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2011 CRF250R Smart Performance piston mod

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I have a 2011 CRF250R which I have revalved myself. It is working really well at the moment (thanks to the Dogger) but I'm trying to figure out how to improve it more. I came across the Smart Performance mod, which you drill bleed holes in the base valve piston... Has anyone done this to their own bikes and noticed an improvement? Or had any experience with it? I just don't want to damage my pistons if its not going to do any good...

Thanks in advance!

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if your mid/base pressures are balanced, you should not need to drill the free piston. but, some other companies (fc) are doing this no matter what. i've not been drilling mine as of late. i have not had issues. however, if the mid/base valving are out of balance, the resulting pressure imbalance will grenade those suckers if you jump something large, or blast the desert at high-speed.

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How can you tell the pressures between the chambers are balanced or not??? Does the amount of air which comes from the air bleeder screw have anything to do with it?

I am willing to do the mod but dont know if it is worth it, or if I will feel an improvement in suspension action?

I can post up my stacks if needed

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I think Smart perf. talk about drill only a free piston. Not a base valve piston.

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2011 crf 250 need drill free piston for bleed all air in cartdrige and not for make difference on pressure

different by 2010 model check the picture

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How can you tell the pressures between the chambers are balanced or not??? Does the amount of air which comes from the air bleeder screw have anything to do with it?

I am willing to do the mod but dont know if it is worth it, or if I will feel an improvement in suspension action?

I can post up my stacks if needed

i've seen some bikes have too much float in the mid (check plate), and the rider used the bike for jumping. that blew out the kyb free piston.

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if your mid/base pressures are balanced, you should not need to drill the free piston. but, some other companies (fc) are doing this no matter what. i've not been drilling mine as of late. i have not had issues. however, if the mid/base valving are out of balance, the resulting pressure imbalance will grenade those suckers if you jump something large, or blast the desert at high-speed.

I'd like to challenge you to explain how you believe what you describe is possible since both the base and mid valves are on the same side of the free piston, and since at full travel, the free piston would release all the pressure in the cartridge through the bleed ports and into the outer chamber.

To the OP: The free piston drilling mod for KYB twin chamber forks is done not for the purpose of making any improvement in ride or damping, but to prevent the piston from being broken. The way they get broken is that if there is a slight leak at the rod seal, oil from the cartridge migrates up onto the spring side of the piston. With an undrilled piston, the air volume above the piston is both fairly small and is sealed in, so if too much oil accumulates, the air pressure within that space can become extremely elevated during compression, and the piston can burst as a result.

The aftermarket aluminum free pistons actually have very large ports built in rather than small holes, and have deleted the top O-ring. Either way, drilling/venting the piston simply allows oil to escape without breaking anything in the process.

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Well lets see. How do you explain a light rider launching and grenading the free pistons and my fast Guy landing to flat on a freestyle ramp, and not? This is what I've come across.

Now another way to explain what happened could be an unbalanced transition from the active to passive valving. I've drilled and not drilled them. I've also seen them blow out when large Guy rides small Guy's bike.

When the right spring and appropriate valve scheme are chosen, I've not had the issue, so why drill?

There are some who drill in the center 6mm. Multiple smaller holes is probably a better way to do this.

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remember drilling kyb piston make the fork more progressive ,the pressure in outer chamber spin on the free piston when landing and help the ics spring............

the pressure of chamber help the ics

that s is standard on tc showa fork and on kyb when you drill hole.the hole have the funcion of safety as gray explain but not forget the progression..

on showa 2011 crf yuo have the same method of old tc!!!!!

the drill is only for bleed air when close the piston on inner chamber.

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All things being equal, drilling the free piston illiminates the "SSS"...

That being said, i have Rate Tech aluminum pistons and honestly didnt look too close at them when I put them in, but couldnt tell a noticeable difference in the feel of the front end afterwards. My understanding at the time (years ago) was that they were for durability only and wernt designed to alter the function of the fork.

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Well lets see. How do you explain a light rider launching and grenading the free pistons and my fast Guy landing to flat on a freestyle ramp, and not?

Oil on top of an undrilled piston, or coil binding the ICS springs at near full travel.

Unless the mid valve cavitates dramatically, the base only passes oil displaced by the cartridge rod, and the free piston is only exposed to the oil passing the base, so it really has no way of sensing an imbalance between the mid and base. And again if the piston travels beyond its design limit without encountering a condition of hydraulic increase in air pressure or coil bound springs, the overfill/bleed ports open to discharge any pressure below the piston into the outer chamber.

