WR450F Rear Shock Rebuild
As with many motocross and enduro motorcycles, the Yamaha WR450F comes stock with a Kayaba rear shock. According to the manual, this shock is maintenance-free and must be replaced in case of malfunction. This is due to the fact that there is no valve with a nipple to control the pressure in the tank, and the bladder itself is only good for 20–30 punctures, so it is impossible to regularly check the pressure. If the pressure drops, first, when the shock heats up, cavitation occurs and the oil fills with air bubbles, and second, the bladder may leak air due to a looser fit against the reservoir body. Professional shocks are equipped with a valve and a piston instead of a bladder, which guarantee tightness and allow you to repeatedly check the pressure.
The cost of a new shock is comparable to the cost of a second-hand motorcycle, so it is cheaper to repair, despite the "maintenance-free" label, and practice shows that it is quite realistic and works well. See below for detailed videos on how to remove and repair a shock absorber, as well as read about interesting moments that were not included in the videos.
Shock absorber leaking
If the shock is leaking, you must first check the pressure in the tank and pump it up if the pressure drops. If the pressure is normal, and the shock leaks, the oil seal needs to be replaced. It may be necessary to bleed air through the compression valve.
Repair kit
Repair kits are different. The video shows the most expensive option: a sleeve with a pre-installed seal, dust boot and guide ring. You can buy the seal, dust boot and ring separately, and independently replace them in the sleeve — it will be cheaper! It is advisable to change the bladder when disassembling.
Oil
It is best to use oil for the shock absorber, which has a viscosity index of around 400. This oil is not sold everywhere, so most people use 5W or 2.5W fork oil (viscosity index 100-150). The viscosity index characterizes the dependence of viscosity on temperature; the lower the value, the thicker the oil becomes when the temperature decreases, and thinner as it rises. In practice, the fork oil thickens in winter and the shock becomes very stiff. If you do not ride in the winter, then the fork oil is ok.
Reservoir cap
On the 2012 WR450F there is a tank cap without a valve. Such plugs need a special needle. We replaced this plug with a plug with a valve from the WR250F 2008 — it fit perfectly and solved the problem with the needle. In fact, finding a plug was not easy.
Rod nut
On Kayaba shock absorbers, the staking on the nut can be ground off completely — this allows you to save the nut. But on some shocks (for example, Showa), the staking holds the adjustment mechanism for the rebound, so it can not be ground completely. As a rule, the nut is secured with high-strength threadlocker. It is important to degrease the thread and let the threadlocker cure before reassembling. The most reliable option is welding.
Rear shock assembly
There are many videos on the internet of a wet shock absorber assembly: it is filled with oil, the reservoir and rod are installed, and the oil overflows. The popularity of the method is incomprehensible, because a lot of oil is consumed, it is very difficult to push the rod plug and install the retaining ring, plus often there is air in it. The only plus is that you don’t have to unscrew the compression valve, but it’s not hard to do. We did a dry assembly, which is simple, reliable, and requires minimal oil.
Air bleeding
After bleeding the shock absorber there should be no air — it can be heard when the rod is moving. If the air remains, it can be released by loosening the compression valve plug when the shock is in a vertical state. The pressure in the tank will drop, so it must be pumped up.
Air or nitrogen
The tank must be pumped with nitrogen. Nitrogen molecules are larger, so it is better retained. Nitrogen has less thermal expansion, so pressure is maintained at different temperatures. Nitrogen does not heat as much under pressure. In reality, 78% of nitrogen is in the air — so there is no tangible difference. Equipment for filling nitrogen under high pressure is very difficult to find. The experience of many motorcyclists shows that air also works fine. It is necessary to regularly check the pressure, and there will be no problems.
It is recommended to service the bearing on the removed shock absorber. Strictly speaking, the shock absorber bearing is serviced along with the linkage, but there is not always time and desire to remove the shock absorber for this.
Hopefully this information is helpful. Send criticism and comments by email or in the YouTube comments!
