EnvoyIROCZ

I'm thinking of replacing the stock springs, struts, and springs . thinking of KYB struts and shocks. wondering what difference between stock replacements and variable rate springs.

Johnny Blaze

iTrader: (15)

Variable rate spring are soft until they are compressed to a certian point then they get stiff.

Not what I would want on a sports car.

They were the hot thing ten or so years ago.

EnvoyIROCZ

87350IROC

iTrader: (2)

The difference is the construction of the spring. Linear springs are just that, linear.

Example: 1000 lb/in spring
If you push down on the spring with 1000lbs of force it will compress 1"
If you push down on the spring with 2000lbs of force it will compress 2"

A variable rate (progressive) spring means that the spring rate is not constant, the curve is not linear. The spring rate increases with spring deflection.

Example: hypothetical progressive spring
If you push down on the spring with 1000lbs of force it will compress 1.5"
If you push down on the spring with 2000lbs of force it will compress 2.0"

The actual stiffness depends on the design. You can design a variable rate spring to be always stiff than a linear spring or always softer, or somewhere in between.

The concept behind using a progressive spring in a vehicle is that when going over small bumps that don't force as much suspension travel, you will get a softer ride because you are in the softer portion of the spring.

Another nice thing about progressive springs is you can design them to protect bottoming out of the suspension, so as you get closer to the bump stop the spring gets firmer and prevents you from hitting the bump stop.

Here is a neat graph from Wikipedia. On the horizontal axis is spring deflection and on the vertical axis is the force on the spring. You can see #2 is a linear spring and #1 is a progressive spring.

sofakingdom

iTrader: (1)

#1 on that graph is what stock springs for these cars, or Moog CC series, will do.

The Moogs are about 50% stiffer than yerbasic typical wore-out sagging stockers that have given up the ghost from being flexed a few million times. Go do that to a coat hanger wire or something and see what's left of it. That's all a "spring" is; a piece of wire. It bends back and forth, the more so when the shocks aren't kept in good shape, and loses its "spring"ness over time.

Constant-rate ones are easier to tune a race car around; especially since their weight doesn't vary beyond a fuel load.

For the VAST MAJORITY of us on here, who just drive our cars around on the street with the occasional passenger or other load, variable-rate is the way to go.

Reid Fleming

iTrader: (2)

Variable rate springs are good for towing heavy loads or installing lots of heavy stereo equipment in a trunk. For a performance car, I'd say go with the constant rate. For a daily driver non performance vehicle, I'd say the variable rate will be fine.

87350IROC

iTrader: (2)

Coat hanger test is not valid. What you are doing to the coat hanger is putting it through a series of plastic deformations. This occurs when stress exceeds the yield stress of the material.

Springs stay in the linear elastic zone. They will be completely bottomed out and still elastic. If you keep the coat hanger in the elastic zone (no permenant deformation) it will last a very very long time as well.

406TPI

iTrader: (1)

You are 100% correct, also it should be noted coil springs do not lose spring rate over time like people think, they just lose height.

sofakingdom

iTrader: (1)

The coat-hanger demonstration IS valid. Bend one of those about a million times even in the "linear" region, it'll behave EXACTLY like a spring. Steel is steel is steel is steel, no matter how it's packaged and what name is on the box, or whether it's associated with "car" and therefore elevated into the mystical mythical realm of the unknowable.

The reason springs "lose height" IS BECAUSE they "lose rate". HOW ELSE do you suppose they compress farther with the same force (weight) applied? What do you propose this new definition of "rate" you would like to create, should be, if not "deflection per unit weight"?

87350IROC

iTrader: (2)

It sounds like now you are agreeing with me. I've never heard of someone arguing a hanger will last forever when you bend it, usually the opposite. My mistake.

Finally, I couldn't disagree more that steel is steel. I think you will find spring steel has a MUCH higher yield_stress/ultimate_stress ratio than "normal" steel.

Google spring steel and you will see what I mean.

Short of specialty applications car steel is "normal steel" as far as I'm concerned. Usually you hear about "aircraft grade" metals, I was not aware that colloquialism existed for the auto world as well.

sofakingdom

iTrader: (1)

No, I'm not arguing a hanger will last forever if you bend it back & forth a million times.

