Torsion bar math question

[Chris Pomares]

Does anyone know how much my rear suspension would be raised if I went from 23mm to 24mm rear t bars. This is assuming all other settings are the same.
Thanks, Chris

[John Z Goriup]

Torsion bar diameter is not a setting. Torsion bar diameter has no influence on ride height, therefore, assuming there is no change in any other settings in the rear suspension in the process of replacing your 23mm bars with 24mm bars, there would be no change in ride height.

Having categorically stated that, you may find that in actuality it may be entirely possible that the increased diameter ( stiffness) of the thicker t-bars would not permit the car to settle as low as the smaller diameter bars, resulting in a minor change in height after all, but that contains far too many variables to allow an accurate prediction / calculation at the degree ( no pun intended ) of any changes.

It must be understood that the t-bar splines engaging the torque tube are indexed differently than the outside splines which engage the splined spring-plate collar. They are designed in that manner to take advantage of the limited range of fine adjustment in ride height inherent in the spring-plates. By withdrawing the t-bar, rotating it, re-inserting it into the torque-tube spline, the relationship of the spring-plate to the horizontal is changed, and you have the capacity to change ride height, always to be followed up by fine-adjustment of the the spring-plates to achieve the exact desired ride height & cross leveling.

( Front ride height is adjusted differently )

The procedure and values are described in most shop manuals.

[Flieger]

There should be about 18.6% less settling with the larger bars, so look how much droop there is when raising the car and then multiply by .186 and you should have about how much higher the car will sit if you index the spring plates at the same angle as they were before. If you know how much ride height change there will be you can divide by the lever arm (I have seen this reported as 480mm) and get an approximate angle to subtract from the installation spring plate angle.

[1QuickS]

SWB cars see a "wheel rate" higher than the "wheel rate" for LWB cars. The wheel rate is the force to move the wheel through a vertical displacement so for a given torsion bar size the SWB car will have a higher wheel rate than a LWB car which means the same torsion bar in a SWB will provide a stiffer suspension than for a LWB car.

The length from torsion bar center to wheel center is pretty close to 16" for a SWB and approximately 18 1/4" for a LWB so the wheel rate would be 14% higher on a SWB.

This applies to your droop question. By the way; torsion bar stiffnesses may be compared by taking the ratio of their diameters to the fourth power. In other words a 24mm bar would be stiffer than a 23mm bar by (24^4/23^4)*100 = 119% or 18.6% stiffer; material properties and length of torsion bar are constants.

[Flieger]

Sorry, I missed that this was a SWB. The 480mm is for LWB. Still there will be 18.6% less droop. But to get the angle change you divide by 16 inches instead.

[Chris Pomares]

You guys are very good at this. Let me pose the question differently. My car currently is at 23 3/4 inches at the top of the rear fender. I understand this is a rather poor way of measuring. I'm wanting to end up in the 24 3/4 to 25 inches at the rear with the 24mm bars, 23's currently. If I index the bigger bars the same will I have "enough less" droop to be close or will I be way off. The front is fairly strait forward to adjust. I'm currently at 24 1/2 front, 23 3/4 rear. I'm wanting 25 1/4 to 25 1/2 front and 24 3/4 to 25 rear. I'm just barely draging my headers on my driveway. It goes up 2 1/2 feet in only 8 to 10 feet.
Thanks again, Chris

[Flieger]

Jack up the rear of the car and see how much the wheel moves down. If you can put a jackstand next to the car, level with the centercap, at static ride height then you should be able to see how much travel there is. Then add 18.6% of that travel to your ride height. Much more than 5" of travel and you will be higher than you want.

[Chris Pomares]

It took 6 to 6 1/2 imches to get the suspension fullly extended. My rear fender is actually at 23 1/4 inches. My spring plate bushings are metal on metal. I have to address this next week.

[Flieger]

The rubber bushings also contribute to spring rate in a not so easy to calculate way, so I can't really say where your ride height would end up after new bushings and bigger torsion bars (not counting the extra .25-.5" just from addressing the bushing sag).

[ibmiked]

I cannot emphasize this enough. Having reindexed more times than I care to remember, I can categorically say that the single best SWB suspension mod I have ever done is installing adjustable spring plates.

Period.

Don't like your front or rear ride height? In less than 5 minutes you can have raised or lowered by 1" any wheel on your car. Totally DIY, even for the mechanically inept. (not implying that of anyone here)

If you are running 7Rs in the rear, the only option is Sway-A-Way. You cannot use Elephant adjustables. Even SAW spring plates need to be modified.