Tuning forks, but not a Yamaha.

Now that I have four sessions of kart track time under my belt and some very good data regarding tire pressure and temperature I figure its time to get out to a big track with the little 250. Throttle/engine performance, braking and heat management are all working well and I know how the bike is supposed to behave. The one thing that isn't sorted is the suspension. The Ninja 250 is designed with someone more svelte and compact in mind than my burly frame. The springs are nowhere near strong enough to hold up the weight of the bike and my portly butt while controlling the movement of the wheels under any kind of track scenario. It was time to fix that. With a big-track day planned in April a little over a month away, I set out to get the front end under control.

I borrowed a front head stand from my neighbor, good friend and motorcycle buddy to get the Ninja up off the ground. I had to strip the fairing to keep the stand from crushing it when the bike was resting on the stand. I remember thinking that I might run the bike naked when I first started prepping the little Ninja for the track. The tail section and overall stance has that 80's look that would go great with a square flat number plate zip tied to the forks.

I unbolted the handle bars from the upper triple tree and let them hang for the moment. Unfortunately I can't run clip-ons under the upper triple with this bike as the bars would hit the fairing. I may be able to mount them on top of the upper triple clamp by lowering the forks in the triple trees a bit. This would have the effect of quickening the steering. It is something I'd like to try after I do the fork springs as I don't want to try too many things at once. If I have too many variables and something isn't right, it will be harder to figure out exactly which one caused the negative change.

After prepping the top part of the fork assembly, I pulled the wheel and fender from the bottom. I had to cut off all of my safety wiring which is a good thing as I want to slowly redo most of it anyway. I want to try a better wire and get all of the twists more uniform. To get the wheel off, I had to jack the bike up with my bike jack under the engine, as the front wheel wouldn't clear the fender. I had to remove the wheel to get to the bolts for the fender. None of the bolts put up much of a fight. I was worried that over the years the bolts would be seized due to all of the road grime and water the bike traveled through. I did clean the fork legs while everything was cleared out and it was easy to do.

Before pulling the fork legs from the triple clamp, I measured the depth that the fork legs penetrate through the upper triple. I got a measurement of 13mm. I will use that measurement to assemble the forks when it comes time to either add clip-ons or drop the forks for better corner tip in. The tool I'm using is just a cheap metric ruler from a Japanese dollar store. It worked a treat for this application so I consider it money well spent.

I loosened the bolts in the upper and lower triple tree, but before dropping the fork leg out I popped loose the fork cap. I used the deep C-clamp shown in the picture above to press down on the fork cap. I then used a pick and a small flat headed screw driver to remove a thin c-clip holding it in the fork tube. I then backed the upper triple tree clamp bolt off to allow the fork spring to pop free from the  the tube. I loosened the triple tree bolts the rest of the way and removed the fork leg from the bike.

Here is the partially disassembled fork. From left to right we have: the c-clip, fork cap, pre-load spacer, washer and fork spring. Behind these parts is the fork leg and in the container is the fluid drained from the fork. I measured the preload spacer at 50 mm. I will use some PVC to make some new preload spacers in several sizes so that I can change up the pre-load after some testing. I am toying with the idea of making a threaded pre-load spacer to replace the OEM fork cap, but my April time limit will keep me from doing that I imagine.

The fork fluid was dark and a bit stinky. Not as bad as I imagined, but definitely time to change it out. The container I used had graduated markings perfect for telling me how much fluid to use when reassembling the forks. In dampener rod forks such as these, there is an air gap at the top of the internal fork assembly. This air gap acts as a progressive spring and changing the amount of oil and the viscosity of the oil in the fork leg can help tune the suspensions performance. I need to know the staring point for any future tuning.

No you are not seeing things. The fork leg is mounted upside down in the triple tree. I was trying to break the damper rod bolt free from the bottom of the fork leg. I have two impact guns from Harbor Freight. One is a battery powered, low torque unit. The other is a higher powered unit that plugs into a 110V outlet. Neither could break the bolt free with me holding the fork leg. I clamped the leg in the triple tree hoping it would hold the stanchion enough to break the bolt free. No luck with either of the impact guns so I packed it in for the day. I'll do some YouTube viewing to see if I can find a method of breaking the damper rod bolt free that works with the tools I have in hand.

Next up is to break the damper loose and finish cleaning the fork assembly inside and out. I also have to order parts including new fork dust seals, oil seals and springs. While I have the bike broken down this far I might as well change out the head stock bearings with some new All-Balls tapered units and give everything a good clean up and lube. Come on back for more for upgrades soon.

See you then,

Blue