YBR Fork Oil Change

[jeffyjeff]

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This is my motorcycle; a 2015 YBR125 purchased new and placed in service in 2019. It has 21000 km on the clock. I use it to explore back country roads here in northern Morocco, and for that purpose it has served very well indeed. I service the bike using the Haynes manual as my primary reference. There are instances where Haynes is cryptically ambiguous regarding service intervals; draining and refilling the fork oil is one such instance. "After a high mileage the fork oil will deteriorate and its damping and lubrication qualities will be impaired." Although I do not personally consider 21k to be particularly high mileage, ten years is about as long as I care to leave any fluid in continued service…so it is out with the old and in with the new.

I am not new to this task, having changed out fork oil my Honda VFR's and PC800. The first time I did this job, I was shocked at how crappy the fork oil appeared when drained from the Honda forks, like sully gray machine oil with a metallic sheen to it. Literally, it reminded me of metallic paint and it was the same on all three Hondas. That is what I was expecting to drain out of my YBR forks, but no. The oil was black as overcooked coffee; no hint of metal suspended in the fluid whatsoever.

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I pumped the forks to evacuate all of the internals, and hung them upside down to drain completely. Then I carefully measured out fresh 10wt fork oil in a graduated cylinder. The new oil is clear and colorless, like vegetable oil. Haynes says 163cc should result in a fluid level of 142mm with the spring removed and the fork tubes fully seated. I drained a few cc's to obtain the required 142mm level, and reassembled the fork with the spring, washer, spacer, and cap. Then repeated with the opposite fork leg.

Since I was in it this far, I proceeded to remove the steering stem and check out the bearings. The bearings were in good physical condition, but bone dry. There was only a trace of dry caked grease on the bottom bearing cage. I smeared the races with Bardahl multipurpose grease and coated the bearings for assembly, then fit the stem and torqued down the adjuster nut by feel (no torque wrench, just a hook spanner). Haynes says 33 Nm to seat the bearing and 22 Nm final setting. I'm confident that I got it close. Bottom line, no wobbles and the bike tracks straight and well balanced.

Assembly went uneventfully, but once I got the bike underway, something felt…off. I initially thought the tire was out of balance, and even went so far as to check the pressure. Finding no abnormality, I hit the road again. It feels like the motorcycle is super sensitive to small imperfections in the road surface; when I struck a speed bump, wham! It hit me. IT'S THE OIL!

Changing my fork oil has made my bike significantly more sensitive to road surface imperfections. Given the conditions of the roads in my area here in Morocco, that might not be such a good thing. I know that I have the right quantity of oil in the forks, and I know it is the recommended viscosity. I'm amazed at the drastic effect that new fork oil has made in the quality of my ride. It doesn't feel unsafe; the bike tracks better than ever. Maybe I just became acclimated to a soft, spongy front end. I hope that after a few hundred kilometers I will become accustomed to a new (improved?) level of front-end performance.
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History shows again and again how nature points out the folly of men - BOC

[Robby]

It could just be that 10W is too thick for your application, and you'd be better with 7.5 or 5W.

[jeffyjeff]

[lingeringstin...]

My experience with forks and fluid is that most telescopic forks use a pretty simple method for damping. It's basically about fluid being forced through a hole in the damping part of the fork usually at the bottom end.

This means thinner oil should make the forks react faster than thicker oil if the springs and everything else are the same. Sometimes you want this, sometimes you don't depending on what you're using the bike for. I would use thinner oil (in longer forks) for a trials bike and thicker oil for racing so it wouldn't dive in corners.

I have no rear suspension (hardtail) other than tyre squish so the way I set my forks up is kind of important and after much experimentation I now use diesel as fork oil on my bike and it works great.

In my experience the actual sort of fork oil doesn't seem to be nearly as important as it's viscosity.

I've also tried ATF and brake fluid just to see what would happen and basically it works. I even used water once and it worked pretty well until it froze one winter morning leaving me with completely solid suspension and a thirty mile journey on ice covered roads, which is a travel tale of horrifying proportions I won't delve into here.