Spring Types

[TimHayosh]

I have a question about the front springs found in VW-style beams. How do these leaf spring packs compare to a "regular" torsion bar in terms of allowable twist (travel), constant (predictable) spring rate, and durability. Also, what are some ball park figures for the spring rates of stock leaf-type suspensions. Lastly, in terms of dampening, which works best with a shock?

Any opinions/idea's are appreciated.

 

[mgobaja]

I'm not sure I understand your question completely. Torsion bar type spings and Leaf springs both work in very similar but yet very different ways.They both provide the ride height needed and some mild dampening but are intended to be supplemented via Shocks.When you use a coilover 99% of the time you get rid of the leaf or torsion bar springs.The front leafs do wear out quicker than a Torsion bar does due to the fact that their is less material tacking an equal load.I also think that a torsion bar remains more predictable and is steady on the spring rate as its life goes by. But when a torsion bar breaks it's a done deal as where with a leaf pack if one breaks you can still keep on racing at a slower pace. Hope this helped.

 

[Bob_Sheaves]

Hi Tim!

As was stated previously, torsion leafs and bars are similar, yet different. In addition to the aforementioned information, you should also be aware that a leaf pack has inter-leaf friction that affects both rate and durability. Torsion bar suspension travel is generally limited to 45 degrees of rotation, where leaf packs are capable of more (somewhere around 75-90 degrees) rotation travel for a given length. Rate predictability (ultimate- not initial) is far better with the bar (either soild or tubular) due to no inter-leaf friction and wear.

Leaf packs, within the space claim available, are far easier to "tune" to a given suspension by adding or subtracting leaf material. The only method I am aware of to tune a round bar is to make it a telescoping tube and bar arrangement with internal splines to "slide" the inner solid bar into the tube, effectifely stiffening the spring rate. This taks space however, and in a VW style front suspension, is, in my opinion, not very practical.

From a production standpoint, I have seen vehicles with as little as 75#/degree twist going up to 7900#/degree twist (M1A1 tank) so I am not sure what you are actuall asking for with that question.

From a damping standpoint (theory only) the single bar will be more easily tuned to the shocks, becatuse of a lack of internal friction.

I hope I have helped....

Best regards,

Bob Sheaves

 

[Bob_Sheaves]

Re: Spring Types- ADDENDUM

Tim,

One other thing you may want to consider is a variable rate torsion bar. There are several companies that grind tapered shaft bars and tubes for this purpose. The catch is- COST. They ain't cheep.

But for a nice progressive ride- they cannot be beat.

Best as always,

Bob Sheaves

 

[FABRICATOR]

TIM,
Stacked leaf torsion springs offer more degrees of twist for a given torque rating. This is because it is a multiple of smaller "bars" added together. Like all springs, if they are not abused or run to their limit all the time (the way we like to do) they are very reliable and consistent. Because the outer leaves get twisted slightly farther and serve to mount the assembly, they are usually the first to go. The trailing arm actually moves quite a bit inwardly and outwardly if you are using the VW style pinch bolts. A lot of builders don't know or allow for it. This is one, but not only, reason why the ends (from the outer pinch bolt dimple) break off. It is helpful to just cage them and let them all float. But you will have to provide another way to hold the trailing arm in. As far as rates, they are not practical for high very high rates. I have looked into this and found that this type of spring gets very bulky as the rate increases very much. A small increase of torsion bar diameter is a big increase in torque rating. This is not true for the stacked leaf torsion spring. This type of spring must be kept VERY long. This type of spring does have friction but much less than a flat leaf spring. It is not a significant factor in shock absorption. Sway-A-Way used to list spring rates in their catalogs.

Tapered torsion bars? Unless these are very long and not twisted much, I don't think so. Perhaps a stepped torsion bar with built in stops and bearing supports if you want to experiment.

 

[Bob_Sheaves]

Hi Fabricator!

Tapered bars are nothing new-they have been used for over 50 years, but untill the MY1987 release (with the design on the AMC Premier) they were not practical for the reasons you state. In 1985 (I believe that is when the patent was issued), AMC designed the "folded" torsion bar, which consisted of a tapered wall tube, and a tapered thickness solid bar that was welded together in the "middle" of the spring effective length. This effectively reduced the overall packaging length to somewhat less than 5/8 the total effective length. Both the anchor and adjustor was mounted concentrically and on the same side of the vehicle. The rate varied from (as I remember- I could be wrong) 147lb/degree to 290lb/degree or roughly 2:1-about the maximum difference within the steel modulus. All and all quite a neat installation. as these transverse bars had an effective length of over 6 feet long, yet he packaging space claim length was only 3 feet 7.5 inches, as I remember.

