Azzkikin

When building a shock, is there any way to get the shock to not have any vacuum (extending the shock) or pressure (collapsing the shock)?
Sealing off the shock after you bleed it, and providing you have not screwed up your O-rings when building it, you should have a tight seal. So no matter how you move the shock, you should create a vacuum or pressure.

((Reason behind vacuum or pressure, no need to read if you understand how it works.
The reason for vacuum or pressure is due to the shock shaft. The shock shaft has a volume to it, it takes up space.
So if you seal the shock with the piston at the top, the shaft is now in the body of the shock and taking up space. This you can see as you bleed the shock it pushes oil out of the bleed hole. So when you extend the shock, the shock shaft leaves the body and you have created a void therefore creates a vacuum. Release the shock shaft and it will get sucked back into the body.
If you seal the shock with the shaft fully extended, you have the maximum amount of oil in the body. Collapse the shock and the shock shaft goes into the body and you get compression from the shock shaft trying to push away the oil and it having nowhere to go. Release the shock shaft and it will get pushed back out. ))

I have seen videos of people building a dead shock, but I don’t think if built right, that it’s possible.

What do you guys think?

Davidka

iTrader: (86)

Yes. It requires that you use a bladder, like 1/8 scale shocks use, and drill a small pinhole in the shock cap so that the air bubble behind the bladder can vent outside of the shock, relieving positive/negative pressure.

Ty/Gord Tessman prepares shocks this way. They use a small piece of scotchbrite behind the bladder to help it hold its shape. I don't know if that's necessary with all bladders.

If the caps have bleeder screws, you can use a set that have been center-drilled so that your caps are not permanently vented.

bubbaslash

iTrader: (1)

If not using a bladder, would it be wise to reopen each bleeder with the vehicle sitting at normal ride height to "split the difference" between pressure and vacuum?

Davidka

iTrader: (86)

I find that shocks aren't air tight. The way I fill emulsion shocks is to fill, open the bleed screw, then push the shaft/piston all the way up (if it's full enough, oil comes out). With the shaft all the way up, I put the screw back in and put the spring and retainer on. The negative pressure pulls in air through the shaft seals to compensate for the shaft volume. I do it this way because the fixed volumes of the shaft decide how much air is introduced and it is repeatable and consistent.

icecyc1

While there may be air leaking in from the seals, there is also always air in solution in oils. So, the way you are describing pushing the shaft in, it will create that vacuum which allows the air to come out of solution. Much like opening a bottle of pop. The amount of air in solution is an equilibrium dependent on the pressure the oil/air is under. Your method reduces the pressure, causing a new equilibrium solution, and since it's in a closed container, that dissolved air now occupies that volume initially occupied by the shaft. It's a good way to be consistent with an emulsion build because you are able to more precisely control the air volume as compared to starting with a partially filled shock.

The air in solution is not visible (the oil will look clear), so it looks like all the air is out. Unless you put your shocks in a vacuum pump (like the one from Tamiya) the air will be in there. The air molecules actually fill the voids between the oil molecules. (think of a jar filled with marbles) I have found, pretty consistently, that if I vacuum pump the shocks twice, I will get most of the air out. Then any more vacuuming does not produce any more or very few bubbles, even after running them on the dyno and rebuilding them.

bubbaslash

iTrader: (1)

This thread is enlightening, considering I've only had experience with bladder shocks in the past. My youngest boy just built his first set of emulsion shocks a few days ago, and it looks like I had him bleed them correctly....installing the bleeder screw with the shaft at the top of its stroke

Davidka

iTrader: (86)

There is no meaningful amount of air in shock oil, otherwise the oil itself would be compressible (Dot5 brake fluid is silicone).

JAE

iTrader: (30)

Is there any value then in vacuuming the air out of the shock of you hen use davids method of bleeding thru the bleeder hole? I'm wondering what The value is if you are introducing air back into the shock anyhow...thx much learning a bit here

Davidka

iTrader: (86)

There has to be air in the shock to compensate for the volume of the shock shaft. My concern with vacuum is that it's another process/variable. I'm satisfied if I can fill & bleed my shocks the same way every time.

Azzkikin

Ya, there has to be air in the shock. If not, then you have turned your shock into a hydraulic cylinder and would have no movement. Air compresses, oil does not.

This is all good info and insight to what we think is happening inside the shocks.

I think I am going to try and build a shock with a bladder and leave the bleed hole open.
When you cycle the piston up and down with the cap off, you can see the oil rise and fall. It's not much, so a bladder and open cap should work with these small volumes and not blow the oil past the bladder.

wittyname

iTrader: (4)

RC is funny ....

Grown man with job spends hours building and rebuilding his shocks to attain perfection , sets down car , hits wall 23 times ...

Kid picks his nose , drops down his b4 , and laps the field .... shocks don't even have oil in them ...

Yet we come back for more ...

Davidka

iTrader: (86)

Yes! Post of the month candidate.

will5879

Yep, I've seen that exact thing. Old outdated car with nothing special. He only showed up with the car, radio, and a charger, ... nothing else. About 30 entries in the class and he smoked 'em bad.

bubbaslash

iTrader: (1)

Way to make beer fly outta my nose, Wittyname. Ya, that's why I just crew chief and let my boys drive. I'd be into the wall 24 times. But my boys have VRC Pro now, and I figure if I practice enough I might be good enough to race Sportsman LMAO. At least in the virtual version I don't have to worry about marshalls collapsing from exhaustion trying to keep me right side up. There's a 10 year old kid at a track we race at, and he keeps up with most of the fast guys, with only about a year under his belt. It's amazing. He has a gift and I hope one day it takes him far.

ThePanda

iTrader: (68)

You can buy shocks that use a through through piston shaft design with no vacuum or pressure. from t shox