The front end of the stock RS50 is under-sprung, meaning the springs are too soft, even for 160lb riders. From our crude measurements, the stock fork springs have a rate of around 0.60 kg/mm. Our performance fork kit include stiffer springs (0.75kg/mm) and heavier fork oil (20W). The heavier springs will allow a more reasonable amount of static sag for street riding and racing. Static sag is the amount the forks compress from the fully extended position to position with a rider on board. The easiest way to measure static fork sag is with a zip-tie. Put one around your fork slider (the chrome shiny part). Slide it to the dust seal. Grab a buddy and get on the bike, put your feet up (buddy holds the bike). The zip-tie will slide. Get off the bike and measure the distance from the zip-tie to the dust seal. Be sure to fully unload the front suspension by lifting up on the handlebars. Rear sag is measured from the axle to a point up on the body. Again, measure with the rider onboard (feet up) and then fully unloaded. Subtracting those two distances is your static sag.Sag can be thought of as how much the springs "sag" under your weight. For the best handling bike, front and rear sag will be very close to each other. On the RS50, we are limited to how stiff the rear spring is. The Bitubo shock we sell greatly improves rear end feel, and has a much stiffer spring with adjustable preload. Setting sag is the most critical first step in good chassis setup. Ideally, we would like to have 30mm front and rear sag, but the rear shock holds us back. Stock static sag with a 175lb rider is well over 50mm of sag. If you can get the front and rear sag under 40mm, you'll have a much better handling machine.
Next comes damping. Imagine compressing a spring and letting it go in free air. It goes "boing-boing-boing". In other words, the spring vibrates, it has a frequency. If you had no damping and you hit one bump, you'd go up and down several times. You've seen those cars go down the road, the shocks (damper) are blown, while the spring is still there. The car hits one bump and then bounces up and down the whole street. The RS50 has basic forks where damping is controlled by oil weight and some holes in the damper rod. We improved damping by using a much heavier oil. Heavier oil slows down the "boing-boing-boing" action. Unfortunately, only one of the RS50 fork legs has a true damper rod. The other side is missing a oil sealing ring. If we can locate this ring for the other fork leg, we can even further improve damping. Aprilia does not seal that part separately.
Step 1: Fork Removal
Raise the entire front of motorcycle in the air. We use an overhead stand and some tie-downs. You can use anything that is sturdy overhead (ceiling joists, steel beams, or even a strong tree (though, you might be a redneck...). Remove the plastic caps on the top of the forks, the front wheel, fork brace and fender, front brake caliper, and remove both fork tubes from the triple trees. Also shown is the small basic set of tools you will need. You will also need a fork oil height gauge, although a ruler can be substituted.
Step 2: Disassembly
With the forks removed, you can begin disassembly. Thread a spare M5 bolt into the top threads (where the plastic fork cap cover was bolted to). With an Allen wrench, push down on the fork cap while holding the slider (the shiny part of the fork tube). You will now see a metal circlip down inside the fork tube. Remove the circlip, while still applying downward pressure to the fork cap. It is very useful to have a second person to stabilize the fork assembly, and help with the circlip removal. Remove the fork cap and the spring. Dump all the of fork oil into a bucket, and leave the fork upside down to drain.
Step 3: Lay out the kit
The kit includes two 100mm pre-cut aluminum spacers, six steel washers, and two 0.75 kg/mm springs. Picture two shows the layout of your new parts. The spring will go in first, followed by the required washer, the spacer, then optional two washers (both are optional), the fork cap, and finally the circlip. The length of the combined spacer and washers is what sets the fork spring preload. Spring preload is the "pre-tension" on the spring when the fork is fully extended. With no optional washers, spring preload is the same as stock. The springs are stiffer, but the spring preload is the same. Going to anymore spring preload makes it VERY difficult to reinstall the cap. Spring rate and spring preload are different. Spring rate is what mainly determines front end stiffness and the static sag. If, after installation at the base settings, you desire more preload on the spring, add the optional steel washers.
