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Air in oil and do we care

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3 hours ago, DEATH_INC. said:

You wouldn't have saved those runs would you? Any chance of posting them?

Myself and Mog have discussed/debated/argued about a couple of things like (and including) this, and the one thing that pisses Me off is the lack of evidence for many of the things that are stated as fact by some in the suspension industry. It's like arguing with conspiracy theorists that quote links that go round and round between conspiracy theorist sites. The suspension industry seems to be the same, though some of it will be due to the secretive nature of a lot of tuners. It's pretty frustrating when you don't just blindly believe something because someone says it, and don't have the gear to test it yourself.

I'm pretty sure I've heard/seen exactly this quoted by a well respected tuner somewhere.

No chance of posting unfortunately. id have to sanitize far too much and still risk getting in trouble.

You dont find a lot of evidence for things in the suspension industry because the data either isnt collected or those in the industry want to keep it a secret. In a lot of cases collecting this data is hugely expensive so no one bothers outside of automotive industry. The tuning industry is generally fairly low tech and tons of tuners dont even have a dyno. Keeping the data a secret means that your competitor cant use that info. This also allows the tuning industry to sell whatever products they want and make up wild claims of performance gains. You ever seen anyone actually publish friction numbers from their "low friction" products?

Ive seen the same links go round and round myself and usually get a chuckle out of them. Its the same with marketing materials from almost anyone in the industry. The fact that your shaft is bigger than someone elses doesnt make your shock better. But the public eats it up, bent shafts arent even generally a problem. 

Its the same thing with companies going back and forth over IFP vs bladder. One shock has a bladder, aftermarket jumps in and sells an IFP. Goes exactly the other way as well. 

The lack of information is generally just enough to puzzle the average consumer who wont even touch the adjusters before sending something in for a revalve. 

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6 hours ago, Budlite said:

I hate to agree with Mog about anything...lol...but he's absolutely right about air in a shock.  The compression gets softer but even worse the rebound damping goes away.  Imagine hitting a pothole on a jump face of a big jump. The shock compress too quickly and the rear kicks the back end over the top leaving the rider going over the bars. Its not a pretty site especially when it's your son dropping out of the sky.   Been there and done that.........the following surgery isn't fun either.  Anything that will lower the amount of air in the shock is a big step in the right direction.  We started changing the oil on a monthly basis when he returned to racing again.....

I actually don't have a problem agreeing with Mog when He replies with a reasonable argument that can be proven. We're here to learn, not fight. Even if it doesn't seem like it at times.

So, a couple of questions. You're sure it was trapped air in the system that caused your issue, not a seal/bladder failure? How much are we talking? Obviously that's hard to quantify, but was it a few bubbles (like a bit foamy) or a big air bubble (like the oil wasn't full to the top). Do you also vacuum the oil and/or vacuum bleed now? And how long had the oil been in there before the accident?

I haven't said more air is a good thing (though mog seems to think I have), obviously bleeding well is important, it's just whether vacuuming is justified or not, which is what this thread is about.

One last question for anyone/everyone, do any of the oil companies suggest/recommend vacuuming their oils before you use them?

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15 hours ago, mog said:

You just said the shock could be run past the use by on the oil and " it will survive ok and work averagly " 

You just make stuff up ! 

 

Damping changes by a good % after working the shock hard for a few runs 

A shock that has bad oil will bottom hard and rebound hard ,that's dangerous 

You say average ,I say your not coming near my bike 

It's a silly discussion with you making up how things work to justify your belief that basic  common sense tells you is wrong 

You can't help some people 

You probably think the earth is flat 

 

 

Generally fluid breaking down at the end of its life will make about a 15% change in damping force. I would equate this to working "averagely". this is about the same force difference that 2 identically valved shocks will make. No shocks ever dyno the same with the same exact valvestack. but they are usually within about +/- 15% of eachother. In the automotive industry this performance is tested over 11 straight days of running a shock on a dyno for a total of about 5 million cycles.

15% can be handled by just turning the clickers in.

