Pictured: ARB OME BP-51 with 295/70R17 Cooper STT Pro + Ekstrom Designs Strut Shims & Rear 1″ Spacer
OME (Old Man Emu) BP-51 Bypass Shocks Techincal Overview, Height Adjustments & Compression/Rebound Adjustments
We recently installed the OME (Old Man Emu) BP-51s. In that post, we provided the full step-by-step install and our initial impressions. Since then we have put well over 5K miles on the suspension and tested it on-road, and off-road consisting of mud, sand, snow, and on the rocks.
In this post, we are going to dive into a detailed review that will cover a technical look at the shocks, compression/rebound damping adjustments, adjusting the preload, and general on/off-road performance.
If you don’t want to read the whole post, check out our quick trail takeaway.
Quick Trail Takeaway:
Straight out of the box, the BP-51s are plush. Without any adjustments, they provide a very comfortable yet firm on-road experience (less nose dive, and less body roll over the factory shocks). If you are coming from a stock suspension, you will notice a huge difference in control, stability, and general on-road driving manner improvements.
The on-road experience is great, and if you need to firm the shocks up a bit to increase handling, you can simply adjust the compression/rebound damping.
So on-road is great, what about off-road?
As the piston speed in the shock increases, the damping firms up quite a bit. That said, off-road the BP-51s have been really nice as well. When handling whoops and washouts, you barely feel big hits at 30-40mph. The internal bypass design does a great job at soaking up harsh hits with ease.
Coming from an Icon Stage 2
We are beyond impressed with them so far. After running the Icon Extended Travel Stage 2 shocks for about 2.5 years, I was relieved to have a suspension that isn’t so stiff on road, yet lives up to the hype off-road. What do I mean by that? I don’t think there is anything an Icon stage 2 can do off-road that a BP-51 can’t.
With a set of Icons, you will NEVER find a soft and comfortable on-road experience unless you have them custom-tuned to be more linear or progressive. Some refer to it as “plush” and others more like a “Cadillac” driving experience. At the end of the day, the BP-51s offer the best of both worlds; a great on and off-road experience. To take it a level further, with the BP-51s, you can adjust the shock body’s compression and rebound damping individually on each corner in order to dial in your exact ride comfort.
Compression and Rebound
For example; the BP-51s come with a 5 compression and 7 rebound setting. I thought that was a bit too soft on-road (naturally coming from the Icons) so I adjusted the compression and rebound both up to a 9 – just to get an idea of what that would feel like.
The 9 setting on both compression/rebound was a little too stiff (much like the Icons) so I brought them down to compression (7) and rebound (8) and that seemed to be the perfect setting for my needs personally. These settings allowed me to get them in between super plush/comfortable on-road all while finding a good firm road-handling balance.
For off-road use, we brought the shocks all the way down to a 0 setting on the compression and rebound. This was amazing! It’s comparable to airing down your tires. The suspension doesn’t have to work as hard and the ride is much more comfortable off-road. Instead of being bounced around, the suspension works so well in its ability to soak up the dips, rocks, and washouts.
Quick Q & A:
- How do you adjust damping? There is a shock adjustment tool in your kit – looks like a spanner wrench.
- To do you adjust preload? The coilover will need to be removed from the vehicle and the spring removed; reference for additional detail.
- Down travel and up-travel specs? Extended – 550.5mm (21.6″), Compressed – 440mm (17.3″).
- Are shocks rebuildable? Yes.
- 3-year/37,000-mile warranty
- Internal bypass shocks
- Progressive valving
- Remote reservoirs with advanced heat dissipation (also allowing increased travel of the shock body)
- Compression adjusters (all corners)
- Rebound adjusters (all corners)
- User-friendly valving adjustments
- Competitively Priced
Find them online:
- 3″ Heavy Kit (Non-KDSS) – OME4RNR10HKBP51
- 3″ Medium Kit (Non-KDSS) – OME4RNR10MKBP51
- 3″ Heavy Kit (KDSS) – OME4RNR10HKBP51KD
- 3″ Medium Kit (KDSS) – OME4RNR10MKBP51KD
The BP-51 Concept
Before you understand the BP-51, first you should learn about shocks.
