Jump to content

Drilling SSS fork "pistons" ?

Recommended Posts

My friend bought a YZ250X.  The stock forks sucked on technical singletrack, even with the clickers open.  They weren't as bad as MX forks, but they weren't up to the task either.   My friend send his forks in to the local suspension tuner.  He did his magic.   The reworked forks were better but still not as good as the forks on my YZ250FX.

My friend sent his forks back in for rework.  The tuner just wanted to change the oil level and weight.  I convinced my friend to ask for more mid valve float at least.  This must have inspired the tuner because he reworked them again.   When my friend picked them up, the tuner told him he made changes to the base valve, midvalve, lowered the oil level and "drilled the piston" for more flow.

Long story short, the forks are excellent.   Maybe magical.  They stand up well, they don't wallow and don't seem to bottom.  And you can hit stuff like logs at speed and they just soak it up like it wasn't even there.

Has anyone heard of a tuner drilling the piston in KYB SSS forks ?  Which port are they drilling ?

Share this post


Link to post
Share on other sites

It's the ics pistons , drilled in the middle with a 5mm hole , you also remove the top o ring , it's for reliability as the standard pistons crack

Share this post


Link to post
Share on other sites

Gold valves come with either a predrilled bleed hole in the piston/valve or they tell you to drill a 1.3mm hole, I think my gold valves were predrilled. So drilling a bleed hole in the piston would be a good idea especially for woods riding. Google gold valve bleed hole. 

Share this post


Link to post
Share on other sites
35 minutes ago, mog said:

It's the ics pistons , drilled in the middle with a 5mm hole , you also remove the top o ring , it's for reliability as the standard pistons crack

If you drill the ICS piston, is it still an ICS fork ?  Or does that convert it to open chamber ?

Share this post


Link to post
Share on other sites
31 minutes ago, tom02cr250 said:

Gold valves come with either a predrilled bleed hole in the piston/valve or they tell you to drill a 1.3mm hole, I think my gold valves were predrilled. So drilling a bleed hole in the piston would be a good idea especially for woods riding. Google gold valve bleed hole. 

Could one do this with the stock piston ?

Damping wise, how is an orifice in the piston different from opening a clicker a bit more ?

Share this post


Link to post
Share on other sites

Yes, and now it got me thinking I should try it in my 125 forks and a friends yz250 forks that we revalved. I think it just allows more bleed than what you already get through the clicker circuit bleed bypass, better bump absorption. 

  • Like 1

Share this post


Link to post
Share on other sites

The stock mid piston already has a 1mm (from memory) bleed hole in it

Share this post


Link to post
Share on other sites

No it's still a cc fork but it's then closer to a showa in setup

Share this post


Link to post
Share on other sites

I just confirmed with my friend that they did replace the pistons in his forks.  So the tuner either added bleed holes to the piston or enlarged the ones that were in it.

There is another chapter to this story.   My friend also has a YZ125.  He had the forks tuned by Tuner A.  He didn't like them.  Tuner A replaced the pistons in his forks.  He is now sending the YZ125 forks to the tuner that did his YZ250X forks to get them set up exactly the same way.  Apparently this involves replacing the pistons that Tuner A put in the forks.  I'll get to look at them once they are out.

What are the advantages of these aftermarket pistons over the stock KYB pistons for woods riding ?

I don't understand how there are rebound bleed holes and compression bleed holes.  If you drill a hole, doesn't it go both ways ?

Edited by MidlifeCrisisGuy

Share this post


Link to post
Share on other sites
1 hour ago, MidlifeCrisisGuy said:

 

What are the advantages of these aftermarket pistons over the stock KYB pistons for woods riding ?

I don't understand how there are rebound bleed holes and compression bleed holes.  If you drill a hole, doesn't it go both ways ?

you need to know a little more about the pistons he changed to, many different style of pistons ie: 2 port, 3 port, 4 port, various different size ports, gold valve pistons which allow stack preloading or restriction etc.

 

a bleed hole effects both rebound and compression, it is free bleed in both directions

Share this post


Link to post
Share on other sites
36 minutes ago, bowser said:

a bleed hole effects both rebound and compression, it is free bleed in both directions

When I Googled "gold valve bleed" I came up with discussions that mentioned drilling separate bleed holes for compression and rebound ?

Share this post


Link to post
Share on other sites
19 minutes ago, MidlifeCrisisGuy said:

When I Googled "gold valve bleed" I came up with discussions that mentioned drilling separate bleed holes for compression and rebound ?

not possible, you can either drill the rebound webbing or the compression webbing of the piston, either way it will be free bleed in both directions. The only way you can separate the flow is to use a rebound check valve

  • Like 1

Share this post


Link to post
Share on other sites
9 hours ago, tom02cr250 said:

Yes, and now it got me thinking I should try it in my 125 forks and a friends yz250 forks that we revalved. I think it just allows more bleed than what you already get through the clicker circuit bleed bypass, better bump absorption. 

What is your thinking about running a bleed hole in the piston ?

More bleed softens things at slow speeds but the bleed flow becomes negligible at high speeds.  So running a bleed in the piston would allow you to run a stiffer stack than you normally would without making the slow speed stuff too stiff ?

The thing about a piston bleed hole is that it doesn't go through any long ports or past any clicker needles, so it is a low drag passage.

I'm usually trying to get less high speed damping to lessen the blow when hitting trail trash at speed.  And yet I want the suspension to firm on low speed stuff so it isn't wallowy.  Isn't adding a piston bleed doing the opposite of what I'm trying to accomplish ?

While discussing this topic, how does adding a piston bleed differ from increasing the mid valve float ?  Mid valve float only allows flow on the compression stroke.  Mid valve float flow is also spring loaded, so it has to overcome a bit of pressure before it opens.  What else ?

I'm trying to figure out which of the changes the tuner made to my friend's forks made the most difference - base valve, more float, drilled the piston and less oil, so a softer spring rate progression.

I'll say one thing.  It was really helpful to have my bike as a baseline from which to compare his bike.

 

 

 

Share this post


Link to post
Share on other sites

Bleeds can be asymmetric depending on inlet/outlet geometry such as lead-ins, radius on the edges, or elbows on entry/exit.

Share this post


Link to post
Share on other sites

Bleed starts before low speed damping , it's not the same as float etc

Share this post


Link to post
Share on other sites

The newer KYBs have the blleed holes in the reb pistons.  They do not have the ICS pistons drilled.  This "third closed chamber" on top of the ICS  was a way around a Showa patent.  ICS are drilled to prevent excessive pressure build up on the back side of the plastic piston and breakage, not performance.  The stock reb pistons with one bleed hole can be valved to be excellent without additional drilling.

  • Like 1

Share this post


Link to post
Share on other sites
On 3/21/2017 at 11:08 AM, MidlifeCrisisGuy said:

They stand up well, they don't wallow and don't seem to bottom.  And you can hit stuff like logs at speed and they just soak it up like it wasn't even there.

 

That definitely sounds magical. I'd love some of that.

Edited by Goatse

Share this post


Link to post
Share on other sites

Do your forks have a bleed stack ? If so its quick and easy to flip it over and give you extra bleed. This maybe what your chasing!

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×