As to whether drilling contributes to an increase or reduction in the progressivity of the fork depends on whether the air pressures in the ICS chamber and the outer chamber of a fork with undrilled free pistons are significantly different under compression. I would suggest that the fact that the pistons tend to burst outward into the outer chamber rather than collapse inward when they fail indicates that the ICS chamber pressure was higher, at that moment at least. A simple change in oil level in the outers would alter any difference, as well.

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the floating piston expload when the fork compress at bottom ,when the chamber leak and oil flow over the piston, up ,in the ics spring zone.

the cc. of oil cant compressed and make high pressure in the plastic piston and broke it

but the higher pressure gray said is that!

not in the inner chamber,at bottom the inner chamber have less pressure than the outer chamber.

right gray?

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With "SSS" meaning "Speed Sensitive System", this is not true in any way.

I dont remember which suspension guru it was specifically that im quoting, but they very specifically said that once you drill the vents, the air volume above the free piston now shares the air volume in the outters and there by illiminates the tuned sealed chamber that "is" the design of the "SSS" KYB intended, no?

One observation I can make with 100% certainty is that my pistons never cracked and I have several hundred hours (somewhere around 1000 hours) on them when I chose to swap the aluminum in (just as a preventative measure). But I change my oils and release pressure regularly. I believe the only time they crack is if you wear out the free piston shaft seal allowing oil to build up. Once you drill holes to allow this oil to drain out, where does it go? In the outter chamber just like the "blow off" holes on the side of the IC?

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The sealed air chamber above the piston may or may not have been carefully sized for some reason, but there isn't anything about it which would be speed sensitive. In fact, any dependency on a captive air volume like that would be position sensitive, same as any other spring, which is essentially the opposite of speed sensitive.

In truth, all "normal" modern damping valves are speed sensitive. The bleed circuit delays any operation of the valve until the oil flow reaches a certain speed. After that, the stack opens to varying degrees in response to increasing speed, dependent on port design and stack configuration. "Float", "blow-off", and crossovers are all speed sensitive elements that are commonly employed. IMO, all the respect for Yamaha and KYB that you know I have, "SSS" was just marketing.

And yes, oil leaking past the free piston rod seal with a drilled piston would drain into the outer chamber oil. I never broke the plastic pistons, either, before or after drilling, and I think most people can say that.

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I ride mostly sand motocross with one hardpack section. I am 165lbs/ 75kgs and 6'2.

I have 0.48 springs on the front and stock 5.3 on the rear.

I felt most comfortable with my forks at this: Comp 12, Reb 14.

390cc outer chamber (380cc is max so might have to bring it back down a bit???)

forks flush with clamps

looks like I will be drilling the bleed holes next time I have the forks apart!

This is my setting:

Base Valve:

14 x 32 x 0.1

1 x 24 x 0.1

1 x 31 x 0.1

1 x 30 x 0.1

1 x 29 x 0.1

1 x 28 x 0.1

1 x 26 x 0.1

1 x 24 x 0.1

1 x 22 x 0.1

1 x 20 x 0.1

1 x 18 x 0.1

1 x 16 x 0.1

Mid Valve Compression:

4 x 20 x 0.1

1 x 18 x 0.1

1 x 16 x 0.1

1 x 14 x 0.1

1 x 12 x 0.1

1 x 11 x 0.1

2 x 11 x 0.3

2 x 17 x 0.3

Spring

thick collar

Mid Valve Rebound:

5 x 20 x 0.1

1 x 19 x 0.1

1 x 18 x 0.1

1 x 17 x 0.1

1 x 16 x 0.1

1 x 14 x 0.1

1 x 12 x 0.1

2 x 12 x 0.2

Are they balanced? how can you tell?

I bought 0.48 fork springs from racetech and they came with 4 preload rings... I didnt know how many to put in so I just put in 1 each side.

How much should the forks be preloaded?

The float from stock was about 0.3mm so maybe adding a 0.1 shim to the mid valve will close up the float to 0.2?

Please feel free to comment on my stacks!

I would like as many opinions as I can get, all the help is appreciated as I am still learning :bonk::smirk:

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IMO, all the respect for Yamaha and KYB that you know I have, "SSS" was just marketing.

:smirk::bonk:

So in reality, once you drill your free pistons you end up with much the same setup as single chambers just with seperate IC and OC oil?

And yes, oil leaking past the free piston rod seal with a drilled piston would drain into the outer chamber oil.