In their simplest form, shock absorbers consist of a body containing working fluid (oil) and a piston connected to a shaft. The piston divides the body into two chambers (top and bottom). When the shock is compressed or extended, oil flows through the piston from one chamber to the other. Oil can flow back and forth through the piston fast or slow. The rate at which fluid flows through the piston is typically determined by the size of the holes in the piston and/or the combination of shims stacked on each side of the piston.
The holes in the piston and the shims stacked on the piston have a job and that is to generate the compression force in one direction and rebound force in the other.
In simple terms:
- More shims on the piston = more restriction to the oil passing through the piston.
- The greater the thickness of the shims = more restriction to the oil passing through the piston.
- More restriction to the oil passing through the piston = more damping force
- More damping force = slow compression/rebound
- Slow compression/rebound = more control
Speed is also important:
The shape of the piston and the way the shims are stacked/arranged both affect damping but in general the faster the piston moves, the more damping is generated. This is known as velocity-sensitive damping and bypass shocks handle velocity-sensitive damping in a very specific manner.
For a better understanding of how shims, shock pistons, and damping works, accutuneoffroad.com has a few good videos that explain it.
When tuning these progressive bypass shock absorbers, compromise is usually reached at a point. The goal is to achieve a soft and comfortable ride that can absorb smaller bumps and daily driving imperfections while maintaining some control, yet firm enough to provide improved control to keep away from harsh bumpstop contacts and top out events when larger bumps are encountered.
How Damping is Generated?
- Piston Design
- Position of the piston in the shock absorbers body
While most shocks only generate damping based on fluid flowing through the piston, the BP-51 offers an alternative path from one chamber to the other around the piston in the form of bypass passages in the body of the shock.
What are the bypass passages?
These passages are essentially tubes around the main piston bore towards the outside of the shock absorber body extrusions. Ports from the main piston bore to the bypass passages allow the fluid to pass around the piston from below it to above it in extension, and above it to below it during compression. Bypass flow during the compression stroke is shown by the blue arrow and through the rebound stroke by the red arrow.
At the end of the bypass passages, the fluid passes a regulator that also has shims to control the rate at which fluid can pass through the bypass passages. The resulting damping is a combination of the flow through the main damping piston and the flow through the bypass passages around the piston. The ports from the main bore that allow fluid to enter the bypass passages are placed at different positions so that different levels of damping can be generated at different positions of the shock absorber stroke.
In the compression direction, there are four passages of different lengths to provide bypass around the piston.
When a bump is encountered and the piston moves to compress the shock absorber, it will progressively move past and block ports that provide access to the bypass passages. As the piston approaches the end of the shock housing, four passages become three, then two, then one, and eventually none, leaving the piston as the only passage for fluid to move from the compression chamber to the rebound chamber.
The result is increasing compression damping as the shock absorber moves towards its fully compressed position.
There are large improvements to the ride/control compromise that result from this bypass design. During normal on-road use (where the piston is situated around the middle of the stroke), the damping force provided by the shock absorber is relatively low so the ride is comfortable as minimal force is transmitted to passages. However, when the vehicle is driven off-road and subjected to much higher impact forces and travel, the damping force provides significant increases to minimize heavy bump stop contact.
In simple terms:
- Slow = softer ride and less control.
- Speed = stiffer ride and more control.
In a similar way, two rebound bypass passages contribute to controlling the extension of the shock absorber. When close to fully compressed, there is no access to any ports, so all the fluid has to pass through the piston.
This provides significant damping to control the potential energy of the compressed spring. As the shock absorber extends back out to its original state, two ports open to provide softer rebound control. As the shock extends further, it moves past one port and then another to ramp up damping as it approaches full extension.
As with compression, in the rebound end-zone, all fluid must flow through the piston shims so the damping reaches its maximum performance to reduce or prevent top out events.
Each BP-51 is tuned for vehicle-specific applications, however, compression and rebound bypass flows can be adjusted independently by the user. This is one of the biggest selling points of the OME BP-51 shock. You can essentially fine-tune the shock piston performance by adjusting the compression and rebound damping.
Adjusting your BP 51
The compression adjustment ring is at the shaft end of the shock absorber body and is adjusted using the adjustment tool provided. For more compression damping, turn the compression ring to the right. For less compression damping, turn the compression ring to the left.
