Do torsion bars 'sag' over time?

[Mark Rinker]

Q: Do torsion bars weaken, i.e. 'sag' over time, requiring that their preload be adjusted to maintain correct suspension geometry?

My '06 K3500 SRW came with oversized rubber and no lift. In the ~40K miles I've since put on, it seems like the front tires are more prone to rub when turning sharp, backing out of the driveway, etc.

Wondering if the torsion bars are the variable?

[Mark Rinker]

Installed new Bilsteins today and turned up torsion bars 2 turns each side. Tire rub is gone, except when backing and full turn to left or right where light contact with fender flare occurs.

New ride is very smooth, no visible/perceptible 'hood dive', even under moderately aggressive braking. Chatter over railroad tracks has disappeared from the rear.

Rides like new again. :D

[cowboywildbill]

I think someone makes a bra for the sagging. Oop's ! sorry I'm on the wrong forum "Ha Ha"
But I would think torsion bars would fatiuge a little in time just like springs would do from the constant weight on them. Do you have to check the alignment after cranking 2 turns?

[Mark Rinker]

We put three turns on when hanging snowplows to counter the additional weight hanging up front.

I have been told that any turns will affect alignment, but not a problem until over 5...:eek: That would be pretty stiff, in my experience.

Never had any abnormal inside edge wear as a result. I like the firmness of a few more turns, no hood dipping, nice cornering, etc.

[DarylB]

I"ve had a winch setup on both my Dmax's and had to crank the bars to help level truck after adding additional weight. I was told that any upward adjustment at least required the toe-end to be adjusted, if not a full front wheel alignment to be on the safe side.

[JohnC]

Adjusting the torsion bar doesn't make the bar any stiffer, it merely changes the neutral position. It may, however, change the angle of the control arm such that more twisting of the torsion bar is required for a given amount of suspension travel.

[carco]

Yes, they will "sag" over time. Nice thing is ride height adjustment is relatively easy when doing alignments with torsion bar suspensions. bob...........

[Mark Rinker]

Adjusting the torsion bar doesn't make the bar any stiffer, it merely changes the neutral position. It may, however, change the angle of the control arm such that more twisting of the torsion bar is required for a given amount of suspension travel.

Makes sense.

I know that if you turn them up (adjustment bolt 'in') fender/tire clearance increases, ride and suspension rebound 'stiffens', and suspension travel decreases.

If you turn them down (adjustment bolt 'out') fender/tire clearance decreases, ride and suspension rebound 'softens', and suspension travel increases.

[DmaxMaverick]

Adjusting the torsion bars ONLY adjusts the spring preload of the suspension. It cannot, under any circumstances, effect suspension travel. The upper and lower limits remain exactly the same. Your perceived suspension travel is determined by the preload (starting point), actual load bearing, and driving conditions. Late model GM light trucks incorporate the bump-stop (jounce bumper, etc.) into the greater compression (higher loading condition) travel of the suspension. In original configuration, the control arm stop is engaged slightly onto the bumper, including it into the initial spring load. Often, increasing torsion bar preload moves the control arm off of the bump-stop at neutral loading. This is why some folks report a "softer" ride when the bars are tightened (later bump-stop engagement). If you adjust the bars to compensate for additional load bearing (plow, bumper, winch, etc.), you can either return the suspension to its original Z height, or a departure from this, lesser or greater. Rebound and stiffness is relative, and dependent upon what you are comparing.

[Mark Rinker]

That explains it. I'll pay close attention from here on to the bounce-stop clearance before and after adjustments, loading of plow weight, etc.

Still seems odd to me that if you have a steel bar, with a finite range of twist (i.e. it can't be wound up like a watch spring) and you increase pre-load, that you don't decrease suspension travel, with the lessened degrees of deflection left in the material.

I have been turning torsion bars up and down since 1986 ('83 2.8L S10 4WD with oversize tires) and always 'perceived' that there was less suspension travel going on...weird.

[OhmEye]

Still seems odd to me that if you have a steel bar, with a finite range of twist (i.e. it can't be wound up like a watch spring) and you increase pre-load, that you don't decrease suspension travel, with the lessened degrees of deflection left in the material.

That's just it, a steel torsion bar is just like a watch spring. The amount of deflection is proportional to the torque applied. There is no theoretical limit to the amount of deflection you can get out of either one (more force = more deflection), and as long as you stay within their elastic limits they will spring back to their original shapes once the loads are removed.

[DmaxMaverick]

Torsion bars have no "finite range of twist". They can be "wound up like a watch spring". A watch spring, however, is not a torsion spring. It's a coil, and has physical limits. A torsion bar can be twisted to the point of failure, but can be twisted, infinitely. The adjustment cams (or "keys", the part you adjust when you tighten/loosen them) are a physical limiting factor. Coil springs, on the other hand, are theoretically finite, as they have a limit of compression (coils contact each other), and extension (make a straight bar out of it). Coil springs operate on the same principal as torsion bars, as they are a torsion bar, of sorts, just shaped differently, with the stored and released in exactly the same fashion. Leafs are similar to coils, in that they have a physical limit. At some point, the ends will meet, in either direction. With any of them, hardware is the limiting factor. We could, theoretically, compress/extend these springs to their absolute limit or point of failure, but we don't have the means to do this with the hardware on the trucks. This is not to say the springs can't/won't fail under normal conditions, short of their theoretical point of failure. They do. We know they do.

[OhmEye]

Key here is that as long as you are operating within the design limits of the component, then a spring is a spring is a spring.

[Mark Rinker]

Okay, I have read your spring theory, and am about to head to the ga-rage with a wrench and tape measure. Three turns in each direction from OEM factory, and start jumping up and down on the bumper while my kid records the results...

(In fact, I was thinking about how to video tape this experiment, and make it more interesting. Maybe in the parking lot of Hooters, using three or four waitresses for the counter weight? Anyway...I digress...)

I lived in Missouri for awhile...in this case, I gotta show me. :D

[OhmEye]

The more tension you put on the torsion bar, the less deflection for a given force. Increase the force sufficiently and you will use the full range of travel no matter how much you preload the torsion bar.

You have Hooters in your neighborhood?!! I love your idea. . .:D