Sprindex adjustable coil spring review

A correctly set up coil-sprung shock can provide more suppleness and grip than an air-sprung shock. The stumbling block has always been getting the shock set up exactly for your weight, which has traditionally meant swapping springs.

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Sprindex’s adjustable coil uses a plastic spacer that can be wound in, altering the number of active coils on the spring, thus upping the spring rate. The advantage to this over simply winding on preload is that it doesn’t pre-compress the spring and reduce small-bump sensitivity.

Sprindex promises a 40lb adjustment range and the notched spacer clicks around in 5lb increments, making it easy to dial in your spring rate and tweak things out on the trail to suit the conditions.

Multiple spring weights, stroke lengths and shock fitments are available. I’ve been trying out the 390lb to 400lb, 65mm-stroke (2.6in) version on a Fox DHX2 shock. Manufactured from high-grade steel, it weighs 417g, so the difference over a standard spring is negligible.

To test the accuracy of the stated spring rates, I paid a visit to suspension-tuning specialists Mojo Rising, where head honcho Chris Porter ran the numbers on his testing jig.

In the mountain bike industry, springs are measured by the amount of force required to compress them one inch (i.e. 400lb of force for a 400lb spring). The results showed that the Sprindex coil was a fair bit softer than it was stated to be – approx. 374lb in the 390lb setting and 394lb in the 430lb setting.

This may sound fairly inaccurate, but put into context, the Fox SLS spring we tested as a comparison was even further out. With MTB brands striving to shed material and thus weight, it’s apparently difficult to mass-produce coil springs to fine degrees of accuracy.

As far as adjustments go, the Sprindex works as it should, but the range isn’t quite as large as stated. In the initial part of the stroke we recorded a 10 per cent difference in spring rate, but this reduced to 2 per cent when the spring was compressed by two inches (75 per cent of the 2.6in stroke on my shock).

This corresponds to what I felt out on the trail, which is that there’s a notable difference early in the stroke, but deeper into the travel, the effect lessens. Part of this is down to the way the plastic adjuster effectively shortens the coil.

We observed on the testing jig that when the forces exerted on the shock reached a certain point, the coil started bending around the edge of the plastic adjuster. This deflection effectively makes the spring longer again and therefore softer. It was a little unnerving to see it flex so much and damage to the paint shows it’s been happening on the bike too, but we reckon you’d have to send an almighty huck to flat to push it past the limit.

While my sample didn’t live up to all of Sprindex’s claims, it’s still a good tool for tuning the sag point and dialling in how a shock behaves on chatter bumps, which does have a noticeable effect on how a bike rides.