Yes.  Besides having an adjustable spring rate, Sprindex is also progressive.  Sprindex is specifically designed to have a progressive spring rate for the last 20% of stroke for big hit support and for bikes with a linear linkage design.  For more detail, go to "how it works / progressive" at the top of the page.

Coil shocks include a threaded ring for preloading the coil. Preloading causes the coil to partially compressed, which requires a higher initial force to start coil movement. Too much preload causes:

1.  Poor small bump absorption resulting in a harsh ride and poor traction.

2.  Poor solution to a coil bottoming out because preload will still be small compared to the load at full travel.

3.  Increased stress reduces coil life.

4.  Sometimes reduces full shock travel.

Spring rate is the force required to compress the spring a certain distance. For example, a 400 lb/in spring rate requires 400 pounds of force to compress each inch of the coil, or about 1040 pounds at 2.6 inches of travel. A 450 lb/in spring requires 50 pounds more per inch or about 130 pounds more at 2.6 inches of travel. Preloading a 400 lb/in coil 5mm (maximum typically recommended) is a 79 pound preload, which will prevent small bumps from moving the suspension (destroying small bump sensitivity), yet probably not prevent bottom out on big hits.

Start by following your shock brand's recommended SAG amount and adjust your Sprindex rate as needed. This is only a starting point. Then slide your rubber bottom-out bumper over so that during your ride, you can look to see how far the bumper has moved. If the bumper is not moving as far as possible, then adjust your Sprindex spring rate lower, move the bumper again, and ride more. Ideally, you should get near full shock travel during your ride without bottoming out.

Your Sprindex range is too low for you. You need a higher range Sprindex.

Your Sprindex range is too high for you. You need a lower range Sprindex.

Consider flipping Sprindex the other direction if that would provide better access to the Dial.

If the Sprindex is less than 2 mm too long to install, then carefully squeezing opposite coils slightly together with zip ties sometimes works. Squeeze as little as necessary and wear safety glasses and gloves. If Sprindex is more than 2mm too long, do not install.

Be sure you are using the correct adapters for your shock. This allows Sprindex to properly fit a wide variety of shock bodies.  Sometimes shocks have different diameters to fit at each end, in which case you should use whichever adapter fits best on each end.

The range depends on a percentage of change.  Twisting the Dial deactivates up to 1/2 active coil.  There are more active coils on the longer stroke springs and thicker wire on our stronger springs.  The range depends on the percentage of active coils reduced and multiplied times the rate of the spring.  Coils with fewer coils have a bigger range and coils with stronger spring rates have a bigger range.  Percentage-wise, all springs within a family have about the same range.

Yes, and in most cases, more so.  Sprindex coils were designed to handle the stress at the highest adjusted spring rate.  To achieve this, we used the highest tensile spring steel available.  When adjusted to its maximum spring rate, Sprindex will last as long as any quality shock spring.  When adjusted below it's highest maximum spring rate, Sprindex will last even longer.

The included Performance Adapters allow Sprindex to fit nearly all mountain bike shocks.  The Sprindex coil is designed to fit the largest diameter shocks (Rockshox) without an Adapter and to fit all the other shocks with an Adapter.  The difference diameter between the smallest shocks (Fox) and the largest is small, but it is important to use the proper Adapter to remove unnecessary play.  The Adapters are made of slippery Delrin to allow small twisting movement of the coil ends, which adds performance.  That is why we recommend using the included Performance Washer for Rockshox when no size Adapter is required.

No, those parts will only fit a custom Sprindex spring.  Our spring had to be custom in order to be designed to be properly stressed at full stroke when adjusted to the maximum spring rate. If you added the mechanism to your existing spring, you would highly overstress that spring. Additionally, the mechanism itself blocks about 1 full coil even when adjusted to the minimum setting, so adding the mechanism to your existing spring would increase the minimum spring rate by a lot. Our spring is made with the same expensive high tensile steel used in other "lightweight" springs.

For Fox shocks, use both their aluminum adapter ring AND the Sprindex Performance Adapters that say "Fox" on them.

If your Dial is hard to turn, it is okay to add a wax lubricant such as White Lightening or wax from Chap Stick lip balm to the wire coil where the Dial contacts.  Usually this helps a lot.  Also, make sure that your preload is not too much (1 or 2 turns of preload ring).  Also, you may find it easier to hold the Dial still and turn the Coil instead of holding the Coil and turning the Dial.