4WD MODIFICATIONS - SUSPENSION & BRAKES
Many of us are familiar with the design of off-road-race shock absorbers, with their external ‘bypass’ tubes. ARB’s BP-51 bypass dampers use the same principle, but with internal bypass technology. They’re vehicle-specific and new models are being added periodically.
Unlike a traditional velocity-sensitive shock absorber, in which damping force is determined by the internal piston speed, bypass technology allows the shock absorber to deliver varying damping
force throughout its stroke. The damping force at full compression or full extension can be up to 300 percent more than at static ride height, helping the shocker from slamming into the bump stop or from topping out.
In traditional bypass position-sensitive shock absorbers, oil is diverted around the shock’s piston and into external tubes, but while that works on off-road racing machines it’s not compatible with coil-over-shock spring designs.
ARB’s BP-51 (‘BP’ for ‘bypass’ and ’51’ for ‘51mm bore’) design team came up with the idea of an internal bypass system, diverting the oil around the piston through holes drilled into the inside of the shock bore and internal galleries in the shocker body.
This is why the BP-51 has an extruded aluminium body and also why it is able to incorporate bypass position-sensitive technology without the tell-tale external tubes of traditional bypass shock absorbers.
The large 51mm bore diameter allows for maximum internal oil volume, but that’s increased by the BP-51’s remote reservoir arrangement, which moves the ‘floating’ piston and gas from the main part of the shock body to a separate reservoir. Moving the floating piston increases the compressed length and therefore the potential travel over that of a conventional monotube shocker.
Another distinctive feature of BP-51 is the ability to adjust the compression and rebound damping, through a range of 10 clearly marked settings, using a C-spanner.
When the shock absorber is in ‘bump’ or compression action, fluid flows through the piston as it would in a normal monotube shock, but it also flows through four bypass tubes in the outside of the extrusion. Oil is pushed through the valving shim stack on the piston, but at ride height it’s also pushed through the four compression galleries that are within the body of the shock, but outside the main bore.
This achieves comfort at ride height, because the oil is not only going through the piston but around it, in the tubes.
As compression progresses the piston passes one hole at a time, shutting off that tube, so it goes from having four bypass passages to three, then to two, to one, to none as it gets to the end zone. In the end zone all the fluid has to pass through the piston, creating additional compression damping.
There is a similar increase in damping force
on the rebound side, as the shock absorber extends.The placement of the holes, drilled between the main bore and the bypass tubes, creates five zones of decreasing then increasing rebound damping.
ARB’s BP-51 has an aluminium shock absorber body and remote reservoir, as this was the only material suitable for the internal drilling required. However the chosen 6061T6 aluminium, typically used in the aircraft industry, proved difficult to extrude, but that problem was overcome.
A by-product of choosing aluminium was this material’s excellent heat-dissipating properties.
It became obvious after the initial release in 2014 that the BP-51 kit needed to be set up correctly by ARB fitters, taking into account suspension height increase over standard and the accessories and load for each application. Poorly set-up BP-51s show their displeasure by being noisy.
There were also issues with stone guards that protected the vulnerable shafts from flying rocks, but design changes seem to have overcome that problem.
The BP-51 range has gradually expanded, from the initial launch of LandCruiser 200 Series and HiLux models to include older, popular vehicles: Toyota LandCruiser 80 and 105 series.