GPZ 900R Anti Dive system

There is nothing more frustrating than replacing fork seals, and then discovering weeks later that the job needs to be done once again. In virtually every instance of fork seal failure, the problem lies elsewhere within the suspension, whether it be pitted stanchions, worn bushes, or seized anti-dive units. You must always treat a blown fork seal as a symptom and not the problem.

Anti dive systems featured heavily on motorcycles designed in the early and mid eighties, and there are two viewpoints as to it's merits…"Love it, or Hate it." Sadly the number of owners who have grown to hate it, has increased in recent years due to the simple fact that the anti dive units fitted to the GPZ range are growing tired, few people comprehend their function, and for those that do, spare components are not available from Kawasaki. Replacement units however are, but carry a £100 price tag. Many of the units have been bolted on to lower fork legs, and served their masters valiantly for over 15 years, without so much as a squirt of WD40. It is important that owners understand these units, and armed with this greater understanding can then help to avoid costly fork seal problems, not to mention handling difficulties. Of course A7 & A8 owners have none of these concerns, as anti dive units are not fitted to these models.

A minority of owners choose to disconnect the anti dive units, mainly through ignorance of how they work, but also because of the very poor availability of spares. This feature aims to address the educational aspect, and also introduce the new anti dive unit repair kits, available solely through the GPZ Zone.

What are Anti Dive Units, and how do they operate?

The 900R's anti dive system is designed to prevent excessive front suspension compression under heavy braking. This allows for later braking without loss of control when entering corners at speed. It also gives greater control during emergency braking procedures.

These anti dive units use a variable control valve in the damping system of each of the fork legs. This control valve is housed in the finned unit which is bolted to the front of the lower fork leg.

When the front brake is applied, hydraulic pressure in the brake line is diverted and operates a plunger in the lid of the anti dive unit, this plunger then closes the control valve in each of the anti dive units. As the braking pressure increases, the control valve closes further and restricts the flow of fork oil, thus stiffening the suspension, and preventing excessive fork travel.

When the brake lever is released a spring returns the control valve to it's resting (off) position, allowing the front forks to return to their normal operation, when they are allowed to respond smoothly to varying road conditions, but once the front brake is applied they become stiffer.
These anti dive units feature a "Dial" adjuster at their base, which is numbered 1-2-3, allowing the rider to select the amount of anti-dive required. 1 being the minimum, whilst 3 is the maximum. (Stiffest).


What goes wrong?

The commonest, and perhaps most severe fault is that of the control valve seizing on, and thus permanently stiffening the suspension unit. This leads to handling peculiarities, and frequent fork seal failure. These seizures can be caused by any, or all of the following:-

a:- In the majority of cases a seized anti dive unit is the result of a weak return spring. (Most of these springs are over 15 years old, and have had a lifetime of being "loaded up" every time the front brake is applied.) Perhaps understandably they reach a point in time when they are incapable of returning the control valve to the resting (off) position. Other causes can be unserviceable O ring seals which tend to grab the control valve and prevent it from returning to the off position.

b:- Air in the brake system's hydraulic line which feeds the anti dive unit. This means that the plunger, in the lid of the anti dive unit is not actuated under braking and seizes up.

c:- Corrosion, and sludge build up within the anti dive unit.

d:- Leaking hydraulic fluid, or fork oil, is almost always down to a failed O ring.

e:- In a few cases, the rotary adjuster simply spins and has no affect on control valve settings.

How can I maintain my Anti Dive units?

It is not difficult.

1:- Always ensure that all of the air is bled from the front brake hydraulic lines, there is a bleed nipple on the lid of the anti dive units.

2:- Whenever you drain the fork oil, or strip the forks, get into the habit of fully stripping, and cleaning the internals of the anti dive units, and check their operation.

3:- Consider overhauling the anti dive unit, by renewing, the O rings, and return spring. Nb: these individual components are not available from Kawasaki, they simply recommend that the unit is replaced. The kits are available from our