Which is exactly what they also do without the holes drilled, which is why the manuals tell you to "stroke" the inner chambers several times to let the inner chamber expell any excess oil before re-assembly so as to not have your outters end up with incorrect oil level no?

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Which is exactly what they also do without the holes drilled, which is why the manuals tell you to "stroke" the inner chambers several times to let the inner chamber expell any excess oil before re-assembly so as to not have your outters end up with incorrect oil level no?
No, and not exactly. Excess oil from the cartridge, whether it was forced in past the rod seals or you just didn't bleed it, will escape through the bleed holes if there is no room for it. But the oil released by drilling the free pistons can't escape otherwise. The only reason it would be on top of the pistons is is the seal at the piston center where the base rod/stem passes through it leaks and lets oil up there. Without adding holes, it can't get out.

So in reality, once you drill your free pistons you end up with much the same setup as single chambers just with seperate IC and OC oil?

I don't really see the similarity. The feature that makes open bath and twin chamber forks different is the separation of air from the damping cartridge, not the presence of two separate air spaces. Open bath forks circulate outer chamber oil through the cartridge, carrying with it whatever foam has developed "outside". TC's don't do that.

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This may help.

The following bikes have a "triple chamber" fork, which means that they have the lower chamber, the upper cartridge and a third isolated chamber that no one told you about.

2010-2012 CRF250R

05-12 YZ125/YZ250/YZ250F/YZ450F

06-12 KX450F

09-12 CRF450R

The forks on these bikes may be regarded as a "dual chamber", "twin chamber", "closed cartridge", "isolated chamber", "AOS" or "SSS" fork. "AOS" and "SSS" are marketing terms and although they were used for certain models or years, the meaning of the terms could apply to any of these bikes, as well as all SHOWA and WP (KTM) closed cartridge forks.

The lower chamber is where the main “fork spring” is, as well as the spring perch, (except the 05 YZ) the bottom out piston and cone, and it this fluid that comes gushing out when the fork seal fails or gets jammed up with junk. When you remove the bleed screw, or press on your quick-bleeders, it is this chamber that is relieved of any excess air. In some rare cases, the bleeding may actually be a vacuum.

Lastly, sometimes oil will come out when you bleed the lower chamber because of a small channel or ring-pocket that resides between the two o-rings in the cap, (long story).

The upper chamber, best known as the cartridge, is where "most" of the hydraulic damping takes place. The third chamber is the area above the free piston, where the IC spring is. And yes, "IC" or "ICS" means "Inner Chamber" or "Inner Chamber Spring", or in some cases, "free piston spring"....which is just a subset of the cartridge.

The CRF250R is technically a SHOWA fork, but it contains a KYB like design, and many internals are the same as the KYB forks. But the free pistons on the CRF250R are not interchangeable with the KYB fork free pistons. The CRF250R fork also has a second bleed screw that allows the user to bleed off any pressure that may have built up on the back side of the free piston, (the IC spring area). This area, is the third chamber as mentioned earlier. So…these bikes have a ”third chamber bleed screw”.

The purpose of the free piston and the spring that sits on the back-side of it, is just to compensate for the volume of fluid in the upper cartridge being displaced when the main damping rod enters the cartridge. All of these bikes have a 12.5mm rod so with 300mm of travel the displacement is about 37cc/ml and with the free piston being about 38mm in diameter, this means that at full travel, (full compression) the free piston moves about 32mm.

With that, most of these forks come with a 20 to 22N (Newton) or 2.0 to 2.2KG spring. The spring is not pre-loaded so when a fork is fully compressed, the free piston has 64 to 70KG of force on it, generating a maximum cartridge pressure of about 79 to 87PSI. This means that the force acting back on the rod (rod piston) is between 14 to 16lbs. In other words, if you convert from a 20 to 22N ICS spring, it will take another 32lbs (14.5KG) of weight to fully compress the fork, with all other factors remaining constant. Times that by 2.

ONWARD!

Q - What causes the free piston on a KYB fork to break?

A - Two causes. The most common is the rod seal leaking (yes, the rod seal).

When the rod seal leaks, you have fluid from the lower chamber that enters the cartridge, via the seal head, leading to an excess amount of fluid in the cartridge, which attempts to purge out when the fork is fully compressed. This purge happens when the lower outer free piston seal (an o-ring) reaches a taper (a larger OD) in the upper body of the cartridge tube. However, when velocities are high, (think faster, harder riding riders) the excess fluid in the cartridge, attempting to purge out, is met with too much resistance and therefore seeks an alternative route. That alternative route is the free piston inner shaft seal, which in turn sends a given amount of fluid up to the top of the free piston, in the IC spring area, which we spoke of earlier as the “third chamber”. Over time, more and more fluid is sent passed the free piston shaft seal until you a substantial amount of fluid in the third chamber, which means less and less air. Then, at some point in time, as the fork compresses, the small amount of air volume remaining in the third chamber is compressed to a point that the pressure becomes so great that the piston explodes.

