Why do springs sag with SOA?

[FAM]

I drive a TJ so I don't even have leaf springs. But I was wondering why people say that springs sag/wear out faster with SOA than they do with SUA. Can anyone explain it to me?

Thanks,
Fred

[crawlinTJ]

I would assume because the loading is different. In a SUA, the points of loading are negative (down) at the shackles and positive at the axle, somewhat counteracting each other in a static condition.

For a SOA, the loading points are all negative, 2 at the shackles and 1 at the axle. So in a static condition alone, the spring(s) are being pushed flat. Combine that with the dynamic loads of driving and flexing, and they would go flat quicker.

I could be wrong, it's happened once before. =)

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Dan
chopped & locked TJ

[Jeeper]

The pressure points on the spring pack are completely different. Picture it this way, with SUA and your wheel travelling upward...it pulls the pack up. With SOA and a wheel travelling upward it pushes the spring pack up and has more travel distance.

Jeeper

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99WJ: loaded
90YJ:SB 400, D60/D44, 4.88's, full Detroits, TH350, Atlas II 4.3:1, 38.5x14.50 SX's, 2.5" Alcan Spring SOA, Revolver shackles and Much Much More!
1950 CJ3-A small project
1975 J-10 Parts vehicle

"The harder you play, the more you pay!

[Jeeps2Live]

"Spring Wrap" that's what kills springs.

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Blake
Slightly Stock Cj-7

AZ EXTREME 4-WHEELERS, Arizona Virtual Jeep Club

[larsbc4x4]

<font face="Verdana, Arial" size="2">Originally posted by Jeeper:
The pressure points on the spring pack are completely different. Picture it this way, with SUA and your wheel travelling upward...it pulls the pack up. With SOA and a wheel travelling upward it pushes the spring pack up and has more travel distance.
</font>

Whether the spring is pulled or pushed up makes no difference. The result is the same. However, with the axle below the springs and, assuming you have enough fender clearance, the springs will be forced into more negative arch because there is now more room between the bump stops and axle/spring. That extra negative arch will fatigue the springs.

Also, because the axle tube is now further from the main leaf, it has longer torque arm between the main leaf and the center of the axle tube. This means that axle wrap is more pronounced.

These two things, in conjunction, cause the springs to sag sooner.

Oh, the last thing to consider is this: stock springs are usually softer than lift springs which is another reason why they sag sooner. However, I have had excellent results with stock springs + an extra main leaf in each pack (3 years on the same packs so far - they've sagged but not by a huge amount).


...lars

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www.bc4x4.com (http://www.bc4x4.com) - Canada's Biggest 4x4 Site

[Jeeper]

Lars, that was kind of my point but you articulated it much better http://forums.jeepsunlimited.com/forums/images/ubb/biggrin.gif

Jeeper

[Blatant]

Did someone say "articulated"? Nice double entendre.
Dion

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Flat-fendered 94 Wrangler Sahara, SOA, 1-inch body, 36x12.5 TSL SXs, Ford 8.8 rear, Lockright/Trakloc, full cage, free Arizona pinstriping.
Member: AZ Extreme 4 Wheelers, Arizona Virtual Jeep Club
Bros. 8.8

[Jeeper]

<font face="Verdana, Arial" size="2">Originally posted by Blatant:
Did someone say "articulated"? Nice double entendre.
Dion

</font>

I hate people that write for a living http://forums.jeepsunlimited.com/forums/images/ubb/biggrin.gif

Jeeper

[TEX]

<font face="Verdana, Arial" size="2">Originally posted by Blatant:
Did someone say "articulated"? Nice double entendre.
Dion
</font>

Good "Bushism" http://forums.jeepsunlimited.com/forums/images/ubb/biggrin.gif

Basically, it's been explained well above with the springs getting more of a workout due to the increased available travel. Also, because the center of gravity is that much higher, you get more spring compression on corners (on the highway) & then there's the axle wrap also mentioned above.....

TEX

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[azyj]

I believe the real underlying cause is leverage. With the axle under the springs, it is further away from the points where the springs connect to the frame. This allows the axle to place more leverage on the springs than can an axle that's over the springs. This extra leverage allows the springs to be flexed more which gets you more travel but also causes the springs to fatigue faster.

I also suspect that SOA spring perches are typically shorter than SUA u-bolt plates so there is less support for the springs. This also allows them to flex more and fatigue faster.

These are just my theories.

-Roger

[larsbc4x4]

<font face="Verdana, Arial" size="2">Originally posted by azyj:
I also suspect that SOA spring perches are typically shorter than SUA u-bolt plates so there is less support for the springs. This also allows them to flex more and fatigue faster.</font>

Actually, the original RE "anti-wrap" perches were about 8" long which is far longer than any factory perch. So much for your theory. ;-)


...lars



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www.bc4x4.com (http://www.bc4x4.com) - Canada's Biggest 4x4 Site

[azyj]

<font face="Verdana, Arial" size="2">Originally posted by larsbc4x4:
Actually, the original RE "anti-wrap" perches were about 8" long which is far longer than any factory perch. So much for your theory. ;-)

...lars
</font>

I wasn't comparing SOA perches to SUA perches. I was comparing SOA perches to u-bolt plates. In other words, I was comparing the parts that go under the spring and apply the upward force during compression (I assume compression causes springs to fatigue more than droop. I could be wrong though.) Most u-bolt plates I've seen are longer that most spring perches I've seen. The RE perches may be an exception but I doubt they represent the majority of the set-ups with sagging SOA springs.

-Roger

[larsbc4x4]

<font face="Verdana, Arial" size="2">Originally posted by azyj:
I wasn't comparing SOA perches to SUA perches. I was comparing SOA perches to u-bolt plates.</font>

Oh, I see what you're getting at. In any case, the RE kit's long perches (longer than the u-bolt plates) still resulted in rapid spring sag so I don't think the length of the spring pressure area is a dominant factor in the springs sagging.


...lars

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