Your first line of defence when battling frost heaves, ruts and cracks in the road is the fork on your motorcycle. The most basic fork is the telescopic fork. It’s a simple device consisting of two steel tubes that fit into aluminum sliders. An axle connects the sliders to the front wheel, and when bumps are encountered, the wheel and sliders move up the fork tubes, reducing the shock you’ll feel through the handlebar and seat. To return the sliders to their original position after hitting a bump, there are springs inside the fork tubes, and to keep the front end from pogoing endlessly, oil damps the movement hydraulically.
As with any lubricant, fork oil needs regular replacement. Here, we’ll cover changing the oil in a damper-rod fork, the most common type of fork used on motorcycles today. It is called a damper-rod fork because damping is controlled by oil passing through holes drilled into the side of a steel tube — or damper rod — located at the bottom and inside the fork. The easiest way to identify a damper-rod fork is by the absence of any damping adjustment on the fork; there are no adjusting screws at the top or bottom of the fork. This is not to be confused with the preload adjuster, which is a larger adjustment located at the top of each fork tube that varies the tension of the spring inside to adjust ride height. Damper-rod forks may have this type of adjustment.
When changing fork oil, you must first safely support the motorcycle in such a way that the front wheel is off the ground. Depending on your model, you may have to remove the individual fork tubes from the motorcycle to replace the fork oil. The Suzuki SV650 used here has no fork-oil drain bolts, so the only way to drain the fork oil is to remove the fork tubes, unscrew the fork caps and turn the forks upside down. If your bike has drain bolts, you can leave the forks in place, but you’ll still have to remove the fork caps (hence the raised front wheel – it relieves the tension on the springs). This lets air in as the oil drains out the bottom. Be very careful when removing the fork caps, because even with the front wheel off the ground, there may still be some spring pressure. It’s also through the top of the fork that you will add the new oil.
With the fork caps off, it’s a good idea to remove the springs – and possibly the factory-installed spacers – and wipe them clean. It’s also easier to refill the forks with the springs removed.
The fork oil capacity can be found in the service manual, though it is often included in the owner’s manual, too. This can appear as a measured quantity, as a measurement from the top of the fork tube, or both (as in the case of the SV, 491 ml per leg or 102 mm from the top of the fork tube with the spring removed and the fork compressed). It’s not uncommon to have different measurements for each fork tube.
Sometimes manufacturers offer a range of fork oil capacity. You can firm up the suspension a bit by adding the larger quantity of oil into the forks. This reduces the volume of air in the fork once assembled, and since the oil does not compress, it provides a firmer ride. Don’t exceed the manufacturer’s recommended fork oil level or you risk blowing the fork seals, which leads to other problems.
Another way to firm up a too-soft ride is to go to a heavier grade of oil. But don’t make extreme changes or you might have to start over again after realizing that the ride has become harsh. The SV’s recommended viscosity is 10-weight; I replaced it with 15-weight and got very good results.
You may want to improve the ride quality further still, so since the fork springs are already out, now’s the time swap them for a firmer or more progressive wind, available through aftermarket suppliers like Progressive Suspension. Always follow instructions provided with aftermarket springs and pay attention to the footnotes; you may have to cut a provided spacer to a specific length for your bike.
If you removed the forks to replace the fork oil, before reassembling everything, give the front wheel bearings a quick check. Place a finger in the centre of the bearing, apply some pressure, and spin the inner race back and forth. If the movement isn’t smooth and if you feel any ratcheting or grinding, it may be time to replace those bearings. Also, when sliding the fork tubes back into the triple clamps, make sure they slide in easily and turn freely in the clamps with the mounting bolts loose; if they bind, the fork tubes may be bent. And always torque the mounting bolts to spec.
Finally, with the front end assembled and the wheel still in the air, swing the fork fully from stop to stop and pull on the axle fore and aft, thereby putting pressure on the steering head bearings. This movement should also be smooth and free of ratcheting (indicating that there are no flat spots on these bearings), and there should be no free play when pulling the axle fore and aft. Free play means it’s time to hit the service manual for the procedure on how to adjust the steering head bearing.
The owner’s manual has recommendations on how frequently the fork oil needs replacing. Adhering to this will maintain your bike’s ride quality even with tens of thousands of kilometres on the odometer.
Technical articles are written purely as reference only and your motorcycle may require different procedures. You should be mechanically inclined to carry out your own maintenance and we recommend you contact your mechanic prior to performing any type of work on your bike.
