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kyb 48 open chamber

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hi i have kyb 48 open chamber forks thats are really bad deflects all over and harsh

bike is hva 310 te 2013 springs 4.6

open them today and what i think its shimd stiff execily the rebound and base valve

stacks are

 

base valve

24x9x0.12

18x1x0.12

22x1x0.15

20x.x0.15

18x1x0.15

16x1x0.15

14x1x0.15

13x1x0.15

12x1x0.25

 

mid comp

24x3x0.12

22x1x0.12

20x1x0.12

18x1x0.12

16x1x0.12

14x1x0.12

12x1x0.12

10x2x0.30

 

rebound

 

23x7x0.12

14x1x0.12

22x1x0.12

20x1x0.12

18x1x0.12

16x1x0.12

14x1x0.12

12x1x0.12

9x2x0.20

 

any ides? iam thinking take away 2-3 face shims on rebound

and maby 1 face shim on mid comp?

and maby smaller xover on bae valve?

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And maby take away 3 face shim on base? And have a 12x0.10 xover?

Do some 1 have any info or stacks to this forks? I think yamaha and kawa had this forks around 2004?

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Ok take away 3 face shims on rebound

And mid valve comp stock

And take 3 face shim on base and smaller xover?

What do you think mog?

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I can offer a slightly different perspective, based on my own experimentation with Showa 47 TC.

Your changes closely mirror my initial revalve attempt. I was also wishing to reduce deflection and harshness, especially from square edge hits while trail riding at moderate speed. I initially softened the BV LS by removing a few face shims and using a smaller crossover. I hated it! The soft LS immediately caved into the HS stack and my forks were harsher. Square edge hits and deflection are high suspension speed events, so softening the LS didn’t work for me. As Mog suggests, you may benefit from making fewer changes to better identify the specific changes made by each alteration.

With this in mind, if I were in your garage with you, and acknowledging my lack of specific experience with your forks, I’d propose the following valving logic.

BV. I’d leave the LS and crossover alone. I’d remove shims from the HS; (20,16,13).

The key for me was to soften the MV compression. Oil moves through the MV at a much higher speed than the BV (apx seven times as quickly). Since hydraulic resistance rises exponentially with velocity (Doubling the speed quadruples the drag/resistance), the MV much more rapidly develops resistance to high suspension speed than the BV. I softened my MV comp and slightly opened the float. I chose not to grind down the shim dowel, and stacked shims at the bottom of the stack to compensate for each floating shim removed. In your case, these “filler” shims would match your MV Clamp Shim OD of 10mm. This not only softened the MV compression valving, but also allowed for greater total shim deflection before being stopped by the Base Plate. This allows for even more flow at full deflection. This configuration allows much greater ability for the forks to blow off the rapidly rising hydraulic resistance from high suspension velocity, such as those experienced during square edge impacts. I would consider removing shims lower in the stack since I don’t know the diameter of your base plate which offers shim support. So, I’d consider removing the 20,16 and 12 floating shims.

I have no suggestions for your MV Rebound. If you’re certain you need to soften it, your suggestions seem logical.

This setup allows for firm initial fork action, which avoids diving and provides (for me) better chassis stability and front wheel precision. However, when the hard/quick hits occur, the forks are much better at blowing off the impacts.

This setup would NOT be good for big, flat landing jumps! When I experienced these on the local MX track, I bottomed more severely than I prefer. I increased my fork oil to the maximum and my bottoming issue was greatly improved.

Just another perspective to consider, especially since you seem to be headed down the same path I initially took with my valving experiments. Maybe this can save you some steps. It’s best to make sure your seals and bushings are fresh so your ride impressions are valid.

Good luck, and please let us know how this turns out!

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What kind of riding do you do?  Did you measure the float?  Are there any bleed holes in the mid valve?

I have good results with removing three of the seven rebound face shims, that will work.  I would do the base valve differently.  Too soft will cause cavitation and poor fork action.  Some tinkering will be in order to fine tune it but I would be inclined to pull three face shims and the crossover.

