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found this on the Traxxion forum; thought it worth posting here:
The Ohlins damper isn't gas charged with nitrogen. It is bleed to have no air in it, but in the top portion of the damper, there is a piece of foam that actually crushes to allow the oil to expand, and then the foam itself re-expands to normal size when cool. This foam is what will cause that damper to fail, but it takes a long long time for it to degenerate. Like a few years.

All of the dampers will "hydraulic lock" which is the simple problem of oil trying to move too fast through an orifice. Any orifice has a limit of how fast it can pass an oil of a given viscosity through it.

Rotary dampers meter oil through needles, seats, and as such orifices just like a tube style damper.

What Geremy called cavitation is actually "emulsion". This is how shocks worked in the period prior to "DeCarbon Style" shocks (nitrogen/oil separated). The oil and the gas charge just mix together to make a foamy sort of oil bath. This yields very inconsistent damping.

For consistency in any damping system, you need to remove as much air as is physically possible.

Cavitation is actually a "bubble" created by having the oil "pulled apart". It is actually a vacuum, not air. The same thing happens to all sorts of pumps (jet skis and boats etc) when they are able to pull more water through the pump than can be fed to it. It just tears the water apart.

Note that none of this is in engineering terms, just ones people will get hopefully.

I don't recommend rotary dampers at all, just because of the air in them. That is fine for dirt bikes, but no good for road riding.

Like I said, I am just trying to put things in terms that most people can easily grasp.

Oddly enough, what most people don't understand, is that a damper is basically SUPPOSED to hydraulic lock. It is is supposed to stop violent swings of the handlebars in your hands. It should make little or no damping at low speed (of the damper shaft, not the bike) and tons of damping at high speed (tank slapper, off-road excursion, bumped by another rider, etc). If it didn't try and lock up, it would only do 50% of what it is supposed to.

The other 50% is that it is supposed to stop oscillations before they start. That is why an air bubble renders rotary dampers useless for this purpose. They don't damp as the unit changes directions.

Note further that you bike should NOT wobble, with no damper on it. A damper is a safety device, not a repair for an ill handling bike.

The reason the air bubble is put into the rotary damper is that there needs to be some place for the oil to expand to as it heats. On an Ohlins, make no doubt about it, there is a piece of foam in it that collapses, not nitrogen. I have no knowledge that WPs are nitrogen charged, I thought they used a mechanical spring.

On Sprint Dampers, there is an oring on either end that the bearings expand against and sorta crushes them flatter. When the oil cools, the orings push back into shape.

This is just static observation, not a formal survey. Some might dispute my figures, but they aren't miles off.

Tobys look and feel cheesy to me. I don't like them, not when you can buy a nice piece like a Sprint for the same money.

Matris has just come to the US last year, and I have only seen one or maybe two at the track, and cannot comment on their function, but they do look very nice and trick.

Ohlins is very nice and typically fit very cleanly.

The Sprints, to me, are the best working dampers and are usually $1-200 less than an Ohlins. Their complete kits sell for $375 "racer net".
Their dampers are made by a guy who used to work for MacLaren F1. They are extremely well made, extremely well executed, and parts are readily available for service here in the US. They are trick, clean, and install in minutes.

I sell Sprints and Ohlins here.

Ohlins has ceased to sell damper parts to independent shops like mine, as they want to do damper service in house. (there was also scuttle about them updating shafts quietly without actually issuing any recall or formal public update). They want the dampers assembled with a vacuum machine that costs $6000, and no one in the US has bought one, so they have to do it themselves.

I would suggest you buy a spare damper before you send one in to Ohlins for service, as it may take a while...


========================
Max McAllister
President
Traxxion Dynamics, Inc.
Axxion Arm Systems, Inc.
 

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Nice info, thanks! Interesting read...

Hadn't thought about the heat expansion of the oil in steering dampers, but it's a problem that has to be dealt with one way or another.
 

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to add one bit of clarity, cavitation is in fact equivalent
to local boiling of the fluid when pressure is reduced
below the vapor pressure at the current temperature. everyone knows that water boils at less than 100 C at
any altitude higher than sea level. imagine then reducing the
pressure on a closed container (equivalent to increasing altitude) until the temperature equals the boiling temp. at
that pressure. in a moving fluid, pressure changes with the magnitude of velocity. in regions of very high velocity (e.g. near the tips of marine propellors) the pressure can be reduced to the point at which boiling will occur .... thus, cavitation. the detrimental effects of cavitation (pitting and erosion of materials in contact with the region of cavitation) occur when the bubbles of vapor collapse back into the fluid state near a solid surface. the presence of the solid surface causes the bubble collapse to become asymmetric resulting in a very high speed fluid jet aimed at the wall. over time, the impingement of these jets causes the observed pitting.

in the context of a damper or shock absorber, pushing oil through an orifice does result in a small region of very low pressure as the oil is accelerated, but the boiling temperature of oil at any reasonable pressure is (i'm guessing) sufficiently high as to mitigate any cavitation except in the most extreme conditions.

sorry for the lecture. :-X
 

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Good explanation, stevey. Sounds like you've spent some time dealing with cavitation... ;)
 

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Interesting reading but I'm always a little sceptical when the story finishes with "I sell brand X and brand Y". Although I guess it discloses any bias towards certain items and is better than being raised by someone else.
 

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Good explanation, stevey. Sounds like you've spent some time dealing with cavitation... ;)
unfortunately, too much time thinking about that stuff and not enough on the duc.
 

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No Sprint experiance here...... I've got a Hyperpro on my ZX12R which works nice and has saved my butt on a few bumpy New England roads. Ive ordered a Matris M4 damper for my S4R. I should have it in 3-4 weeks. The build quality of the Matris looks to be top notch.
 
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