Torsion Bar tech 101

Discussion in 'Tech Info' started by bowtiefreak, Oct 19, 2011.

  1. bowtiefreak

    bowtiefreak No Quarter Supporting Member

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    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.

    Code Torque Rating (ft/lbs) GM Part #
    XK 4553 15048307 (LH)
    15048308 (RH)
    XL 4626 15048309 (LH)
    15048310 (RH)
    WX 4863 15712407 (LH)
    15712408 (RH)
    XM 5638 15048311 (LH)
    15048312 (RH)
    GF 5826 15528955 (LH)
    15528956 (RH)
    YH 5913 15058267 (LH)
    15058268 (RH)
    WY 5913 15712409 (LH)
    15712410 (RH)
    GG 6707 15528957 (LH)
    15528958 (RH)
    GH 7161 15528959 (LH)
    15528960 (RH)
    WZ 7267 15712411 (LH)
    15712412 (RH)
    GK 8615 15528963 (LH)
    15528964 (RH)
    GL 8782 15528965 (LH)
    15528966 (RH)
    XG 9054 15732338 (LH)
    15732339 (RH)

    *(LH) means left hand side (driver) and (RH) means right hand side (passenger)
    Name Indexing Part #
    “Green Key” Middle 15592573
    (GM)
    “Purple Key” Less 15045171
    (GM)
    “Ford Key” Most XL3Z5B328CA
    (Ford)
    Belltech Torsion Key (88-98) Least 3920
    (Belltech)
    Belltech Torsion Key (99+) Least 3921
    (Belltech)


    Standard Torsion bar length = 54”
    Hex head size = 1.5"
    Weight = ranges from bar to bar – heavier the torque rating heavier the bar
    Torsion bar bolt size = M12 1.75 x 60
    Torsion bar retainer part # = 15737373
     
  2. bowtiefreak

    bowtiefreak No Quarter Supporting Member

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    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.
     
  3. bowtiefreak

    bowtiefreak No Quarter Supporting Member

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    Ok now how do I actually do the replacing/cranking
    Tools needed
    18mm 1/2” drive socket* 1/2” drive ratchet
    18mm 3/8" drive socket 3/8" drive ratchet
    1" 1/2" drive socket
    WD-40 Hammer
    Jack sufficient to jack your truck up GM torsion bar removal tool OR 7+ ton 2 jaw gear puller
    Pair of 6 ton or greater jack stands Tape measure


    Step 1
    Measure the distance from a point on the front of your truck to the ground on both sides and record this number


    Step 2
    Jack up the front of the truck using a floor jack, bottle jack, whatever works best for you. Make sure both tires are off the ground or just touching it. Properly block the vehicle and set it down on jack stands so the tires are still off the ground. Even for cranking this is important. If you crank it while it is on the ground you are lifting the truck up actually by cranking, and putting way more stress on the bolt than you would if the truck is on the ground. You will be running the risk of breaking the head off the bolt, which is no fun – trust me.

    Torsion bar crossmember

    Torsion bar bolt Step 3
    Now if you are going to crank, its time to do just that. If you are replacing the keys or bars you can skip this step. Locate the torsion bar adjusting bolt and tighten it a few turns with a 18mm socket. Turn each bolt an equal amount. You can then lower the truck and drive it around a little. The torsion bars will settle down some after driving so make sure to drive it before measuring. When you have driven it around and bumped it around some, measure the height at the same place you did before and compare. You can redo steps 1 and 2 to adjust it to where you want. If one side is higher just adjust that one bolt so both sides until both sides are equal. If you were just adjusting your bars you are now done, if not read on.
    Step 4
    Now it is time to unload your torsion bars. This can be very dangerous because there is a decent amount of pre-load on them and if not done properly you can be injured. I HIGHLY recommend using an actual GM torsion bar unloading tool. A 2 jaw gear puller will also work. The next step is for a GM tool, the one after is for a gear puller. Once you locate your torsion bar cross member spray some WD-40 on the adjusting bolts and let it sit a few minutes. It will make things much easier.