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I took 3 face shims off base

And 12x0.10 xover

And i took 3 face shims off rebound

And mid comp haved huge float so i closed it to 0.75 ish? And i drilld a bleedhole in mid piston 1.5mm

I think it workt to low before

And it was packing

 

So will fill it whit oil and try it

About oil level? 110-120? 

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.75 is still a lot of float but it should be better than it was before. 

1.5mm bleed is what I use as well.  Works great.

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Do some 1 have any info or stacks to this forks? I think yamaha and kawa had this forks around 2004?

I have the 48mm OC KYB forks on my 2004 YZ250. Used for bush trail riding. Medium pace single trails. Mostly 3rd gear. Some rocks and roots and some fast corners with edges to hit hard.  The forks work really well, with some mods to the stock MX valving. I drilled the base piston ports bigger to 4.0mm (were 3.5mm), that softened the VHSC damping a lot. I also removed one face shim from the mid comp. So now (according to my old notes) I have 2x27.1 20.1 16.1 over a 14mm clamp (yes just 4 working shims). Beware that the float gap in these OC forks is supposed to be large compared to the smaller CC mid valves.  Sadly I didn't measure my float when I had them apart in 2017. Those two mods softened the comp damp so much that I had to beef up the base stack by 17%. I also changed the base x-over shims from a 12.12 to a 20.1, so almost single stage base comp, and this was a really good improvement. I use Dex 6 ATF in these forks. I really like them. No deflection at all when the clickers are dialed in. C=9 and R=11.

Looking at your stock valving my guess are:  Your reb stack is not too slow for your stiff 4.6 springs. It actually looks too fast. So I don't understand your packing down (low in the stroke) issue.  Your stock reb stack is a lot softer/faster than the stock Yam reb designed for 0.44 springs.  I use the stock Yam reb stack with my 0.42 springs and it feels good - very connected to the dirt.  Your base looks just a little too firm, so I would remove 2 face shims and one of the 0.15 shims.  Also check your base ports are at least 3.5mm and consider going to 4.0 to let the fork really move on fast/sharp bits.  Your mid comp is very stiff and I suspect your main problem. I'd remove one face shim in your mid and probably do softer again. But you need to set a the correct float. Sorry I don't have a number for you. I think it should be over 1mm. 

It's an OC fork, so my thinking is that if you ask for too much comp damping from the mid, then the oil will cavitate and foam up too easily.  Then the fork damping will fade and action will get unpredictable.

I took 3 face shims off base

And 12x0.10 xover

And i took 3 face shims off rebound

What was it like??  I guess you'll not like it, feel too much spring on the reb. Also the softer base and much softer base first stage might make it ride lower than before. And your oil will foam up faster.

What do you weigh?  46 springs are quite firm for trail riding. I'm 165 lbs (without gear) and like to keep my weight over the front find 0.42 kg/mm springs hold the front up just fine. In the rear I have a soft spring.

What's your spring preload?  I have 5mm and tried 2.5 and 7.5mm and it made a significant difference to the "connectedness" of the front tire.

Are you sure your shock is working well enough?  I find my shock setup matters more than the forks. When the rear is hooked up and smooth the front doesn't matter so much.

 

 

Edited by numroe

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20 hours ago, numroe said:

I have the 48mm OC KYB forks on my 2004 YZ250. Used for bush trail riding. Medium pace single trails. Mostly 3rd gear. Some rocks and roots and some fast corners with edges to hit hard.  The forks work really well, with some mods to the stock MX valving. I drilled the base piston ports bigger to 4.0mm (were 3.5mm), that softened the VHSC damping a lot. I also removed one face shim from the mid comp. So now (according to my old notes) I have 2x27.1 20.1 16.1 over a 14mm clamp (yes just 4 working shims). Beware that the float gap in these OC forks is supposed to be large compared to the smaller CC mid valves.  Sadly I didn't measure my float when I had them apart in 2017. Those two mods softened the comp damp so much that I had to beef up the base stack by 17%. I also changed the base x-over shims from a 12.12 to a 20.1, so almost single stage base comp, and this was a really good improvement. I use Dex 6 ATF in these forks. I really like them. No deflection at all when the clickers are dialed in. C=9 and R=11.

