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Discussion Starter · #1 ·
So I got a vacuum bleeder, but no shocks need servicing right now. So I did what any reasonable person would do, and started vacuuming random crap around the garage.

Mostly, I wanted to see just how much air was trapped in a regular old off the shelf bottle of suspension fluid. I filled a glass jar up with 100ml of maxima plush 3wt and started pulling a vacuum.

It bubbled violently and splashed around. A LOT of air came out. I think it took about 10 minutes to get air to stop bleeding out of the fluid, at max vacuum at sea level.

Shimano brake fluid was similar.

To eliminate the possibility that I was somehow just pull air through the fluid from somewhere else, I removed the 100ml of fluid from the vacuum chamber and left it on my workbench over night. I tried pulling a vacuum on it the next day, but no more air came out.

So what does this actually mean? The air is definitely dissolved in the fluid, but since its dissolved, does it actually matter? Is this useful to know? Can I improve damping performance with more air-free fluid?

People have reported shim rusting in dampers. I wonder if the excess air is causing that.
 

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So I got a vacuum bleeder, but no shocks need servicing right now. So I did what any reasonable person would do, and started vacuuming random crap around the garage.

Mostly, I wanted to see just how much air was trapped in a regular old off the shelf bottle of suspension fluid. I filled a glass jar up with 100ml of maxima plush 3wt and started pulling a vacuum.

It bubbled violently and splashed around. A LOT of air came out. I think it took about 10 minutes to get air to stop bleeding out of the fluid, at max vacuum at sea level.

Shimano brake fluid was similar.

To eliminate the possibility that I was somehow just pull air through the fluid from somewhere else, I removed the 100ml of fluid from the vacuum chamber and left it on my workbench over night. I tried pulling a vacuum on it the next day, but no more air came out.

So what does this actually mean? The air is definitely dissolved in the fluid, but since its dissolved, does it actually matter? Is this useful to know? Can I improve damping performance with more air-free fluid?

People have reported shim rusting in dampers. I wonder if the excess air is causing that.
In theory, removing that air would help suspension performance as it wouldn't heat up and expand. Same reason you're supposed to pull a vacuum on DOT fluid for cars, and MTB brakes.

Sent from my iPhone using Tapatalk
 

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So I got a vacuum bleeder, but no shocks need servicing right now. So I did what any reasonable person would do, and started vacuuming random crap around the garage.

Mostly, I wanted to see just how much air was trapped in a regular old off the shelf bottle of suspension fluid. I filled a glass jar up with 100ml of maxima plush 3wt and started pulling a vacuum.

It bubbled violently and splashed around. A LOT of air came out. I think it took about 10 minutes to get air to stop bleeding out of the fluid, at max vacuum at sea level.

Shimano brake fluid was similar.

To eliminate the possibility that I was somehow just pull air through the fluid from somewhere else, I removed the 100ml of fluid from the vacuum chamber and left it on my workbench over night. I tried pulling a vacuum on it the next day, but no more air came out.

So what does this actually mean? The air is definitely dissolved in the fluid, but since its dissolved, does it actually matter? Is this useful to know? Can I improve damping performance with more air-free fluid?

People have reported shim rusting in dampers. I wonder if the excess air is causing that.
Thats why we vacuum bleed all suspension and all suspension fluids in our workshop...
 

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Discussion Starter · #5 ·
They're incredibly expensive if you buy a commercial unit. The race tech unit is $2000.

I made my own, its primarily used for stabilizing wood (other hobby), but it works fantastic for suspension. Probably a couple hundred bucks as a bare-bones setup. I used an AC vacuum pump and a wood stabilizing chamber from amazon.
 

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Interdasting. I'm not sure of the value of de-gassing a suspension fork oil because it will be constantly mixed with air during use and air would just get put right back in.
 

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Discussion Starter · #9 ·
I'm not even sure it's air. Or that the fluid absorbed it at all actually.

It could be outgassing from the fluid itself. If that's the case, it's not going to mix with an open emulsion damper anyway.

Anyone know for sure where that gas is coming from?
 

