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Zenith "Donut" Nose Gear Suspension Installation and Testing
At Airventure, I expressed interest in the new Zenith "Donut" nose gear suspension system and Roger recently provided me with a kit to retrofit my STOL 750 so I could provide an early evaluation and testing (Zenith has been testing this in-house for a year). This system will also be available for the Cruzer and the 601/650. The 701 system will follow, but apparently it will need some modifications from this design.
The original bungee system works well, but it does have a single-point failure potential (the bungee!) and apparently the last few years the production process has changed and bungee failures are occurring more frequently. In addition, the bungee is non-adjustable for pre-load and induces some torsional resistance when the nose gear rotates.
My kit arrived Friday and was very complete - the only additional material needed was some white lithium grease to lubricate the area where the donuts are located (3M silicon grease is now recommended - doesn't dry out like white lithium grease does). A detailed drawing and step-by-step instructions were included (note correction on Page 2 about spacer above last puck). The total weight of the installed parts was 3.5 lbs (this is with one steel collar - the second collar is removed after pre-load adjustment). The bungee and bungee pin removed were 0.5 lbs for a net weight of 3 lbs. Here's what's in the box:
I removed the nose gear by cutting the bungee and detaching the steering rods and lower bearing. I had the stubs that hold the bungee on the upper end of the nose gear cut off and the resultant holes welded shut. (You can modify your own nose gear, send it to Zenith for modification, or purchase a new nose gear.) I powder coated the lower, exposed portion of the nose gear (not required, but something I had wanted to do the next time the nose gear was off!), painted the area from the steering arms up to 10" from the end of the upper gear leg, and ground and profiled the welds to provide a smooth surface.
I polished the upper exposed 10" with a #80 aluminum oxide abrasive disc backed by a foam pad (so as to conform better to the curvature of the tube). It is important to polish the tube and profile the welds so the donuts can slide smoothly.
The kit includes 10 spacers and 10 rubber "donuts" or pucks that are stacked above the 2 steel shaft collars. I found the spacers and donuts to be a tight fit, so I opened them up slightly with an oscillating spindle sander. The sander removed very little material from the donuts, but easily opened up the spacers so they could slide on the tube without binding. The spacers and donuts are then stacked on the nose gear (start with a spacer, then a donut, and alternate, finishing with a donut). The rubber donuts fit snugly, but will slide with a little lithium white grease for lubrication (white lithium grease was initially recommended, but now Zenith recommends 3M silicone paste grease). I then drilled and riveted the front and rear angles to the upper stop.
These 8 rivets were drilled out in the forward firewall gusset on each side and opened up to #12 holes with the upper stop cleco'd in place.
After deburring and Cortec application, the upper stop is bolted in place with 16 AN-3 bolts - heads inboard and nuts outboard. The nose gear is then reinstalled and the stack is pre-loaded by tightening the lower shaft collar, prying up the upper collar with screwdrivers on each side, and then tightening the upper collar. I then loosened the lower collar, moved it up, and repeated the process for a total compression of between 3/8"-1/2". Some pre-load is necessary to permit the self-centering of the nose gear in the lower bearing.
Apparently I got the pre-load about right - when the aircraft sat back down on the nose gear, the steering arms rode approximately 5-6 mm above the bearing block, allowing for easy ground steering. As I mentioned earlier, once the pre-load is adjusted, the second steel collar can be removed. (One is sufficient and they weigh 1/2 lb each!) Zenith will eventually have an adjustable tool to adjust the pre-load and the second collar will no longer be necessary at all. With one shaft collar, Roger recommends Loctite on the securing machine screws.
My original bungee system worked great. It was smooth and I couldn't even detect the self-centering "notch" as I swung the rudder from one side to the other. (IMHO, most rudder smoothness problems are due to over-tensioning the cables.) However, I was amazed at the difference after installing the new "donut" system! The suspension feels more compliant and is quieter. Steering effort on the ground was reduced and in the air, the rudder pedals were extremely light. However, when the rudder was centered, it seemed to hold it's position well. The best way I can explain the difference in "feel" is it is similar to the difference between manual steering and power steering - it feels like the nose gear is turning on ball bearings - there is absolutely no torsional resistance! I always felt my finger-tip dual stick forces were much lighter than my rudder, and now they are equally light. After flight testing and bumping along on a turf strip, I checked the bearing marks on the grease on the strut below the bearing and it appears the gear was deflecting about an inch during landing and taxi, which is fairly similar to what I saw with the bungee.
