MX-TECH took delivery of our 2015 KXF450 on Friday last week 2nd Week of July. Like everyone I was eager to get my hands on the parts and see how they were constructed and how the adjustments work. Showa and Kawasaki arranged for some major changes to the damper side, like 12mm shims on the base valve! Additionally the base valve is being fed by a 14mm rod and the midvalve remains the same as previous models. Ok, so yes that’s all a big deal and should make for some good tuning opportunities, the real interest for me was the air side (spring) side of the fork.
So what is in there?
There are 3 pressure regions, all separate from each other.
There is an inner and an outer compression region and then a “balance” spring (region). Note this fork is very similar to the Suzuki RMZ450 TAC, but it is different from the CRF250 TAC. See notes below.
The Inner Cylinder is highest pressure and has an internal oil volume that will control the rate of progression in the pressure.
The outer chamber is the lowest initial charge, and can be adjusted by internal volume and pressure as well.
The Balance chamber adjusted at the lug. This is a pressure region that counteracts the force produced by the 2 pressure regions above it. It can also have its volume adjusted to control the rate at which its pressure drops as the fork compresses.
What does this mean, and with that let me offer the disclaimer, I’m very early in my understanding of the variables. I am getting my head around it and it will take time. As with most of my approaches to problems I will take 2 paths. One being quantify, measure and test the other be observe the performance in the field and cross validate.
So with that here we go on my initial technical review.
The idea is that with different pressures and volumes you can break the stroke into more segments and better tune the force verse displacement arriving at something that looks like a linear spring. That’s the idea that we all assume. I’m not sure, if that’s possible given the construction. So it’s no wonder the verdict is still out, and pro’s with direct support from Showa are still looking for the magic combination of volumes and pressure in each region.
So how did it ride? The track was deeper than normal, but JMX does not develop big breaking bumps like Walnut, Ki, Redbud, Moto Land or Glen Helen. That’s what I’m waiting for. (Thursday) The action is sticky on the floor but seems decent when you ride. The wheel really follows the ground and the bike gets great traction. The front seems to really dive under braking, and seems to be like the PSF lacking spring rate in the middle of the stroke. I read my manual for clues, it has very little tuning info. It also notes that the balance spring is the method by which a stiff or soft setting should be achieved. But what is weird is that soft setting is the low pressure balance chamber setting, and the stiff setting is the higher pressure balance setting. Can’t say that’s what I think it should be and that’s not what we felt when we rode it! Waiting on Kawasaki to offer a correction. Needless to say I started playing the balance spring settings and found softer balance spring and stiffer inner seemed to get more compression, but it did not add much in the middle, more stiffness at the top and bottom but not so much in the middle. This is volume issue.
Based on my first review of the parts and some initial tuning and observation here is my breakdown of each region.
Outer Compression: It has some impact on preload, and it is used to impact the bottom 1/3 of the stroke. More oil makes it come in sooner, less; later, more pressure more preload more overall force in the bottom 1/3. I think Showa wants to keep this low. It makes the fork more reliable. The outer chamber having significant pressure makes the fork a liability. *Note CRF TAC Notes.
Inner Compression: It has impact on preload and over all spring rate. It is the driver in terms of overall spring rate. Its volume has the most linear pressure change, and therefor closest to a spring. Plus by isolating the “Gas Spring” inside the fork is way more reliable! Rock ding or seal nick the fork will not lose spring rate. Maybe the driver for the middle of the stroke. Have not seen it yet.
Balance spring. Changes the top-1/3 the most.
I can see some things I’d like to try already. But initially I am just going to map spring combinations.
Sorry no picture of the Air side piston. I need to make a tool to remove it safely. I need to ride the fork and can’t risk damaging it.
Fork Seals: SKF Kit49S works awesome and massively reduces stiction.
Inner 50mm x 1mm x 20mm
Outer 52mm x 1.5mm x 15mm
12mm Shims for the base valve.
*No sub valve.
MXT T1009 is perfect for the bottom bolt on the gas side. (22mm end socket.)
**Honda TAC notes for the CRF250**
Honda has an advanced TAC. It uses no external balance tank because it’s on the inside of cylinder, as a tank or bottle attached to the piston rod.
The outer chamber is not charged. It just uses oil to control progression in the last 1/3.