It is that simple, and that complicated.

This is also why some riders break free pistons, and others don’t. In other words, if you can break a KYB free piston you most likely are a very good and fast rider.

Now…making matters worse are two other issues.

The first is a build-up on the fork damping rod that is caused from a reaction from the fluid. This build-up can be bad enough that it prematurely destroys the rod seal, allowing more and more fluid from the lower chamber to enter the upper cartridge. The other issue is the plastic that the free piston is made from reacts with the fluid causing the piston to become yellow and brittle. In fact, you’ll often see free pistons yellowed in an area where the fluid has been pooling in the cup of the free piston, much like a soap line in a bath tub.

That said, the other cause of free piston failure (but not at all common) is from the front or leading edge breaking or cracking. This is mainly from the fact the plastic is not strong enough, or has become brittle, in combination with the fact that you ride the bike fairly hard. Running an SPI-Racing EPNP kit may cause the leading edge of a weak free piston to break whereas it may not when using a stock IC set-up.

Q – So…to drill or not to drill?

A – For all 06 to 08 YZ bikes and 06 to 08 KX’s, you don’t have a choice. It needs to be drilled for the only reason of surviving. We recommend drilling the piston so that any fluid that builds up in the cup of the design will drain back down. In other words, not much sense drilling the hole mid-way or up top.

For 09 to 12 YZs and 09 to 12 CRF450Rs and 09 to 12 KXs it’s a little more of an option. The reason? Starting in 09 KYB started using a better free piston shaft seal. AND…on the 10-12 YZ450F and 10-12 KX450F and the 11 to 12 CRF450R, they switched over to a better rod seal.

Q – What about the CRF250Rs?

A – For the 2010 CRF250R, you don’t drill the free piston unless you need to change the performance of the fork. For the 11-12 CRF250R you drill the free piston only for the purpose of bleeding the cartridge. In other words, nearly all 10-12 CRF250Rs don’t have the issue of free pistons breaking. I didn’t say “all”…just most.

Q – What about metal or aluminum free pistons?

A – Available for the 06-12 KYB forks. There are not available for the 10-12 CRF250R forks. However, if you race for a living, I would replace the plastic free pistons in your CRF250R with something that is not plastic.

Q – What else can be done?

A – When replacing a broken free piston with a new free piston, it’s always best to get one that has the updated free piston seal. Not all shops are aware of this so…buyer beware. Even some of the aluminum free pistons on the market use the older free piston seal, but…all things considered, it doesn’t matter when the piston is “vented”.

You can also update the older free piston shaft seal to a new one.

AND…you can also convert the seal head to the updated rod seal design.

Q – What about the 05 YZ?

A – Seldom do you see broken free pistons on an 05, in part because they actually have a better rod seal (seal head) design than the 06 to 08 YZs. But…it can happen. By the way, there is not a 05 aftermarket aluminum free piston available as there readily is for the later model bikes.

Q – What changes in the fork when you drill the free piston?

A – Two things. First you introduce about 75cc/ml of volume to the lower chamber, as you have now vented or opened up the third chamber to the lower chamber. In other words, when you drill the free piston you now have a true “dual chamber” fork, just like the 47MM SHOWAs (think 04 to 08 HONDAs, most RMZs….etc.).

Second thing – you open up the pressures in the lower chamber to the backside of the free piston, meaning that you now have the force of the IC spring pressing on the free piston as well as the force of the compressed air from the lower chamber (which increases as the fork compresses). Of course, the pressure in the lower chamber, (progression and maximum pressure), is controlled by how much fluid you put in the lower chamber. And now with the third chamber vented, whatever the air pressure is in the lower chamber, is now the oil pressure in the cartridge. So if you have 200PSI in the lower chamber, you now have 353 pounds of piston force on the other side of the free piston (which is a number we don’t care about) but this translates to an additional 37 pounds of force acting on each rod, (total = times 2 since it's a fork). In other words, you don’t just add in another 75ml of fluid to compensate for the difference between a triple and dual chamber KYB design, when considering end-of-stroke dynamics.

Hope this helps.

P.S. – I did the math in my head so don’t be surprised if I screwed something up.

Edited by DaveJ
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