I'm arguing THE TRUTH: which is, that A SPRING will also not last forever after being bent back & forth a million times, JUST LIKE A COAT HANGER. Just ask the guys with solid roller cams that have to change their valve springs regularly. Same set of facts, just located in a different spot in the mythical mystical magical "car" thing in which physical principles seem to vanish and turn into black magic and made-up Friday-night McDonalds parking lot monkey-spank.

Yes I'm aware that there are differences in types of steel, to a point. Yes I'm aware that spring steel is designed and processed to produce a "much higher" level of various properties than coat-hanger steel. However, "much higher" is NOT AT ALL the same as "infinite" or "forever" or anything of the kind; more like, if you can bend a coat hanger back & forth some small amount some number of times and get some particular result, it might take 2 or 3 or 4 or even 10 times as many times to get the same result from doing the same thing to spring steel; but sooner or later, the same failure mechanism will occur. It's just a question of when, not if. I'm also aware that no matter what alloy, what heat-treat, or whatever else, it ALL shares certain characteristics. Which in fact, ALL metals share to a certain extent. One of those characteristics being, the act of bending back & forth changes the crystalline structure, in ways that affect its physical properties.

Which is why, when you see those old hooptie cars driving down the road with the one wheel flopping up and down every time it hits a bump because the shocks are wore out (might I suggest the rear of Xler minivans as particularly blatant examples of this phenomenon), that corner of the car will virtually always sag more than the other corners of the same car; because that one spring has been bent back & forth farther and more times than the other springs. How convenient that you can literally watch the principle in action on your morning commute. Only a closed-minded person could ignore such obvious demonstrations of such a simple physical principle.

87350IROC

iTrader: (2)

Yes it is true that all steels have an endurance limit. If you do not exceed that stress, you will NEVER fail (yield) the material. I refuse to believe any competent engineer would design a spring with a working stress higher than the endurance limit. Springs by application are designed to be high cycle components. All spring steel buys you is a higher yield stress/endurance limit compared to common steels.

I would be the cause of spring sag is creep in the material from constant stress over many years. That is not the failure mode you have outlined with coat hanger example.

Regarding your Chrysler Minivan example. I think you will find that many of these came with Nivomat shocks stock in the rear. Nivomats actually help hold the car up unlike standard shocks. When they fail, that corner will drop significantly. Also, often when they fail they will be replaced with conventional shocks and that corner will be lower.

Finally, since you have resorted to name calling, I am not going to waste my time any more with this thread. If anyone wants to continue with an intelligent conversation, send me a PM.

John

sofakingdom

iTrader: (1)

No name-calling here; only FACTS and TRUTH.

Yes springs are designed to be "high cycle" components. Of course. But "high" is not "infinite"; and the failure mode is exactly as I described.

We're not talking about "yield". At least I'm not. Injecting some other "straw man" into your argument won't change the basic facts that you're ignoring.

"Creep in the material from stress" is just an artful dodge for avoiding admitting that THE RATE GOES AWAY as the material loses its initial temper and other properties due to crystalline structure changes resulting from repeated bending. No amount of double-talk can allow you, or anyone else, to escape the reality that springs lose their rate over time and repeated flexing.

Bottom line is, springs DO lose their rate, aka "wear out"; whether they're the ones that support the car's weight, the ones that close the valves, or ANY OTHER springs in ANY OTHER application besides "car" where they're subject to repeated cycling. "Car" doesn't somehow remove the particular ones at the wheels from the world of known engineering and scientific principles and send them off into some alternate universe of mythology and mysticism. It is not helpful to anyone attempting to educate themselves about their "car" to pretend otherwise.

406TPI

iTrader: (1)

Suspension coil springs do not lose their rate, they lose their height. One only needs to look at the formula for spring rate to see why this is true. Every suspension spring manufacturer will tell you the same thing.

InfernalVortex

iTrader: (4)

Ive never heard of a spring failing by breaking. Even the ones that end up looking like curled over pretzels are from being cut/heated with a torch.

sofakingdom

iTrader: (1)

I've actually had one that did, in my own car.

Had a sway bar that broke too.

Not to mention more than my share of valve springs.

The idea that springs are somehow immune to failure is pretty laughable after a few decades of swapping em out.

Needless to say, I have several million miles on me.

