Shock oil

[_im_from_earth]

look no further: http://www.peterverdonedesigns.com/bikesuspension.htm

 

I will soon be running a wiki style tech section on Rushsports.co.za will be a great place to keep and search for all of this info.

 

morewoodkid2009-05-27 06:42:12

[Johan Bornman]

Oil can damage a shim in the following ways.

Too thick for the shim stack and it creates to high a pressure on the shim surface since the oil will not flow through the bleed ports as quickly.

I believe the shim are being bent back and forth' date=' the material work hardens and the fail.

 [/quote']

 

Therefore, on a shim with a bleed port on only one side, you'd see shim damage as a straight line broken off one side of the shim? Unless the shim can rotate freely as some do, then you'd theoretically see a washer breaking off the shim, leaving it with a little remaining dot around its fixture point?

 

Possibly the break is not complete, with just an bent, but intact shim?

 

What did it look like?

 

 

Too thin and it can cavitate as the oil heats up. I've seen eviddence of pitting on some shims' date=' suggesting cavititation is occuring. This couild happen in any fork actually, depending on the condition of the oil.

[/quote']

 

This I have definitely not seen. All the shims I've seen were perfectly smooth. I don't get to see downhill or motorbike forks, I'd imagine it could happen there.

 

Why do you say they cavitate once the oil heats up? How does the temperature come into play? Viscoscity and therefore speed related?

 

 

[_im_from_earth]

Lefty you like the link?

 

I think what we are confusing here is that oil is not oil when it comes to most bicycle forks and shocks. You must remember that the volumes used in bicycles [generally speaking] is so tiny that even the slightest contamination can have a severely negative effect on suspension performance and damping. Some suspension MFG's who I have spoken to have scoffed at many of the "leading" bicycle fork and shock designs because of the small amount of fluid used. Secondly there is another significant factor to take into account, despite the fact that most bicycle suspension MFG's use fluid within the legs as lubrication for the lowers/ stanchions and bushings, the forc cartridge itself is without lubrication save for what should be damping fluid. This means that forks and shocks are relying on their damping fluid to provide lubricating properties in addition to superior damping given the tiny volume and flow. this = MAJOR PROBLEM

 

I think given the above there is huge value in spending a little extra money on suspension fluid, do your research, get the correct weight and ensure when you are working on your fork or shock you are using immaculately clean surfaces, lint free rags and residue free cleaners and solvents.

[GoLefty!!]

Yes i like the Link, Peter is  excellent in this topic. He should be since he tunes motorcycle suspensions

 

I do tend to agree with you around how hard a bicycles suspension can work and agree with statements around low volume. That 125ml of oil works hard for it's money. Its constantly being circulated through a compression stack and then drawn through a rebound valve, anything to slow it down but not stop it completely. Add a lock out and suddenly the shim stack and valving needs to stop that flow. Only to repeat the whole cycle again in a matter of seconds. Motorcycles suspension has a much higher volume so the oil gets a chance to cool before being shoved around again.

Hence I stick roughly to oil change guidelines, because even if the oil is still clean, the Viscosity improver additives get sheared and broken down by being forced through tiny valves. In a motorvehicle engine there is 4.6L of oil and it is exposed to mostly hydrodynamic lubrication regimes. There is some shear in the oil pump and if you have a turbo. So we really should not underestimate the amount of work a simply bicycle suspension's oil has been exposed to.

 

Johan,

 

the shim exposed to too viscous oil had split from its internal diameter to it's outer diameter.

 

the pitting due to cavitation I have only seen on big DH forks (Manitou Dorado) and MX forks to date. Oh and one Manitou XC fork where the oil was not changed for 2yrs!

This occurs when the oil is warm because under these conditions the oils is less viscous and flows faster. Add contaminants like water, through in some rough terrain to get the fork working really hard and you've got a recipe for cavitation.

Most rider don't push their bikes hard enough for this to be a factor in 90% of cases

 

I would still like to understand the failure of the shim. I do recall in around 2001 I worked on a Heashok DL70 cartridge where the shim had failure from internal diameter to outside diameter as well as some cracking on the  circumference. In was in the compression stack and  below the  lock out disc.

 

 

 

 

GoLefty!!2009-05-27 12:36:06

[_im_from_earth]

don't believe it cavitation? check out the links below to some AMAZING video's by Roehrig, leading suspension dyno MFG's out of North Carolina:

 

http://www.suspensionhotline.com/dyn...ear_shock2.wmv

 

and

 

http://www.suspensionhotline.com/dyn...ear_shock1.wmv

 

I'm sure you'll love that Mr Lefty

[Bos]

Hahaha!! What is that all about?? It looks like one of those Mythbusters videos!

 

Morewood kids, Please explain? is there suspention oil in there? or is it smoke coming from that damper rod?

[GoLefty!!]

don't believe it cavitation? check out the links below to some AMAZING video's by Roehrig' date=' leading suspension dyno MFG's out of North Carolina:

 

http://www.suspensionhotline.com/dyn...ear_shock2.wmv

 

and

 

http://www.suspensionhotline.com/dyn...ear_shock1.wmv

 

I'm sure you'll love that Mr Lefty

 

 

 

 

yes i like a lot. I don't have any visuals of similar tests I conducted on the damper units of an F1 car. esentially the customer had some custom platform dampers in their car but the damping was fading after around an hour into the race. We set up their system in similar apparatus and found that the platform valving led to severe cavitation. we changed the oil and the problem went away.

