Torison Bars

[1BadBowtie]

What are the differences if any in the k1500 to the k2500 and the k3500 torison bars? Are the k2500 and k3500 considered HD torison bars? If not what are HD torison bars and how do I find them?

 

[Swims350]

the spring rate on them is higher for the bigger trucks and swapping in heavier bars aint the easiest thing either some say they won't work, others say the main thing is to switch sides so they'll load, and from what I've read you use your stock 1/2 ton keys or else it won't lift it as much as the others?

 

[WFO4X4]

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.



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)

 

[1BadBowtie]

Well I've read that before. It doesnt tell you what is considered HD bars. Is that 3/4 ton bars? or are they the new 2500 HD ones? Im not sure. If I can Ill just order brand new 3/4 but if theres not difference then why should i do that! Im so lost!

 

[WFO4X4]

the GL bars are about as stiff as i'd go in any half or tres quatro, the very last one is for heavy duty ass duly.....

 

[Swims350]

why does it matter which ones are HD? if you have a 1/2 ton then almost any of them are HD compared to yours. It just depends on what bars you currently have.

 

[MOBS]

hahaha....XG on 1/2tonner would be just like a rigid suspension!!!