Looking at shock details can be a bit daunting at the first. Picking the right shock details can be a bit overwhelming if you’re not familiar with what the numbers really mean. Below we’ve outlined some basics of a rear coil shock. Much of the same applies to air shocks as well. If you’re trying to switch to a different manufacturer’s shock from your stock one, make sure these numbers are the same as your old shock and that the shock will fit in your frame properly throughout the shocks entire range of motion.
As an example, we’ll be looking at the FOX DHX 5 Coil shock for this example:
Basics to coil shocks
A – Mounting Hardware
B – Eyelet
C – Preload ring
D – Bottom out bumper
E – Shaft
F – Eyelet
G – Mounting Hardware
The Shock Body
Most shock bodies will be listed in either inches such as 9.5″ x 3″ or in metric such as 241.3 x 76.2. There are many sizes out there and it’s important to use the proper length shock and correct stroke.
The first number (9.5″) represents the shock length. This is a measurement from the middle of one eyelet to the middle of the other eyelet.
The second number (3″) represents the travel/stroke that the shock can move.
Shock Length – A
Stroke Length – B
The Shock Mounting Hardware
With use, the mounting hardware/bushings can often develop play. This is normal as it protects the more expensive shock. If you notice your shock feeling loose, it is possible your DU Bushings or mounting hardware needs replacing.
Mounting hardware presses into each shock eyelet. The piece that typically wears out is the DU bushing. It sits inbetween your mounting hardware and the shock. You need a DU bushing press (CTS also makes one) to do this properly or make a homemade setup.
Marzocchi, FOX, Progressive take a 12.6 x 15.2 x 12.75 (mm) DU Bushing
Manitou takes a 12 x 14 x 12 (mm) DU Bushing
Mounting hardware is sold in a variety of sizes and its imperative you get the correct size for it to work correctly. Marzocchi, Cane Creek, Fox, Rock Shox Vivid, and Progressive rear shocks use 1/2″ (12.7mm) mounting hardware.
If you’re unsure of the hardware size you need, you can measure the eyelet of your shock body (make sure there is no mounting hardware in there though).
The next piece you’ll need to know is the bolt size (M6 or M8). This can be measured as well by measuring your bolts. M6 bolts will be 6mm in diameter at the threads, M8 will measure 8mm in diameter at the threads.
The last piece of information you’ll need is the length of the hardware. If you don’t know what your old hardware measured (the width of the hardware), you can measure this by measuring the where the shock would sit or consult your bike shop/manufacturer.
Take note that you need to measure both ends of the shock/frame as they are not always the same width.
The second major piece is the spring. The spring will also be labeled similarly to the shock body by defining the spring rating followed by the maximum stroke the spring can handle (like 500lb x 2.8). Fox for some reason labels their springs slightly differently and states the second number as the maximum spring compression at shock bind which can confuse you.
The first number in bold represents the spring rate. (500lb x 2.8)
The second number in bold represents the maximum stroke (or spring compression at shock bind in FOX’s case) (500lb x 2.8)
As an example, A FOX labeled with a 450 x 2.8 spring is the same numerically as everyone else’s 450 x 2.75 spring as they label their springs with the maximum spring compression at shock bind. This can cause some confusion when trying to source a non FOX spring for their shock (like a titanium one) as often people will be looking for a 2.8 spring, when it is a 2.75 spring they need if they’re not getting a FOX labeled spring.
A FOX 1.65 labeled spring = 1.5″ stroke
A FOX 2.35 labeled spring = 2.0″ or 2.25″ stroke
A FOX 2.8 labeled spring = 2.5″ or 2.75″ stroke
A FOX 3.25 labeled spring = 3.00″ stroke
If you’re sourcing a new steel spring, its best to get the same brand spring as you will need to pay attention to the inner diameter of the spring and the outer diameter of the spring as well as the free length (total length of the spring).
If you’re sourcing a new ti spring (or are using a spring from a different manufacturer), this can be be a bit daunting as you need to check that the inner diameter, outer diameter, stroke, and free length of the spring will fit your shock properly. If you do not, it could cause damage to you and/or your shock.
If you’re in a pinch, you can also use a longer rated stroke spring (but not shorter) in your shock so long as the spring meets the inner diameter, outer diameter, and will fit in your shock body.
example: Say you have a FOX shock that is 8.75 x 2.75 and you need a 450lb spring. You can use a FOX 450lb x 3.25 spring on your shock so long as the spring will fit inside the shock body. Using a different brand spring than the shock manufacturer, you will additionally have to make sure that the inner diameter and outer diameter are correct. It is important to remember that you can use a longer rated stroke spring, but not shorter. In this example, it would NOT be ok to use a 450lb x 2.5 spring. The spring stroke must be at least the same as the shocks stroke length otherwise damage can occur.
Lastly it is important to not over tighten your spring. Consult your manual to see how many turns you can do when preloading the spring. If you have to do more than 2 turns to get proper sag on your rear shock, you most likely need to step up to a higher rated spring (i.e. from a 450lb spring to a 500lb spring).
There are many spring calculators you can use to help determine the correct spring for you, but these can only give you a general idea especially if you have a linkage actuated shock. There is no substitute for real testing as even though a spring may say it is rated at 450lbs, spring tolerances (especially steel ones) will vary more.
Hopefully this has helped you understand a bit more about your shock and the numbers behind them to get the most out of your riding experience. Make sure to take your bike to a professional to perform any changes and ensure proper fitting.