The Fox Shock Perch Adjustment Mod

[MarkT]

frog... I'm not really sure what you are trying to debate here....

You kinda summed it up when you said:

if you have progresive springs and you add weight they get stiffer...

So unless you are adding weight to the Raptor at the same time you are moving the perch, the progressive spring will have exactly the same amount of compression as it did before you moved the perch.... therefore, even if you have a progressive spring the "stiffness" or rate will not change from the original perch position until you get close to the suspension bottoming out point.

A few words about progressive and stacked springs... (warning: most people should stop reading here to prevent boredom! lol)

Let's start with a basic coil spring... a coil spring is simply a steel bar... well... "coiled".

What makes one round steel bar (of the same heat treat, material, etc) more resistant to bending than another?

Diameter: A thicker bar will be harder to bend than a thinner bar.

Length: A longer bar is easier to bend than a shorter bar.

That's it. The "secrets" of a coil spring revealed.

A true "progressive" spring works just as your link described... but the way I like to look at it is that as the progressively closer spaced coils begin to touch, it's exactly like "choking up" on a straight bar... you have effectively shortened the bar which has made the spring stiffer. On a progressive spring this happens gradually as the coils "progressively" touch as the spring is compressed. True progressive springs are rare and very expensive.

Stacked springs (usually with "crossover spacers" as coil bind is VERY bad for a spring) are far more common.

I'm having trouble following your example unless you are talking about two springs stacked on top of each other... (if the 2000 lb was enough weight to coil bind one half of the spring on the first measurement, your analysis is flawed... "average" spring rate is not very important for suspensions. But the effective spring rate at each point throughout the travel is very important)

Remember how I said the longer the steel bar, the easier it is to bend? So what happens to the rate when you stack two identical 100 lb/in springs on top of each other? Twice the wire length = twice as easy to bend = half the spring strength. The rate of the two stacked 100 lb/in springs becomes 50 lb/in.

Using your example, a 500 lb/in spring stacked on a 2000 lb/in spring has a combined rate of (only) 400 lb/in.

(stacking springs ALWAYS results in a new spring rate of something less than the rate of the softest spring)

Add your 2000 lb weight and the assembly has compressed a total of 5", not 4" as you stated.

Eventually the 500 lb/in spring reaches the point of coil bind. Then the rate of the stacked springs instantly jumps from 400 lb/in to 2000 lb/in!!!! That's a sudden five-fold increase in spring rate when the suspension hits that point of travel! Hardly "progressive"...

Maybe your "average" spring rate throughout the travel would be something like you calculated... but I can guarantee from experience that you would notice the sudden jump in spring rate when the first spring hit coil bind...

.

 

[frogslinger]

not arguing any of your data...

to start I was only pointing out that some springs do get stiffer when loaded, without breaking any laws of physics...

as to the two springs being softer... that is also true... it does not show up in my example... directly, but can be surmised from my 2k example...

to work out total compression of two springs stacked you assume that they are both mass-less... take the total deflection of the first spring and add the total deflection of the second spring....

if you use my example (which for simplicity can be seen as a pair of coil springs welded end to end, or as a spring with a greatly reduced diameter at one end ) and add 1k lbs you will have a total deflection of 2.5 inches (2 in the soft section .5 in the stiff section) giving a total off 1 inch per 400 lb (as you calculated)

HOWEVER each half of the spring has a total ability to only compress 3 inches (remember the spring was a foot long could compress only to 6 inches)... so when you double the load the compression goes to 3 inches for the top (all it can do coils are touching each other) and the bottom half will compress 1 inch. So total compression WILL be 4 (3+1) not 5 (your calculated 4... which is defying the laws of physics... the coils are fusing into each other +1). As I stated...

Now this spring was not designed to model a true progressive spring... it was simply to show that it is quite easy to build a spring that increases its stiffness with load.

To paraphrase what you said, a progressive spring would be more like stacking a 200 on top of a 250 on top of a 300 on top of a 400 etc all the way up to 2000... I did not feel like doing the math on all that when this simpler example showed my point.

Now... I do not think that the raptor has a progressive rate or dual rate spring... which is why I said "not relevant"...

As to the rear suspension... I know WAY less about leaf springs than I do about coil springs... I do recall though that several vans (especially overseas) use a progressive rate leaf spring... each leaf is a different stiffness and as load increases each additional leaf is engaged... stiffening the overall spring pack... that is a case of a 4 or 5 stage spring.

