GM Torsion Bar 401
By Stephen Bradley (Dark Eternal)
To start out with, lets first discuss what a torsion bar is. A torsion bar is a type of spring, just like a coil spring or leaf spring. To be exact it is a type of rotary spring. A spring can be defined to be an elastic member which exerts a resisting force when its shape is changed due to an applied force and returns to its original shape when the force is let off. There are many different classes of springs, but I will only discuss a few that are relevant to us. The first is the most common, a linear spring. A linear spring requires the same amount of force all the way through its range of compression. The next class is a step linear. A step linear requires different forces at different positions of compression through the range, requiring more force the more you compress the spring but in steps. This means it might be easy to compress for half of the compression range and then very hard for the second half. The last class is the progressive spring. It is similar to the step linear but instead of being in steps and having a set number of different amounts of force required to compress the spring, it is progressive. This means that every little amount you compress the spring its gets harder. There are no steps, but instead it is a constant rate at which the force needed to compress the spring increases.
To define how a torsion bar works we can compare it to other springs. A coil spring uses compression and extension and depending on the design of the spring can be a linear, step linear, or a progressive rate spring. A leaf spring uses flex and is a progressive spring. The torsion bar on the other hand uses resistance to torque and is a linear spring. A torsion bar is essentially a steel bar with some form of splines on each end. On our GM torsion bars it’s not the type of splines you are used to, like what you would find on an axle shaft, but instead a hexagonal head, similar to a bolt head. In fact you can slide a socket over it if you have a big enough one. Now when one side is fixed to an immovable object and the other is twisted, torque is applied. The torsion bar resists this torque and like any other spring it returns to its original position when the torque is let off. Now let’s apply this to our trucks. The immovable object is going to be our frame, or rather the torsion bar cross member on our frame where the torsion bars are anchored. The other end is connected to the lower control arm (LCA). The LCA pivots on a fixed point on the frame and creates twisting motion on the torsion bar the torque needed to make it a spring. For example, when the vehicles front suspension compresses, the LCAs move upwards and exert force onto the torsion bars, so they push back onto the LCAs, just like a coil or leaf would.
Now that you have a basic idea of how a torsion bar works lets discuss why it is what GM chose for our 4wd suspension. One answer is that it is cheap and maintenance free. Some torsion bars do use bushings that need greasing on other vehicles but GMs do not. Second, they are out of the way. A leaf spring can’t be used on IFS and a coil is in the way of the half shaft used to transmit power to your wheel in the front. The torsion bar is totally out of the way. Finally, it is adjustable. The fixed end of the torsion bar that is anchored in the torsion bar cross member is anchored into a “key” which can be turned with an adjusting bolt, therefore effecting the resting position of the LCAs and the height of the front of your vehicle. Sounds like the perfect suspension doesn’t it? Not quite. The fact that torsion bars are a linear spring effect the ride quality in a negative manner. Luckily, with modern technology and the advanced design of independent front suspension (IFS) ride is still going to be far superior to that of a progressive leaf spring and a solid front axle. Also, you may notice that the LCA on the GM IFS seems to be resting on its upper bump stop. This is not only a bump stop, but what is called a progressive dampner. It compresses at a progressive rate, simulating a progressive rate spring, and letting the torsion bars achieve close to a progressive feel. Another con is as you increase the ride height by adjusting your keys, the ride gets worse. Now it’s a common misconception that there is more of a load on the bar and this is the cause. That is impossible since the LCA moves as well, so there is no more torsion on the bar than before, the spring rate does not actually change like many belive. The twisting action does not change the pre-load on the bar directly, so cranking your bars is just like adding a block in the rear or a coil spacer. It is simply changing the position of the spring. Just to clear things up, pre-load is the amount by which the torsion bar can't return to its original (unloaded) position due to be installed in the truck. The reason for the bad ride quality is instead a combination of things. First of all, when you adjust the height up, the angle between your LCA and the ground is increased which effectively does reduce your pre-load by reducing the leverage your LCAs have on your torsion bars. This negativly effects the ride by taking the torsion bars out of the preload range they were designed to work in. In combination with that, if you crank too far your UCAs have very little room left between them and the lower bump stops, reducing downward travel. You also pull the LCA away from the progressive dampner, making it ineffective and putting yourself back into the position of having a linear spring. These are the reasons the ride gets worse! One last disadvantage is that like all other springs, torsion bars will wear out over time causing your front end to sag. The more weight on the front of your vehicle, the faster they will wear. Also, the more your suspension travels the faster they wear.
Now let’s start looking at the technical side of the GM torsion bars. GM makes a variety of different torsion bars with different torque ratings. The torque rating # is in ft/lbs needed to move the spring. The numbers seem huge, but remember there is leverage and a lot of weight on them! Obviously the heavier bars are in heavier trucks. Also, the different bars have different indexing of the hex ends. Here is a table on the torsion bar keys. To ID your bars there is a tag on them with a 2 letter code, just match it to the code in the table.
How bad is it to “crank” my torsion bars for more lift?