 

10 hours ago, Budlite said:

I hate to agree with Mog about anything...lol...but he's absolutely right about air in a shock.  The compression gets softer but even worse the rebound damping goes away.  Imagine hitting a pothole on a jump face of a big jump. The shock compress too quickly and the rear kicks the back end over the top leaving the rider going over the bars. Its not a pretty site especially when it's your son dropping out of the sky.   Been there and done that.........the following surgery isn't fun either.  Anything that will lower the amount of air in the shock is a big step in the right direction.  We started changing the oil on a monthly basis when he returned to racing again.....

air in a shock isnt as big of a problem as you make it out to be. its really only a problem if the air has to go through any valving. Emulsion shocks are still a thing and 99% of yall have shocks on your cars/trucks that have air sitting directly on top of oil. If you have enough air in your shock to lose that significant amount of damping then something else is wrong with your suspension. this doesnt happen from not degassing oil. It happens from either starting with a massive air bubble in your shock from the fill process, or you have lost nitrogen pressure and its migrated to your oil side where it no longer helps suppress caviation. Once you lose nitrogen pressure the shock cavitates making compression substantially softer. Once cavitation happens you no longer have rebound damping until you close the air bubble. An air bubble will form regardless of degassing oil.

15 hours ago, mog said:

How do you assess how much is too much ? 

How do you know the anti oxidants last more than a say 10 hours ? 

 

 

Because we have industry tests that specifically look at oil oxidation and can be used to judge the lifetime of the oil. ASTM D2272 is one such test. 

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So how do you know how much air is ok ? What method ensures you have the same amount of air each time ? Pot luck ?

 

Each oil is different for oxidation , is 10hr too much ? You half answer which makes me wonder what the result is rather than the test process , you quote the test but not the answer 

So here is a scenario 

You have air in oil ,you have oil break down 

You have nitrogen migration over time which can be a lot on a bladder shock 

I don't think you saw that in 11 days in your test ? So you have less pressure ,you haven't mentioned that in your posts 

 

And your saying that will give an " average " working shock 

 

I say you have a poor working shock that will cavitate and your 15% less damping is meaningless as you now have zero damping until the cavitation is over 

Is that not true ? 

Again your testing hard over a short time 

This isn't how riders do things, they leave a shock for over a year and the gas pressure goes ,the lower the pressure the more important it is to have no air

The KTM shocks rebound is marginal when hot due to the dual cross over , worn oil and air will not make that shock work average 

 

 

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11 hours ago, mog said:

 

So how do you know how much air is ok ? What method ensures you have the same amount of air each time ? Pot luck ?

 

Each oil is different for oxidation , is 10hr too much ? You half answer which makes me wonder what the result is rather than the test process , you quote the test but not the answer 

So here is a scenario 

You have air in oil ,you have oil break down 

You have nitrogen migration over time which can be a lot on a bladder shock 

I don't think you saw that in 11 days in your test ? So you have less pressure ,you haven't mentioned that in your posts 

 

And your saying that will give an " average " working shock 

 

I say you have a poor working shock that will cavitate and your 15% less damping is meaningless as you now have zero damping until the cavitation is over 

Is that not true ? 

Again your testing hard over a short time 

This isn't how riders do things, they leave a shock for over a year and the gas pressure goes ,the lower the pressure the more important it is to have no air

The KTM shocks rebound is marginal when hot due to the dual cross over , worn oil and air will not make that shock work average 

 

 

10 hours is not too much for any decent hydraulic fluid. You can send out your own oil to verify using the astm test i mentioned earlier.

11 day tests simulate expected life for shock absorbers. if you have significant migration during the test your shock will fail due to reduced damping force. Bladder leakage rates can be measured and are a relatively known value. It can be an issue in some architectures. If migration due to the bladder is a paramount concern for you, then switch to an IFP. 

YOU say that there will be a poor working shock after durability testing, without any testing history. These arent even anecdotes at this point because you dont have any experience with durability testing of shocks. 

Testing hard over a short time is about the only thing that can be done. Do you expect someone to keep a shock on a dyno for 2 years stroking it every once in a while? Do you think that anyone does the same with their engines or is it more likely that they run them hard for a short time and correlate that to expected life? 

If you have no gas pressure then you will cavitate regardless of how much air is in your oil. Air in oil is not a significant problem as long as it remains in solution, see any modern car shock or even an emulsion shock. They are proof that the simple presence of air does not render a shock useless.

If the KTM shocks rebound is marginal when hot then it needs a revalve or a higher VI oil to start with, its got problems beyond worn oil. Degassing oil will not solve poor valving.

