The author stated a torsion bar is a linear spring. This means a certain amount of force, say 100 lbs, will move the suspension 1 inch. The second 100lbs will move it again, 1 inch and so on. He then says after the torsion bar is preloaded:

"VUALA! the spring will now take much more force to get it to intially move than an un-preloaded spring of the SAME rate."

Something is not right here. If the spring IS linear, the preload force will flex it some, but initial movement of another inch will take - that's right - 100lbs, not "much more force". So either he's wrong in that assumption, or the spring is not linear. The answer my friends is that it is not linear, but actually progressive - meaning the spring takes more force to move it each additional inch than it took to move the first inch.

So, the spring is not linear. On cars, the use of progressive springs is uniform to prevent bottoming out and the damage it can do. A fairly trivial point just to warm me up, compared to the next one.

Preload. There is no such thing as preload in the context of an automotive spring. You can preload your Glock 10mm, your Mossberg 12 guage, and your retirement account, but you cannot preload your torsion bars.

The torsion bar on a stock truck sitting on the ground has force acting on it. That force is the weight of the front end of the truck. Let's say its 2400lbs split equally on each tire at 1200. Sitting still, that 1200lbs has twisted the torsion bar a certain amount. We'll use 30 degrees of twist for our example. O twist if the car is on a lift and the tires are hanging down. As the car settles, the springs are twisted 30 degrees before they are generating enough force to offset the 1200lbs and stop the car from sinking on the suspension.

Now, when you twist a torsion bar to jack up the car, you are using a rotating mount on the back end of the torsion bar where it is firmly held against turning in normal driving by a chassis mount. The suspension end twists up and down as the car encounters bumps, but this end simply sits still. The rotating mount will move with tools however, so let's do that.

We'll rotate the torsion bar so the front end lifts one inch (this was referred to as preloading by the hapless author). Now how much twist in the torsion bar is there? Still 30 degrees and 1200lbs of force. Well, we started with 30 degrees measured from the back end where it's fixed to the front end where the suspension and car's weight twisted it those 30 degrees. Now we rotated the torsion bar's fixed end, and Viola! we also rotated the suspension end by the same amount - that's how the suspension arm moved to lift the vehicle, and there's still 1200lbs acting on each spring. So there was no more twist put into the torsion bar! None. Zippo. Why is this true? Because the force that torsion bar is holding up is still the same 1200lbs, that's why. It's simply holding that same load an inch higher.

Look at an analogy. A guy catches a 5 lb fish on his fishing pole and is admiring it hanging out there from the pole's tip. Of course the pole has a deep bend in it. Now he decides to lift it a couple feet higher for a better view. The pole still has the same bend in it because it still has the same strength, and the same weight acting on the end. If his pole hit a tree branch on the way up and he kept lifting agains it, it would put a deeper bend in the pole just like the suspension stopper would in our torsion bar example.

The initial spring rate of the torsion bar acting on a bump would be identical to the stock height (minor geometry differences in the higher suspension arems excluded for simplicity). No ride difference.

The only way you could create a preload (additional load) on the torsion bar would be to continue cranking until the suspension has topped out against the suspension limiter. NOW when you turn the torsion bar more, you are applying more force because the suspension end can no longer move and you are pushing harder and harder against the suspension limiter. Just like the fishing pole analogy. Maybe you keep cranking until you have 40 degrees of twist on the torsion bar. And, of course a bump would have to overcome the normal torsion bar spring rate, plus all that extra force holding the suspension against the stopper before it could move the suspension. That would be a much stiffer ride, my friends.

So, to recap.

#1 Torsion bars are not linear, they can be manufactured in many ways.

#2 Lifting an IFS torsion bar truck with the bars does not make the ride appreciably stiffer.

#3 There is no way to preload a torsion bar suspension if the goal of the suspension is to sit between the bump stops and be able to go up and down freely to react to bumps.

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DougM

1999 Monty, rear locker, muddies, Hellas, rear
suspension air bags,
3-way shocks, Hella twin
beam backup light, 2
broken crap original
mudflaps.

I checked my post, and I think I got a little windy there.

Here's the basic summary regarding adjusting the torsion bars to lift the front end (variously referred to as 'twisting', 'preloading' ,'tightening' , 'torqueing', etc on this list): it does NOT change the spring rate. It does NOT preload the springs. It does NOT subject the springs to any added stress or breakage. The spring does NOT change the way the truck rides on the road, other than you've raised the vehicle and it might be a little tippier. In essence, and in fact, you are doing the EXACT same thing as a guy who puts a spacer under his coil sprung IFS to achieve a little free lift. The coil spring doesn't get any stiffer or change anything about its action in suspending the car.

I have spent hundreds of hours on and off the track evaluating vehicles as part of my career, and even got to work on the Active Suspension Corvette program in the 80's. My bookshelves are packed with engineering references and last week I delightedly received a text book as yet unread titled "Fundamentals of Vehicle Dynamics" by Gillespie for my birthday to show you what a wacko I am for vehicles.

While the author that Joe found above makes a valiant and well intentioned effort at understanding some things, his conclusions are wrong. The minor changes in ride are NOT due to the springs themselves.

Since you are considering a lift, it may be of use to you to know that it's quite simple to achieve a little front end lift on a torsion bar vehicle, and the downside is negligible. The downside includes but is not limited to tire alignment changes, handling changes from a raised vehicle, and what to do about the bump stops since you will now bump against the lower one more often.

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DougM

1999 Monty, rear locker, muddies, Hellas, rear
suspension air bags,
3-way shocks, Hella twin
beam backup light, 2
broken crap original
mudflaps.

Thanks for clarifying that issue for me.Oh and Happy belated Birthday. One more question then about lifts...

If I make a lower drop bracket will it ease the smacking of the lower bumpstop or am I better off getting a low profile bumpstop for the lower control arm.

I absolutely must ask what is your second passion? Feel free to pass on this if it is not suitable for a family forum..

On the drop bracket vs low profile bumpstop, clarify what you mean by the former. I'll also say that several guys on this and other forums came up with what I think is an excellent bump stop mod. Instead of cutting them down, they've drilled them with holes like Swiss cheese, so they compress easily. I'd think some combination of a mild cut and a few holes would be good, but let me know more about your 'drop bracket'.

That four part book sounds like great reading, and reminded me of a possibly similar book I lent to someone and missed so much I bought another. Called "The Design of Every Day Things", and it details all the dumb things designers do (lobby doors with those big plates that are obviously to push the door open, that turn out to be something you pull on, etc) and how to design in line with the user's internal intuition in mind.

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DougM

1999 Monty, rear locker, muddies, Hellas, rear
suspension air bags,
3-way shocks, Hella twin
beam backup light, 2
broken crap original
mudflaps.