Bad hydrosorbs

[Zac Weidner]

I am in the process of working on my landing gear, and I have a question about the hydrosorbs. The "owner's handbook" says that they are not servicable, and must be replaced when worn. I fear that mine are "worn" and would like any help on distinguishing "worn" from "airworthy". As I hold the top end firm and pull them apart (without bungees on), they are quite weak for about the first inch and a half (probably the most travelled section), then more what I would classify as being "normal" the rest of the way out, with a few "rough spots". With Univair's $300 a pop, is there any other source for them, or am I gonna' get suckered into spending the $600? Thanks

[Steve Pierce]

The hydrasorb acts as a resistance device. You should have resistance in both directions. If you bounce it lessons the rebound or slingshot effect of the bungees. Univair is the only source I know of.

[JohnW]

Please allow me to expand on what Steve just said... The HydroSorbs are nothing but shock absorbers (indeed...the Piper HydroSorbs were MADE BY Monroe-but are no longer available from them under that p/n!). And just like the shocks in your CAR, they allow your suspension (MLG, in our case) to be deflected "UPWARD" when encountering upward momentum while in forward motion, against the "spring" (or "bungee"). What is DIFFERENT about "our shocks" is that in a car, this deflection occurs when the shock is "collapsed" during this "upward motion", and by nature of the geometry, the airplane shock is EXTENDED during this "upward input". In other words, "they work upside-down"! What the shock DOES is to restrict the "rebound action" of the springs (and/or "torsion bars", in a car) or... the "rubber bands" (bungee cords) in our Pipers. Even your car shocks exhibit MUCH LESS resistance when "deflecting" than when they are "rebounding". Same-same with "out gear" vs "in gear" on the airplane. The brand new HydroSorbs (actually original PMA'd replacement ones for SuperCubs, and including PA-22s during certification -unless another p/n has relatively recently been offered) that you can purchase have this same noticeable "easier OUT than IN" action. Many Hydrosorbs have been replaced "out there" because the installer noted how they seem "easier to extend than retract", when in fact this is what they are DESIGNED to do! Fly Markets often have "worn out HydroSorbs" in EXCELLANT serviceable condition (but you HAVE TO "operate them" to check 'em out)!

That being said... You're indicating that the FIRST PART of the travel "outward" seems considerably easier than the latter part of the "stroke". So, it is indeed possible that you may have an issue with them. Usually, however, they are "loose as a goose" ALL THE WAY through their "stroke" when they are "bad" (and USUALLY this in in the "rebound" direction). This is because how the damping action is acheived, and "worn out" or "damaged" parts SHOULD effect the entire "stroke" in that direction. So, you have something of a "judgment call" to make. But please DO consider that NO shock restrict travel in the "compressing fluid" direction (unless maybe it is for some special application", and all shocks by design will offer "noticeable resistance", but not "freeflow") but ALL shocks "meter flow" to dampen "rebound". I guess you better "seek a Professional opinion" for a "hands on" evaluation of your specific HydroSorbs unless you are thoroughly convinced yours are definitely "toast". SOME people just "stand and deliver" for new, but you have exhibited something of a "concern" over their value and worth (yeah, me too). Any denting, bending, leaking or rusting is cause for rejection, but these things actually work THOUSANDS of TOs and Lndgs and still actually work. Credit THAT to someone having changed the bungees before they didn't "spring any" at all. If they drag effectively when pushing the stem back in...they are USUALLY still good (again, yours MAY still have continued cause for concern...). How does the "rebound" feel???

[rmalone]

Look for a good used one. I found one used for 150.00.

[Gilbert Pierce]

Clippers do not have Hydrosorbs and land just fine. After flying Cessna's with spring gear I find my Clipper never bounces. So what is the effect of a worn out hydrosorb in a Pacer or Tri-Pacer. Does it just effect the tricycle gear airplanes or do taildraggers have some bad characteristics when landed with a worn out hydrosorb?

