FOX Steel Coil 550 lb/in 3.25″ Stroke 6.7″ Total Length


This is the 550 lb/in FOX Steel Coil, 3.25″ Travel and 6.7″ total length
FOX Part Number: 039-09-005
BikeCo PN: 550 # cofox1044 3.25*6.7

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SKU: cofox1044 3.25*6.7 Categories: , , Tags: , ,


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Getting the right coil on your bike will make a huge difference in performance.  Read on to learn more about the importance of the spring rate, stroke, total length as well as how Pre-Load settings can produce similar sag settings on different coils allowing for modifications to bottom out resistance.

This is the 550 lb/in FOX Steel Coil, 3.25″ Travel and 6.7″ total length
FOX Part Number: 039-09-005
BikeCo PN: 550 # cofox1044 3.25*6.7

FOX Steel Coil Spring black

Getting the right coil on your bike is the difference between the perfect setup and something that’s never quite right.

Click through these tabs to learn about the physical dimensions to take into account for Coil Spring Fitment, Tuning a Coil: Spring Rate, Pre-Load and Ramp Rate as well as the ability to compare spring rates to help understand which is the best setup for your needs!

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).

This calculator DOES NOT take into account bike specific details such as leverage ratios, rising / falling rates, etc. It is available to easily compare cross-over as well as spring specific output.

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.

Here’s a look at some coil tuning basics from a previous blog here on

Coil Spring Tuning Basics

With the launch of the new Yeti SB165 we’ve had a lot of questions. Here we go with some Coil Spring Tuning Basics.

Some very basic principals to start with here.

Coil springs are rated by the weight that creates an inch of spring compression. So a 400lb spring requires 400lbs to compress the first inch and would take 800lbs to compress two inches. However, coil springs are not produced to “exact” standards. That is to say if a coil is offered in say 50lb increments ( example: 350, 400, 450) 400lb spring may actually we anywhere from 376 to 425lbs. That’s a pretty wide range and most quality spring manufactures will have a slightly tighter tolerance, but that’s the concept.

Coil springs are also measured by total length as well as available stroke. Running a coil that is too long or short in either measurement can create problems.

A coil spring provides support based on its diameter, the actual coil’s diameter, how much elasticity the material has, how many active coils it has as well as the angle of the active coils.

Less active coils increases the spring’s resistance to compression. As the spring is compressed the furthest coils contact the wrapped ends. (most MTB springs have closed and ground ends for performance) The furthest active coils will have a lesser distance to the end coils which is why they make contact and, well, work.


Coil Spring Active Coil Contact during compression

Coils interaction with spring ends or the decrease in active coils is highlighted, left to right, in this illustration as the spring is compressed. Which leads us into the next basic concept tuning coil springs.


Preload on a coil spring applies additional tension to the spring. It uses the same theory as above by decreasing the distance between the end coils and active coils.

Coil Spring Preload as first coil is contacted.

The illustration above shows 2.5mm of preload on the left image. Tighten the upper spring perch to increase Preload. Loosen it to decrease Preload.

As a note – all springs should have at least 1 turn, or 1mm on Fox shocks, of preload to keep the spring in place. Don’t dial in tons of preload turns as it can stress the damper as well as create problems with coil bind if the spring’s freelength to full bind is smaller than the stroke of the damper.

So when do you use preload? Well, again assuming you have a spring with enough travel capacity, you can utilize preload to adjust your sag setting or modify the spring’s resistance.

Wait, you can adjust a steel spring’s resistance? OK, not exactly. You’re shifting how much force is applied at what point in the stroke.

Let’s look at a 450lb spring compared to a 400lb spring with 1mm of preload as well as 3.5mm of preload.

Graph illustrating 400 450 coil spring rate comparison and potential crossover with preload

So unlike an air spring a coil produces a linear ratio of force to compression. When we apply preload we have a force at 0mm of travel. Remember, Fox coils require a minimum of 1 turn which produces a small amount of preload.

The black line represents a 400lb spring while the purple line represents a 450lb spring with 1mm of preload.  The red and orange illustrate the same progression rate, ie the lines are parallel, but the additional preload forces the line further to the left of the graph.

The higher force allows for a heavier rider to set the sag at the same percentage as a lighter rider using the same spring.

Perhaps more interesting, in fact, to me 100% more interesting, is how we can find a crossover point between two different coils at sag with a relatively minor preload adjustment.

The difference between the red 400lb 3.5mm preload and the purple 450lb 1mm preload at sag is quite minimal. However as more of the shock stroke is used you see a widening digression between the coil’s force. The heavier spring will still ramp harder than the preloaded lighter spring. So while your sag is the same how you run through the travel is not. The lighter spring might be too linear for the compression controls to correct. Or, you may leave travel on the table if the heavier spring ramps too hard in combination with the suspension design.

Got it? Coil Spring Tuning Basics done right? Well, you’ve got what the coil is doing at this point. But there’s some other factors we need to touch on…

Angle Correction Factor

The angle of force applied to the spring changes how much force is transferred from the spring to the suspension. This is the Angle Correction Factor which is the cosine of the angular difference. So, the only instant that your spring is actually returning 100% efficiency is when the linkage is at 90 degrees to the shock.

