HELP! Explain coilover/linear v conventional/progressive spring rate differences

[phenryiv1]

I am researching suspension for my B15 and thought that I might come to the well of information over here. Anyway, in looking at suspension for my SpecV, I am really leaning toward coilovers with a custom spring rate. After doing some conversions, I started comparing rates in an attempt to select a rate that will result in an improved stance, better handling, and a lower center of gravity, all while keeping the most reasonable ride possible (I have a baby due in a month and don't want to break her back in the Spec).

Anyway, when trying to compare the rates, I noticed a trend that made it difficult to compare. This raised a few questions.

The first question is, are the spring rates of coilovers and those of conventional springs actually/directly comparable? If the rates between a coilover and a conventional spring are the same lb./in rating, are we really comparing the same thing, based on the linear windings on coilovers v. the progressive rates of most conventional aftermarket springs?

Example: With Tein B15 applications, the Basics are 336/336. The Tein S-Tech springs are 170(f)/270(r). Are the front on basics really nearly twice as stiff as are the S-techs? And are the rears fairly similar? Or does the linear/progressive nature of the different springs result in the same lb./in ratings, but a different feel on the road?

Next, why do most conventional springs have front rates that are lower than the rear rates, while most coilovers have higher front rates v. lower rear rates? It seems odd that the stiffer springs are consistently in the front on coilovers, and consistently in the rear on conventional springs. Can anyone explain the reason for this?

I read the following over on b15sentra.net, but am still very confused:

Quote »Just adding a bit of info for some suspension newbies since this is a sticky anyways.

Spring rates are the key factor in balance of the complete chassis. This leads to the debate between "linear" and "progressive" spring rates. There's no mystery about progressive springs: A progressive spring has a variable rate increase throughout its compression stroke. For example a progressive spring with a starting rate of 200 pounds per inch for the first inch of compression and an end rate of 400 pounds per inch for the next two inches of compression would then equal a load of 1000 pounds.

A linear spring rate has one rate throughout its deflection. This means, if you have 300 pounds per inch spring rate, it takes 300 pounds to compress that spring one inch. A 300 pounds per inch linear spring, compressed three inches, would equal a load of 900 pounds. As you can see, one progressive spring can do the work of two or more linear springs. This is a big advantage in modern automotive chassis design, fulfilling the needs of today's discerning customers.

Springs with a high linear rate would be used on a smooth racetrack, while on a rough or bumpy road course; you would use a softer spring rate. Since many racetracks have different road surfaces a suspension that is adaptive to changing road surfaces is desired. Progressive rate springs can offer a chassis tuner the means to achieve a compliant suspension in the rough and a tight suspension for high-speed turns.

Another issue that adds to the debate between "Linear" and "Progressive" rate springs, is that when most spring manufacturers say that their springs are progressive they are not! Springs may be wound progressively, but that does not mean that they function progressively. Some suspension springs are wound progressively but function as a linear spring. These springs can be called "dual-stage" coils, but are generally referred to as springs with "dead" or "inactive" coils. Dead or inactive coils are coils that are in contact with adjacent coils at loaded height. Inactive coils do nothing but give the spring enough free-length to stay tight in the spring perches at full rebound (when the tires and wheels are hanging in the air like when the car is on a lift). A spring that is wound with inactive coils and no progressive coils that are active, is actually working as a linear-rate spring. This is why when you call a spring manufacturer for spring rates for your application you must ask, "What is the actual working spring rate?" This ensures that you do not just get numbers quoted from a design sheet. For example: A design sheet may have rates of 69lbs. per inch, to 160lbs. per inch, to 220lbs. per inch. When the actual rate is 170lbs. per inch to 220lbs. per inch. As you can see, getting the correct information is important in making a true comparison.[/quote] To add to this, why would the B15 Nismo "coilover" setup have a 185/300 spring rate (F&R) when most coilover suspension carry a higher front rate than in rear? The Nismo setup seems to be the only "coilover" that breaks this trend. The more I think about the Nismos being an anomoly, the more I realize that they are more of a spring&strut combo than a true coilover, which would explain why they buck the "coilover" trend, but still leaves to question why there is the different trends in the first place.

At the end of all of this, what I am asking, is if I have ridden in a car with springs that are a given rate, and I am looking at coilovers with the same rate, should the ride feel similar (assuming comparable damping settings)?

[Q45tech]

PUBLISHED Spring rates MUST be corrected for mounting angle and the inboard distance from the wheel hub.

If the springs sit midways on the lower suspension arm they will move 1/4 the wheel distance movement and will have to be 4 time stiffer [1" of wheel movement only changes the spring 1/4"].

Ideally the as installed stiffness will be measured with electronic weight scales and a jack to raise and lower [compress] over the +- 3 inch range from static.

As installed springs will typically [on a performance car] have a front to rear stiffness ratio equal to the static weight distribution:

Say a 4000 pound [Q45] with 54/45 will have a front spring 54% [ 146 lb/in] and a rear spring 46% [123 lb/in].....................the spring will be 16" unmounted ans compress ~~ 8" with the body weight on them..........they will have 4" of springiness left before they BIND UP.

All that matters is the REAL stiffness as applied to the wheel [tires are springs [1500 lb/in] in series and typically reduce the spring stiffness by 10% [depending on sidewall and psi].

The ratio of front to rear stiffness is fine tuned based on the wheelbase to optimize the critical speed [Interstate 60-75 mph] to minimize the pitching so the numbers may vary +-10% vs the real weight distibution.

Front seat passengers are near the center of gravity vs rear and trunk and gasoline weight so you might tune the rear springs a little stiffer to compensate........... most don't.

All these published numbers 600, 400, 300 must be corrected to know what you are really dealing with.

The problem with linear fronts and progressive rears is the normal understeer ratio decreases as the rear get stiffer so handling [understeer, neutral, oversteer] will be different as the G load increases.