    Holes in top of crossmember

    Step 5
    If you have the actual GM tool you will notice it looks like a real beefy C clamp. There should be a nipple at the very top on the inside. If you look at the top of your cross member there will be a hole above the torsion bar key. The nipple fits into there and on the bottom of the torsion bar key there will also be a dimple that the bottom adjusting screw of the tool will fit into. Tighten it hand tight so the tool will not fall off.
    Step 6
    If you have the gear puller you need to find the lip on either side of the cross member. This is where you will be putting the arms of the puller. The adjusting screw on the puller will go into a dimple that is located on the bottom of the torsion bar key. Tighten it hand tight so the tool will not fall off.



    Step 7
    Now make sure you’re not under the tool when you do this because you will be putting the entire pre-load onto the tool and off of the bolt. If the tool slips you won’t want to be under it. You can start to tighten the tool up now, but only tighten it about one turn. Now take your 18mm socket and loosen the torsion bar bolt. You might have to use a 3/8" drive to make it fit with the GM tool. The tool should be holding the key in now. You can remove the bolt and push out the metal block that it goes into. Use a wrench or something so you don’t have to stick your fingers in there.



    Step 8
    Now slowly loosen the tool until the point where all the pre-load is gone and you can remove the tool. You can now grab the torsion bar and pull/push it towards the LCA. Watch out because the key is going to fall off of the cross member. If you cant get it to move, use a hammer on the LCA end and push it through the key then try pulling it back out.
    Step 9
    Repeat for the other side.
    Step10
    Obviously you are at least replacing your keys if you have gone this far. Whether its just adding keys, or putting in new bars and keys you can do this now. Make sure you put the left bar on the left side and the right on the right side.

    Placing key in

    Making sure its centered


    Step 11
    Time to reload the bars. Place the tool back onto the new key the same way you did before. Make sure the key is centered in the cross member. Now tighten the tool slowly just enough so you can re-insert the metal retaining block. You can then put the bolt back in, make sure that it goes into the dimple for the bolt (not the one where the tool is) on the key. If your bolts are at kind of a funny angle its ok, as long as its not to extreme. This is common with ford keys since the dimple is in a little bit of a different place. Make sure you tighten the bolt so it is touching the key. You can then remove the tool and continue to tighten the bolts to where you want them.
    Step 12
    Make sure to do this to both sides! You can then lower the truck and drive it around a little. The torsion bars will settle down some after driving so make sure to drive it before measuring. When you have driven it around and bumped it around some, measure the height at the same place you did before and compare. You can redo steps 1 and 2 to adjust it to where you want. If one side is higher just adjust that one bolt so both sides until both sides are equal.
    *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

    By Stephen Bradley (Dark Eternal) (GMFS
     
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  4. gregsb

    gregsb Member

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    Great information as usual! Thanks for posting this.

    Someone should sticky this...
     
  5. the big blue bus

    the big blue bus Full Access Member

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    one thing to note is the 4dr tahoes t bars are 52" truck bars can be swaped in as well as the crossmember
     
  6. ArcticYukon

    ArcticYukon Full Access Member

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    If using the Torsion bars to lower the truck, is there a way to adjust the UCA's for proper camber?
     
  7. dorisk

    dorisk TYF Newbie

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    I appreciate this thread, was very helpful.
     
  8. Donnie Yukonie

    Donnie Yukonie That sounded expensive Supporting Member

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    awesome thread bowtie was just about to start a thread asking a question about this but now there is no need
     
  9. dm600

    dm600 TYF Newbie

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    Yes, you will have to do an alignment. The alignment shop will adjust the camber/caster by the bolts that go through the UCA at the frame. The bolts are keyed to an acentric which when turned will pull or push the UCA in or out. There should be plenty of room for proper adjustment. Not sure on how many inches you can go or if the bolt falling out from the key is the first limiting factor.
     
  10. rican4lfe

    rican4lfe Full Access Member

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    For replacing the bar itself, are there any other steps to follow after taking the key off???? Does anything needs to be done at the arm to completly remove swap a bar. In my 96 hoe is very difficult to lift the driver side screw amost all in an maybe an inch. Still 1 inch below the pass side with screw maybe halfway in. I have some 97 kon bars and keys. Should I swap them?? Needs advice!!! Thanks
     

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