Fork oil level - KTM Duke 390 Forum
  • 3 Post By Diploman
  • 2 Post By jettatdi
  • 1 Post By bahawton
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post #1 of 5 Old 01-06-2017, 08:26 AM Thread Starter
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Fork oil level

Any tips on what a good starting point for fork oil level? I mixed up 450ml of Redline 4wt, but can't seem to find a spec for level. What weights have you tried for street use,average roads? I haven't removed the forks yet,is there anything unusual about doing that? Thanks
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post #2 of 5 Old 01-06-2017, 07:31 PM
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Jettadi, To preface this discussion, you should download this link to a very useful compendium of manuals and other information compiled by Forum member Luc Lopes. Extremely handy and relevant reference material.

If you open the Repair Manual, then go to the table of contents in the upper right corner, in Chapter 6 you will find a step-by-step illustrated guide to disassembling the front forks. Changing the fork oil is covered on pg. 21. Once the old oil is drained, each fork leg requires 450 ml of new fork oil. The manual calls for SAE 4 weight oil. If you pour in the specified quantity of fork oil, the correct oil level will be achieved automatically. The manual says you may have to pump the forks several times in order to accommodate the full 450 ml. There has been relatively little discussion of this operation on the Duke forum, but on the RC forum, (whose competition-oriented members are highly focussed on performance issues, in this case suspension performance) the general consensus seems to be that a heavier weight fork oil than the stock recommendation will improve fork damping. For racing purposes, they speak of oil in the SAE 7-10 range. For a street-ridden Duke, which lives in a less demanding speed/performance envelope, I would say something in the SAE 5-7 range would work well. Just drain the old oil then fill with 450 ml of new oil and the level will take care of itself.

Cheers, Will

"If you don't know where you're going, you might wind up somewhere else."
--Yogi Berra

2015 390 Duke
1980 Moto Guzzi V50 II (becoming a cafe racer)
1984 Moto Guzzi SP 1000

Last edited by Diploman; 01-06-2017 at 07:37 PM.
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post #3 of 5 Old 01-10-2017, 09:23 PM Thread Starter
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Thanks for the info. Between the Duke Forum and RC Forum I should get plenty of ideas setting up the fork. With my other bikes, I would start at stock fluid spec then change from there. Increasing viscosity to increase compression and rebound. Increasing fluid height to add "spring rate" at the end of fork travel (by reducing air volume in top of fork). If that doesn't do enough, then Gold Valves (or YSS PD valve) allow a large range of tuning choices. I also have a 97 Ducati SS, and a 2014 Honda Grom, both with Racetech Gold Valves,and Racetech springs. The Grom fork is quite similar to the Duke, with its spring located under the valve,in the lower fork tube. A lot of work to change springs, and preload spacers, but hopefully it won't come to that on the Duke. Sorry I took a while to respond, but here in the Midwest, temps are single digits, blowing snow,etc, shoveling snow comes before the bikes, I hate to say.
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post #4 of 5 Old 01-11-2017, 11:51 AM
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I've been thinking about adding Racetech Gold Valves to my forks but hate the idea of taking them apart every time I need to make an adjustment. Definitely affordable so I would pull the trigger if I knew exactly what settings to go for my weight, riding style, etc... I spoke with the guys at Racetech last year and they didn't even know what part # fit our bikes so I put the project on hold.
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post #5 of 5 Old 01-11-2017, 07:09 PM
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There's a set of RC390 fork internals for sale on the RC forum for $20. I'd put a couple of bucks towards someone purchasing them to ship to Racetech. I bought a similar set here for more but probably cheaper to sort this out in lower 48 rather than introducing cross border shipping from me...
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