InfernalVortex

iTrader: (4)

I would figure valve springs are different, they clearly are forced to articulate far more of their total height constantly compared to vehicle springs. Something like a car spring doesnt see nearly as much movement. Other springs I've known to have issues are magazine springs in guns, but that's another example of a spring that's forced to articulate much, much farther than a car spring. Maybe that matters, maybe that doesn't? Regardless, as springs sag their number of active coils go down and the spring rate stays the same.

Joe Tag

iTrader: (4)

Valve springs see the exact same amount of movement every time, other than decreasing with lobe wear. Suspension springs movement varies due to load, roughness of road and shock wear. I've seen both break myself.

87350IROC

iTrader: (2)

Valve springs blow through cycles a bit faster than a suspension spring. They will initiate crack growth and lead to fracture failure much quicker than a suspension spring. Their average stress will also be much higher as pointed out by several posters above.

SlickTrackGod

Dean here,
Someone asked me to come in and give input to this discussion.

Spring do NOT loose rate when they sag.

In a real world sinerio, a coil spring will increase diameter slightly as it looses height or "sags" . Ill give examples.

1)Lets take a 0.750 wire and a 5" outside diameter. With 8 active coils and a 12" free height the spring will rate at 719lbs.

2)Now make another spring out of the same length of .750 wire that is 11.5" free height. In order for it to remain the same rate it will have to change wither diameter or active coils. lets start with how a srping compresses in dynamic form. The spring will generally gain width throught he mid section of coils because of bind friction of the uper and lower mount surfaces (the lower is usualy indexed so it really does not move) Thus, to remain 8 active coils, the spring width will grow to about 5.1" to remain the same rate of 719lbs...this is what happens when springs sag (partially)

3) now we look at active coils. to remain the same rate and width but decrease freeheight, a spring will have to increase active coils. So in other words, if a .750 wire 11.5" freehight spring that is 5" OD, the spring will have to increase from 8 to about 8.15 active coils (the top of the spring twists as it sags- again partially/ it does a little of both twist and increase width) in order to remain the 719 lbs.

Now if you were to take the original .750 wire, 5" OD , and 8 active coils and make a spring only 11.5" tall instead of 12" tall it would decrease rate because of the coil ramp angle. Although, less length of wire is used wihich generally increases rate, the leverage angle of the wire coil ramp decreases and is less effective in a vertical load.

Bye.

TEDSgrad

iTrader: (4)

Ron Sutton quote from his sticky on lat-g:
Part 3 of 3

From this, three other revelations came, that are more important to you & me, since we’re avoiding coil bind. They are …
1. Energy dissipation through springs, suspension flex, bushings, etc … reduces the net loading of the tires.
2. The length of the wire used in making a spring directly affects its energy dissipation characteristics … and grip to the tires.
3. The hardness of a bump stop directly affects its energy dissipation characteristics… and grip to the tires.


I’ll expand on each of these, but in a different order …


#2 The amount of energy lost or “dissipated” … through the springs … is directed related to how long the spring wire is. The more wire required … the more energy lost. For years, if a spring had a rate of 500#, the amount of wire to make it was the same from brand to brand … because for years all automotive spring manufacturers, racing & oem, made springs from the same cheap spring steel. But automotive spring technology … which was stagnant for years … has come a long, long way in the last 15 years.

Today, the spring companies on the leading edge, have learned better processes and use higher grade (and more costly) materials to produce springs that last longer, don’t sag, settle or deform … and use less coils to achieve the same rate. These springs have less energy dissipation than older design, cheaper springs made with inferior metal & processes. That means they apply more net load on the front tires … for more grip … during braking & cornering.

How much more? To know that, we’d have to know how much energy was being lost/dissipated through your current suspension, which is beyond my scope. But the simple math is this:
• A 500# spring of old design & cheap spring steel … with dimensions of 5-1/2” od x 10” tall with ½” wire … requires 9 full coils (approximately 141” of spring wire). That is 141” that energy dissipates through.
• Comparing to a new design spring, with higher quality steel … in the same size … but requiring only 6 full coils … takes approximately 94” of spring wire. That is 1/3 less wire … 1/3 less energy dissipation … and 1/3 less grip lost.

How do you know the quality of springs?
Visually, you can see a higher quality spring takes less coils and has more gap between the coils … when comparing the same rate & size. (See photo below for example.) But that’s only part of the equation. Material is key, so ask what they‘re made with. A simple rule of thumb is the higher the number, the higher the quality. Where 4000 series steel was a step up 10 years ago, it is low end today. 5000 series works better, and 9000 series outshines them all.

