02 KTM520SX - Playing with valving (part 2)

[Yoken]

For part 1, please follow this link:
http://www.dirtrider.net/forums3/showthread.php?threadid=75162

So when I finally hit a faster track, I realised 2 things:
1.

Originally posted by dbrace
Those two .1x14 in the comp stack = waaaaaaaay softer. To the point of being harsh (too divey). I think. (sometimes)

...dbrace was right. the fork was diving at an alarming rate! In fact, the first track I tried was too slippery and too slow. I now realise that I was not using the front brake hard enough and the fork felt fine! On the faster track however, I had a better grip I was 4th gear WOT in some sections. Stiffening with compression clickers and increasing the external fork pre-load to 4 turns IN helped a bit but not much. Grabing the front brake at high speed in braking bumps resulted in a RAPID and DEEP front end dive, making the bike totally out of balance.

2. Under the following link,
http://dirtrider.net/forums3/showthread.php?s=&threadid=68714
Jeremy debates on independent vs non-independent valving. The way I understand it, the 0.2mm cross-over makes the LS and HS valving VERY independent (please tell me if I am wrong here).

What I felt on the bike was a lack of consistancy in the damping. It felt plush and smooth on some bumps at a certain speed. But it felt much stiffer and even harsh at a higher speed in the same section (I almost lost the bar when entering too fast in breaking bumps!)

Unfortunately, I didn't have the shims to try a non-independent valving stack. So instead, my 2nd valving mod went like this:

BV stack (2nd trial)
(3x)24.1
14.1 (instead of 2x14.1mm)
22.1
20.1
18.1
16.1
14.1
11.25

MV compression stack (2nd trial)
lift=0.3mm (unchanged, 0.5mm was a measurement error)
(3x)24.1
14.1 (instead of 2x14.1mm)
22.1
20.1
18.1
16.1
14.1
10.5 (spacer)

MV rebound stack (2nd trial)
(4x)D.1 Delta shims
20.1
18.1
16.1
14.1
14.1 (back to where it was)
10.3
16.25

... and it felt a bit better. Diving was much reduced. Although I still believe that stiffer springs are necessary. Bottoming resistance is great too. But I lost high speed plushness. The bar has now a tendency to 'vibrate' in braking and accelerating bumps... the ride is a bit more tiring.

Well, I believe this setup is better than stock, but it can certainly be better. Next, I'd like to try stiffer springs and a non-independent valving set-up...if I can get shims...

It's just fun to play with those things...
Kind regards to all,
Yoken

 

[dbrace]

Keep us up to date on your progress.

 

[Bud-Man]

Yoken,

I think you are on the right track with what you are doing. I think you will have the best performance using the MV compression with a low lift value, like whatyou are using (.20mm~.30mm). I'm a little confused on the 10.5 spacer on the bottom of the MV compression stack. Did you make this spacer or is it a typo? Is there another shim below this, such as a base plate or is the spring acting on the larger diameter shims? Just curious. Definetly try experimenting with a non independent crossover setup on the BV, such as 16 or 17mm diameter. You will need to reduce the number or even the diameter of some shims in the 1st stage stack as the larger diameter crossover shimm increases the strength of the first stack. A very common primary stack is:
24.10
20.10
16.10
With a lot of MV compression in the mix you will need to reduce the strength of the BV or the forks will start to feel harsh. (As you have noticed) In your initial tests the large crossover gap softened the BV enough to give decent plushness but it also reduced the lowspeed compression necessary for good control. I think this is where the non-independent stack has it's strengths. Done correctly, it can give you the best of both worlds. On another note, you should really look into the springs in the forks. If the rate is really far off for your weight and intended use, you will wind up chasing your tale with the valving. Also, check how much preload is on the springs. I've notice that large bore 4-strokes like a good bit of preload on them. On my own '01 520 SX, I've had good success with as much as 15mm of preload. Good luck and keep us posted!

 

[Yoken]

Bud-Man,

Thanks for your post.

The 10.5mm shim is a spacer. I found it convenient to reduce the amount of float without overloading the spring. It sits on the bottom, inside the spring. It is basically a bottoming shim and it doesn't disturb the spring action. The spring in acting directly on the next shim 14.1mm (the last shim of the HSC).

You are right about the springs. I must use proper rates before going any further in valving mods.

I've got 2 problems though:
1- Since the fork feels better, the rear end feels strange! I use to service the Kayaba shock on my previous KX with no problems... but here I'll think twice before opening the PDS!
2- I will probably move to your continent (in Tenessee) within 6 months and I am not even sure I can import my bike...

Enjoy your riding and thanks again

 

[Bud-Man]

Yoken,
So the shims will bottom out against the 10mm diameter spacer? If so, that is nice because it gives you a small clamping shim.....which can soften the strength of the MV. One thing to watch is the height of the spacer when installed. If it sticks up too far, the shims of the midvalve wont have any thing to stop them from overbending. Most MVs have a base plate or a large diameter that will stop the shims from overbending. Unfortunately, I've seen MV shims actually fracture from this, which among other things defeats the rebound! Give me an email at: info@scheirer.com and I can give you some more tips to try on the forks. As for the shock.....well that's another story. Your wise to respect the WP shock. It's a lot more work than a typical KYB/Showa shock. If you are mechanically inclined, it's still possible but you defenitely need some special tools to do it properly. Good luck and keep us posted!

 

[Yoken]

Bud-Man,

Thanks for your valuable input. I 'll drop you a mail from work probably by Monday. Am OFF tomorrow... long week-end in view... riding and spear-fishing are on the menu!