little_joe
Former Admin
Reged: 01/01/00
Posts: 10859
Loc: Holly Springs, North Cackalack...
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Suspension: Torsion bars demystified - Very long!
09/12/00 07:23 PM
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As a follow-up I promised to DougM, here is a scientific explanation of torsion bars and spring rates.<P>I saved this from the T4x4Pickup Board, but I cannot recall who authored this. I really wish I could give the guy credit, this is great information.<P><B>Once I figure out how to convert his Excel spreadsheet to properly post, I'll get it online. The spreadsheet is an excellent supplemental, graphical representation of this and is required to help understand the explanation. I'll try and get that posted asap.</B><P>This is entirely unedited.<P>Enjoy!<BR>******<P>Ok folks, check out the attached file (in m.s. excel) and use it along with the following explanation. After reading much on the subject of torsion bars and the torsion bar adjustment affecting ride quality etc., I decided to look into the subject and come up with some real, scientific, answers! This is what happens when a guy with a 4x4, a master's degree, and a whole lot of free-time over the Thanksgiving holiday gets you! If you don't like, take it up with Sir Isac Newton! Ive read about "preload" and how it is'nt the cause the harsh ride quality achieved by adjusting up the torsion bars, but is caused by moment arm changes and "A" arm angles, well I beg to differ!<P>1) A torsion bar is a linear spring, and as such, has a constant spring rate. Therefore, its spring curve is a straight line A spring's rate is determined as force/displacement which is equal to the slope of the spring's curve (a straight line for a torsion bar).<P>2)On the figure, you will see the spring curves for a stock bar, a H.D. (25mm SwayAway if you like) and a preloaded stock bar. Note they are all straight lines because they have linear spring rates. Note that they are all different, a thicker torsion bar has a higher spring rate than the stock bar (larger slope), a preloaded stock bar has the same rate as the stock bar (because its slope is EXACTLY the same), but is shifted to the right, I will explain this later!<P>3) The X-axis is movement of the spring (displacement) or you can think of this as susp. movement of your truck. Note there are both ( ) movements for "up" travel of the susp. and (-) movements or "down" travel of the susp. The Y-axis is the force required to move the spring (or truck susp.) a particular distance.<P>Note that the truck's own weight exhibits a force on the spring and moves it a distance. Follow the horizontal line across the graph from the Y-axis with your finger, where it intercepts the spring curves, a red dot has been placed. This is how far the truck will sag under its own weight with a given torsion bar. Note that the H.D. torsion bar does not allow as much "sag" therefore the truck will sit higher, the "lift" achieved by the H.D. torsion bars! Now, you can say that a bump in the trail is equal to a point on the Y-axis above the trucks weigth, trace your finger in a straight line across from any point on the Y-axis and you will see that the stock spring will move (truck susp. will move) further for any bump than the 25mm bar. In order for the 25mm bar to move the same amount, you would need a bigger bump or a point higher on the Y-axis (more force = THE HEAVIER BAR WILL FEEL HARSHER!!!)<P>4) Now, before we say the "P" word (preload!), lets talk about spring limits and truck limits for spring movement. Your torsion bars, like any spring, can only move so far. This is the physical limit of the spring. Your toyota allows the spring to only move so far by the use of bump stops, your bumpstops are placed such that the spring will never reach its physical limit. On the graph, the physical limit of the spring is represented by the arrow on the end of each spring's curve. The truck limits (bumpstops) are shown by the vertical line on the left (lower bumpstops) and the vertical line labled "0" on the right (upper bumpstops).<P>5) Now, the infamous "P" word....PRELOAD! Note that the word preload is really two words, pre meaning "before" and load meaning "to load the spring". When you are in your stock configuration, jacked up so the wheels are hanging and the "A" arms are topped out against the upper bump stops, your bars are unloaded (this is easier to use because it is simpler, we know their is a certain amount of preload on them from the factory). When you sit the truck down, the torsion bars twist and are "loaded". When you jack up your truck and turn the adjustment bolts, your are PRELOADING the bars, you are loading them before you load them, to use the definitions. On the graph, this is represented by the preloaded bar's "0" being in the (-). Now follow the "0" line down until it intercepts the preloaded bar's spring curve..and VUALA! the spring will now take much more force to get it to intially move than an un-preloaded spring of the SAME rate. It is indeed..PRELOADED! Now this is where it gets good! Follow your trucks weight across again and the red dot where it intercepts the preloaded bar's spring curve is where the truck will sit in its travel. Note that is sitting higher than the unpreloaded bar....WAM...LIFT! Also note that it sits exactly as high as the truck with the 25mm bar. But oh my god! If you look at the spring's phyical limit of movement (the arrow on the end of the preloaded bar's sping curve, it lies BEFORE the trucks limit!!!!!!!!! The preloaded bar will not let your suspension move through its full travel. Even though the 25mm bar will take a larger bump force to reach the bump stop, it still CAN reach the bump stop, the preloaded bar can not. Also note that the preloaded bar still has a lower spring rate than the 25mm bar.<P>WOW! that took a lot out of me, i hope you have gained some insight into all of this, I will however agree that because the "A" arms are at a greater angle at rest when heavier bars or preloaded stockers are used, that the truck doesnt have as much leverage as before (when the "A" arms are flatter at rest) but the harshness of preloaded or larger bars can be explained best by good ol' Newtonian physics! I welcome all comments, I had a good time working this out! I guess the answer is that if you put a small amount of preload on your stock bars, you will note some intiall stiffness, but you should be fine, cranking em' up to gain a few inches of travel will most likely give a harsh ride and limit your travel.<P>------------------<BR>Joe Micciche<BR> Joe@4x4Wire.com <BR>94 Toyota SR5 V6 Xcab, 33" MT's, locked and dented.
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4x4Wire's Toyota Section
1995 FZJ80 LandCruiser - 3 locking diffs, +200k mi, JD HIR's, 285/75R16 Nitto TG's
Olde North State Cruisers
Edited by DougH (08/12/04 11:31 AM)
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