As you have probably noticed I am fairly contrary... I generally tend to make flip comments that appear a little off but are accurate if you study the very specific frame of reference to which I am referring... productive? meh... educational... not really... I generally know exactly what I am talking about, and am just making a point... fun?... hell yeah...

Nothing but love... appreciate being able to talk to someone with a technical mind and the ability to point out that my examples may be cases of reductio ad absurdum.

 

[MarkT]

At the risk of :deadhorse:

not arguing any of your data...

to start I was only pointing out that some springs do get stiffer when loaded, without breaking any laws of physics...

Actually, I think what I said was something to the effect of "changing the preload (lower perch position) on the spring could not make the spring stiffer without breaking the laws of physics". And I still stand by that statement whether or not the spring is "straight rate" or "progressive rate".

I never said that there was no such thing as a progressive spring that changes rate as the load increases. The point is the load on the spring does not increase when the perch is moved one position.

SNIP

HOWEVER each half of the spring has a total ability to only compress 3 inches (remember the spring was a foot long could compress only to 6 inches)... so when you double the load the compression goes to 3 inches for the top (all it can do coils are touching each other) and the bottom half will compress 1 inch. So total compression WILL be 4 (3+1) not 5 (your calculated 4... which is defying the laws of physics... the coils are fusing into each other +1). As I stated...

I noticed this... but I just couldn't imagine why you would try to make a point about progressive springs by completely collapsing the 500 lb spring with the first 2000 lb load??? Do you know what the half of the spring that collapsed completely in your example is called? Let's see.... it's a special technical term... hmmm... what do they call it?

A spacer! That's it. You made half of the spring into a spacer when you added the initial 2000 llb load.

Now this spring was not designed to model a true progressive spring... it was simply to show that it is quite easy to build a spring that increases its stiffness with load.

While I never said that a spring cannot change rate throughout the travel, your example is flawed and didn't prove your point at all. Let's look at your results:

add another 2k lbs and the spring will only compress 1 more inch
add another 2k lbs and the spring will only compress 1 more inch
add another 2k lbs and nothing happens your spring is bottomed out.

the effective overall stiffness when measured with 2k lbs was 1 inch per 500 lbs
the effective overall stiffness when measured with 4k lbs was 1 inch per 800 lbs
the effective overall stiffness when measured with 6k lbs was 1 inch per 1000 lbs
the effective overall stiffness when measured with 8k lbs was 1 inch per 1333 lbs

The first set of numbers shows what is important. After you turned half the spring into a spacer, what you had left was a spring that compressed 1 inch for each 2000 lb of load. That is the definition of a straight rate 2000 lb/in spring.

The "effective overall stiffness" you present in your second set of numbers are useless when it comes to suspension design. Springs are not rated this way and the data you presented is pretty much meaningless in terms of real-world usefulness. (or proving the existence of a spring that can change rate)

Use this example. Put a 6" long 2000 lb/in spring on coilover shock with one foot of shaft travel. This spring has 3" of travel before coil bind and spring is positioned so that the shock is completely collapsed at the point of coil bind. Now add your 2000 lb weight. The shaft collapses nine inches (freely) and then the spring compresses 1". So that would mean (by your definition) the "effective overall stiffness" of this coilover shock is:

the effective overall stiffness when measured with 2k lbs was 1 inch per 200 lbs
the effective overall stiffness when measured with 4k lbs was 1 inch per 363 lbs
the effective overall stiffness when measured with 6k lbs was 1 inch per 500 lbs

So by adding "air" to the equation we just made another progressive spring!

Not really. This spring, just as the one in your example (since the initial load turned half the spring into a spacer) is a garden variety straight rate 2000 lb/in spring.

Simply put, in suspension design it doesn't matter what the " effective overall stiffness" is... What's important is that in my example for each of the first 9" inches of travel the spring rate was "zero" and then instantly went to 2000 lb/in. Or in your example, (if you wouldn't have overloaded half the spring immediately), in the first couple of inches of travel the rate was 400 lb/in and then "crossed over" to 2000 lb/in for the remainder of travel when half the spring reached coilbind.

To paraphrase what you said, a progressive spring would be more like stacking a 200 on top of a 250 on top of a 300 on top of a 400 etc all the way up to 2000... I did not feel like doing the math on all that when this simpler example showed my point.

No. That's not what I said or meant. But I did describe both a "progressive rate" and a "dual rate" spring in detail if you re-read my posts.

