KDX Replace Fork Springs

From DirtyTorque

This article describes how to replace the front fork springs on a Kawasaki KDX. The steps described in this article could also be used to service the fork oil.

Tools

30mm wrench
17mm wrench
Syringe with tube and ruler OR oil gauge (to measure oil level)
Torque wrench
Mitre box and hack saw or table/band saw (to cut fork spring spacer)

Spares

Loctite thread locker
Approximately 1100ml 5W fork oil (Approximately 530ml is required per fork)
New/different springs (Optional if you are planning on replacing the fork springs)
Spacer stock/PVC piping/aluminum tube (to make new spacer)

Calculate Spring Rate

Calculate Spring Rate to determine which springs should be used in your forks

Stock KDX Springs and Spacers

The stock spring and spacer setup has a lot of preload applied and a shorter spring than most replacement springs. The stock spring should be around 470mm and the stock spacer around 90mm.

Setting Fork Spring Preload

In stock form the 1995 to 2002 KDX200/220s suspension is out of balance. The 5.0 kg/mm shock spring is well suited for riders between 170 and 200 lbs while the fork springs are better suited for riders up to 130 lbs. This out of balance conditions raises havoc with the KDX's suspension performance and detracts from an otherwise good handling bike. Many KDX owners will replace the stock fork springs with stiffer units to bring the suspension back into balance and vastly improve handling. Because, in most cases, replacement fork springs aren't the same length as the original units new preload spacers will need to be installed to properly set the preload.

What is preload?

Fork spring preload refers to the amount of tension applied to the fork springs. Preload tension is applied by compressing the spring from is fully extended length. The difference between the springs fully extended length and the compressed length is how much "preload" is applied to the spring. Typically KDX owner will use about 8 to 10mm of preload.

Determining Preload Spacer Length

Determining how long to make your preload spacer is very easy once you know the fully extended length of your fork internals. Here is a picture of a 1995 to 2002 KDX's fork internals. They are fully assembled and the total distance between where the bottom of the fork spring and the top of the preload spacer seats is indicated. Once we know this measurement we can easily calculate how long you'll need to make your preload spacer to get the desired 10mm of preload. The total length is 539mm - the springs uncompressed length = the length of the preload spacers.

Preload Spacer Formula

Assuming your new spring was 505mm in length and you wanted 10mm of preload the formula to calculate the length of your spacer would look as follows:

539mm + preload - spring length - washers (x 2) = spacer length

therefore...

539mm + 10mm - 505mm - 4mm = 40mm

Preload Alternative Explanation

In this example, we will assume that the fork spring is the perfect length for ZERO mm of preload. The top of the fork spring will have a washer on it, as the spring spins a little when compressed. After the fork cap is tightened to the proper torque, the bottom of the fork cap will barely touch the top of the washer. The bottom of the cap does not compress the fork spring at all. To get 5mm - 10mm of preload with this magic length spring, a 5mm - 10mm (minus the height of another washer) length of PVC tube is inserted between the washer and fork cap. This compresses (preloads) the fork spring 5mm - 10mm from its free length. When mounted on the bike, the weight of the rider and bike will compress it a little more, but we are just worried about the fork off the bike.

The fork spring you buy will not be the magic length. It will be shorter. You must make up this distance by cutting a piece of PVC tubing. The length of the spacer will be the length required to provide 5mm - 10mm of preload after the fork is reassembled.

Measure the distance between the bottom of the fork cap and the lip where the fork cap seals onto the fork. I want to say it is about 15mm or so. When you stack the fork spring, then washer, then tube, then washer, it should be about 5mm or so below the surface. When you set the fork cap on top of the spring/washer/tube/washer, the sealing lip should be 5mm - 10mm from the top of the fork tube. I recommend 10mm.

I think the stock springs are a different length, with a spacer length that won't provide the exact height you need. The stock preload is not 10mm. They were not afraid to load it on down "back in the day". :)

Things to Remember

Remember to measure the total length of your spring with the two end caps installed if you plan to use them. Most aftermarket springs have a larger ID than the factory springs so you'll need to expand the three tabs slightly so they'll clip into the spring and be held in place.
Remember to include the two spacer guides in your spacer measurement.

KDX Fork Spring Diagram

Prepare New Fork Spring Spacers

Most new springs come with some "spacer stock" (tubing) in the packaging which is used to make a new spacer based on the size required.

Make sure that you cut the spacer stock straight by using a mitre box or suitable electric saw and cut the spacer to the measurement calculated above, e.g. 40mm

Remove The Suspension

Remove the front suspension from the bike completely before going any further.

Fork Oil Draining

Loosen the fork cap bolt using a 30 mm wrench until it comes out of the inner tube. Slowly compress the outer fork tube downwards until it stops.

Push down on the fork spring and insert the spring holder along with a 17 mm wrench to hold the piston rod nut. If a spring holder is not available you can use a wrench smaller than 17 mm, it is only used to hold the fork spring away from the piston rod nut.

Use a 30 mm wrench on the cap nut and a 17 mm wrench on the rod nut while holding the spring holder tool.

Remove the fork cap bolt and inspect the O-ring.

Remove the fork spring, fork spring seat, spacer and washer along with the fork cap bolt.

Pour the old fork oil into a container. Move the piston rod up and down several times to remove all of the fork oil from the fork.

Pull the piston rod up so it is above the inner fork tube. Tighten the rod nut finger tight and compress the piston rod and inner tube all the way down.

Fork Oil Replacement

Use KAYABA 01, SAE5W fork oil or RACE TECH suspension oil when refilling your forks. Slowly pour in the correct amount of fork oil.

(Specification for front fork oil capacity is 530 ml)

Use your fingers to push the piston rod in and out slowly several times to prime the piston rod with fork oil. Wait about five minutes for all the air bubbles to release and check the oil level.

Measure down to the oil level with a suitable tool and add or remove oil until you get the correct amount. Set the fork oil level to specification with the fork spring removed and the fork completely compressed. (Specification for fork oil level is 85 to 89 mm)

Extend the fork and install the fork spring, fork spring seat, spacer and washer into the fork tube.