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Why do we have to change the oil in out forks so frequently?
When you think about it, engine oil lasts a lot longer in much harsher conditions, an hour on the highway has got to subject it to much more stress & temperature extremes than in a suspension fork, freeriders aside. ;-)
I realise that a car has an oil filter but is contamination the only reason we replace our oil? Would properly filtered used fork oil be as good as new?
These questions came about after noticing how, after a few months sitting in an old pop bottle I keep my used oil in, a lot of the contaminants had sunk to the bottom leaving only slightly cloudy oil in place of what used to be a black soup.
Cost savings aren't really worth considering when you look at the small quantities of oil in a fork & the cost of a quart of Belray from your local motorcycle store (more than paid back by the few minutes you get to spend ogling the latest sportbikes & dreaming of buying one) but environmentally re-using old oil would be a good thing.

Anyone who knows about oil care to answer?
 

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2 things

contaminants

and the breakdown of the oil, after a while it looses it's viscosity. It is "flexible" and it "wears out" just like most other natural things do. Think of it like metal fatique, after a while it can not be cycled any more and just breaks down.
 

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Discussion Starter #3
That's my point.
1- Contaminants: What if you filter them out (I'm thinking get a good oil filter, take out the filtering element & filter the oil through it)?
2- Breakdown: I have trouble believing that what goes on in a mountain bike suspension fork can cause all that much breakdown over the period it takes for the oil to get contaminated enough to require replacing. The oils used are motorcycle fork oils (at least in the case of my Bomber) & they last much longer & under more stress (higher temps, heavier damping) in a motorcycle fork than in an MTB fork.
 

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"El Whatever"
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My guess is that it's just the problem with contaminants.

Only way to avoid contamination is thru better sealing and doing some kind of oil flushing each time you load oil (especially the first time). Other than that, a filtering system makes no much sense as actually the oil is not moving but the pistons and shims are moving inside the oil.

For the breakdown, yes you can have more breakdown from a bike fork than on a moto fork. The damping devices basically are much smaller (orifices) and then the oil it's forced much more rapidly thru them.That causes more wear than on a moto fork.

We must accept that sealing technology in bike forks is a bit behind moto forks. Besides, there's another reason why there's a bit less dirt entering a moto fork. Almost all of them are inverted, as dirt can only come inside by the seals, a wiper helped by gravity will do a much better job than a wiper working against gravity as on a conventional fork (it will 'eat' dirt as it goes thru its travel).

All of those are educated guess and I would like to know the truth from a designer.
 

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Much of those "contaminates" are aluminum oxide and the oxides of other metals from the forks. Sealing a fork so that no air can get in is pretty difficult (partial pressure of 02 in air, and what happens when metal oxidizes (partial pressure goes down)), and for air-shocks you'd have to use nitrogen instead of air to pressurize it so you could avoid oxidation. Oxidation is what creates the contaminated oil for the most part.
 

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noMAD man
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Oil molecules shear.

Surestick Malone said:
That's my point.
1- Contaminants: What if you filter them out (I'm thinking get a good oil filter, take out the filtering element & filter the oil through it)?
2- Breakdown: I have trouble believing that what goes on in a mountain bike suspension fork can cause all that much breakdown over the period it takes for the oil to get contaminated enough to require replacing. The oils used are motorcycle fork oils (at least in the case of my Bomber) & they last much longer & under more stress (higher temps, heavier damping) in a motorcycle fork than in an MTB fork.
Granted, the environment in a bicycle fork doesn't rival that of internal combustion engine or a transmission, but the oil molecules in suspension oil do indeed shear and break down. The molecules are gathered together in little chains that sacrifice themselves over time to lubricate the suspension internals and provide the hydraulic action for suspension control. I'd agree that contamination occurs faster than the breakdown, but it's there. And when you're talking something as relatively cheap as suspension fluid and in relatively small amounts, why bother?
 

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Surestick Malone said:
Why do we have to change the oil in out forks so frequently?
When you think about it, engine oil lasts a lot longer in much harsher conditions, an hour on the highway has got to subject it to much more stress & temperature extremes than in a suspension fork, freeriders aside. ;-)
I realise that a car has an oil filter but is contamination the only reason we replace our oil? Would properly filtered used fork oil be as good as new?
These questions came about after noticing how, after a few months sitting in an old pop bottle I keep my used oil in, a lot of the contaminants had sunk to the bottom leaving only slightly cloudy oil in place of what used to be a black soup.
Cost savings aren't really worth considering when you look at the small quantities of oil in a fork & the cost of a quart of Belray from your local motorcycle store (more than paid back by the few minutes you get to spend ogling the latest sportbikes & dreaming of buying one) but environmentally re-using old oil would be a good thing.

Anyone who knows about oil care to answer?

I agree about the oil being stressed in a mtb. It just isn't an issue. The contamination issue is from the wearing parts in open bath forks. Manitous with their wear strips and plastic damper caps don't really have the problem, my four year old Xvert DC still has some of the original oil in there (it's been topped up and split a few times). The oil is still bright and clear.