Looking at your stock valving my guess are:  Your reb stack is not too slow for your stiff 4.6 springs. It actually looks too fast. So I don't understand your packing down (low in the stroke) issue.  Your stock reb stack is a lot softer/faster than the stock Yam reb designed for 0.44 springs.  I use the stock Yam reb stack with my 0.42 springs and it feels good - very connected to the dirt.  Your base looks just a little too firm, so I would remove 2 face shims and one of the 0.15 shims.  Also check your base ports are at least 3.5mm and consider going to 4.0 to let the fork really move on fast/sharp bits.  Your mid comp is very stiff and I suspect your main problem. I'd remove one face shim in your mid and probably do softer again. But you need to set a the correct float. Sorry I don't have a number for you. I think it should be over 1mm. 

It's an OC fork, so my thinking is that if you ask for too much comp damping from the mid, then the oil will cavitate and foam up too easily.  Then the fork damping will fade and action will get unpredictable.

 

What was it like??  I guess you'll not like it, feel too much spring on the reb. Also the softer base and much softer base first stage might make it ride lower than before. And your oil will foam up faster.

What do you weigh?  46 springs are quite firm for trail riding. I'm 165 lbs (without gear) and like to keep my weight over the front find 0.42 kg/mm springs hold the front up just fine. In the rear I have a soft spring.

What's your spring preload?  I have 5mm and tried 2.5 and 7.5mm and it made a significant difference to the "connectedness" of the front tire.

Are you sure your shock is working well enough?  I find my shock setup matters more than the forks. When the rear is hooked up and smooth the front doesn't matter so much.

 

 

This is amazing I was about to respond to this topic before I read this post, Numroe knows what he is saying, I've done alot of experimentation with these forks too so I understand what he did.

Let me try to explain what he did some more.

First of all DO NOT reduce your rebound damping, one of the main reasons you feel like the fork is packing/chattering is because on compression stroke these OC forks partially empty the rebound chamber of the cartridge due to do piston rod displacement, which is ok, but in reality what also happens on multiple sharp edge hits is that the MV moves so quickly that it would push fluid out through the BV leaving an air pocket in the rebound chamber leaving a void in rebound damping causing cavitation etc.

So to help fix that he increased MV float, reduced MV compression and increased BV damping. This allows more fluid to easily get past the MV on compression to keep the rebound side of the chamber full with fluid for more consistent rebound damping and also reduce high speed compression damping.

The increased BV damping will help compensate for the whatever slow speed damping the MV offered.

I hope this helps because these girls could be a pain if u don't know what u r doing

 

 

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41 minutes ago, ROD Kerry said:

This is amazing I was about to respond to this topic before I read this post, Numroe knows what he is saying, I've done alot of experimentation with these forks too so I understand what he did.

Let me try to explain what he did some more.

First of all DO NOT reduce your rebound damping, one of the main reasons you feel like the fork is packing/chattering is because on compression stroke these OC forks partially empty the rebound chamber of the cartridge due to do piston rod displacement, which is ok, but in reality what also happens on multiple sharp edge hits is that the MV moves so quickly that it would push fluid out through the BV leaving an air pocket in the rebound chamber leaving a void in rebound damping causing cavitation etc.

So to help fix that he increased MV float, reduced MV compression and increased BV damping. This allows more fluid to easily get past the MV on compression to keep the rebound side of the chamber full with fluid for more consistent rebound damping and also reduce high speed compression damping.

The increased BV damping will help compensate for the whatever slow speed damping the MV offered.

I hope this helps because these girls could be a pain if u don't know what u r doing

 

The mid can only push oil out of the cartridge if the base valving is too soft.  This fork is using a lot of float and a reasonably firm base valve which makes cavitation unlikely.  The damper rod entering the cartridge can not cause cavitation, it only displaces fluid.  Base valving is key as it regulates the amount of pressure the damper rod builds inside the cartridge under compression.  Cavitation cannot occur unless pressure in the rebound chamber drops below zero.

I always run faster rebound in these, with lighter stacks and additional bleed.  If you have cavitation occurring it won't matter what valving you have in there as the rebound stroke will be completely unimpeded until the voids in the rebound chamber are compressed and the valving can build pressure. 

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