eMTB & MTB FOX Coil Springs

Shop the FOX SLS (Super Light Steel) and Steel Coil Springs here.

Narrow available springs by shock size and spring rate.

Wondering which spring is for you? Want to learn more? Scroll down to learn about eMTB and MTB Coil Fitment as well as how Pre-Load settings allow spring cross over and thus more or less progressive spring rates through the coil’s travel!

eMTB and MTB FOX Coil Spring Fitment

Here are some physical dimensions to take into consideration when shopping for the right eMTB or MTB Coil Spring.

FOX Coil ID: Inner Diameter

Current FOX Coils are Inner Diameter: 1.385″

The correct inner diameter ensures proper clearance to the damper body and proper upper and lower “spring perch” sizing.

Coil Stroke versus Shock Stroke

We’ve listed the published stroke or coil travel for each part number.

The Coil Stroke should be greater than, at least slightly, than the Shock Stroke (taking into account that some coil stroke is used with the Pre-Load adjustment – more on that on the next tab).

If your Coil Stroke is equal or, much worse, LESS THAN the shock stroke you will have Coil Bind.

Coil Bind

Example of Coil Bind on MTB or eMTB suspension

Example of Coil Bind on MTB or eMTB suspension shown above.

Coil Bind is when a spring is compressed completely sitting each coil against its neighbors.

Coil Bind may cause damage to the damper body, suspension, bike and rider.

When a coil is completely compressed it is no longer a spring, so the spring rate goes to infinity (like a hard tail bike). This typically results in a bounce which then unloads the spring to “pogo” making the entire situation worse. Coil Bind should be avoided by ensuring that your spec’d Coil Stroke or Coil Travel is greater than the Shock Stroke.

Coil Total Length

The coil capable FOX dampers have an adjustable ring for Pre-Load as well as to accommodate various Total Length coil springs.

FOX DHX2 Coil Total Length Fitment Chart DHX
FOX DHX2 Min & Max Spring Total Length per Eye to Eye Measurement
Eye to Eye Min Spring Length Max Spring Length
7.5″ 4.32″ 4.38″
7.875″ 4.32″ 5.21″
8.5″ 4.82″ 5.83″
9.5″ 5.32″ 6.84″
10.5″ 5.82″ 7.83″
185mm Trunnion 4.48″ 5.30″
210mm 4.49″ 5.60″
205mm Trunnion 4.88″ 6.09″
230mm 4.88″ 6.39″
225mm Trunnion 5.27″ 6.88″
250mm 5.28″ 7.18″

Read onto the next Tab for more information about fine tuning your FOX DHX2 Coil using Pre-Load and Spring Rate to adjust your coil spring setup’s progressiveness!

Tuning with Pre-Load & Spring Rate

We’re going to use the FOX DHX2 damper as an example to show you how multiple spring rates will produce similar resistance at a SAG measurement while allowing riders to fine tune the available resistance later in the stroke, often quite notably.

We touched on Spring Rate in the Fitment tab – it’s measured in LBs/IN – how many pounds it takes to move the spring some portion of an inch. Since this is consistent it creates what’s considered a linear spring rate (it will graph in a line) compared to a progressive spring rate of an air shock (which graphs in a “J” shape).

First let’s look at Pre-Load

FOX recommends a minimum of 8 clicks of Pre-Load on a spring and allows for a maximum of 26 clicks (to minimize the chance of coil bind as well as controlling the forces on the spring perches, threads, etc)

So you install the coil, tighten the Pre-Load perch down to contact and then adjust it between 8 and 26 clicks to fine tune your SAG setting, which we’ll say is 30% of the SHOCK TRAVEL. If your coil is sized appropriately – ie more coil travel even with Pre-Load added than available shock travel the coil will be at less than 30% of it’s travel. So remember, the measurement you’re looking for is a difference that equates to 30% of the SHOCK stroke.

example: 55mm SHOCK stroke = 16.5mm of SAG at 30%. so a 210x55mm shock will have an eye to eye endearment of 210-16.5= 193.5mm. If you measure off another component adjust your math as needed (such as spring perch to spring perch, etc depending on accessibility of the shock in your frame).

If with the minimum of 8 clicks your sag setting is LESS THAN 30% sag you’d benefit from a lighter spring rate.

Should 26 clicks find you at MORE THAN 30% sag a firmer spring rate is required.

Spring Rate Cross-Over with Pre-Load Adjustments

Here’s where it gets interesting: Variations in Pre-Load settings create overlap at SAG PSI – meaning you likely have a choice if you want to use the higher spring rate at a lower Pre-Load (which will have a more aggressive spring rate deeper in the travel) or use a lower spring rate at a higher Pre-Load (which will have less aggressive spring rate deeper in the travel).

Here’s an image from our calculator as a quick reference:

BikeCo eMTB and MTB Coil Comparison Example

You can see there is cross over at sag, highlighted in light yellow, between the higher Pre-Load Settings on the 250# coil as well as the lower Pre-Load Settings on the 275# coil.

Looking at the rest of the charts you can see that the higher spring rate coil will provide higher resistance as you pass the 30% SAG measurement. This is kind of like how volume spacers are used to adjust an air spring’s ramp rate.

Want to explore the crossover more? You can use the google sheet FOX Coil Spring Rate calculator here (opens in new tab).

Quick Details on Rising / Falling Rate Suspension

How much of the spring’s power is actually applied into the linkage varies in part tied to the angle of the linkage compared to the spring’s “push”. As this angle is variable with suspension linkage the percentage of the springs actual output to the suspension system will vary.

eMTB and MTB Rising and Falling Rate Suspension Concept

Rising / Falling Rate Suspension Illustrated

Above you’ll see a basic illustration showing two rising then falling rate suspension linkages.

What makes this important – a coil spring (or air spring) is only 1 to 1 effective when the spring’s power is pushing at 90 degrees to the linkage arm. On the upper graphic this is shown in the Orange details.

Continuing to reference the upper graphic: at full extension the spring’s effectiveness will be slightly less than the spring’s power rating. As the linkage rotates (clockwise in this case) to the 90 degree angle (shown in orange) it is a Rising Rate suspension.

As the linkage passes the orange 90% point until full compression it is a Falling Rate suspension.

How does this matter or effect you?

Well if you’re read this far I hope you’re learning something and it’s interesting right?

Let’s compare the red details between the upper and lower illustrations now.

In the upper illustration the angle between the spring and linkage is more exaggerated than the lower illustration.

This means at full compression the upper spring is exerting a percentage LESS of it’s rated power compared to the lower red detail which is closer to the 90 degree position, thus it is exerting a HIGHER percentage of the spring’s rated power to the linkage.

If I was considering two spring rates I would likely consider using the HIGHER Spring Rate with less Pre-Load in the upper concept and the LOWER Spring Rate with more Pre-Load in the lower scenario.

Is this somewhat splitting hairs? Of course. But, the data is out there, the components are out there so what harm can the knowledge do? (and, I hope that a disclaimer like this eliminates the snarky comments, or at least some of them!)

Final Thoughts on the Above Calculator

So, the calculator won’t tell me what spring to use? Correct. There are too many individualized factors in each bikes’ design to make that really feasible. This calculator ideally gives you a comparison point if you’ve started with some understanding or data point on your setup.

Your bike dealer or manufacturer should be able to provide you with some basic setup concepts and the calculator would provide a reference to compare options around what they suggest.

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