SNIP
As you have probably noticed I am fairly contrary... I generally tend to make flip comments that appear a little off but are accurate if you study the very specific frame of reference to which I am referring... productive? meh... educational... not really... I generally know exactly what I am talking about, and am just making a point... fun?... hell yeah...

Nothing but love... appreciate being able to talk to someone with a technical mind and the ability to point out that my examples may be cases of reductio ad absurdum.

I agree with the "fun" part... not so sure about some of your other final comments though! :nana:

 

[FSM06]

I don't know if this will help, but......................................

By raising the spring perch, you are not compressing the spring any more that it is right now. The overall hieght of the spring remains unchanged. The only thing you change is the distance from the bottom of the lower spring perch to the center point of the lower control arm mounting point, therefore raising the ride hieght.

You two have burnt too many of my brain cells over this one! phew!! LOL!

FSM

 

[MarkT]

I don't know if this will help, but......................................

By raising the spring perch, you are not compressing the spring any more that it is right now. The overall hieght of the spring remains unchanged. The only thing you change is the distance from the bottom of the lower spring perch to the center point of the lower control arm mounting point, therefore raising the ride hieght.

You two have burnt too many of my brain cells over this one! phew!! LOL!

FSM

LOL... leave it to you FSM to condense pages of my ramblings to one concise, easy to understand, statement!:thumbsup:

 

[frogslinger]

I don't know if this will help, but......................................

By raising the spring perch, you are not compressing the spring any more that it is right now. The overall hieght of the spring remains unchanged. The only thing you change is the distance from the bottom of the lower spring perch to the center point of the lower control arm mounting point, therefore raising the ride hieght.

You two have burnt too many of my brain cells over this one! phew!! LOL!

FSM

That is all well and good (also accurate, relevant and practical)...

but giving good accurate information here is not constructive... I know exactly what MarkT is talking about, and I guarantee he knows exactly what I am talking about... we are just poking miniscule holes in each others arguments, despite the fact that we are saying that same thing... It is the kind of thing I have wasted hundreds of hours at IHOP and starbucks doing... that and coming up with my theory of universal creation from hyper speed prophylactic projectiles (that is "proving the big bang theory was created by two faster-than-light, flying condoms).

 

[FSM06]

That is all well and good (also accurate, relevant and practical)...

but giving good accurate information here is not constrictive... I know exactly what MarkT is talking about, and I guarantee he knows exactly what I am talking about... we are just poking miniscule holes in each others arguments, despite the fact that we are saying that same thing... It is the kind of thing I have wasted hundreds of hours at IHOP and starbucks doing... that and coming up with my theory of universal creation from hyper speed prophylactic projectiles (that is "proving the big bang theory was created by two faster-than-light, flying condoms).

Frog........you crack me up! I guess as long as you aren't poking miniscule holes in each others prophylactics, all is well!!

FSM

 

[frogslinger]

K... to start at the end and end at the middle;

If it hadn't been for my horse, I would never have spent that year in college...

Reductio ad absurdum... to brake an argument down to such a small or impractical stage that it is ridiculous...

Some classic examples would be "you are either with us or against us"... "let them eat cake"(based on the explanation that the peasants were in revolt because they could not get bread)... or describing to springs welded together as progressive...

now to refute your refutations...

Actually, I think what I said was something to the effect of "changing the preload (lower perch position) on the spring could not make the spring stiffer without breaking the laws of physics". And I still stand by that statement whether or not the spring is "straight rate" or "progressive rate".

I never said that there was no such thing as a progressive spring that changes rate as the load increases. The point is the load on the spring does not increase when the perch is moved one position.

Ok... I totally agreed that it would not change the spring rate in this application... I was simply arguing with your laws of physics thing... and I can back it up...

take a progressive spring... add preload... keep adding preload... ok now a little more... we have added so much that the spring cannot reach its uncompressed length (essentially your shock is topped out)... now add some more... as it is a progressive spring and it is being compressed, it is getting stiffer...

RaA? Of course... no laws of physics broken... several of good sense ans suspension design though.

I noticed this... but I just couldn't imagine why you would try to make a point about progressive springs by completely collapsing the 500 lb spring with the first 2000 lb load??? Do you know what the half of the spring that collapsed completely in your example is called? Let's see.... it's a special technical term... hmmm... what do they call it?

A spacer! That's it. You made half of the spring into a spacer when you added the initial 2000 llb load.



While I never said that a spring cannot change rate throughout the travel, your example is flawed and didn't prove your point at all.