This design, while good, was an expensive one-the bars cost roughly 3 times what equivelent 1 piece bars did, but it was the only way to accomplish what was in the design parameters. If you can locate one of these cars for a "look-see" note the amount of wheel travel-over 8 inches, jounce to rebound, compared to a Chrysler C body (comparable vehicle) of 4.9 or a Chevy Caprice of 6.75 inches.

The ride was such that the vehicle was very "poised" and it took a LOT of vertical deflection to induce "pitch".

Stepped bars are another possibility, but remember that the "step" will cause a load concentration at that point and form a twist load failure mode.

Best regards,

Bob Sheaves

 

[FABRICATOR]

Hello Bob,
I'll keep an eye out for an AMC so equipped. I'll definitely go to the AMC section next time I'm at the junkyard. AMC was into experimenting, unfortunately they experimented with body design too...

The stepped bar I was referring to would have radiused ends to each torsional section, just like all bar ends. These would not only be at the ends but at a few points throughout the bar's length. The larger diameter section between each torsional section would contain a single or pair of short bearing surfaces (to hold the bar in alignment) and some protruding stop tabs. There would be no stress risers along the entire length of the bar. The smaller sections would be protected from over torqueing by the corresponding stops. This has been done with 2 or so seperate bars placed end to end. But to make a system which is more position sensitive, it needs to be, perhaps, at least 4 sections. This bar would be of significant expense and somewhat longer overall. It just seems more capable than a tapered bar.

It's good to have you back here, providing the more scientific explanations.
Take care

 

[Dylan]

The dual rate torsion bar set up with stoppers is a neat set up. We used to make them for Toyota’s. It had a splined section near the middle of the bar that carried the stopper arm. From here there was a tube with a corresponding stop that went over the bar and splined onto the same spline as the adjuster. In the first stage both the front and rear torsional sections would twist giving you a soft initial rate (series spring). When the stopper came in contact it would stop the rear half of the bar from twisting (assuming that the tube is sufficiently stiff to act ridged) leaving you with the sort stiff first half of the bar. This is the same in theory as a dual rate coil-over. This set up fit in the stock location and the point that second rate occurred was adjustable. We don’t make them any more because they were to expensive. I believe Downey off road used to carry them, if you look in an old catalog you could see them.

Fabricator’s explanation of leaf packs as multiple small bars is right on. The damping that you get out of the inter leaf friction is called Coulomb damping and is usually not something you want. Coulomb (friction) damping was used on cars in the 10’s and 20’s.

Bob: A tapered bar is not variable rate, it is variable stress however which is usually something to avoid. The advantage of a tapered bar is to minimize the required length (you made me get off my butt and get my SAE HS-796 reference manual for this one) as the ratio of Dmax/Dmin increases L decreases for a given rate bar, limited by the stress on Dmin. I’m guessing that on that AMC thing they stopped the outer bar at some point in the travel to get the rate increase. P.S. A tapered bar actually would change rate but only when Dmin began to yield and this would be a falling not a raising rate one time only bar.

 

[FABRICATOR]

Dylan,
I'm with you on this one! Bob may be able to mow us down, but a tapered torsion bar seems like a bad thing no matter how you look at it! The only way it MIGHT work is if it had something to progressively protect it from self destructing. A tapered coil spring on the other hand might not be a bad idea...

 

[Bob_Sheaves]

HI guys (good to hear from you again Dylan!)

I forgot to add that there is a second stop mechanism on the welded end of the folded bar. Sorry!!

BTW- The design is used on the 87-88AMC Premier, and 1988-1992 Eagle (Chrysler) Premier/Dodge Monaco.

Best as always,

Bob Sheaves

 

[TimHayosh]

Gentlemen, sorry to have posted a question then split! Unfortunately, I can't afford any of your collective talent to prep my car (I'm racing this Saturday night in Mexico) so, I've been quite busy.

Thank you for your opinion/facts in regard to torsion leaves vs. torsion bar. I have learned much from this thread. In fact, this thread will probably save me $$$$$'s in machining and development!

Thanks again for being so willing to share your hard won knowledge.

Cheers, Tim