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I'm not sure of the value of de-gassing a suspension fork oil because it will be constantly mixed with air during use and air would just get put right back in.
I agree, but it does make sense for brake fluid. You don't need a fancy tool. You can get a lot of air out of your brake fluid with a syringe. Fill it, put your finger over the hole and pull the plunger. I realised a few years ago that if you do this the brakes will bleed right first time and stay bled. If you don't, you need to bleed them again later.
 

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Degassing brake fluid helps to get a good bleed. It's specified for most DOT brake systems but I also do it for mineral oil brakes. Pull a vac on mineral brake fluid and see how much gas comes out. When it's degassed, it's less likely to outgas when heated in braking and it will absorb any small bubbles that are left in the system after bleeding.

IMO, vac bleeding of sus mainly helps ensure there's no trapped air in nooks or crannies of the damper. Structures and fluid paths in dampers are more complex than what goes in brakes so there are more opportunities for trapped air. Degassing of the fluid is all good too.

There's a phenomenon in dampers where the local dynamic pressure of the fluid moving through orifices or around shims becomes less than the vapor pressure of the fluid causing it to boil on a very small scale. This is called cavitation. Degassing might help, but the principle means of suppressing this is high IFP pressure which prevents the local dynamic pressure from going that low. Cavitation is distinct from aeration, which is mixing of the fluid with air or gas forming bubbles or foam. In a sealed dampers (IFP or bladder) there should be no air/gas to mix with so aeration is not possible. Cavitation still is possible.
 

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Discussion Starter · #12 ·
I think the marketing department got a little too aggressive to push fork bladders. I dont think forks cavitate or even aerate significantly. Bladders more help hit a target weight than help performance, I think. Shocks, maybe, but not forks.

Old marzocchis used 10 gallons of suspension fluid and they worked pretty well. It was just a lot of fluid to slog around, and when RS started lightening up they got left behind pretty fast.

But still! It goes back to the premise that the fluid has dissolved air in it at all (my fault, I assumed that for the title).

If a brand new bottle of suspension fluid is filled with some weird reaction gas from the initial blending of the fluid, one bleed will get rid of that gas forever and it wont be reabsorbed in a fork exposed to normal air.

Its been a while, I'll vacuum bleed another 100ml of fluid. If it boils out again, its definitely sucking up air. If nothing, then vacuum bleeding open bath fluid could be beneficial.
 

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Yes there is. Oil sitting there at room temperature and pressure has about 8% air (by volume) dissolved into it.

Increase the pressure and it can hold even more. Decrease and it can hold less. Which is why a foamed up shock is like opening a shaken can of coke!

This is also why mineral oil brakes are no good when they get hot. The air expands out of the fluid, creating air bubbles and making your brakes all mushy. They need bled.

Moto forks (and Manitou Dorados) have air bleeders for the same reason.
 

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So, how do you get the de-gassed oil into the fork legs without introducing air?

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Once you have degassed fluid, it takes either time or agitation for it to absorb more air.

Putting degassed fluid into a fork leg with air is a temporary thing. It'll agitate and reabsorb pretty quickly. You need a damper that is either sealed or has minimal air-gap for the oil to stay degassed.
 

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Does anyone remember a time your open bath fork foamed up? Or is that something people just say? I thought the anti-foaming additives prevent that.
Open bath forks don't because there's no air entry. Cartridge dampers can and do foam up. Mostly from air ingested through the shaft seal.
 

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By no air entry do you mean no critical air entry? Cause I would assume they can still suck in air around the wipers. They’re just designed to have some air in them as a commentator anyway. Right?

Do self bleeding cartridge damper foam? If so would use of open bath oil in them help mitigate it?
 

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By no air entry do you mean no critical air entry? Cause I would assume they can still suck in air around the wipers. They're just designed to have some air in them as a commentator anyway. Right?

Do self bleeding cartridge damper foam? If so would use of open bath oil in them help mitigate it?
To suck air properly you need lower pressure than whatever is on the other side. That doesn't really happen with fork wiper seals.

A lot of self bleeding damper cartridges (and ones that aren't supposed to be sealed) suck in air. Sometimes by design, sometimes by accident.
 
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