There is no further amount of explanation that i or anyone else can give you that will convince you otherwise. Its like wrestling a pig in mud. This started as a degassing question and expanded into a million different "what if"  and "I feel" scenarios and anecdotes from people who have never even dynoed a shock. The honda factory service manuals mention nothing about degassing oil or even vacuum filling your shock for a rebuild. My ktm manual on the other hand mentions vacuum filling the shock and degassing oil. So if you follow manufacturer guidelines its a wash. If one way was clearly better than another then the manuals would say the same thing. Id tend to follow honda's advice more carefully as they own a significant stake in Showa/hitatchi who is one of the largest shock absorber producers in the world over KTM/WP who changes fork architectures every 3 years as they cant seem to get anything to stick.

 

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13 minutes ago, rawr said:

10 hours is not too much for any decent hydraulic fluid. You can send out your own oil to verify using the astm test i mentioned earlier.

11 day tests simulate expected life for shock absorbers. if you have significant migration during the test your shock will fail due to reduced damping force. Bladder leakage rates can be measured and are a relatively known value. It can be an issue in some architectures. If migration due to the bladder is a paramount concern for you, then switch to an IFP. 

YOU say that there will be a poor working shock after durability testing, without any testing history. These arent even anecdotes at this point because you dont have any experience with durability testing of shocks. 

Testing hard over a short time is about the only thing that can be done. Do you expect someone to keep a shock on a dyno for 2 years stroking it every once in a while? Do you think that anyone does the same with their engines or is it more likely that they run them hard for a short time and correlate that to expected life? 

If you have no gas pressure then you will cavitate regardless of how much air is in your oil. Air in oil is not a significant problem as long as it remains in solution, see any modern car shock or even an emulsion shock. They are proof that the simple presence of air does not render a shock useless.

If the KTM shocks rebound is marginal when hot then it needs a revalve or a higher VI oil to start with, its got problems beyond worn oil. Degassing oil will not solve poor valving.

There is no further amount of explanation that i or anyone else can give you that will convince you otherwise. Its like wrestling a pig in mud. This started as a degassing question and expanded into a million different "what if"  and "I feel" scenarios and anecdotes from people who have never even dynoed a shock. The honda factory service manuals mention nothing about degassing oil or even vacuum filling your shock for a rebuild. My ktm manual on the other hand mentions vacuum filling the shock and degassing oil. So if you follow manufacturer guidelines its a wash. If one way was clearly better than another then the manuals would say the same thing. Id tend to follow honda's advice more carefully as they own a significant stake in Showa/hitatchi who is one of the largest shock absorber producers in the world over KTM/WP who changes fork architectures every 3 years as they cant seem to get anything to stick.

Fair enough 

But how do you assess if it's a reasonable amount of air or too much ?

Do you believe the "allow some air" has zero consistency ? Do you feel consistency is important in building shocks ? 

I'm glad you accepted your torture test isn't showing how Mx owners actually operate there suspension , you accept the shirt comings in lab Vs real life 

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Interesting topic, i vacuum fill all my shocks but i was messaging a highly respected and well known suspension builder/shop/tester in the USA who's name i did type but decided to remove on this subject some time ago and he said real world a properly hand bled shock is as good as a properly vacuum bled shock, he only went to vac bleeding as when you have different people doing the hand bleeding, you can get different results.

I would take his word over anyone in the industry but ill continue to use a vac system for consistency, time and cleanliness. 

Edited by chelder
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Everyone knows less air in the shock is the goal, the method used is the question. Hand bleeding, properly, seems to work for quite a vast majority of people, so who am I to dispel their method? I would just argue to make sure you do your absolute best to remove as much air as you possible with whatever method you use. This will give the best result, and most consistent performance I suspect.

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6 hours ago, MANIAC998 said:

Everyone knows less air in the shock is the goal, the method used is the question. Hand bleeding, properly, seems to work for quite a vast majority of people, so who am I to dispel their method? I would just argue to make sure you do your absolute best to remove as much air as you possible with whatever method you use. This will give the best result, and most consistent performance I suspect.

I agree ,and the best and most consistent method of doing it is....and were back to the beginning lol 

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On 6/10/2021 at 2:41 AM, mog said:

 

I say you have a poor working shock that will cavitate and your 15% less damping is meaningless as you now have zero damping until the cavitation is over 

Is that not true ? 

 

 

Do you have zero damping while it's cavitating? I'm not sure that is true. Cavitation happens on the low pressure side of the piston.

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9 minutes ago, jjy130 said:

Do you have zero damping while it's cavitating? I'm not sure that is true. Cavitation happens on the low pressure side of the piston.

I believe it's zero or close to , it's a gas , how can it damp ? 

 

If the shock cavitates on the comp ,then the shock starts to rebound ,isn't that the most dangerous situation? 

