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Home , Car suspension, Weight transfer

Torsion Bar Tango Torsion Bar Tango | By Andy Finkbeiner - AR Engineering

G- suspension tech, Mopar flavored A handy rule of thumb is to pick a bar that has a rate that is 1/10thof the front-end By Andy Finkbeiner Photography by Ron Valera weight. For instance, a 3600 pound 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 . 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 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 illustrated by a 1965 Coronet. Chrysler used this same basic design A fairly common question people have is if they can use the drag race bar on street driven from 1962 until the late 1970’s. Look at all the room! This same basic design has cars. Obviously, this low rate bar is going to have difficulty controlling a heavy car during reappeared on the 2002 Dodge Ram so you know it is a proven cornering so a person wouldn’t want it for any type of daily driving. But the real reason to design.Torsion bar suspensions have been used in everything from sprint cars to avoid such a bar on the street has to do with fact that this bar could be dangerously over the years, but their use under Chrysler passenger cars during the 1960’s overloaded in such situations. Remember, the torsion bar is nothing more than a spring. and 1970’s is what interests us g-machine guys. Rather than copying the And much like a valve spring that is overloaded, an overloaded torsion bar can fail. With a design that Ford and GM were using on their passenger cars, Chrysler went with a spring rate of only 92 pounds per inch of travel, the drag race bar has to be severely twisted torsion bar design. With the torsion bars tucked down between the frame and to support the nose of a heavy car. Such a dramatic amount of twisting sends the internal engine, the Mopar engine compartment was roomy enough for a Hemi. And as stresses sky high. If the car hits a large bump and bottoms out, this overloaded bar could anyone who has ever busted their knuckles changing plugs on a big block Ford snap from the stress. Even if it doesn’t snap right away, the life of such an overload spring Mustang knows, getting rid of the spring towers in a tight engine bay is a great idea! 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 .Bar size Twist required to support 1000 lbs Even though the Mopar torsion bar front suspension has been around for over 40 Stress in bar 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 .840 52 degrees 112,000 psi 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 .920 36 degrees 85,000 psi left behind when the road gets twisty. 1.00 26 degrees 66,000 psi Identification As you can see, the .840 diameter bar must be twisted twice as much as the 1.000 bar in Chrysler passenger car torsion bars came in several different lengths but only the order to support 1000 pounds of weight. This severe twist increases the stress in the bar by A-body and B-body bars are still available new from Mopar Performance. The A and almost a factor of 2. Bottom line is that the drag race torsion bars are just too small for B-body bars share 1.250 diameter hex shaped end dimensions, but they have street use on the heavy B body. Minimum bar size should be the 0.920 bar in order to keep different lengths. The A body bars are 35.8 inches long, while the B body bars are the stress level within reason. 41 inches long. E body cars also used the 41 inch B body bar. One item to consider when using drag race bars is to use a heavier bar as the car gets Pictured are torsion bars from C,B and A body cars. Lengths in this picture are 47, faster. You’ll want to look at this as a way to control the front end of the car and prevent it 41 and 35.8 inches. Several other lengths exist but these are the most common. All from wheel standing or hanging up in the air at the fast end of the . A heavy car bars use a hex shaped forged into the ends as a mounting location for the probably needs a softer spring in the front to get some to the rear but suspension and frame. Notice the smooth radius from the hex end to the bar as your car gets faster and faster, you’ll want more control up front. diameter. Installation 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 Removing and replacing the torsion bars is quite easy when compared to the work required size and location. The left side bar had two splotches, the right hand side just one. on a coil spring car. The service manual contains complete instructions, but we can quickly These splotches are located about 12 inches forward of the anchor end of the bar. summarize them here for those of you who haven’t picked up that important item yet. The Orange splotches on this set of 41 inch bars identify them as being from a B body front suspension needs to be unloaded in order to remove the torsion bars. Best way to with a slant six engine. unload the suspension is to get the car up on jackstands with the front hanging free. Fully release the adjuster mechanism in the lower control arms so that the bars are The end of the bar contains several different marks including the part number, the unloaded. After removing the retaining clip at the rear of the bar, the bar can be driven out manufacturing date and a mounting location code. with the proper tool. Sometimes the rubber supporting the upper will need to be removed in order to allow the suspension to fully droop and unload the bars. Here is an illustration of the ID marks on a typical torsion bar. The 890 is the last The factory tool is the ideal one to use, but substitutes can be purchased or made. Be very three digits of the part number. In this case, the full part number is 2535890 which careful not to damage the bar in any way during removal or storage. Any surface defect on decodes as a 0.850 diameter passenger side torsion bar for an A body car. The R these highly stressed springs can cause instant failure.At the bottom is a home made means right hand, or passenger side application. You’ll notice that the part number torsion bar removal tool. This aluminum block was quickly whittled out on a milling machine. is even and the bar is right hand application as is typical practice for Chrysler By clamping this tool onto the bar and then striking the tool with a hammer, the torsion bar engineering. Parts which mount on the driver’s side of the car typically have an odd is safely driven out. The upper tool is the one that is recommended in the service manual. part number. The 165 is the date code. In this case, it stands for the 16th week of Miller Special tools carries the correct torsion bar removal tool under part number MLR-C- 1975 although it could also represent the 16th week of 1965. 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 Preset not know who Miller is.

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

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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 . 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.

Selection

Mopar Performance currently offers 6 non-production bar diameters to fit the 1962 to 1972 B body . When you combine these 6 aftermarket bars with the 4 Here the Miller supplied tool is clamped in place on the car. A few quick blows from a small production diameters, the B-body based Mopar g-machine has a total of 10 bar 5lb sledge hammer and the torsion bar will come right out. Providing of course, that you sizes to choose from. As the chart shows, these bars range from a little less than removed the retaining ring located at the rear of the bar! 100 lbs. per inch to over 400 lbs. per inch. Aftermarket Torsion Bars Spring rates for torsion bars are measured at the lower so they are representative of what the wheel sees. On cars with coil springs, the wheel rate is There have been a lot of complaints about aftermarket torsion bars not fitting correctly. much less than the spring rate due to the geometry of the suspension. So don’t try This is a real problem for a few of the vendors because they do not build their bars with the to compare a 200 lb/inch torsion bar with a 200 lb/inch coil spring unless you know correct hex offset. The hex ends on the torsion bar are typically aligned with each other you are comparing wheel rates. when the car is at a normal . The hex in the torsion bar crossmember is arranged with the points in the vertical direction. At normal ride height, the points in the The stiffest torsion bar available on production B and E-bodies was the 0.920 LCA (lower control arm) are also arranged in a vertical direction. But, the torsion bar has to diameter “Hemi” bar. Today, the 0.920 bar is the limpest bar that Mopar twist in order to support the weight of the car so the hexes on the bars need to start off mis- Performance offers outside of the super lightweight drag race bar. Evidently, Mopar aligned with each other. Ideally the bars are mis-aligned by just enough so that when they Performance now recommends much higher wheel rates than the factory did 30 twist under load, the hexes are aligned in a verical fashion. For some unknown reason, years ago. 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. Optimal wheel rate is going to be different for each car since it depends on factors such as overall vehicle weight, , anti-roll bar diameter, wheel size, The amount that the hexes need to be “clocked” depends on the bar rate and the weight of and driving style. But most g-machine B body cars are going to want to start with the car. The stiffer the bar, the less they need to be clocked. Likewise, the lighter the car, either the 0.920 bar or 0.960 bar and go up from there. 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”

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