For instance a 20 degree input angle changes the force by 6%. More input angle change means less “efficiency” from the spring. This isn’t a direct fault of a coil – but there are design considerations for it.

 Coil Spring Angle Correction Factor from rising or falling rate linkage design


Both images above show rising to falling rate suspension.

The upper image shows a substantial falling rate compared to the bottom. Illustrated by the greater angular change.

The lower image shows a suspension that has a rising rate past the sag setting as illustrated by the acute angle of the gold lines.

What’s this mean on your setup? Well, it’s important to have an idea of the suspension design in order to quantify what’s really happening where. Let’s say you’re debating the 400 versus 450 spring based on the sag crossover point. A setup with a substantial falling rate will benefit from a stiffer spring such as the 450lb with no preload illustrated above. The lower suspension doesn’t lose as much mechanical advantage on the coil and the preloaded 400lb spring would work better.

The ratio that a suspension loses mechanical advantage from the angle correction factor is a big, big reason that some bikes will work excellent with a coil while others become wallow-y garbage.

A bike designed around a coil like the new Yeti SB165 factors the coil’s personality with the suspension’s mechanical design.

Want to learn more about rising and falling, progressive versus linear? You can read more about suspension rates here as well.

Yeti SB150 Custom Build by profile


This is where coils really get good in the modern application. I keep mentioning the Yeti SB165, so I’m going to stay with it… The frame is spec’d with the Fox DHX2 damper. This shock features high and low speed rebound control as well as high and low speed compression. Oh, and a 2 position pedal platform. So you can “lock out” (they’re never true lock outs because that detonates shocks) your 165mm coil travel Enduro / Park slayer for the climbs? Ya. You can.

But more than that you’re able to compensate with these controls for the performance of both the coil system as well as the mechanical suspension design of your bike.

One of the most popular BikeCo Pro Tunes are the Fox DHX2. There are a couple reasons. First our Pro Tune service personalizes the damper settings for your weight, riding style, etc. Second, and equally important really, you have access to our team of suspension experts to help you dial in the shock after the tune! Our team lives MTB.  There are few things in our world as satisfying as the “OH MAN, it’s dialed” phone calls with clients.

Tuning the low speed compression helps the coil spring’s efficiency under power without hindering the ultimate small bump compliance. High speed compression compensates for ramp rate. This is particularly good if you’re putting a coil on a bike originally designed for an air shock.

In Conclusion

Well, at least conclusion for this blog – this is an amazing rabbit hole to go down if you’re interested in suspension by the way. I’ve worked with automotive, motorcycle and bicycle suspension and you’re always going to learn something. Coil Spring Tuning Basics? With how much these systems all interact it might seem like there is nothing basic about it!

What did you learn from this? Coil springs aren’t exact. Coil springs can have notable cross over in performance setups. Your suspension system changes how the coil behaves.

What can you take from this? Well, candidly – hold off on the expensive spring at first. We suggest clients start with the inexpensive option first. In fact its very common for our clients to order two springs at the time of purchase. This allows riders to fully understand the setup. Besides, if you decide one is leaps ahead of the other, a coil spring always makes a cool pen holder for your desk at work…

Questions? Like we said these are just coil sprint tuning basics. Reach out to our expert staff for the ultimate buying experience. Our team will ensure you get the best spec, setup, tune and pricing in MTB.

See you on the trail!

Yeti SB165 Frame detail in wash basin

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Check out a quick video to learn more about how your MTB suspension functions and how each setting interacts with the others.

Buy with BikeCo: Advantages

Shopping suspension? There are notable reasons to work with The Bike Company. We care about Expertise & Experience: Yours!

Our staff is excited to help you get the right product, setup for your riding. Whether you’re looking to understand your product front to back or simply want to know you got the best product for your needs dialed in for your riding we are here for you. Pro Tune Suspension

Ready to take your Fox Factory DHX2’s performance to the next level? Then it’s time for BikeCo’s Pro Tune Suspension.

BikeCo’s suspension tuners take Fox’s excellent performing forks and shocks and personalize the tune parameters to your specifics.

We will contact you for a riding bio to assess your riding level, terrain, riding disposition, ground speeds, etc. Our proprietary modifications then narrow the fork’s performance window, and thus improve the adjust-ability for you!

Out of the box Fox makes a great product. But it has to “fit” everyone. From the lightweight, smooth rider to a biggest, burliest basher. And everything in between. Well, chances are you’re somewhere in a narrower window with your riding… Let’s dial it in for you.

After Sales Service from

Another aspect that sets BikeCo apart is our after sales service. The best components for each budget is the first step. Dialing in the exact setup combination is the next step. No other shop has the experience to help you understand and setup modern mountain bikes.

Our team will reach out with a variety of loaded questions to help you find that “ah-ha” moment with your bike. A click here, a turn there, a bit of stack, a couple PSI – it can be intimidating. But not for BikeCo clients. We’re here by phone, email, chat or in-store to help you enjoy your MTB.

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Coil Spring LB/IN









550# 3.25" Travel 6.7" TLG



Coil Spring Compatibility

225×75, 250×75, 9.5×3