Fork oil, heavier or lighter? Which is best.

[andyvh1959]

I see the question come up often in many motorcycle forums. Debate over heavier oil or lighter oil, fork setup. Actually, oil viscosity is only a small part of how to make the forks and ride better. Its really more about oil VOLUME more than the oil viscosity. Although oil viscosity is a factor as well.

Back in the late 70s and 80s, the motorcycle magazines did great articles on modifying the damper style and cartridge style forks, which are still very common to the Vulcans. Basically the mods are greater volume of lighter oil, progressive wound springs, preload spacer to reduce static sag, and in some cases adding air valves to adjust for changing loads (like a fairing, carrying a passenger, etc). The mods details are:
1. Progressive wound springs, which start with softer rate on the bottom and get more stiff as the forks are compressed further.
2. Preload spacers, adding spacers reduce the amount of sag when you sit on the bike. Proper preload should be less than one inch. Look up "satic sag fork preload". Back in the late 70's some used VW Beetle engine valve springs as preload spacers. The valve springs are stiff enough to reduce fork static sag, but still offer spring travel at full fork compression.
3. Oil volume, the factory stock volume is the LEAST amount of oil put in for the least cost to provide lube and damping control. Kawasaki only put in the minimum amount of oil needed to make the forks work for whoever rides the bike. Best practice is to remove the fork springs, fully compress the forks, oil fill the forks to within 3" from the top. Then reinstall the fork springs after the fork is extended.
4. Oil weight, go to 10wt or 15wt oil, instead of heavier oil. The bigger gain in ride quality is not heavier oil, but MORE oil. Lighter oil lets the forks react better to small bumps. More oil in the fork legs aids the forks to handle bigger bump by reducing the air volume above the oil. The reduced air volume above the oil acts like a secondary progressive spring, greatly reducing fork bottoming and dive upon braking. When reacting to a big bump, the reduced air volume very quickly becomes a High rate spring to reduce fork bottoming. Adding oil volume (up to the 3" from the top of the compressed legs) will NOT blow out the fork seals.

Now back in the late 70's it was also common on damper rod style forks, to drill out the damper rod oil passage holes about 0.010" larger, and add another cross-drill set of holes. Doing this would also increase the fork responsiveness significantly. That may not be necessary in the more modern Vulcan forks. So perhaps upgrade your forks with the mods listed 1 to 4 above. If that doesn't fully do the trick then over the winter tear the forks down again and drill out the damper rod holes. Some also drilled out the fork leg caps and tapped threads into them to install fittings and Shrader air valves to adjust the air pressure in the forks.

I'm doing some work on my buddy's VN900 to mount some hard saddlebags, and I'm also doing the fork mods listed above. Hos bike has over 34,000 miles on it so the forks are well overdue to get the fork oil replaced. He weighs about 250lbs and I mounted a frame-mount fairing to the bike a few years back so the front end needs to be updated.

[Mechaniac]

I'm interested to see how it turns out. When I redid my Mean Streak forks, which are a bit different than the other Vulcans, I installed Progressive springs and upped the oil weight to 12.5, all else being stock. No complaints and better than it was before. It doesn't impress me though.

Vernon

[andyvh1959]

Curious Vernon, better "how" though it still did not impress you? What was your goal to go with heavier weight oil? Did you use the stock oil volume?

Can't get to sleep at night, well, study this and off to La-La land. But there is a ton of tuning advice here from Race-Tech:
https://racetech.com/page/title/Emul...%20They%20Work

[Mechaniac]

Better as in it's a bit more controlled in the high speed turns but it's no "just send it and it'll go" sport bike. Just "good" for a cruiser, not fantastic.

Vernon