GHEA pistons (which way up!)

[Rebelrc]

My manner sometimes may read a little short. But it's not ment that way, I am just posting in a hurry to try and be helpfull whilst trying to also work with my customers usually watching me
Sorry if it's come across that way

[samd]

Originally Posted by Rebelrc

You also understand it wrong grelland
Cause they cannot return faster cause that's impossible! They are linear in or out
Cheers

I don't think this can be correct.
However small the differance is, it must be impossible for a piston a different shape to move through oil in the exact same way.

[flipside]

First read this:
http://users.telenet.be/elvo/3/2.html

Then you know how a normal piston works, and that it's all about 'pack'.

Ghea pistons have one tapered side. Oil flowing around this tapered side isn't as turbulent, so it will result in less pack. Oil flowing around the non tapered side will be more turbulent and this will result in more pack.

So, if you put the tapered side DOWN, the damper will have more pack when you compress it quickly, than when you decompress it quickly.

On a smooth track with big jumps, you will want to choose pistons with small holes, with lighter oil, to have enough pack when landing from the big jumps so your chassis doesn't slap the ground.

On a bumpy track without big jumps, you will want big holes with thicker oil, so you have less pack, and it will soak the bumps a lot better.

But what if you have a bumpy track, AND big jumps? Then you want to have enough pack while landing from the jumps, and less pack on the bumps.

One way to achieve this to a certain degree is by tapering the bottom of the holes, or the side of the piston (=GHEA). The shock will lock up enough when landing, but it will be able to extend with less lock to go over the bumps (so not quicker, but with less lock, so further). Of course it will not be as good over the bumps as with bigger holes + thicker oil, but setup is a compromise! With big holes your car won't be as good on the jumps...

For example with losi pistons:
You want to use red pistons for the bumps, but black pistons for landing jumps. Then choose GHEA black pistons, and you will have the pack of a losi black when landing, and a 'similar' feeling of losi red when riding the bumps.

Imo, the ghea pistons excell on high grip, bumpy astro tracks with jumps. Like almost every track you have in the UK, or for example the euro's track in Bilbao.

That is my theory And if this is wrong for some scientific reason, I don't care, since I tried it numerous times on track, and that is simply how they behave

[Rebelrc]

But the only problem with that theory is they pack the same either pushed in or out , both directions. The only way to do what you have explained is with a valve. It's all documented in the link that dazzler posted earlier in this thread ( including gheas info ) it's a mystery to me too flipside. Would be nice if ghea made some valved pistons
I can't comment further as I have no more time cheers

[flipside]

Ok I see, so then I cannot understand why you should make them tapered. Just make pistons with in between hole sizes and you will have the same subtle effect...

To me, the test shows that the taper has no effect on damping, otherwise it should show a difference between in or out... (why not make them tapered on both sides, or not tapered at all). Running losi pistons in your asso shocks will also be better over bumps, since losi pistons are rounded on each side and asso pistons have a sharp edge.

I believe it is impossible that a different shaped piston on each side will let the oil flow exactly the same, and give the same pack in both directions.

Ok if you compare a std losi piston vs a ghea one, of course it will feel different, they are completely different shapes (losi's are rounded on each side).

I have made losi pistons with tapered holes, and you can feel the difference in and out even by hand! Don't need a machine for that. If you pull and push the shaft very quickly, you really feel that pushing in locks earlier than pulling out. (with taper on the bottom).

Just try to run ghea pistons with the taper on top, I think you will be amazed. If not, it proves that this kind of taper does nothing. With my selfmade tapered hole pistons the difference is huge.

[elvo]

Originally Posted by flipside

First read this:
http://users.telenet.be/elvo/3/2.html

I should really delete the paragraph about turbulence there, it's wrong. Pack is actually caused by another effect, which is difficult to explain in words.

[reflexuk]

Go on then elvo, if pack is not explained by the relatively easily pictured turbulence vs laminar flow argument then please have a go, help us picture what might really be happening.

[Gonzo]

I witnessed a few of the Elvo-RC-therapy-sessions, so I will have a go:

Imagine 1 hole in the piston with 2 90 degree edges. When the piston moves veeeery slow through the holes, the oil coming from the sides needs to go around the 90 degree corner. Because it moves slowly, it can do this without leaving the piston's surface.

When the piston moves faster, the oil moves faster, and the oil from the sides has more difficulty taking the corner: it "rounds" the corner and 2 vacuum pockets arise. As a consequence of this rounding, the hole size that can be effectivelly used narrows, and is at a minimum half way the hole.

When you chamfer the 90 degree corner into 2 45 degree corners, 4 smaller vacuum pockets arise instead of 2 . The narrowing effect of the hole is less now.

Till here, pure pack, I think it's more inertia of the oil, and not turbulence, as Elvo mentioned.

So in theory, chamfering only the top or only the bottom results in the same pack, because the effective hole size is the same, only the minimum diameter is positioned a little higher (only bottom chamfered) or lower (only top chamfered) instead of half way the hole (none or both chamfered).

But as Flipside mentioned, it clearly does not feel the same both ways when you chamfer 1 side of the holes! The damper feels softer when the oil enters the chamfered side.
The cone-shape of oil that is created at the chamfered side will probably create a more efficient flow through the narrowest point, because the pressure can build up nicely in the cone. When the oil needs to come from the unchamfered side, most of the oil that was in the helping cone before, is pushing against the piston now, so resisting instead of helping. But more important, the "guiding" cone is non-existing, so the flow at the narrowest point will be less effective.

So when explaining chamfering, then I think you could call it a sort of laminar/turbulence at play, like Reflexuk did.

Why a Ghea works the same with chamfered piston side on top or bottom, I really don't know. Like Flipside, I find that hard to believe to be honest. Maybe not noticable (I dont know how much oil passes via the sides compared to through the holes...), but there must be a difference.