Every shock controls movement by pushing oil through valving, and that turns motion into heat. What separates the FOX designs is how each one handles the gas that keeps the oil under pressure, how much oil it carries, and whether the damping changes with shaft position. This guide walks through each design FOX uses on trucks and SUVs, then shows them side by side on one Jeep fitment.
The designs at a glance
| Design | What it does | Where you find it at FOX |
|---|---|---|
| IFP (internal floating piston) | A piston inside the body keeps the oil and the nitrogen apart | Most Performance Series shocks |
| Remote reservoir | Extra oil and the gas charge in a separate canister, mounted with a clamp or bracket | Performance 2.0 reservoir shocks, most Performance Elite, many Factory Race |
| Piggyback reservoir | The reservoir is fixed directly to the shock body | Many Elite and Factory Race rears |
| HTO (Hydraulic Top Out) | Extra damping at the very end of rebound | Many 2.5 Performance shocks |
| Internal bypass | Damping changes with shaft position inside the body | Factory Race 2.5, 3.0 and 3.2 truck kits |
| External bypass | Bypass tubes outside the body, each with its own adjuster | Built-to-length race shocks and two Raptor rear kits |
| Live Valve | Electronically controlled internal bypass shocks | Selected Factory Race kits |
IFP: the baseline FOX design
FOX says most standard steel shocks have no physical barrier between their gas and their oil. When the suspension works hard off road, the two can mix and the oil foams. Foamy oil is thinner, so the shock gives less resistance and the truck feels loose. An internal floating piston fixes that. It is a sealed piston that keeps the nitrogen charge on one side and the oil on the other, so the oil does not aerate. FOX uses it in most Performance Series shocks and in its truck bump stops.
Reservoir shocks: more oil, more cooling
A reservoir adds oil volume and moves the gas charge out of the main body. FOX lists "External Reservoir System Dissipates Heat Fast" as a feature on its reservoir shocks. More oil takes longer to overheat, which is the reason to choose a reservoir for long washboard roads, desert running or a heavy load. The data sheet names two layouts:
- Remote: the reservoir sits apart from the shock on a hose, so it needs a place to mount. Many FOX remote shocks come with a billet reservoir clamp or a universal bracket (fit-guide notes).
- Piggyback: the reservoir is fixed to the shock body, so there is no separate canister to mount.
For a general look at when a reservoir is worth it, see remote reservoir shocks vs standard shocks.
HTO: a softer top-out
Hydraulic Top Out is FOX's simpler form of position-sensitive damping, borrowed from its internal bypass shocks. FOX says it reduces the "clunk" when the shock reaches the end of its rebound stroke. That happens just after a speed bump, or when a front wheel droops on a rocky descent. In our range, HTO appears on many 2.5 Performance shocks.
Internal bypass: damping that depends on position
An internal bypass shock changes its damping with the shaft's position in the stroke. FOX describes the result as smooth over small bumps, with more control and bottom-out resistance when you drive hard. Some of its JL, Gladiator and GM 1500 kits list "Multiple Internal Damping Zones". Other kits list "Quick Switch Compression And Rebound Adjusters". FOX sells internal bypass in Factory Race truck kits from 2.5 to 3.2 in.
External bypass: race-shop territory
External bypass shocks carry the bypass in tubes outside the body: three, four or five tubes on the race shocks we list, with left and right versions. Almost all of them are built-to-length race shocks for custom suspension. The exceptions are two Ford Raptor rear kits, such as 883-09-141, which FOX lists with "Quick Adjust Bypass Knobs". These are not a bolt-on choice for a tow rig.
Live Valve: the electronic version
Live Valve puts an electronic solenoid in each Factory Race internal bypass shock. FOX says the system uses an inertial sensor and the vehicle's CAN bus data and senses terrain and driver inputs 400 times a second. It has On-Road, Off-Road and Custom modes. An example is the 2018-2025 Wrangler JL kit 883-26-160: 3.5 to 4.5 in of lift, requires a front driveshaft, and excludes 4xe and 392 models (FOX label and fit guide).
One Jeep fitment, five designs
| FOX part | Series | Body | Design | Extended | Compressed | Travel | Sold as |
|---|---|---|---|---|---|---|---|
| 985-24-177 | Performance | 2.0 in | IFP | 26.42 in | 16.8 in | 9.62 in | Each |
| 885-24-183 | Performance | 2.0 in | Remote | 26.52 in | 16.4 in | 10.12 in | Pair |
| 987-24-012 | Performance | 2.5 in | IFP + HTO | 26.82 in | 17.54 in | 9.29 in | Each |
| 883-26-050 | Performance Elite | 2.5 in | Remote, DSC | 26.51 in | 16.34 in | 10.17 in | Pair |
| 883-26-057 | Factory Race | 3.0 in | Remote + internal bypass, DSC | 24.54 in | 15.78 in | 8.76 in | Pair |
Which design for which truck
- Tow rig or daily driver: an IFP shock, 2.0 or 2.5, is the simple choice with nothing extra to mount.
- Heavy or long-distance hauling on rough roads (truck camper, overland trailer): a reservoir shock, for the extra oil and cooling.
- Fast trails and mid-travel builds: internal bypass, or Live Valve if you want it to adjust itself.
- Custom long-travel or race car: built-to-length external bypass, specified by the builder.
How the series compare is covered in FOX Performance vs Performance Elite vs Factory Race, and body size in FOX 2.0 vs 2.5 vs 3.0. Shop the FOX collection by vehicle. Found it cheaper? We'll beat it.
FAQ
Is a reservoir shock always better than an IFP shock?
Not always. It carries more oil and cools better, but a remote reservoir needs a mounting spot, and on a mostly-pavement tow rig an IFP shock may be all you need.
What does "emulsion" mean on some FOX race shocks?
FOX's data sheet uses "Emulsion" in the reservoir field for some built-to-length coil-overs, such as 981-20-100. FOX lists them as emulsion rather than IFP or reservoir shocks. They are race parts, not truck replacements.
Do bypass shocks need maintenance?
FOX describes its race shocks as owner serviceable and rebuildable. For service intervals, follow the FOX manual for your part.