First Look: Sprindex's Adjustable-Rate Coil Spring

If coils are so great, why aren't we all using them?

Given the popularity of coil-sprung suspension these days, along with the undeniable benefits of less friction and more traction, isn't it a bit odd that the vast majority of full-suspension mountain bikes come with air shocks? I mean, if these coils truly are better, why aren't they spec'd on all of our bikes? There's the fact that air is lighter than steel, but it's mainly because an air shock is far more adjustable. Using only a shock pump, a shop can have any of its air-sprung full-suspension bikes work for potential customers who weigh 100 pounds or potential customers who weigh three times as much.

As you can imagine, that makes it a whole lot easier to sell those bikes, and the shop doesn't need to keep dozens and dozens of different coil springs on hand.




Dialing back the cynicism a notch or three, you wouldn't be wrong if you also said that dead-simple spring-rate adjustment is air suspension's greatest asset. If you want just a nip less sag, all it takes is thirty-seconds to add 10 psi before your next ride, and the fact that it's so easy means that more of us will take the time to set-up our suspension properly. Or at least get closer to it. I suspect there'd be a ton of wildly over- and under-sprung bikes out there if everything used coils.

It's almost like an adjustable-rate coil spring would be a great idea, which is exactly what the folks at Sprindex have come up with.


How does Sprindex work?

To understand how Sprindex works, we have to understand how a coil spring works. They're just squishy, right? Well, yes, but also no; if you look closer, there's a bit more to it than that.


So while they look like boring, straight steel rods, a torsion bar is basically just an unwound coil spring. And the shorter the rod, the more force required to flex said rod. That means that a shorter torsion bar is stiffer than a long one made of the same material and gauge. Because a wound steel coil twists in the same way a torsion bar does, a shorter coil spring will be more difficult to compress the same distance than a longer one made of the same gauge steel. So if you could somehow lockout or deactivate a section of the coil spring, even just half a coil, you'd effectively be making the spring stiffer because it's effective length is shorter.

The easiest way to understand Sprindex might be to think of it as a simple way to adjust the effective length of the steel rod that, in the case, happens to be wound into a coil.




The glass-reinforced polymer Sprindex unit comes pre-installed on one end of their coil springs - it can't be purchased on its own. The bottom half is held in place in the last coil, while the top collar half turns and has fourteen indexed positions. The inner face of that collar also interlocks between the coils, and when the collar is turned all the way to the left, it's only working with a bit of a coil. Turn it all the way to the right, though, and it aligns with the bottom section to lockout half a coil and essentially makes the rod (coil) shorter.

Pretty damn clever.




There are probably more than a few people out there having their own 'Why the hell didn't I think of that?!' moment, as well as more than a few questions...


What's spring-rate and why does it matter?

It's the amount of weight required to compress the spring by one inch, so if it says 400 lb/in on your spring, it means that you'll need 400 lb to compress it by a single inch. Two inches? That'll be 800 lb, please. The spring-rate is what holds the rider up, and it's also what largely determines how much travel gets used at each impact. If you're not using enough travel, it probably means that your spring-rate is too high and vice versa, and it's the first thing you dial in (before damper adjustments) when setting up your suspension.

Four factors determine the spring's rate: material, the overall diameter of the coil, the diameter of the steel wire used and, most relevant to our convo, the number of active coils. To alter the rate, Sprindex is effectively letting you activate or deactivate a small section of the coil spring.


How big is the adjustment range?

It depends. ''There are more active coils on the longer stroke springs and thicker wire on our stronger springs,'' Sprindex says on their website. ''The range depends on the percentage of active coils reduced and multiplied times the rate of the spring.'' Just some chill math, then. You're looking at somewhere between 30 and 60 lb/in of adjustment range, depending on the spring's length and rate. That's not enough to have a single spring work for everyone, of course, but it's more than enough for tuning purposes.




Neato, but how is this different from preload?

The threaded preload collar on your coil-sprung shock doesn't affect the spring-rate because it's only partially compressing the coil, not altering the length of the coil (your wound steel rod). If you dial on a ton of preload, the only thing you'll be doing is raising the ride height of your bike and increasing the starting force required to activate your shock. More preload means less suspension sensitivity, and the general consensus is that you only want to tighten the preload collar enough to keep the coil from moving around.

If you want, you can combine your preload adjustment with Sprindex and it will work fine. That said, they recommend that you use their adjustment and modify the spring-rate to achieve the correct ride height instead. Less energy stored in the spring equals more supple suspension.


What kind of shocks does it fit?

With the included Delrin adapters and different diameter coils offered, you can get a Sprindex to fit shocks from all the names that matter, including Fox, RockShox, DVO, MRP, Marzocchi, Cane Creek, Öhlins, RockShox, X-Fusion, and Push models. Maybe some others, too, and you can check out the very long compatibility chart on their website to see if your shock is on the list.





But does it work?

The spring-rate-O-meter that I ordered off Amazon hasn't shown up yet, but a dozen donuts got me access to a very neat custom-made version assembled within a burly steel frame. A small car jack that serves as the lower spring perch is bolted to the bottom, while an adjustable upper perch is used to compensate for different length coils or add a nip of preload. A digital caliper is attached to that lower perch, and all you need to do is adjust the jack until it reads zero, then bring down the upper perch with just enough preload to hold it in place. The caliper tells you precisely how far you've compressed the spring, while a load cell on the lower perch tells you how much weight you're applying to the spring while compressing it with the car jack.

It's a clever machine, and while it's not lab equipment-precise, it's more than adequate for the day's job: Figuring out if Sprindex's claims are true. Can their little device, which consists of just two simple pieces, really offer an effective, tool-free coil spring-rate adjustment?

It sure can.




With the Sprindex set to 340 lb/in, and 10 lb/in of preload applied to roughly replicate it being installed on a bike, the spring-rate-O-meter's digital display read 348 lb/in when compressed by a single inch. Rotating the collar up to 350 lb/in saw the display change to 363 lb/in (don't forget about that preload), and each of the indexed 10 lb/in spring-rate settings tested after that corresponded to an 8 to 15 lb/in increase in the force required to keep the spring compressed by an inch.

At the test spring's stiffest 380 lb/in setting, and with the preload backed off to see how accurate the numbers are, the display read 374 lb/in. The spring-rate-O-meter probably isn't quite as accurate as a commercial-grade machine, but those numbers are pretty damn close. It also proved that Sprindex's adjustable, two-piece system does actually work as advertised.




Out of the lab and back in the workshop, the Sprindex unit is installed just like any other coil - it slides down over the shock body, and don't forget to use those red Delrin washers that let the spring twist freely as it's being compressed. I installed it on two bikes, a Yeti SB165 and a custom-made 180mm-travel enduro rig that you'll get to see real soon; measuring the sag on both bikes throughout the Sprindex's adjustment range backed up the spring-rate-O-meter's findings.




We're probably not going to see air suspension disappear anytime soon, and the Sprindex doesn't offer enough adjustment range to see a one coil work for any rider on any bike, but it's certainly a good tuning option for those who like to tinker. Wet day? Dial back the spring-rate a bit for more compliance. Fast day in the bike park? You might want to run a firming spring-rate for all those jumps you're going to overshoot. It could make sense for bike shops as well, with them no longer needing to stock countless coil springs.

I guess that leaves me with a single question: Why didn't I think of this?

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