Oxidation etc aren't problems for mtb forks, oil needs a decent temperature to start oxidising.

I don't see why you couldn't filter and reuse fork oil. You'd just need to devise a pressure or gravity fed system to do it easily and cleanly.
 

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Dougal said:
Oxidation etc aren't problems for mtb forks, oil needs a decent temperature to start oxidising.
You got to be kidding? Oil doesn't oxidize, the AIR causes the metal to oxidize, parts of the fork that are exposed to the atmostphere (inside the fork), and then it gets carried by the oil. You should know this.

Change the oil on 1998-2002 boxxers and you'll see horrendous effects of oxidation (that are not just from simple wearing, in fact boxxers have a lot of plastic peices in the damping-rods anyway.
 

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"El Whatever"
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I don't know how bad or good oxidation is. But being the internals of a fork mostly aluminum and mag, these metals form a kind of protective layer which indeed is some kind of oxide which actually looks like grease. Rub a clean rag against some aluminum part and you'll see. Friction accelerates this process, so the reasoning behind the internals to oxidize is kind on the spot. Part of the gunk in fork oil should come from there.

Besides, as said, bike fork put more shear forces in the oil due to the smaller orifices the oil has to go thru causing breakdown. This is slower as heat in a fork is nothing for oil, so it virtually will last a very long time.

Besides, there's the material used in seals, o-rings and stuff. Oil attacks this elements (some more than others) as degrades them and form deposites of this plastics and rubbers in the oil. This is a little contribution factor, but it counts anyway.

It is the combination of all those factors which make us change oil so frequently. And yes, closed cartridges Manitou's can last a long time with the same oil.
 

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Warp2003 said:
. And yes, closed cartridges Manitou's can last a long time with the same oil.
While they chew through their bushings :D This is their achilles heel, semi-open bath helps, but many still report that they still have to grease the fork to keep it "smooth"...
 

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"El Whatever"
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That's why ....

Some clever guys at manitou put the Microlube ports.... so constant regreasing is a breeze. No disassy or messing around with caps, bolts, grease and stuff......

I still have a '00 SX which has the original oil in it and has no appreciable wear. some grease from time to time and off you go... clever forks. There's very little in there that could fail. They might not be the best forks out there, but for sure they are the easiest forks to service. Only thing that can break is the sliders (and they do :D )
 

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Jm. said:
You got to be kidding? Oil doesn't oxidize, the AIR causes the metal to oxidize, parts of the fork that are exposed to the atmostphere (inside the fork), and then it gets carried by the oil. You should know this.

Change the oil on 1998-2002 boxxers and you'll see horrendous effects of oxidation (that are not just from simple wearing, in fact boxxers have a lot of plastic peices in the damping-rods anyway.
Nothing oxidises in an oil bath.

Aluminium oxide is completely stable, impervious to oxygen and quite hard (anodising is coloured aluminium oxide). The only way to remove it is with acid or to mechanically rip it off. If you've got mechanical wear happening (like many open bath forks do) then oxidation is the least of your worries.

Steel bolts do not oxidise in an oil bath.
Magnesium does not oxidise in an oil bath.
Nothing oxidises in an oil bath. Pull apart an engine or gearbox and see for yourself.

My 00 Xverts still have their original bushings. No problems here.
 

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Dougal said:
Nothing oxidises in an oil bath.

Aluminium oxide is completely stable, impervious to oxygen and quite hard (anodising is coloured aluminium oxide). The only way to remove it is with acid or to mechanically rip it off. If you've got mechanical wear happening (like many open bath forks do) then oxidation is the least of your worries.

Steel bolts do not oxidise in an oil bath.
Magnesium does not oxidise in an oil bath.
Nothing oxidises in an oil bath. Pull apart an engine or gearbox and see for yourself.

My 00 Xverts still have their original bushings. No problems here.
You are failing to see the obvious facts here.

Many forks have magnesium lowers. You better believe that magnesium oxidizes. Theres probably also some steel hardwarre in there. There are most definitely steel springs in there, steel negative springs too in many forks. As you know, the fork and spring are not always immersed in the oil bath, there are generous metal sections that are not "immersed" in oil (due to the space that is allowed for the fork to compress). The other fact has to do with how partial pressure works. Once you've oxidized some of the metal and it's been suspended in the oil, it will keep happening. The partial pressure of oxygen will remain the same because as some is used up in the oxidation process, new oxygen will move in to take it's place.

I'm glad that your Xverts still have the original bushings. As someone who's worked in a shop (and I am going to start a new shop-job next week), I have seen and worked on many manitous with shot bushings. It's not a big leap to figure that one they chew through their bushings is because of a lack of lubrication. If you are relgious about greasing your fork, then great. Not everyone does that though (they probably owned a mazocchi before hand :D). Manitous eat their bushigns much faster than many other forks that I've seen.