[Zac Weidner]

Going back to what JohnW mentioned about good "rebound", my hydrasorbs are weak enough on the "rebound" that I can "retract" them with my little finger alone. I was hoping that this was normal when posting this, but after what you have said, I'm pretty sure that mine are undoubtedly "worn out". I didn't clarify it before, but these are on a Tri Pacer, just for the record. Thanks a lot. (again)

[pistoncan]

Sterve or JohnW, I have a related question. IF the hydrasorbs are bad, then is the gear system basicly the same as a cub or vagabond system where you have just bungies reacting on a sliding tube?

[Steve Pierce]

As far as I can tell. Just more added weight.

[Glen Geller]

I did bungee changes on three PA22's last year, all at once, and noticed that they seem to recover 'full-stroke' performance if you exercise them full-stroke. Try clamping the small end in a solid bench vice and then run them in and out full-stroke several times to get the oil circulating and lube up the seals etc. They should improve with less than a dozen strokes. For extra fun get someone else to do it for you while you criticize their technique.
And regarding the Vag/Pacer L/G difference, I can think of lots of other things I would spend the $300/each on. Better to have fresh strong and relatively inexpensive bungees says I.
I suspect for Piper the Hydrosorb Shock Absorber is not technical razzle dazzle, but more of a marketing breakthrough. They had to compete with Luscombe, who had a hydraulic shock on their planes.

[taildraggerpilot]

I did bungee changes on three PA22's last year, all at once, and noticed that they seem to recover 'full-stroke' performance if you exercise them full-stroke.

I experienced the same thing. After the old bungees were removed, the hydrosorbs were very easy to move in both directions, but after about a dozen strokes, they firmed up and had what I would qualify as adequate damping.

[Zac Weidner]

I have worked them several dozen times and the "rebound" stroke is unchangingly weak.

[andya]

many years ago I did the same exercise and although they weren't dead when I removed them they seemed better after exercise, I thought I was just imagining things.

[pistoncan]

As far as I can tell. Just more added weight.
So, If I understand it correctly, in an experimental taildragger situation, the hydrosorbs could be removed and replaced with a lighter tube within a tube connector and just bungess and it should work just like a cub, clipper or vagabond system?
Believing in the KISS priciple, If that is true, it would seem like a good idea, to save maybe 10 lbs and 600 dollars, for somebody to come up with a STC for it, considering how many 22/20's there are out there. Or am I just nuts?
Gary

[Steve Pierce]

Without redesigning the gear it would have to be a pretty strong tube to take the place of the hydrosorb.

[Glen Geller]

I did a Google search of hydrosorb and stumbled across this, the Alpha Omega Suspension System:
http://www.burlac.com/AOSS.htm and http://www.aoss.net
These are for PA18 Super Cubs. Instead of a separate spring (or bungee) and a motion dampener, it contains a solid, deformable polymer inside the shock body. This acts as both a suspension and shock absorbing medium. Put simply, it's a metal tube filled with a very stiff spongey plug of magic plastic on either side of the piston head.
I wonder if the extension arm were removed and an appropriate end installed, would they fit and correctly function on a PA22 or PA20?
Pretty spendy however: $2200/pair!

[Steve Pierce]

They are the cats meow but much longer than the hydrosorb. You could convert the pacer to Super Cub gear and use them. We converted a new Husky to Super Cub gear and Burl's AOSS to the tune of about $12K. :shock: You can drop it in and she just lands, no rebound. Burl had a video of the drop tests that were very interesting. The Super Cub was dropped from several feet above gross weight on greased plates. The stock hydrosorb/bungee setup bounced up and down but the AOSS just stopped. Would be neat if Burl could make a version for the Pacer/ Tri-Pacer gear.

[Bultaco Jim]

Taking the hydrosorbs off and working them full stroke sounds like a worthwhile effort. It used to be well known, in motorcycle circles, that shocks from the 50's and 60's (which look amazingly similar to hydros), could be brought back to life by removing the springs and moving them fullstroke about a dozen times. I have done this myself at least five or six times, and it really worked. It removes the air bubbles.
The only thing I would add, is, don't let them get horizontal after bleeding. Keep them vertical until installed. Jim