Internal Bypass Coilovers Compared

Are you confused by the different kinds of IBP coilovers and not sure which to choose?
Here's a guide to the Pros and Cons of each 2.5" internal bypass coilover setup:

1. The Why and How of IBP's
2. ARB BP-51
3. Bilstein 8112
4. Fox IBP
5. King IBP
6. So which one to choose?

1. The Why and How of IBP's

External Bypasses let you quickly tune each of your zones with the twist of an adjuster, so why have an internal bypass at all? The short answer is, it just doesn't fit. The coils of a coilover ride very closely to the shock body itself and there is no room for adding tubes on the outside of the body of the shock. This is an interesting engineering challenge, and almost every brand making an IBP has a different solution for it.

Shocks operate based on a simple principle, shaft speed is directly related to damping force (not dampening force, unless reading this makes you sweaty). That means that if the shock moves at a certain rate closer to full bump or full droop, the valving will be the same. A bypass differs from a normal shock because it dampens based on shaft velocity and position. With this you can have a soft and comfortable "ride zone" while having firmer bump and droop zones.

What issue are they trying to solve?

A shock that is tuned to do well offroad will ride poorly on-road, and vice versa. ARB put's it well when they say "there are always compromises between comfort and control."

What they mean by this is that tuning your shocks for one scenario will make them worse at another. If you make a shock firm enough so you aren't blowing through your travel quickly off-road, you're going to feel potholes and expansion joints on-road transmitted to the chassis as jarring or chattery. A shock that corners with great control will lend itself to having poor comfort, another reason why anti-sway bars are a staple of any performance suspension. If your shock is comfortable and floaty on-road, you can infer poor handling and off-road performance. This makes intuitive sense, you wouldn't expect a cadillac to soak up whoops.

With Bypass shocks you can have a soft initial ride zone that will cruise over low velocity inputs like potholes or rough bits on the freeway, without being wobbly and uncontrollable around turns or evasive manuevers. When you take them off-road the softer initial valving can get the tires moving quickly over small stuff and with the compression and rebound zones you don't blow through your travel too fast or have harsh top outs from not controlling spring force. It truly is the best of both worlds for performance in any situation. You enjoy better ride quality and handling than a traditional shock absorber in any situation.

2. ARB BP-51


  • External Adjustability for both compression and rebound
  • Good warranty at 36k miles/3 years
  • Aluminum body has excellent resistance to rust and corrosion
  • Rear incorporates a rock guard for the shaft


  • Small 2" working piston can run out of damping especially at a motion ratio like IFS.
  • External Adjusters are hard to use as they sit behind the coils which gets in the way of the tool, also unclear what speed of compression or rebound is being adjusted
  • Rear setups use the same twin wall design and 2" working piston despite an external bypass with full size piston fitting in the same location
  • Can only be rebuilt by ARB, they will not send out seal kits for this product so you may be waiting if yours leaks or is damaged from harsh top outs when paired with their leaf springs.
  • No universal option.

The 51 in BP-51 stands for a 51mm main piston which translates to roughly 2". This is the biggest drawback of the twin-tube design that Fox and ARB use, is that it greatly reduces the size of the main working piston to that of a smaller shock. ARB uses an inner sleeve with holes in it to create bypass zones in a similar manner to Foxes design. It features externally adjustable compression and rebound, though it's unclear what part of the valving range these adjust (low, mid, or high speed, zone). The front adjusters are almost impossible to turn when on the vehicle as they are covered by the coil springs. The rear setups are also internal bypass which is a bit odd as an external bypass fits in the OEM configuration. When i say "OEM fit applications" I'm referring to a shock that bolts into your stock mounts and is valved to your application. OME/ARB have a great warranty on these at 36k miles or 3 years, however they cannot be rebuilt by any normal shop and must be sent in to be swapped or sent to their authorized rebuilder. I have installed these on a few vehicles and find the fit and finish to be great despite their limitations. This is a great product for someone who wants more comfort for mild trails.

3. Bilstein 8112


  • Full size 60mm working piston
  • 3 seperate pistons create 5 zones of damping
  • 2 stage Jounce Cutoff with a telescoping sleeve eliminates the need for an external hydraulic bump stop
  • Rebound Cutoff engages the 3rd piston to prevent harsh top-outs and eliminate the need for limit straps
  • Truthfully since it is not "bypassing" the main valving but rather using the additional 2 pistons to work in conjunction with the full size piston and telescoping hydraulic JCO, it's not technically an internal bypass shock. It's probably unfair to have these in this IBP category, however this is the category it most closely resembles and there isn't any direct competition in the Zone Control category yet so i'm lumping it in here.
  • Rear 8100 series shocks are 2 tube external bypasses with 60mm pistons and adjustable zones.
  • Offered in 3 way external adjusters, with high and low speed compression and an adjustable compression zone. When the rebound zone adjuster hits the market at the end of this year these will be 4 way externally adjustable.
  • Bilstein states these will last an average of 100k miles before needing rebuilt, excluding any physical damage to the shock.
  • Every single 8112 is Dyno Tested before shipping to ensure each zone is working to their specs at every speed.


