A Little Harshness at Top of the Stroke . . .

Yamajeb

Member
Feb 20, 2006
25
0
After doing a little work on the KYBs on my '02 RM250 to start making the suspension suitable for trails/harescrambles (bladder removal, LS/HS comp stacks) I took the bike to a local tuner to just "get'r done". The improvement is dramatic, he did forks/shocks and the way it tracks on the trails is much improved. No more ricocheting off of rocks and roots, holds a line much better now. The clickers actually change things now.

However, the one thing that annoys me is that it isn't as smooth as I'd like it to be on the smallest of the bumps. Once I'm in the stroke with the bigger stuff or taking faster hits things seem to work well. The problem is apparent when I'm riding on well-traversed trails that are very rocky. I seem to take more of a beating than I ought to. I must admit, though, I took a brief spin on an '09 Husaberg 450 that had some suspension work done on it and it was dreamy smooth so I've been spoiled.

I did notice two things that made for an overall improvements that were surprising: shock adjustments and fork rebound stiffening. I softened rebound and low speed compression on my shock and it actually improved fork performance . . . a balance thing?

Back to the fork, I moved my compression and rebound adjustments to the middle of the range and started adjusting rebound first. I ended up about 10 clicks out on rebound (stiffer rebound) and ended up softening compression a little from midrange which, overall, was a stiffening of the compression. Before I had compression and rebound at the softer end, but where the adjustments are now is . . . better! Does that make sense?

With all the rain I had time to pry into the tuners work just a bit. It seemed reasonable to make some minor changes on the compression stacks to see what the effect is (primarily because it's easy) so pulled the bolts to make the changes. Here's what the tuner set me up with:

2X25.10
1X13.10
3X22.15
1X22.10
1X18.10
1X15.10
1X13.10
1X12.10
1X11.10

He had four very thick washers at the bottom of the stack, presumably to shim up what I believe is a short stack (does that sound right?) I was told he used 5wt oil.

Here's the way I changed things, good or bad:

1X24.10
1X22.10
1X13.10
2X22.15
1X20.15
1X20.10
1X18.10
1X15.10
1X13.10
1X12.10
1X11.10

Used 5 wt oil. Rode it briefly today and I think I mighta helped the top of the stroke some. Still not super-supple.

Should I start considering the midvalve? Would a 2.5wt oil be an easier move that may work? Or am I dreaming that I could even begin to get performance like what one would feel with on the Husaberg?

I do have springs fore and aft that are suited for my weight and riding type/skills which would be eastern woods / novice.

There's lots of questions imbedded here, comment on what you feel comfortable with, hopefully Jeremy will make some comments as well.

Thanks!
 

kawamaha

Member
Mar 16, 2005
179
0
first, I don't think it's a good idea to run 2.5 wt oil. it has more influence at rebound and you are able to change your shim stack, so this is the way to go.

I don't have experience in trail stacks, but what I would do first is get rid of the 0.15 shims. (for almost equal stiffness you have to use 3 0.10 shims to replace one 0.15 shim).

and, I noticed 4-stroke suspension is more comfortable due to the sprung/unsprung ratio - nothing will beat my XR600 ;)

nevertheless I am sure the RM250 can be dialed in at least like the Husaberg
 

Yamajeb

Member
Feb 20, 2006
25
0
kawamaha said:
first, I don't think it's a good idea to run 2.5 wt oil. it has more influence at rebound and you are able to change your shim stack, so this is the way to go.

I don't have experience in trail stacks, but what I would do first is get rid of the 0.15 shims. (for almost equal stiffness you have to use 3 0.10 shims to replace one 0.15 shim).

and, I noticed 4-stroke suspension is more comfortable due to the sprung/unsprung ratio - nothing will beat my XR600 ;)

nevertheless I am sure the RM250 can be dialed in at least like the Husaberg

Thanks for the feedback.

2.5 wt oil . . . has more influence at rebound - so the lighter oil will impact rebound more than compression? What would you expect would be the impact on rebound? This goes back to my question about the midvalve - maybe I need to start looking there.

sprung/unsprung ratio - are you referring to preload? I don't see how I could change preload in my forks unless I got shorter springs. I DO have the stock springs which are .43 kg/mm vs the .44 kg/mm I've got in there now.

I didn't know about the 3 X 0.10 stiffness = 1 X 0.15 stiffness. Seems like a lot!

Again, thanks.
 

kawamaha

Member
Mar 16, 2005
179
0
Yamajeb said:
Thanks for the feedback.

2.5 wt oil . . . has more influence at rebound - so the lighter oil will impact rebound more than compression? What would you expect would be the impact on rebound? This goes back to my question about the midvalve - maybe I need to start looking there.

sprung/unsprung ratio - are you referring to preload? I don't see how I could change preload in my forks unless I got shorter springs. I DO have the stock springs which are .43 kg/mm vs the .44 kg/mm I've got in there now.

I didn't know about the 3 X 0.10 stiffness = 1 X 0.15 stiffness. Seems like a lot!

Again, thanks.

so I never paid attention to 2.5 wt oil because there's no sense if you want to soften compression by using lighter oil (and 2.5 wt doesn't change much) and then re-shim the rebound... (just saw that mog would give it a try - so try it, maybe you like it)

sprung/unsprung weight ratio means wheels, tires, axles, lower fork tubes, swingarm compared to the rest of the bike. the heavier the rest of the bike compared to the wheels... , the better the suspension works. I always felt 4-strokes are plusher than 2-strokes, but power delivery is also a factor...

shim thickness: imagine ten 0.10 face shims, easy to bend. now imagine one 1.0 "shim" - same overall thickness, no way to bend it.
the reason we use many thin shims instead of one thicker shim, maybe a 0.25 or 0.30, is, the bunch of thin shims provide an almost linear damping curve, while one thick shim has a progressive curve. means too much high speed compared to low speed.
 
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