You have fallen into the classic trap of assuming I am trying to make one point when I am in fact making another... I was not trying to demonstrate ow a progressive spring works... I was simply trying to show a detailed model of how a spring works... I was only trying to show that a spring could get stiffer as load increased... which coupled with my example above simply disproves your laws of physics comment...

Let's look at your results:

'k

The first set of numbers shows what is important. After you turned half the spring into a spacer, what you had left was a spring that compressed 1 inch for each 2000 lb of load. That is the definition of a straight rate 2000 lb/in spring.

ok, yes... assuming that the hypothetical vehicle has 2000 lbs on each wheel... which i am not... if you go through my second tome you will see the discussion of a 1k load... which would give the suspension a nice plush ride for normal loads and plenty of load handling capacity if a large load is put on it...

remember this hypothetical spring was created in my mind for two reasons... simplicity of math and to show an example of how preload in a ridiculous case can cause increased spring tension... however...

The "effective overall stiffness" you present in your second set of numbers are useless when it comes to suspension design. Springs are not rated this way and the data you presented is pretty much meaningless in terms of real-world usefulness. (or proving the existence of a spring that can change rate)

OK to be practical and accurate yadda yadda yadda... there is only one real world reason I can think of to know my numbers... that is that it allows you to guestimat how far in the air your nose will be sticking when you load a 4 tons on the hood of your truck... it does demonstrate the rate thing though even if you do not want to admit it... those numbers would be accurate and if you plotted a graph of average spring deflection vs load it would show a nice curve that anyone could point to and say... yup getting stiffer...

Will the curve in any way reflect ride quality... not in the slightest... will its smoothness give a false impression of what the truck will do if you hit a bump at speed? Absolutely... but I was trying to proove on very small and unimportant point...

about the preload and the stiffness... and physics...

continuing...

Use this example. Put a 6" long 2000 lb/in spring on coilover shock with one foot of shaft travel. This spring has 3" of travel before coil bind and spring is positioned so that the shock is completely collapsed at the point of coil bind. Now add your 2000 lb weight. The shaft collapses nine inches (freely) and then the spring compresses 1". So that would mean (by your definition) the "effective overall stiffness" of this coilover shock is:

the effective overall stiffness when measured with 2k lbs was 1 inch per 200 lbs
the effective overall stiffness when measured with 4k lbs was 1 inch per 363 lbs
the effective overall stiffness when measured with 6k lbs was 1 inch per 500 lbs

So by adding "air" to the equation we just made another progressive spring!

'k now that is just crazy talk...

I do get your point though...

And i respect that you took my impractical hypothetical suspension and turned it into one that whilst technically not impossible, is at the least wildly irresponsible...

You have also created a massless truck... so way to go on that too... (we will assume my truck weighed 1 lb)

Not really. This spring, just as the one in your example (since the initial load turned half the spring into a spacer) is a garden variety straight rate 2000 lb/in spring.

Simply put, in suspension design it doesn't matter what the " effective overall stiffness" is... What's important is that in my example for each of the first 9" inches of travel the spring rate was "zero" and then instantly went to 2000 lb/in. Or in your example, (if you wouldn't have overloaded half the spring immediately), in the first couple of inches of travel the rate was 400 lb/in and then "crossed over" to 2000 lb/in for the remainder of travel when half the spring reached coilbind.

Again... put it in my ridiculous preload example... my spring gets stiffer with preload yours doesn't....

No. That's not what I said or meant. But I did describe both a "progressive rate" and a "dual rate" spring in detail if you re-read my posts.

So you do not agree that is how a progressive spring operates? Cause it is...

I agree with the "fun" part... not so sure about some of your other final comments though! :nana:

Meh... just wanted to make sure you were not taking it personal.

 

[Xjrguy]

That is all well and good (also accurate, relevant and practical)...

but giving good accurate information here is not constrictive... I know exactly what MarkT is talking about, and I guarantee he knows exactly what I am talking about... we are just poking miniscule holes in each others arguments, despite the fact that we are saying that same thing... It is the kind of thing I have wasted hundreds of hours at IHOP and starbucks doing... that and coming up with my theory of universal creation from hyper speed prophylactic projectiles (that is "proving the big bang theory was created by two faster-than-light, flying condoms).

Reminds me of Ocean's 11 and these guys. LOL

 

[NoCaDiver]

A lot of info flying around, hypothetical or not....I'm sure I got lost somewhere around page 2.