torsional leaf springs linear? / preloading arms myth!

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gcorrado
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torsional leaf springs linear? / preloading arms myth!

Postby gcorrado » Sat Oct 20, 2007 7:22 pm

are packs of torsional leaf spings linear, as in obey F = -k*x (or rather Torque=-k*theta)?

if they are... then this whole thing about pre-loading your front end by counter rotating your upper and torsion arms is BOOOGUS. you'll take the slop out if there was any, but you won't increase your spring rate AT ALL.
kdf
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Postby kdf » Sun Oct 21, 2007 5:41 pm

I've always thought that the more you preload the springs, the more you preload the bearings and every other place that causes friction. Friction equals stiffness?

I also mentioned this in an earlier thread, only real response I got was Ozzie's invitation for a beer...
http://www.shoptalkforums.com/viewtopic.php?p=777255
Bruce2
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Re: torsional leaf springs linear? / preloading arms myth!

Postby Bruce2 » Sun Oct 21, 2007 8:09 pm

gcorrado wrote:are packs of torsional leaf spings linear, as in obey F = -k*x (or rather Torque=-k*theta)?

No they are not. A torsion bar is progressive. Meaning that the next degree of rotation requires much more force than the previous degree of rotation.
k is exponential.
Duus
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Re: torsional leaf springs linear? / preloading arms myth!

Postby Duus » Mon Oct 22, 2007 4:54 pm

Bruce2 wrote:
gcorrado wrote:are packs of torsional leaf spings linear, as in obey F = -k*x (or rather Torque=-k*theta)?

No they are not. A torsion bar is progressive. Meaning that the next degree of rotation requires much more force than the previous degree of rotation.
k is exponential.


No. Solid torsion bars like the ones in our rear suspension are linear:

T = 0.5*theta*G*Pi*r^4/l

T - torque
Theta - angle(rad) of deflection
G - Shear modulus (80GPa for steel)
r - radius of torsion bar
l - lenght of torsion bar

I think kdf is on the right track about the friction thing... even though the friction is dampening it might give the feel of a stiffer suspension.
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gcorrado
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Re: torsional leaf springs linear? / preloading arms myth!

Postby gcorrado » Mon Oct 22, 2007 5:53 pm

Duus wrote:
Bruce2 wrote:
gcorrado wrote:are packs of torsional leaf spings linear, as in obey F = -k*x (or rather Torque=-k*theta)?

No they are not. A torsion bar is progressive. Meaning that the next degree of rotation requires much more force than the previous degree of rotation. k is exponential.

No. Solid torsion bars like the ones in our rear suspension are linear:
T = 0.5*theta*G*Pi*r^4/l


Solid torsion bars are DEFINITELY linear, as Duus suggests. Sway-A-Way goes so far as to quote the spring rates for their rear torsion bars. http://www.swayaway.com/TechRoom_VWguides.php So, Bruce2, you're definitely off that "a torsion bar is progressive", but it still might be the case that packs of torsion leaves (which are not single solid bars obviously) are in fact progressive.

I know for fact that leaf strings used in carriage suspension ARE progressive - but in that case it comes from having many leaves of different lengths. The torsional arrangement might be totally different.

So if torsion leaf packs are like torsion bars, they're linear... if they're like like leaf spring stacks, they might be progressive

Duus wrote:
kdf wrote:I've always thought that the more you preload the springs, the more you preload the bearings and every other place that causes friction. Friction equals stiffness?
I think kdf is on the right track about the friction thing... even though the friction is dampening it might give the feel of a stiffer suspension.


(1) I don't believe it.
(2) If it *is* true, it's a lame solution.

Re 1: The needle bearings in the torsion housing and the ball joints in the arms are *designed* to produce minimal friction under the load of the whole stinking car. The few dozen extra pound of force you could add by preloading would have a tiny effect on friction. If your bearing are SHOT, maybe, but then it's just going to be a creaky sticky mess.

Re 2: If you want more damping, put on stiffer shocks. Shocks are *designed* as dampers, do the job really well, and are available in a wide variety of stiffnesses. If you could overload the bearings so much that you were seeing significantly more friction at the bearings, you'd just be wearing out your bearings and your springs - asking them to do a job they were never intended to do.
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Ozzie
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Postby Ozzie » Mon Oct 22, 2007 5:56 pm

Beer invite still stands. :D
I don't know what the phisics book says about the stack of leaf springs. What I do know is that the more you pre-load one against the other, the stiffer the front end gets. Plenty enough to feel the difference.
Your results may vary (but I doubt by very much).
If you are studying it worms eye, don't forget that not only are you twisting the spring, you are wrapping them around each other helix style. The more you twist, the more it wants to shrink in both diameter and length. It can't shrink, it's solid.
"When the people find that they can vote themselves money, that will herald the end of the republic."
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gcorrado
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Postby gcorrado » Mon Oct 22, 2007 7:34 pm

ozzie wrote:What I do know is that the more you pre-load one against the other, the stiffer the front end gets. Plenty enough to feel the difference.


hmmm... well maybe the leaf sets really are progressive...

ozzie wrote:not only are you twisting the spring, you are wrapping them around each other helix style. The more you twist, the more it wants to shrink in both diameter and length. It can't shrink, it's solid.


a plausible explaination for how they achieve a progressive spring rate.
kdf
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Postby kdf » Mon Nov 19, 2007 8:27 am

ozzie wrote:Beer invite still stands. :D

If you are studying it worms eye, don't forget that not only are you twisting the spring, you are wrapping them around each other helix style. The more you twist, the more it wants to shrink in both diameter and length. It can't shrink, it's solid.


This same happens in a regular torsion bar too. Tiny steel molecules being wrapped around each other in helix style.

So the beer invitation made everyone silent again...

I'm still betting on friction.
Testing this is quite high on my big list of what to do when I really get the spare time and inspiration to do it, which is probably never.

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