  • Lead times can be longer
  • While the digressive piston does not necessarily have digressive valving, it can struggle with shaft speed over small inputs like large washboard.
  • Customer education and engagement is lacking even though Zone Control has been around for almost a decade (that's why it's here in this IBP comparison).
  • No universal fitment, the design doesn't lend itself well to a universal shock because the bump zone is engineered for a motion ratio.
  • The JCO engaging can be noisy
  • Coilovers aren't currently offered with external adjusters (newest version will have 3 adjusters)

The 8100 series shocks have been out for several years now and have been well received despite poor marketing efforts. The front is a coilover design that features a full size working pistons and 5 total zones of damping, while the rear is an external 2 tube bypass with adjustable compression and rebound zones.

The long service intervals for these coilovers is due to their seal head assembly which was originally designed for tactical vehicles from one of Bilsteins military contracts. The JCO assembly itself is made in 1 facility in Germany that also produces parts for supercars. These are serviceable by any competent shock retailer, though I have yet to have a set in the shop for a rebuild.

Bilstein invented the modern shock in the 50's and has been slow to innovate since then, however the 8112s are a huge leap in shock technology that has no direct competition. Nevertheless the digressive piston can struggle with shaft speed over small inputs like SxS chop, granted the few 8112s i've opened up have had linear/close to progressive compression valving. I believe if you look at these products objectively with no brand bias, these are the best OEM fit 2.5" coilovers on the market by a wide margin... if you can find them in stock that is.

4. Fox IBP


  • Offered with dual speed compression adjusters which greatly help fine low speed control.
  • Practically limitless options for tuning by adding or subtracting zones on the inner cylinder. The universal versions have flats machined across the entire length of the inner cylinder in order to accomodate end user fine tuning
  • Piggyback options feature a Rotating Reservoir Bridge that allows you to position the reservoir in any position 360 degrees around the shock body.
  • Great selection of 2.5 and 3.0 coilovers in many lengths
  • Foxes default JM92 oil outperforms the competition


  • Dual cylinder design creates a small working piston
  • Due to small working piston size this is not offered in OEM fit 2.5" coilover applications outside of UTV's
  • As with all IBP's, tuning can be difficult, however the low speed compression adjuster helps quite a bit.
  • Aftermarket support from Fox can be frustrating, although there are often a few lengths in stock with distributors.

Foxes universal fit 2.5 and 3.0 coilovers are offered in lengths from 10-16" for 2.5s and 12-16" in 3.0s. They use high quality JM92 oil that is rated to 400 degrees, the seals and shaft hardness are generally above industry standards. These are a great option for solid axle swaps and what i prefer to use in the link kits i have designed. I always recommend folks run these with the dual speed reservoir adjusters to give some control over low speed damping, which can otherwise be an overwhelming part of tuning IBP's. The 2.5 series coilover uses the standard valving kits for their 2.0 shocks and 3.0 uses the 2.5s, so having a good valving kit on hand that covers all normal stacks for them is easy. The rotating reservoir bridge makes running piggyback coilovers in a universal fit application feasible, as you dont have to stress about fitment issues. These are a great product for solid axle swaps where real estate is at a premium or for budget builds that want the functionality of a coilover and bypass with a cheaper upfront cost.

5. King IBP


  • Full size working piston
  • Final stage of the compression zone acts as a hydraulic bump stop
  • The tapered metering rod can be modified to change the bypass flow through the ported hollow shaft which will fine tune compression zone engagement.
  • Mid-range adjuster available
  • Offered in both universal and OEM fit applications


  • Least amount of bypass zones of any damper in this article, only damper without a top-out zone
  • There are some examples of metering rod reliability issues, if any foreign material gets into the hollow shaft, or if there's too much side loading, catastrophic failure can occur
  • Shaft hardness and seal quality average below industry standards

Kings IBP design is the oddest of the bunch we are talking about today. They use 2 seperate pistons both with compression and rebound valving, and a tapered rod that cuts off flow to the second piston to create a bump zone. During normal use both pistons are engaged and hydraulic oil can "bypass" the main piston to the secondary smaller piston. At full bump as the damper is transitioning to its rebound curve, only the main pistons rebound valving is working, you could design this to re-accelerate the unsprung mass of the wheel quicker to get into the extension stroke faster however from the modeling I've seen, it appears as if it uses initially firm rebound valving to dissipate stored energy in the springs which lends itself better to heavier sprung applications. This shock is a great option if you are racing in a class that doesn't allow external hydraulic bump stops and are looking for a budget friendly option.

6. So which one to choose?

Note: I attempted to have a section with an objective overview of customer service for all these shocks. I decided to email each company from a private email address and ask questions about the shocks. No company responded within the 2 day time constraint. I'll attempt this at a later date and hopefully have better results.

I sell, rebuild, an tune all reputable brands of shocks. That doesn't mean I don't have my favorites for certain applications though.

For OEM fit applications you cannot beat Bilsteins full size working piston and 5 zones tuned to your vehicle, no other brand gets close.

For Universal Fit applications like a 3 link SAS, Fox stands out as a clear winner with their universal IBP coilovers with dual adjusters.

For a Universal Fit application at a motion ratio (like IFS), you might be better off looking into King's 2.5" IBP with a full size working piston to dissipate the increased spring energy.

Whichever shock you choose, we are here to help!
Reach out to us at any time for unbiased objective advice, from builders who specialize in suspension.