Fork Oil Weight

Can anyone help?

I have my forks as stock with 5W fork oil but have the compression damping set to max and it still dives too quickly.
I was advised at the weekend by a suspension 'guru' to replace the oil with 15W to put my adjusters back in range.
Putting thicker oil in makes sense but to go from 5 to 15W is a helluva jump.

Any advice?
Anyone had their forks rebuilt by a suspension specialist? what did you end up with?
Any tricks or traps?
I hear that there is a dirty oil trap somewhere in the WP forks; is this true and how do you make sure all the cr4p is out of them?

What oil levels / volumes do you put in the fork legs?

N.B. Track use only. Rider is 260lb.

Your help much appreciated

I was never happy with the damping range on my 05 forks. It was mostly rebound though. I'm about 195-200lbs fuly kitted.

My bike had lead an easy life, but the guy didn't have a clue so it was running with the damping at almost full soft wich had burnt the oil. Was pretty dirty and smelt off when i changed it.

Changed it to fresh 5wt oil with 450ml per side but this was too much and you could feel the forks starting to hydraulic lock at the bottom of the stroke under hard braking.

Changed it to 10wt and 420ml of oil, very happy with it now. I run my bikes with a fair bit of front rebound damping, just the way i like it as i trail brake a bit and it feels like it helps it hold a line as you get off the brakes.

As for the oil trap. I didn't want to fully strip the forks so i just undid the top cap, took forks off and drained from there. Spent a bit of time working the forks through their fulll stroke while tipping them up and down. You'll find it seems empty, then you hold them horizontal and work them and you get a good bit more oil out. Just keep repeating this until its empty. When i emptied out the new 5wt oil it was stil like new so i'm sure i got all the dead oil out with this process.

It's an easy job so i would say just go up in 5wt increments until you find the right one.

A quick thought, but are your rider sag settings in range 1st. At that weight i'd imagine you might be a bit heavy for the standard springs.

The suspension 'Guru' was the guy provided FOC at the Performance Bikes gig @ Rockingham
I have got his card somewhere but the way he talked he seemed to have a lot of experience of Superdukes.
He claimed to use 15W instead of 5W which put the suspension settings into the middle part of the range.

15W does sound like a huge change to me however I am running maximum compression damping.

You need stiffer springs!

I'm 250lbs and am running 1.10kg/mm fork springs and 5w fork oil @ 120mm from top fully collapsed. Got the forks and shock pulled and going in for service at which time I'm going to have them set level at 115mm from top to help with the last third of stroke. My forks have been revalved to increase HS reb. damping among other things.

5w to 15w seems like a big jump but if you don't like it you can always drain it and refill with 10w.

John

Thanks for all the help and comments so far.
Although the bike is fitted with the springs it came with we think the spring rates are about right because we are looking at about 20mm of dynamic (or rider) sag front and rear.
Just to be clear: there is 12mm of static sag at the rear and 33mm with me sat on the bike.

You want the spring to counter act the diving, not the valves. They keep the spring from bouncing around like an old hooptie and keep the unsprung weight from gaining momentum causing the tyre to leave the ground over bumps. If you're forks are diving and you've got the preload cranked in all the way, you need stiffer springs my friend. With your weight centered on the bike you may somehow magically end up with the correct sag (but the springs have all their range "preloaded" out) but when your girth transfers forward under braking the forks' RATE can't take it.

Try stiffer springs first, it's worth the money. You can't set the valving up properly until you do.

FWIW I've seen other racers use 30W motor oil in SV's.