About the only negative is the additional weight over the bungee, but that's a small penalty to pay for eliminating the potential single-point failure of the bungee and eliminating regular bungee replacements. It was a fairly easy retrofit since the Jabiru is a light engine and there is plenty of working room between the engine and firewall. I understand Zenith is going to make this system standard with new kits.
John
N750A
(Disclaimer: No business or financial affiliation with Zenith Aircraft.)
Replies to This Discussion
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Permalink Reply by John Austin on
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Geoff, you'll also have to consider the Shore durometer ("hardness") of your material vs the Zenith material.
John
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Permalink Reply by Geoff Storey on
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Thanks John do you have any figures on the hardness required
Geoff
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No, perhaps Zenith could supply that information. You might also inquire as to the status of them developing a 701 puck kit - if you find out anything, let us know!
John
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Yes I do that I have the 750 nose leg on my 701 so no problem fitting the kit already made .
but because of this virus problem in the world get parts sent here to Australia takes long time to get anything here so I thought I would make my own .
Geoff
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Permalink Reply by John Mazzolini on
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I'm strictly guessing but 25-30 shore. They are softer then I thought they would be.
They must be an off the shelf item...somebody like McMaster Carr might have various in stock
getting them to Australia in a timely mater is another issue
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Thanks John I have various sheets of 20 mm thick neoprene rubber different softness just cut some out and try them till I get the best to suit my situation.
cheers Geoff
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Permalink Reply by Roy Eichendorf on
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Hi Geoff, Did you make your own donuts from 20 mm neoprene sheet? What dimensions did you use -- inside and outside diameters? Which hardness did you use? How well do they work?
I'd like to get a donut system for the 601HD, which I'm building, but I can't find out whether the 650/750 kit will work on a 601HD.
Thanks for any help,
Roy Eichendorf
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Hi Roy I ended up buying the kit for the 750 and installed it on my 701 I already had 750 nose leg on my 701 so just had enough clearance for the pucks standard 701 setup is to narrow to fit the pucks in ,couldn’t get the right density rubber here in Australia it was to soft so better off just buying the proper pucks from Zenith so far working perfectly.
Cheers Geoff
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Permalink Reply by David J. Beaulieu on
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Just finished up with retrofit install of the system. The instruction sheet still has a typo/misinformation instruction line. For 750 aircraft (not sure about the 650) you will not have to do anything with the A5 rivets of the forward gusset (C75F9-1). The upper stop (C75L3-1) does not make contact with this gusset. There is no interference from the front angle (C75CL-2) riveted to the upper stop and the forward gusset.
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Update!
When I did the initial installation, Roger advised me to lube it with white lithium grease and that worked OK. About a year later at the '19 Homecoming at Zenith, Steve said they were now recommending lubing the assembly with 3M silicon grease as it wouldn't dry out. Sure enough, a few months ago I started to get some squeaks from the pucks. I'm doing an annual right now and decided to re-lube the assembly with the 3M silicon grease - I had been dreading doing it, but it wasn't as nearly as bad a process as I thought!
I first measured the puck stack height (with the plane still sitting on the nose gear) from the underside of the upper stop to the bottom of the last spacer so I would have a reference when I pre-loaded the pucks after re-lubing. On my plane this measurement was exactly 200 mm.
I loosened the retaining ring at the bottom of the stack and let the weight of the plane push the puck stack down until the lower bearing block rested on the welded steel ring on my nose gear strut. I then tied the tail down, raising the nose, and could pull the nose gear down a few inches, exposing the strut at the top for a few inches where it had been previously covered with the pucks.
I then proceeded to generously lube the exposed strut, upper and lower faces of the pucks, and the upper and lower surfaces of each spacer and push a puck and spacer back up till it touched the upper stop. I continued this process until all of the strut, pucks, and spacers had been lubed and pushed up and then loosely fastened the upper retaining collar and leaving a slight gap, tightly fastened the lower or 2nd retaining collar.
After lowering the plane back on the nose gear, I then pried up the upper retaining collar to pre-load the stack, compressing the stack until it matched my initial 200 mm measurement as described above. Then, after tightening the upper collar (I used Loctite on the screws as recommended by Roger if you're going to remove the lower collar), I removed the lower collar and as I had previously done, put a drop of torque check paint across the underside of the collar and onto the nose gear strut. This allows me on pre-flight to glance up through the lower cowl opening and verify the collar has not shifted.
Now all that is left to be done is see how long this lube job lasts! Ha!
John
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John was it the dry spray lubricant or the brush on paste from 3M
John
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