Shock Piston (valve) "Porting"???

[gosdrcrgo]

I service a shock for a guy last week & found that the piston was modified. It looks like it was "ported" I guess would be how to describe it. The angles have been ground down & smoothed out. The inlet side have been ground down to aprox 45 degrees, instead of 90 degrees. The orifice is the same size, just had the angles ground & smoothed. I guess so the fluid doesn't have to make such a sharp turn. I took some pictures but I can't figure out how to post them. The ower says his bike is much smoother over sharp edge accelleration bumps than his friends bike (witch is the same bike only stock). This is an '02 honda crf450. The shim stack is pretty much stock. So if the shim stack is the same, the piston orifice's are the same, only the angles on the piston is different is this a way of taking out some harshness on sharp edges or this some garage monkey trying to find a use for his new grinder? Without being able to ride the bike myself I have to go by what the owner says. He is the third owner of this bike & doen't know who did this modification. So have any of you tried "porting" your piston? I'm wondering about the theory of this.

p.s. I'll post the pictures if i can figure out how.

 

[NO HAND]

Send them to me and I'll post them for you.
motofreak5yahoo.com

 

[NO HAND]

at last, sorry for the delay!

 

[georgieboy]

Yes this will reduce harshness. I don't know to what extend.
Your friends says it works but did he ride the bikes back to back? It will reduce cavitation more, as there are no ledges that creates low pressure spots.
Btw i did the same to my pistons, i just feel it is smoother and i just happen to have a dremel, what else can you do with it. ;)
No ledges is always better when it comes to flow.

 

[gosdrcrgo]

Yes, the guy said he & his friend where at a track & they switch bikes because his friend was saying how bad the acceleration bumps where. He said his bike was much smoother.

As far as piston design goes, I can understand how less restriction on the inlet can help, but on the outlet side does this change the pressure on the shims like using a large port piston or since the port is the same size the pressure is the same just smoother flow? Will the shim stack need to be changed much for this? The shim stack on this bike wasn't much different from stock. It had a little more low speed & smaller diameter crossover shim with the second crossover shim was thicker (.30 insead of .15) the highspeed stack was stock & the rebound stack was stock.

What more can be done to the piston? I don't see any more ledges or & I missing somthing?

 

[georgieboy]

Ok, so he did something to the stack aswell, probably including oil heights. Then there are more variables to take in mind.
In beginning there is no need to alter the shimstack for this, as the total amount of flow is the the same. You don't alter the ports wideness.
But the difference in smoothness between the forks can be addressed to the differents in shimstack and crossover. They play a big part in plushness.
Copy his fork step by step. And you will know what plays the biggest role in plushness in the end. But promise to keep us informed, oke? Ciao George

 

[WWR]

Georgiaboy,
he is talking about a shock here, not forks. Cavitation should not be a problem at all when the system is pressurized (Nitrogen charge)

And yes, minor changes on the shock shimstacks can make a very big difference.

 

[georgieboy]

WWR you 're right. My mistake. I was completely mislead by the photo's. Look just like mine fork pistons.
No, cavitation shld not be a problem then.
But above goes also for the fork piston, nevertheless.

 

[gosdrcrgo]

I'm more interested in theory of the piston design. If you look at most aftermarket pistons they have smooth inlet side angles but not the exit. If smoothing the angles on the outlet side prevents low pressure areas, & low pressure areas are bad than why don't you see anybody else designing pistons like this.

 

[georgieboy]

Ahaa an other soul to talk to. Azmxyz, when you take a look to the penske website you will see the different types of pistondesigns according to the flow.

 

[DEANSFASTWAY]

Often I see where guys have cleaned up the stock pistons or turned shim base down or concave or drilled bleeds .As for the flashing I think it has alot more to do with production than anything else . Have you ever seen a late model KYB fork base valve where the sideports are drilled? there very codgy at best and more than a few occasions drilled way off center Thats something I would maybe adress.Leanient production tolerances I reckon could account for this , The guy making the aftermarket stuff likely pays more attention to detail then the big factory stamping out hundreds of parts at a time.AZMXYZ; Do you think youll really have a low pressure area right at the shim seat where fluid is exiting at high velocity. I think the low press will be a bit away from the seat area where it has been sort of disbursed .Imagine a dam or a levee right at the water exit there is high pressure but a bit downstream lower pressure water .I can sort relate to what youre saying about ledges . but in my eyes a ledge would be more in the affect of the mid valve base plate washer size(backup washer) or the bottom washer on a shock It would seem that if it werent there or smaller less intrusive the shock or fork might work better . It seems theyre just there pushing or impeding oil flow right? On some woodsie bikes we tried running the smallest diameter washers we could and felt that it helped produce a bit of plushness but actually you could attain this just by valving .Funny some of the WP fork stuff doesnt even really have a Mid Valve Base or backup washer and they dont really have a shim breakage problem . Delta shims maybe .One big company was selling trick convex base washers for fork stacks but I didnt really sway that far as to start selling much they looked trick with the parts dissembled on the bech but I bet the average rider would never feel the benefit. but getting back to the main question I doubt you might feel much of a difference on a bike with a cleaned up vs. stock piston . Maybe on a dyno in a controlled enviornment . maybe the cleaneed up piston shock might not heat the oil as fast or as high but I bet the comparison would pretty much be a wash. Penske has a cool shock at their seminars, the body is made of a glass or lexan cylinder It shows the oil flow and the affects of air presence or low or high Nitro pressure. You can work the shock with different circumsances and view the results . Penske has some cool stuff . Gooday DEAN

 

[jaguar]

Race Techs "Gold Valve Shock Kit" basically uses a piston with wider openings for compression flow which is better for hitting things like square edged holes.
I plan to "port" mine once I find the time. I'll round off the intake edges and make the holes bigger.

 

[dbrace]

A friend of mine once purchased a kx 250 that had the shock piston modified the same as that. The fork comp. pistons also had the outside feed ports drilled bigger.I think the previous owner said it was set up for enduro but i didn't keep a record of what the valving was but it was way too soft for pro mx. Probably more to do with the valving than the piston mods.
I have a question, do current model mx bikes have fork and shock pistons that are restrictive when it comes to oil flow causing dampning problems?

 

[Jeremy Wilkey]

Hey Guys!

The porting does nothing on these pistons.. When you have so much area available, and such small deflection values, it’s a waste of time.. It’s possible that if the shock moved fast enough that it could matter but with speeds being what they are and the again the area of the port this transition does nothing. The damping is not the piston or the shim stack but the area of the opening created by the two. So they have a relationship in that the piston controls the deflection characteristics and the shims provide the resistance to that potential area. One impacts the other but damping is created in between...

I suppose you could make the argument that it decreases viscous damping, but again with so much area this is not really an issue..

Next time your buddies bike works better over bumps on the gas reduce your rebound..

BR,
Jer