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Street Race Truck More... The Toolbox

Installing Torsion Bars in Street Rods

7/1/2019

In today’s world, there are a large variety of suspension options to choose from when building your street rod or custom car. The possibilities are almost endless from stock-type coil springs, leaf springs, coil-overs, air ride, and even torsion bar suspension. Though torsion bar suspension isn’t as common as a set of coil-overs, it is popular in certain circles. With that being said, I would like to show you a few things to keep in mind when designing and installing your torsion bar suspension.

First thing’s first: You can do this! Anyone with a little fabrication experience can design, set up, and install their torsion bar suspension. Now, a few things that you should know before you start. Know the weight of your vehicle, how you want to tie in the torsion arm to your suspension, and the length of that torsion arm. You may also want to start planning a position to weld on your torsion tubes, which will house the torsion bars. Their orientation, parallel or perpendicular to the frame, may vary depending on what kind of front end you are utilizing.

For example, if you use a straight axle the torsion bars would most likely run perpendicular to your main frame rails, much like a dirt track sprint car. Whereas, if you run an independent front suspension they would more than likely run parallel to your main frame rails and tie into your lower control arms. This type of setup would be a little more complex.

Once you know where you want to put your torsion bars and how you want your arms to tie in to your front end, the next step is to start making your parts list. Below you will see a starting guideline of parts needed to complete your torsion suspension. Some additional parts may be necessary depending on setup.

1 - Torsion Bar Tubes, 28” x .120” wall, part #916-37033 (Sold as a pair)

4 - Torsion Tube Bushings, .120” wall x 1-1/8”, part #910-37218

2 - Torsion Bars, 30”, such as part #940-2395-SIZE

2 - Torsion Bar Arms, recommend part #910-37223

2 - Torsion Bar Stops, recommend part #940-37012

1 - Torsion Bar Reamer, part #940-81035

Miscellaneous Hardware (Heims, bolts, etc.)

This parts list is a guide and not the perfect list for every torsion setup. We have multiple arms to choose from and depending on your front end, you could weld the torsion tubes together with a spacer in between them. I like to recommend these specific Aluminum Torsion Arms, part #910-37223, because they are universal and can be machined and adapted to your specific application. You can also drill and tap the end for a heim, or machine it down to sit on top of a straight axle.

One thing that will have to be decided by the installer is what they plan to have the torsion stop ride against. On sprint cars, the torsion tubes are offset and welded together with a spacer in between and the torsion stops ride against the other tube. Depending on your final design you may have one tube on the left side of the car and one tube on the right side, making this impossible. You would then have to install a piece of metal that the torsion stop bolt can be adjusted against, which would hold the stop in place.

The bushings installed in the torsion tubes must be reamed for proper clearance to the torsion bar. If they are not reamed, the torsion bars will bind up and not function properly. Sometimes the torsion bars won’t even slide into the bushings without them being reamed. You can do this with our Torsion Bar Bushing Reamer, part #940-81035. This reamer will allow you to ream the bushings in a straight line, allowing for a smooth installation and operation of the torsion bar. Added benefit- this is a reusable tool!

You will have to replace the torsion bar bushings eventually just like control arm bushings, king pin bushings, and hub bearings. The bushings have to be reamed every time new ones are installed and the bars greased where they ride on the bushings. There is no set recommended mileage for replacement of the bushings. However, I do recommend you occasionally take out the torsion bars and re-grease them at least once or twice a year, depending on how much you drive it. At this time, you can check fitment of the bar to the bushings. If you notice the bars are very loose in the bushings, it may be time to install new ones and ream them.

Now that we’ve gone over the physical components, let’s get into the math. Two common questions I get are, “What bar size do I need?” and “What spring rate do you recommend?” The issue with these great questions is that there are a lot of variables. Some of the variables are given and don’t usually change, however, one of them makes a big difference. That variable is the torsion arm length, from the center of the torsion bar to where it rides on the axle or where it bolts into the control arm. I mentioned at the beginning of the article that the arm length would be one of those measurements you need to know first. Though this is true, you can also have a range that your arm could be, for example, 12”-16”. With the information obtained, here is the formula you can use to estimate the spring rate you will achieve from a specific bar size.

K = Spring Rate

D = Diameter of bar in decimal form (i.e. 1.025)

L = Effective Bar Length (24” for sprint car bars)

A = Torsion Arm Effective Length

**All measurements must be in the same base unit (i.e. inches, cm, etc.)

An example of this formula in use would be as follows:

  • 1050 torsion bar diameter
  • 12” torsion arm length

K = 1,129,000 x (1.0504) 24 x 122

If you complete the equation, it produces a spring rate of approximately 397 lbs. As you can see, you can use this formula to help estimate how long you need your arms to be, based on the bars that are currently available, or the bar you want to use. With this spring rate information, you can make an educated guess as to how big of a bar, or how short of an arm you need to have, in order to support your ride. The shorter the arm measures, or the stiffer the bar is, the higher the spring rate will be. Unfortunately, there is no set chart to go off of for torsion bar setups as there is with coil springs. But remember, with torsion bars, if it rides a little low, either adjust the torsion stop bolt, or rotate the torsion stop on the bar.

Just to recap, you can make it happen! Just take measurements, and do your homework. Draw out your design on paper, and follow the parts guide to make your own parts list. Always ream your bushings, use the formula, and follow your plan. But don’t forget the most important thing of all, have fun!

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