If a fork was filled to the top-cap with oil, your theory would make sense.
 

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Jm. said:
You are failing to see the obvious facts here.

Many forks have magnesium lowers. You better believe that magnesium oxidizes. Theres probably also some steel hardwarre in there. There are most definitely steel springs in there, steel negative springs too in many forks. As you know, the fork and spring are not always immersed in the oil bath, there are generous metal sections that are not "immersed" in oil (due to the space that is allowed for the fork to compress). The other fact has to do with how partial pressure works. Once you've oxidized some of the metal and it's been suspended in the oil, it will keep happening. The partial pressure of oxygen will remain the same because as some is used up in the oxidation process, new oxygen will move in to take it's place.

I'm glad that your Xverts still have the original bushings. As someone who's worked in a shop (and I am going to start a new shop-job next week), I have seen and worked on many manitous with shot bushings. It's not a big leap to figure that one they chew through their bushings is because of a lack of lubrication. If you are relgious about greasing your fork, then great. Not everyone does that though (they probably owned a mazocchi before hand :D). Manitous eat their bushigns much faster than many other forks that I've seen.


If a fork was filled to the top-cap with oil, your theory would make sense.
I have never seen a spring, bolt or other steel part in an oil bath fork go rusty. If you can show me one it would be much appreciated.
Magnesium lowers have no wearing surfaces (because it's quite soft), hence the oxidised outer layer doesn't get disturbed.

I grease my forks every 3-6 months. But yes I have met fork owners who didn't know they needed greased, ever.
 

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Dougal said:
I have never seen a spring, bolt or other steel part in an oil bath fork go rusty. If you can show me one it would be much appreciated.
Magnesium lowers have no wearing surfaces (because it's quite soft), hence the oxidised outer layer doesn't get disturbed.

I grease my forks every 3-6 months. But yes I have met fork owners who didn't know they needed greased, ever.
You have to be kidding. Magnesium lowers have plenty of surface inside, that is not in the bath, that is "open" to air, and because of the partial pressure of oxygen, you can rest assured that the oxidation is a continuing process. Hence, oxidation of magnesium (something that magnesium is pretty famous for).

I can show you plenty of fork springs that aren't coated, negative springs that usually are never coated... You aren't going to "see" it go rusty either with oil being spashed on it either. It's a cycle. Leave fork sitting, oil drips down, springs and other parts oxidize. Go riding, oil spashes up and oxidation gets suspended in oil.
 

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Discussion Starter #16
If that was the case the surface of the steel springs would become pitted over time. I have never seen this.
When you get oil on something it stays coated in it for quite a while & this prevents oxidation. Oil does get splashed over pretty much everything inside an oil bath fork.
If oxidation was sucha problem wouldn't air forks & shocks have serious problems with oxidation? The pressure (and therefore the partial pressure of O2) is much higher in them which should speed up the oxidation reaction.
 

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im not saying it's a huge problem, I am just stating what the process is.

Forks vary quite a bit from manufacturer to manufacturer. I've had ones that were just rediculous at oil-changes (boxxer) and ones that would look fine no matter how long it seemed that I waited in between changes (stratos with all-annodized internals)...
 

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1) MTB shock oil is light and less viscous than Moto Shock oil. There is also less of it so the oil's working temperature is lower.
MTB's fork oil comes very close to boiling n many decents it's working that hard, this means tha oxidation does take place readily

2) The sealing on MTB forks is not as tight as on Moto forks. The total weight of rider and bike is less so to reduce stiction and sliding friction, seals are designed to fit moreloosely. This measn that as the fork compresses and extends it breathes. When you stop riding it, it cools which draws in more air which contains water vapour. when the bike gets cold the water vapour condenses and settles in the oil. This makes the oil cloudy andmore sensitive to temperature fluctuation and oxidation occur more easily too.

3) With the up and down sliding, dirt does get trough and into the oil.

These factors make it necessary to service the fork every 40hrs of operation.
 

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PS:
although the oil molecules are sheared as they pass through the damping orifice the effect is not permanent. Permanent damage to the molecules only starts to occur when the oil temp is high. This is when the molecules breakdown and bond with oxygen inthe oil and from the adsorbed water molecules.
 

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Oil does oxidize...

Jm. said:
You got to be kidding? Oil doesn't oxidize, the AIR causes the metal to oxidize, parts of the fork that are exposed to the atmostphere (inside the fork), and then it gets carried by the oil. You should know this.

Change the oil on 1998-2002 boxxers and you'll see horrendous effects of oxidation (that are not just from simple wearing, in fact boxxers have a lot of plastic peices in the damping-rods anyway.
... that is, burn, but not at temps seen in bike forks, at least, not at the speed where you would notice anything for a long while. Higher temps, faster oxidization.

What also happens in a fork is water. There is vaporized water in the air you pump in with your shock pump. Water contamination in oil make it look milky.
 
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