Only occured on one of their cars. The two drivers had vastly different styles.

This problem also occurs in DH forks and I suspect in XC forks as well but I think the units being pushed hard enough are serviced often so it's not an issue.

[GoLefty!!]

BOS1000,

 

the mist you see int hat video is tiny gas bubbles formed by boiling oil. the oil is nto boiling due to temperature but rather due to a very low pressure.

When the bubbles collapse they collapse wuth enough force to tear tiny fragments of material out of the shims and valves.

 

This is cavitation.

 

Use the wrong oil and this is what you can expect.Out on the bike it will feel like the fork suddenly has blown through some travel for no reason. Potentially dangerous and certainly performance robbing

[_im_from_earth]

thought you may appreciate that. now I just need to find out how much Nissan SA will sell me their dyno for... hmmm...

[Johan Bornman]

This is the nicest visual of cavitation I've ever seen. Usually it is shown with a boat prop and the spinning confuses things - for me in anyway.

Thanks Morewood

 

I'm surprised at the relatively low speedof the piston the cavitation happens.

 

I can't figure out what the voice says in the video where he says "This is the ???????" same shock, same gas pressure".

 

What are the two discs floating on top of the oil? Is there gas in the first cavity looking from the top? What's between these two discs? Do they move slightly or is it the shaking camera?

 

 

 

[straatvark]

Well done you Hub boffins 

 

Very interresting, dont think i blinked for 5 pages, all of a sudden i have a lot more respect for my shock

[GoLefty!!]

This is the nicest visual of cavitation I've ever seen. Usually it is shown with a boat prop and the spinning confuses things - for me in anyway.

Thanks Morewood

 

I'm surprised at the relatively low speedof the piston the cavitation happens.

 

I can't figure out what the voice says in the video where he says "This is the ???????" same shock' date=' same gas pressure".

 

What are the two discs floating on top of the oil? Is there gas in the first cavity looking from the top? What's between these two discs? Do they move slightly or is it the shaking camera?

 

 

 

[/quote']

 

 

Thanks for the questions Johan, keeping me on my toes is a good thing

 

re Cavitation: Very understandable as most people associate the visual of cavitation on spinning propellors.

Did you know that a slow spinning nuclear submarine prop can also cavitate. A huge problem since it's basically a very loud "hello!! we're over here with our nuclear missiles.Don't shoot us please!!"

 

in a suspension shim stack the pressure difference on either side of the shim can be huge and cavitation is fairly common, perhaps not as severe on slower moving XC bikes but present because vertical acceleration is still there.

 

I'm not sure what he's saying in the vid, I don't speak Americanese but In the set up I used it was a pressurised gas chamber above with a floating piston to allow for oil movement and expansion. Its sort of a spring for the damper similar to the nitrogen charged chamber in a Fox RP23.

Interestingly, the cavitation started to set in as the oil aged, indicating that its capacity to do teh work required was low. So we opted for a much higher VI oil in the dampers and these lasted more than one GP.

 

So AlstuBlief, don't underestimate whats going on inside a bicycle fork's hydraulic damping circuit. we may be going slowly but the fork is moving in a vertical direction very quickly with quite a bit of acceleration. Its a harsh environment and it pays to look after the internals by using appropriate oil of a very high quality (high Viscosity Index would be the quality measure here).

 

Thanks for those visuals Andre. Brilliant.

[dollar]

Before they sold "fork oil" I used to use ATF on my motorcycles (road & MX) with no problems. It must be pretty good stuff to keep an auto box in one piece?

 

Does anyone still use ATF on bike forks?

[GoLefty!!]

ATF is essentially hydraulic fluid. However some have pretty special base oils such as

 

Some ATF's are still used. Castrol  LMS or something to that effect is one such ATF that  can be used in motorcycle forks. In fact the stuff was designed for use in Citroen elevator cars (those ugly things that seem to raise on their suspension when you turn on the ignition- brilliant horse racing TV platforms)

There are one or two others but really, these are harder to come by than dedicated fork oils.

[Bos]

BOS1000' date='

 

the mist you see int hat video is tiny gas bubbles formed by boiling oil. the oil is nto boiling due to temperature but rather due to a very low pressure.

When the bubbles collapse they collapse wuth enough force to tear tiny fragments of material out of the shims and valves.

 

This is cavitation.

 

Use the wrong oil and this is what you can expect.Out on the bike it will feel like the fork suddenly has blown through some travel for no reason. Potentially dangerous and certainly performance robbing

[/quote']

 

Woooowhee.. This is new to the BOS.. I had no Idea this happened.must have something to do with air forming vapour but because its under such pressure It does this weird Mist thing. My minds working overtime. I want a shock & foer with clear sidewalls.. so I can see it happening for myself

[Johan Bornman]

The bubbles you see is oil vapour, not air. The process is called cavitation.

It happens when a substance is suddenly exposed to severe low pressure and it vapourises, hence the fact that it happens behind the piston you saw in the video.

 

It has a destuctive side too (useful when opening beer, it seems). When the pressure is suddenly returned, as happens when the piston/propellor/whatever moves away from the low pressure zone, the bubbles collapse with great force back to liquid and damages the hardware.

 

I use cavitation to clean small parts in my workshop by way of an ultrasonic cleaner. Have a look at Wikipedia, they will have an explanation for cavitation. Cavitation is also something that eats away your swimming pool pump impeller.

Johan Bornman2009-05-29 05:49:36