We are testing 0.80 kg/mm springs, and will soon have a spring exchange program. We will exchange any 0.75kg/mm spring or springs for a 0.80kg/mm spring or springs for just $10 to cover shipping and expenses. Testing on the 0.80 kg/mm springs for heavy riders should be done soon. The spring rates of each fork leg do not have to match. For example, you can mix a 0.75kg/mm and a 0.80kg/mm spring in the front, and end up with an average of a 0.775 kg/mm setup. We were initially concerned that a too stiff front setup may make handling worse, since the rear shock is so softly sprung. You need the front and rear to be sprung evenly (similar sag) for best handling.
For people who want to experiment further, you can cut your own pre-load spacers with a tubing cutter. Be sure to de-burr the ends, and clean everything very well. Do not exceed 115mm in total spacer length. You will run into extreme difficult installing the fork cap, and the spring could internally bind onto itself.
Step 4: Cleaning, bleeding, and filling
Clean all the fork kit parts with brake cleaner solvent. Set them aside. We will rinse the fork tubes with fresh fork oil. Pour in approx 1/4 of one bottle of fork fluid. Pump the slider up and down. You will hear and feel the fluid being pumped around. Rotate the tubes all around to rinse the whole inside with fresh fluid. Dispose of this oil into the waste oil bucket. Pump the fork upside down to get out all the old dirty fluid. Let them drain upside down for a few minutes.
Filling the forks:
Compress the forks as far as it will go. Fill the fork up with fresh 20W fluid. Now you must bleed the fork. Continue to pump the slider fully up and fully down until all the air is out. It may take ten pumps or so. Next, we need to set the fork oil height (also referred to as fork oil level). Fork oil height refers to the length of air space above the fork oil measured to the edge of the slider down to the oil. This is done with no spring and spacer inside, and the forks fully compressed. We use a special tool although a ruler can be used. We first overfill the fork tube, and then suck out the oil to the desired level. Stock RS50 front fork oil level is 135mm. We are running 110-115mm of oil height. Remember, oil height is measured from the top down with the fork fully compressed......bigger number = more air = less oil........and.......smaller number = less air = more oil. Measure oil height only when the fork is fully compressed with no spring, spacer or washers inside.
Oil height affects the damping when the fork is almost fully compressed. It also affects "bottoming out" characteristics. The air at the top is compressed when the fork is compressed. Air is far easier to compress than the oil, so running more oil (smaller number) will better resist bottoming. Spring rate also affects bottoming. You want the fork oil height to be the exact same fork leg to fork leg. Do not run a oil height of less than 100mm. Stay in the range of 100-140mm. Some experimenting maybe necessary to fine tune the suspension. We run 110-120mm of fork oil height. Spring rate and spring preload are more important than oil height for overall feel and stiffness.
After the desired fork oil height is achieved continue with re-assembly.
Step 5: Final assembly including spring, washer, spacer, and cap
First, extend the fork all the way. Then, the spring goes in and the required washer. This washer is REQUIRED! Next add the spacer, and then any optional washers you want. You can use none, one, or both optional washers. The fork cap goes back on next, just like it came off. Re-install the circlip and ensure it is fully seated. Clean the outside of the forks with solvent and re-install back on the bike. Ensure that both forks extend up through the triple tree the exact same amount. We set ours to 10mm extending through the top. Re-install wheel, fork brace and fender, brake caliper, and your done. Bounce on the front end to insure smooth operation. Note: if you pumped the forks when they were off the bike, they may have felt different. This is due to the RS50 having different damper rods in each fork leg. One fork leg is actually doing more damping than the other. This is completely normal. We hope to source another damper sealing ring to further improve damping in the other fork leg.
If you do not feel comfortable doing any of these procedures, please seek the advise of a professional. Forks are a critical safety matter, and you do not want to hurt yourself.