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Old 06-06-2009, 06:14 AM
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Default Do torsion bars 'sag' over time?

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?
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Old 06-06-2009, 03:02 PM
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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.
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Old 06-07-2009, 08:50 AM
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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?
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Old 06-07-2009, 09:24 AM
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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... 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.
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Old 06-10-2009, 11:27 AM
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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.
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Old 06-10-2009, 12:51 PM
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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.
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Old 06-10-2009, 05:51 PM
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Yes, they will "sag" over time. Nice thing is ride height adjustment is relatively easy when doing alignments with torsion bar suspensions. bob...........
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Old 06-11-2009, 07:46 AM
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Quote:
Originally Posted by JohnC View Post
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.
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  • Previous owner of two 1994 6.5L K3500s, '01, '02, and '05 6.6L K2500s, '04 C4500, '06 K3500 dually, '06 K3500 SRW, '09 K3500HD SRW
  • Total GM diesel miles to date : ~950K

Last edited by Mark Rinker; 06-11-2009 at 07:58 AM.
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Old 06-11-2009, 11:35 AM
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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.
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Old 06-11-2009, 11:38 AM
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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.
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2005 GMC Yukon Denali 6.0L gas (hey its paid for)
  • Previous owner of two 1994 6.5L K3500s, '01, '02, and '05 6.6L K2500s, '04 C4500, '06 K3500 dually, '06 K3500 SRW, '09 K3500HD SRW
  • Total GM diesel miles to date : ~950K

Last edited by Mark Rinker; 06-11-2009 at 12:09 PM.
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