A spring doesn’t have to coil bind to fail. Low quality automotive springs fail in some form or another … from too much dynamic loading or simply from over use (cycles). High travel suspension set-ups with softer front springs work & test those softer rate springs much harder than stiffer springs see in low travel set-ups. I know from my racing experience … running soft front springs, in heavy track cars … transferring a lot of load onto the springs with high g-forces from braking & cornering … lap-after-lap … works these soft springs much, much harder.

Most common quality & brands of racing springs being ran … failed. They failed in different ways. Some would lose height. Some would lose rate. Some would lose shape & distort. Some would do a combination of two or all three things. When springs lose rate, height or shape … they have failed … and no longer behave the same way. That’s worth repeating. If a spring changes it shape, height or rate, it has failed and will not perform the same.

It is hard for most of our brains to get wrapped around this concept … that springs can have a given rate … and yet not act the same. But this is fact. If a 350# spring loses height, but still tests at 350# in a spring rater … what’s wrong with that? It has failed. To lose height, the spring coils must “bend” or “distort” to some degree. They will never act the same. The responsiveness will decrease. We call this a “dead spring.”

In my NASCAR Modifieds, when we tried running 4000 series springs, they would fail before the weekend was over. Some would distort, which was easy to see. Some would lose height, which could be determined by measuring the car height. If a corner on the car “sagged” … we would remove the spring & measure its “free height.” Sure enough, it has lost free height. Toss it.

The 5000 series springs were good for 3 races. But the 9000 series springs held rate, shape & height all season and were still perfect after a season of racing. They cost 3 times as much, but well worth it in my opinion. Just for reference, typical 4000 series oval track racing springs cost $50-60, 5000 series springs were $100 & 9000 series springs were $150-175.

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#3 When a suspension has fully compressed & loaded a bump stop … the energy dissipation decreases substantially … increasing front tire grip. The energy dissipation doesn’t go to zero, like coil bind does, so it doesn’t have quite the load & grip a coil bind set-up does. But it’s close. How close is directed related to how hard the bump stop is.

The harder the bump stop is, the less energy lost. For this reason, pros use harder bump stops, which are less forgiving and act closer to coil bind setups. Less experienced tuners prefer to use softer bump stops, so they have a wider sweet spot range and a more forgiving suspension. Even with the softest bump stops … the energy dissipation is very low … and the load & grip on the tire is much higher … than a spring only set-up.

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#1 The springs are not the only thing in a front suspension that can cause energy dissipation leading to less load & grip. Suspension bushings, if used, cause a lot of energy dissipation. Rubber is the worst offender and harder bushings less so. Race suspensions with rod ends or mono ***** have minimal energy dissipation. For us in the Pro-Touring/track Car world, you need to decide where your priorities are & chose accordingly. Is reducing NVH important to you or is getting every last bit of grip more important. It is a trade off for sure.

Suspension component flex under load can be another source of energy dissipation, but a minor one. If you have upper and/or lower control arms flexing, I’d be more concerned about the negative geometry changes that occur than energy dissipation. Ideally, you want to utilize upper and lower control arms that are just strong enough to minimize flex and yet still light enough for optimum suspension control & responsiveness.

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Summary:
• Springs with modern technology & high quality steel … with less wire … have less energy dissipation … and load the tires more, providing more front end grip.
• Springs with modern technology & high quality steel … handle higher dynamic loads & cycling … without failing, losing rate, height or shape.
• Control arm bushings & suspension component flex figure into energy dissipation … and effect tire loading & grip too.
• Running bump stops achieves almost zero energy dissipation from the spring … and loads the tires even more, providing the highest front end grip this side of coil bind.
• Combining bump stops … with springs of modern technology & high quality steel … in a high travel/low roll set-up … produces the optimum cornering performance available with current technology.



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sofakingdom

iTrader: (1)

Hey TEDS, this is a cheerleading session among the "sports car" {sorry, I don't have the right accent to do the guy in "Back To School" that was Rodney Dangerfield's English teacher...} types, all about telling ME all about how "coil springs don't lose rate", in defiance of all everyday experience all of us, THEMSELVES INCLUDED, have accumulated. You're not supposed to introduce FACTS!!!! That just ruins .... everything.

JamesC

iTrader: (5)

Brian,

While I appreciate your words, they don't seem appropriate for an automotive forum.

JamesC