Forks contain oil which creates damping. You can change the characteristics of the fork by changing the height of the oil inside.
If you increase the oil height, then the fork will become slightly stiffer and more progressive. If you lower the height, then the fork becomes less progressive. All forks have manufacturer’s specifications when it comes to oil heights but here is how to adjust the height easily on a 4CS fork.
Step 1: REMOVE THE FORK
Firstly, you will need to remove the fork from the bike. On some forks, it is possible to adjust the oil height with the fork still on the bike but, for the most accurate measurement, remove it from the bike.
Step 2: UNDO FORK CAP
Undo the top fork cap and slide the outer fork tube down so you can access the spring.
Step 3: HOLD THE LOCKNUT
Take a spanner, place it between the coils of the spring and hold the locknut on the damper rod.
Step 4: UNSCREW THE CAP
Unscrew the top cap from the rod and set it aside on a clean bench.
Step 5: REMOVE SPRING
Slide the spring out of the fork tube and set it aside. Be careful, as some oil will come out on the spring and if you pull the spring out too quickly you will wear the oil.
Step 6: STEEL YOURSELF
With the fork compressed, slide a steel ruler down into the fork tube until it touches the oil, this gap between the oil and the top of the fork tube with the fork collapsed is the oil height.
Step 7: SUCK IT UP
If you want to lower the oil height, take an oil-height adjusting syringe and set it to the measurements that you are after. Slide the tool inside the fork and use the syringe to suck out the extra oil until you reach your required oil height. These tools can be easily made at home with a piece of rubber hose and a syringe.
Step 8: POUR IT IN
If you want to raise the oil height, then pour some extra oil into the fork tube and then suck out the excess with the syringe until you reach the required oil height.
Step 9: TEST RIDE
Once your oil heights are set, slide the springs back into the fork, refit both fork caps and slide the fork back into the bike before taking it out for a test run.
4CS TUNING TIP
For beginner riders or riders who weigh under 80kg, I recommend running an oil height of 100mm but for experienced riders, riders who are riding at higher speeds like desert racing or riders over 80kg then I recommend an oil height of 90mm.
- Use clean tools and a clean work space while working on a fork.
- If you are adding oil, use the same weight oil as that already in the fork.
- Always check the manufacturer’s specifications on oil heights before adjusting.
- Over-tighten the damper rod. It only needs the lock nut to be nipped up snuggly.
- Use any other oil, apart from specific motorcycle fork oil.
- Drop any spring preload washers when removing the spring from the fork.
Technical Editor Mat Boyd