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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 Gary Welch on
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Ok, thanks John!
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I've spent the last two weeks (not full time!) installing the donut system in my 601 XL. I didn't think it would be possible due to the restricted space available from my O-235 engine. Then I got a great deal on the kit and decided to try the install. I don't want to repeat the stuff that's already been documented so I'll just add the issues that I had not seen before.
Things were going swimmingly. It was not as hard as I thought it would be to get access. An hour of pulling things apart and I thought I had it as good as it was going to get. Got the strut out, cleaned it and found someone on the field to remove the bungee posts and re-weld (he asked for $50, I gave him $80 and was happy to do it!).
Got the strut back and test fitted the donuts and spacers and noticed a concern. The inside edge of most of the donuts had a ring of plastic/neoprene burrs on one side. It looked like these would not be good for the smooth compression of the donuts, and they were making it hard to push the donuts on the strut. I used a round grinding stone on my drill and ground them off (just on the very edge, not in the middle of the donut).
With some swearing and patience I was able to get the 16 old rivets drilled out and installed the 16 new bolts. One difference from the install diagram. The install diagram shows the rivets set directly across from each other. On the design drawings for my 601 XL, the rivets were staggered. This meant that only 6 holes on each side fell within the new upper bearing bracket. I was able to get the two extra bolts on each side by adding them, staggered, towards the front of the plane. I don't have a pic.
The next thing that concerned me, due to limited access, was getting the new upper bearing plate aligned with the original strut hole. Once I drilled the new upper bracket I knew there was no going back. I came up with the idea of stuffing something thru both holes before I drilled the new bracket. It turns out a Mouse Milk bottle was exactly the right size.
This worked like a charm. When everything was bolted together the strut was perfectly centered thru the 2 holes.
I test fit the strut with no donuts and everything looked good....
And then disaster struck! The strut could not be inserted in place with the donuts on, due to the engine mount cross-brace! This began a long stream of cursing and ranting at my rotten luck and lack of foresight. All I could think of was there was no way around this. It's hard to see in the pic but the blue tube is the engine mount and there was no way the donuts were going past this.
After calming down I was able to come up with a solution. Remove all of the donuts off the strut. Push the strut up 3/4 of the way in place. Then, one at a time push the spacers and donuts down on the strut (above the cross brace).
Right here I was starting to feel confident... I should know better, its a damn airplane! They just have to screw with you. Ok, the next one was was my mistake, but still.
I was able to pre-tension the donuts, using the screwdrivers, by about a half an inch. That sounded about right from the previous posts. I thought I was ready to start putting things back together when I decided to check one last thing. Was the top donut tight up against the new upper mount? It was impossible to see without a mirror (or a camera jammed in there). And then I saw this:
Argghhh!!*$%! The upper donut was 3/4" from the top bearing bracket AFTER I pre-tensioned the donuts. After I calmed down I solved this by using a combination of screwdrivers, awl, a kitchen spoon and others to, one at a time pry the donuts up so the whole stack was tight against the upper bracket.
I then re-tensioned the donuts hoping that I had gotten that part right,
After connecting the steering rods up I lowered the plane on its new nose tire and, expected the worse. I just knew there would be a problem with the position of the lower stop and the lower bearing block.
And this is the moment my wife chooses to walk into the hanger - arrghh! I just know I'm going to be swearing once the weight is on the tire and she's going to witness my meltdown...
Hesitantly I shined the flashlight on the lower bearing, with Linda watching, - and the gap with the weight on the gear was about 1/2" - perfect!
I looked at Linda and said, "All done honey, piece of cake".
I don't think she bought my lie.
Still have to grease the lower bearing block and safety wire the bearing bolts, but its basically done.
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Permalink Reply by John Austin on
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Looks good, Gary! Where there's a will, there's a way. ;>)
John
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Permalink Reply by Jim and Amelia Isaacs on
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Gary, what that setup you have for your steering rod firewall penetration? It caught my eye, I’m not happy with mine.
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Hi Jim
It's the standard detail that was in my 601XL plans.
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Permalink Reply by Don Morrisey on
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Jim check out Skytek’s steering rod swivel kit:
https://store.skytekaircraft.ca/store/p35/Steering_Rod_Swivels_Asse...
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Permalink Reply by John Mazzolini on
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Hello All
I just received the doughnut/puck kit along with having the front fork modified back from Zenith. I have a question on how much resistance the doughnuts should have sliding onto the tube. I see that many people have noted they clearance the inside of the doughnut....Should they fall on with little to no resistance or some fight to get them on?
thanks
John
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In my experience, the donuts were quite tight despite having smoothed and polished the strut tube. I also had slight resistance from some of the aluminum spacers. So, I used an oscillating drum table sander to smooth the i.d.'s of the spacers and the donuts. Obviously, the sander easily opened up the spacers, but it was less effective on the rubber ... I probably removed a very slight amount from the donuts, but not much.
Afterwards, the spacers would easily slide down the strut tube, but the donuts still provided quite a bit of resistance despite also lubricating them. The resistance was on the order of what you feel when you push down a hydraulic bottle jack with the pressure valve open. I was concerned it would be too stiff, but after pre-loading and testing with taxi and flight, it seemed perfect! It's just a hunch, but I think it's better for the donuts to be a little stiff on the strut vs loose - It probably won't take very many take-off/landing cycles to more-or-less "bed-in" the donuts. I also think the donuts would work best if a little tight since the pressure on the donuts' i.d. is progressively increased and decreased as the donuts load and unload, providing an action almost like a hydraulic shock. If they were too loose, you might lose some of that action. I've accumulated quite a few hours and many take-off/landing cycles and I can't detect any difference in the system's performance from the initial installation. Works great!
John
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Mine were fairly stiff as John describes. Got better after greasing them well, but still took some pressure to move them up and down on the strut.
Make sure you grease in-between the donuts and not just he holes.
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Thanks John & Gary
I think I'll make sure the metal spacers fit on smoothly and leave the doughnuts as they are unless I run into one that is way to tight. Liberally greasing for sure...is white lithium the correct lubricant?
John
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When I did my installation, Roger at Zenith recommended white lithium grease. However, at the last Zenith Homecoming Fly-In, Steve at Zenith recommended silicon grease - he said it would be more resistant to drying out.
John
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Permalink Reply by Carl Orton on
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I lightly "adjusted" the aluminum washers, coated the donuts with DC-4 silicone stuff, and assembled. The next day (not yet installed), l could barely budge any of the donuts. So I removed them all, painfully, cleaned, and re-coated with white lithium. Let them sit overnight. STILL tight as can be. I figured, "oh well" I followed instructions, so someone must know what they're talking about. Not yet flying, nor is the engine mounted, so I have no idea how they'll do.
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