the hex ends need to be clocked. Another factor is the desired ride height. If the car is going to be set up with a super low ride height, then you might want to reduce the amount of initial hex clocking.
As mentioned earlier, the factory engineers clocked the hexes by 30 degrees on all of the factory B-body bars. The 30 degree clocking works just fine for the smaller bars, but this is too much rotation for the thicker torsion bars. With 30 degrees of clocking, a large diameter torsion bar will force the ride height up too high for performance use. The bigger the diameter of the bar, then the less initial rotation is needed since the bar does not twist as much when loaded. We have worked with the guys at Firm Feel in the past on this issue and we know that they understand the correct amount to clock the hex ends. So if you have aftermarket bars that do not fit correctly, consider giving the guys at Firm Feel a call. They should be able to set you up with the correct hex orientation. To help get the correct bars the first time, it is important to take a few measurements and to make some notes. Install a base line set of torsion bars of a known size. Set the ride height to where you want it with the new bars, and then carefully note where the adjustment screw is. Once you know these two things, you are ready to order new bars. Our shop vehicle is a 1965 Dodge Coronet. With the Mopar Performance .960 bars installed and the ride height set where we wanted it, the adjustment screws were right in the middle of the range. We wanted to move up to a set of 1.080 diameter bars which we knew were substantially stiffer than the 0.960 bars. In order to get the same ride height with the adjustment screw still in the middle of its travel, we calculated that the hexes needed to be offset by 20 degrees. It makes sense that the initial clocking is less with a stiffer bar, since the bar will twist less when the vehicle is lowered back down onto the tires.
This article was published in the March 2002 edition of “Popular Hot Rodding”