Warning: Do not adjust the shocks while they’re hot.
Shock absorbers must be clean before adjusting. This will help prevent dirt and grass from entering the shock. To start the compression adjustment process, park your car with the corner you are trying to adjust in a drooped condition to gain access through the coil spring. Simply jack up the 4Runner and droop the wheel hub. You may want to remove your tire to gain access although it’s not needed.
Compression damping troubleshooting
- Suspension bottoms out easily on larger bumps: Increase compression damping
- Suspension feels harsh and does not soak up small bumps: Decrease compression damping
- Shock absorbers regularly bottom out, even off big drops: Decrease compression damping
- Shock absorber is not using all of its available travel: Decrease compression damping
Adjusting damping for weight or towing
Adding a load to the 4Runner will typically require more compression damping, particularly in the rear of the 4Runner. If you run your 4Runner unloaded, decreasing compression damping will result in improved comfort. If you often tow or run a heavy rig, consider adjusting/increasing the compression of all shocks but more so – the rear shocks.
Adjusting damping for surface conditions
Different driving conditions require different levels of compression damping. Driving in sand dunes for example would require significantly increase compression damping to prevent suspension bottoming out. Comfort is usually less of an issue while on the stand as tire pressures are typically reduced. Hard-packed surfaces comprised of small bumps will require less compression damping.
Rebound damping controls the rate at which the shock absorber returns after it has compressed. You will probably find your perfect rebound setting overtime as it changes with load, driving style, and driving conditions.
Rebound damping troubleshooting
- The vehicle bounces in carries on repeatedly after large bumps: Increase rebound damping
- Suspension tops out too hard: Increase rebound damping
- Wheels lose traction on gravel Corners with corrugated surfaces: Decrease rebound damping
- The suspension feels harsh: Decrease rebound damping
- Packing and repetitive bumps: Decrease rebound damping
- Loss of traction when braking or accelerating on corrugation: Decrease rebound damping
Ride height adjustments
The BP-51 spring preload is adjustable. Depending on how heavy your rig is will depend on what preload setting you need in order to achieve the appropriate ride height.
To achieve “optimal performance” from the BP-51s, ARB recommends that you have 60mm (2.5″) of travel available in the rebound droop. They also recommended that ride height adjustments are performed by a qualified shock installer.
My biggest problem with this suspension that ARB really pushes the fact that you can’t and should not adjust the shock pre-load on the ground. ARB recommends that you completely remove the coilover in order to adjust the preload. I don’t know about you, but removing a coilover on a 5th gen is not what I wouldn’t necessarily call it quick process – especially with KDSS. Removing a coilover just to adjust a quarter-inch of preload is very frustrating. This is honestly the only downside of this suspension.
As with all suspension components, periodic inspection and maintenance can help prevent premature suspension or chassis component failure. You should keep your suspension clean for optimal performance. Clean your 4Runner after use in the mud, sand, or any other abrasive conditions.
Avoid using a pressure washer directly on the shocks as this can drive dirt past the shaft seal and O-rings. Also, check for damage to the stone guard after extended periods on gravel roads.
The only downside of this kit is that ARB recommends removing the coilover for adjusting the pre-load. It’s really pretty irritating that you can’t just droop a corner and adjust your threaded shock body – you literally need to take the coilover out and adjust the shock. Not a deal-breaker but it’s something to consider.
But as with everything, all things considered – this is an absolutely outstanding suspension system for the money. If you are looking to run a shock that is fully adjustable in terms of ride height and compression/rebound damping all with remote reservoirs on all corners for around $3k – it doesn’t get better. This is the most feature-packed suspension system for the 4Runner (for the money) to date. We have been running this on Jade (The MGM 4Runner pictured) for over 6 months now and it’s done everything the Icon Stage 2 has done and much more.
I love the adjustability in the compression/rebound, and if you wheel often, you will too. This system really lets you dial in your ride quality on and off-road.
The best part about these shocks is that they focus on both the daily driver and the off-roader. Whatever percentage that is for you, you can really fine-tune these to be the goldilocks of shocks for your specific needs.
Questions and Comments? Leave them Below!