Motorcycle Suspension System Maintenance
The front forks contain oil and, as you know, most oil needs changing occasionally. Several months of street riding or about 40 hours of dirt riding will usually contaminate the fork oil to the point where it needs changing. Fortunately, this often neglected maintenance step is a simple matter of opening a drain plug to let out the oil, closing the drain plug, then pouring new oil in through a fork cap hole. Safety dictates stable elevation of the front wheel to eliminate spring compression.
Changing Dampening Rate
There may come a time when the front suspension of your motorcycle doesn't satisfy you, even after you have changed to fresh fork oil. The suspension may feel too soft and mushy or it may be too stiff and harsh. The problem could be in the viscosity or type of fork oil you used. Thicker oil will often stiffen a mushy set of forks, while thinner oil will usually soften the ride of harsh forks. You can use the same procedure to change the fork oil when switching to a different grade of oil. Remember, never vary the amount of oil used, only the viscosity rating.
Alignment of Twisted Forks
A slight spill or bump can cause the front forks to twist, resulting in misalignment. That is especially the case on lightweight motocrossers or enduros with the huge 23 inch wheels. The sensation experienced in this instance is much like having bent handlebars. That is, to get the bike to go straight you must hold the bars either to the left or right. A quick, simple way to correct this problem is to grasp the front wheel securely between your knees and twist the handlebars to align at a right angle to the wheel. If the misalignment appears to be severe, you can loosen the triple-clamp fork pinch bolts a turn or so and retighten them after aligning the forks.
Straightening Bent Tubes
Your upper fork tubes may become bent in an accident, but they can often be straightened again with a good hydraulic press and some "V" blocks. Be realistic, however, because straightening forks is an unreliable corrective approach if the forks are bent 5 degrees or more. If you do the work yourself:
1. Support the bent fork on "V" blocks with the apex of the bent section upward.
2. Press the bend until it is straight, being careful not to permit the tube to rotate in the "V" blocks.
3. Release the press and rotate the tube to find the "high" spot again.
4. Press the tube straight again, or a bit "past" straight if the tube exhibits any tendency to spring back to a bent configuration after you've straightened it.
5. Gently repeat steps 3 and 4 several times if necessary until the tube is straight.
6. Examine the tube well after you have finished. If the fork is dimpled, dented, pockmarked, but straight, it will work... but you'd better be prepared to change fork seals often because a damaged surface will accelerate wear of the front fork seals. If you have strong doubts buy new tubes. Better to be safe than dead.