The answer is it all depends. The further you crank the worse the bad effects will be. The bad effects include a worse ride for the reasons described previously, and more wear and tear on your IFS components. These components include your ball joints, CV half shafts, and tie rods. The reason these wear faster is that the LCA and UCA are pushed down creating a bigger angle at which these components must operate at. All of these wear out faster when operated at a steeper angle. Therefore the further you lift your truck, the faster your will wear components and the worse it will ride. Also you might have heard of someone breaking a CV shaft or you might have even done it yourself. A CV shaft can operate at a certain angle before it binds. When it reaches its binding angle and power is applied (as in your in 4x4 and gas it) the force has to go somewhere, since the joint cant turn it blows up! This can happen even if your torsion bars aren’t adjusted way up by having your suspension at max drop and wheels turned very sharp. The catch is, adjusting your bars up makes it much easier to reach max drop since your already closer to it, and in turn makes it a lot easier to snap a CV!
What are ford, green or purple keys and key indexing?
Well we already discussed what a key is and how it works. Now lets talk about the indexing of keys. The key has a hex shaped hole in it that the torsion bar goes on. How this hole is indexed or clocked effects how much you can turn the adjusting bolt before the key bottoms out on the top of the cross member or you run out of bolt. The different kind of keys are indexed different, which allows some to make some bars be adjusted further. To make things even more complicated the hex heads on the bars don’t line up with one another, and some bars are different. But as a general rule, all bars you will find on a half ton truck are about the same and all bars on an HD truck are about the same. The half ton trucks have the “green” keys in them and the HD trucks the “purple” keys in them. We call them green and purple because when the keys are new they have a green or purple mark on them. After about 2 weeks in the real world you cant see this anymore though because it will be a big brown rusted key! The only way to tell them apart then is to determine what truck it came off of or compare it with a known key. Now where do Ford keys come into play? Ironically ford uses the same torsion bar design with the EXCAT same hex head. The keys are exactly the same dimensions except the indexing is different. A ford key is indexed more than either the green or purple keys. This means when you put it in a GM truck it allows the torsion bars to be cranked further up. This works on both half ton and HD trucks BUT since the purple key is the least indexed you will never be able to get the Ford key to load in a HD truck, its just too indexed. If you did get it to load your CV, tie rod and ball joint angles would be so bad it wouldn’t be drivable. Don’t worry though your not left out if you have an HD. He green keys will work for you! They are indexed in the middle, less than ford keys but more than purple, so they work the same way as the Ford keys do on the half tons. Now if you put purple keys into a half ton truck you would not be able to crank as far as you originally would have because they are indexed less. Ford keys are easy to identify because of the extra hole in them.
Can I replace my half ton bars with HD ones?
Yes, you can. The advantage to this is that it will take more weight to get your front end to sag and the bars will last longer. This is good if you have a heavy aftermarket bumper, winch or brush guard. The bars are not a plug and play swap though, the bars themselves are indexed differently and there is no way you can load the HD bars with green keys, at least on the GL bars I used. Lighter duty bars may load easier. To solve this problem you can get the less indexed purple keys. Even then you will have a hard time loading them and they will give you a lot of lift when not cranked at all, which still has the negative effects. If you want the heavier bars with no more lift over stock you can use ford keys and rotate them so they are under clocked and they will load very easily this way. It gives you maybe even less lift than stock though and I would not recommend this there will be almost no preload on the bars and they wont ride very well. If you want to just gain a little lift or stay at the stock height with heavier bars you could take a set of purple keys to a machine shop and have some keys made they are indexed less. I would index them about the same amount less that the purple are from the green. In other words say there is a 10 degree difference between purple and green, I would do 10 more degrees from purple/20 from green.
Where can I buy these keys or parts?
To buy the GM parts (all of the bars and the green or purple keys) you can either give your local dealer the part numbr and have them order it or go online to http://www.gmpartsdirect.com and type in the part number and order it there. GMPartsdirect is typically cheaper. For the ford keys once again you can ask the dealer for them or order online at http://www.fordpartsonline.com/.
Can I use my torsion bars to lower the front of my 4x4?
Yes you can, all you have to do is loosen the bolts. Make sure to keep them in enough so the botls is all the way through the retaining block and then some. you dont want teh key just flat out resting on this. Also make sure to keep some load on the bars! If you have a half ton you can use the "purple" keys as well to go even lower, or use the Belltech torsion keys.
I have a Tahoe, Avalanche or Suburban, what is for me?
Your vehicle is almost like a half ton truck unless you have a 3/4 ton avalanche or suburban, then I am not sure, I am looking for conformation that it is like an HD truck or if its like a half ton truck. The only difference is that your torsion bars are shorter on a tahoe for sure, and im checking to see if the suburbans are shorter also. This means you cant use the heavier torsion bars. They keys will work the same way though.
Ok now how do I actually do the replacing/cranking
*Note - after any height adjustment of more than about an inch, you should get a new alignment, things will be all thrown off!
Where exactly can I get this GM torsion bar unloading tool?
Well you have a few options, sometimes the dealer will let you borrow one for a night if they have a nice tech. Some auto parts stores also rent them out for a small charge or even free! If your thinking about buying one the best price I have found is here: http://www.sjdiscounttools.com/otc7822a.html