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Just now, mog said:

I believe it's zero or close to , it's a gas , how can it damp ? 

To be fair, you can have partial cavitation and full cavitation. 

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10 minutes ago, mog said:

I believe it's zero or close to , it's a gas , how can it damp ? 

 

If the shock cavitates on the comp ,then the shock starts to rebound ,isn't that the most dangerous situation? 

I'll agree with that based off the 2nd part of your response. Strictly on the rebound portion.

 

From a practical standpoint, how long do you think it takes for the cavitation "gas" to redisolve in the oil under 150psi? I also consider that unless you really messed up (bottomed out really bad) then the shock shaft slows until it reverses direction instead of going instantly from very fast compression to very fast rebound. Is it possible that the cavitation will be gone before it ever matters?

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Well lots of shocks that cavitate dont have 150psi 

Many will be down near 120 ,with worn out oil 

As the whole Shaft movement is super fast , the bike moves at up to 10m/s 

 

If you lose damping for a fraction of that time it's still a big percentage of the total damping time ,and the first part of the rebound stroke is the most critical 

 

I'm summery I think every part of your post is the opposite of correct 

The scenario you describe  is exactly the one that causes big accidents 

It's the one we aim to avoid , it may not happen much , but it's the one to do everything to stop 

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Every part of my post is incorrect? So the shock doesn't smoothly transition from high speed compression to slower speed until stopped for an instant and then back to rebound instead of instantly going from high speed compression to high speed rebound?

The rest of my post was just questions to get you thinking...

 

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6 minutes ago, jjy130 said:

Every part of my post is incorrect? So the shock doesn't smoothly transition from high speed compression to slower speed until stopped for an instant and then back to rebound instead of instantly going from high speed compression to high speed rebound?

The rest of my post was just questions to get you thinking...

 

I don't believe anything is slow when it cavitates ,little damping dictates faster movement does it not?

It seems like an obvious thing that damping is the slowing of movement, cavitation is little damping 

You will still have zero movement for a millisecond , but then you have max spring tension and minimal damping 

You asked for my opinion,I have given it ,it could be wrong 

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Cavitation happens on the low pressure side of the piston though, so I don't think cavitation will affect compression damping while you are landing (compressing). The oil on the high pressure side is what is resisting the piston from letting the shock compress and that will never cavitate under compression.

I don't know for certain either but I know I've been on a bike with a blown bladder and didn't know it because the air stays at the top away from the main piston. You certainly know it as soon as you turn the shock upside down to service it though when the air is at the piston.

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On those videos of clear shocks I'm sure they cavitate both directions, eg  on stutter bumps , you lose damping both directions 

Even if it doesn't affect the compression ,the rebound is the bigger issue ,it is the main reason we see riders thrown off 

 

I've said this about 3 times now , the situation you describe is the most dangerous we encounter 

 

I'm not sure how this is progressing the discussion , i might be misreading but you appear to be minimising the situation with statements like " it's only for a tiny amount of time , it's only on rebound " etc if a shock doesn't damp for a brief time that's exactly the worst case, it's the very reason a shock exists, it's it's purpose 

Edited by mog

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55 minutes ago, mog said:

 

On those videos of clear shocks I'm sure they cavitate both directions, eg  on stutter bumps , you lose damping both directions 

Even if it doesn't affect the compression ,the rebound is the bigger issue ,it is the main reason we see riders thrown off 

 

I've said this about 3 times now , the situation you describe is the most dangerous we encounter 

 

I'm not sure how this is progressing the discussion , i might be misreading but you appear to be minimising the situation with statements like " it's only for a tiny amount of time , it's only on rebound " etc if a shock doesn't damp for a brief time that's exactly the worst case, it's the very reason a shock exists, it's it's purpose 

I think what He's getting at is the shock doesn't go from compression to rebound without slowing down, then stopping, then accelerating again. In theory the cavitation will stop and the bubbles get recompressed/dissapear as this happens. It would then cavitate again as the rebound speed increases rather than staying cavitated throughout the whole change of direction.

That is of course if you still have enough gas pressure in there to keep the oil pressurized, which seems to be a much more important part of it. I could definitely see your scenario happening if the pressure was too low and the oil didn't recover before the change of direction was over.

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Death inc thats exactly what I'm saying. By the time the shock has slowed down enough to stop, the pressure on the low pressure side should be plenty to get rid of any cavitation unless the shock isn't charged correctly.

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