How do I replace the fork oil?
Amount needed: Buy a quart (or two pints) for ~$10 and be assured of having enough.
Oil change interval: The service schedule calls for the changing of the fork oil every 18,000 miles. Experienced suspension people in the club recommend 10,000 miles or two years, whichever comes first. Most people who do a lot of long distance riding, and put on more than 10,000 miles a year, will just change theirs annually. The more "sporty" your riding technique, the more important changing your fork oil is.
Important: It's not recommended to change fork oil with the forks on the bike. You can drain most of the oil, but the REALLY nasty stuff that you want gone manages to sit below every drain bolt on every fork. Even using an oil extraction kit leaves the cleaning a bit neglected because you just can not get it all.
Simple job, takes about an hour from rideable to rideable for a competent mechanic. About 3 hours for someone doing it for the first time.
Fork oil level
The Kawasaki Service Manual has, from time immemoriam, recommended a fork oil level of 226mm for the 86-87 and 205mm for the 88-07, but we're here to say that that number is not correct. Rich Desmond of Sonic Springs, who works with this stuff every day, says that there should be more oil in your forks (remember that, since you are measuring down from the top, a smaller number means there is more oil/less air space). He recommends a level of 115mm, and several of the guys in the club have done so and report no ill effects. Instructions for Race Tech GVEs specify 130mm. Others have gone with 200mm and say that that level works fine. The FAQ recommendation is to choose one of these figures and then do some tuning if you have any doubt.
Note from Payne: I've historically (60,000 miles) run a fork oil level of 205mm per my manual, but based upon the FAQ I raised it today to 160mm and I've got to say, Wow! I could tell an improvement as soon as I left my driveway. Rode for 60 miles, and I definitely prefer this feel. The front feels "happier" and not quite so "nervous" and harsh. Seems to give a better feel for what's going on up there. This is with 15 weight oil and no modifications.
Tuning the front suspension
Tuning your front suspension is not difficult and is actually a good idea, as everyone has a different riding style and rider weight. The worst that can happen is you might bottom out the forks, which is just hydraulic lock: nothing touches. You can determine "bottom" by fully compressing the forks when the springs are out and making a mark on the fork tube just above where the dust seal comes to. Brian likes to use a center-punch to put a good-sized ding in the tube. This will never go past the fork seal in normal use; you just have to be aware of it when installing new seals. Whatever you do, just make sure you'll be able to see your mark after you ride it for a while. Then, when you put the forks back together, make sure you put that mark in a place where you can see it. When you start testing, run a zip-tie around that fork leg. This is an indicator: If the zip-tie is pushed up while you're riding and covers the mark, then you've bottomed.
Start with 200mm of fork oil level. Try to bottom with hard braking, hitting sharp bumps, and cornering hard. If you don't bottom, it's not a worry. If you bottom the forks, add 10ml to each leg and try again (or measure 5mm more oil; it should be close to the same). Keep doing this until you bottom on intentional maneuvers, but don't often bottom in daily riding. The goal is to use as much of the suspension travel as possible, as much of the time as possible. If you don't bottom out though, or even get close... then there's too much oil.
What kind of fork oil should I use?
In the past, it has always been the recommendation of the FAQ to use only oil marked 'For Damper Rod Forks'. After researching many of the major suspension fluid companies in late 2007, we have found that they all carry oil labeled as 'for damper rod or cartridge forks'. So, make sure that the label at least mentions that the oil is compatible with damper rod forks. It appears as though there is no longer a distinction between cartridge and damper rod fork oil, probably because it added unnecessary confusion when there's absolutely no cross-over. Anything less than 10 wt is too light for damper rod forks (the kind used in the Ninja 250). Cartridge forks use oil that is lighter than 10 wt.
Weight is determined by a few things, but mostly the fork build. Because of the design of the fork internals, the weight/viscosity of the oil determines the damping. Generally you look to tune the rebound damping with the fork oil, but the oil will change its properties slightly as temps get colder. So, most riders tend to go with an oil that is just a little too light for summer but a little too heavy for winter, thereby just changing the oil once a year. Here's where a clear advantage for cartridge forks comes into play; lighter oil, with more consistent results year round.
You can probably see that we're not giving you a specific number for what weight of fork oil to use. You're going to have to experiment to find the right weight for you. Experienced riders say to use no less than 10w and no more than 20w. You'll probably be better off starting with a light oil and moving to a heavier one if you feel the need to. A heavier weight will give more damping, but also a slightly harsher ride. Different manufacturers have different standards for weight, so a 15w from one company may feel slightly different than a 15w from someone else.
Oil volume (or level, which is FAR more accurate) is determined by the manual. See the notes above on oil level. What it does, though, is set the air spring, or the captive air volume in the top of the forks. For the most part, all this does is adjust how easy it is for the rider to use the full travel of the forks (which is desirable). So, a heavier rider, who may bottom forks out more easily, would want to add a little (just a couple mm worth of height, never more than 5 at one time). A lighter rider may want to remove some. It has practically no effect on "stiffness".... it's an advanced tuning technique for those on the racetrack. You really shouldn't deviate from the recommended amount for a streetbike.
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