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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 Roy Eichendorf on
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Hi John,
Can anyone tell me whether the 650/750 kit will work on a 601HD? I am building the firewall now, and would like to know what changes might be required to the vertical stiffener channel 6F9-4. If anyone has complete dimensions for all the parts in the kit, that would be great.
Thanks,
Roy Eichendorf
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Permalink Reply by Michel Ruf on
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If you do not have yet an angle drill you will have one after fitting the puck system. Process to install was quite easy but a tight fit in my case. But I did not have to remove any FWF parts. I like the system very much. Bungee gone and ruder movement much smoother. I am glad I did the update.
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Permalink Reply by charlie rosenzweig on
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So, for those of us who are not lucky (yet) to have built a Zenith, how do the two choices compare. The Zenith vs the Viking. From outside looking in (so to speak) it looks like the Viking is a spring device and the Zenith is a set of rubber donuts with washers. Sound like they are very close in price and weight?
Just curious.
Charlie
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Permalink Reply by John Austin on
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There is negligible difference in weight between the two systems and although the Zenith puck system costs a little more, I like the progressive dampening that occurs with the pucks (due to the squeezing of the pucks, the i.d. of each puck grips the nose gear strut progressively more as it compresses and progressively less as it releases - the effect is similar to a hydraulic shock). All that being said, I'd go with the puck system if installing during a build (I think it's going to be, if not already, "standard" on new kits anyway!), but in retrofit situations, one needs to look at what components may/may not be obstructing the installation of the puck system - in some situations, it'll be easier to install the "steel bungee" system. In my case, the Jab 3300 is so light that there are oodles of room between the rear of the engine and the firewall, so it was no problem to install the pucks. An 0-200 or o-235, however, is quite tight against the firewall due to the weight and access might be a problem for the puck system - I understand the "steel bungee" system might be easier to install in those or similar instances. Either system should be a "once and done" vs periodic replacement of the original bungees.
John
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Permalink Reply by Todd Scruton on
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Just as a PSA I installed the system with my O-200 without an issue. The only “special” tool I used was an offset drill to drill out the top rivets.
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Permalink Reply by Joe Hopwood on
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Morning John,
I have the Viking system and still a little catchy in the air. Would installing puck system still be a viable option for me to pursue or do you think something else is the issue? Engine is Jabiru 3300A.
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What do you mean by "catchy," Joe? I know with the original bungee some say it seems to "catch" or take more force to get the rudder off-center due the notch in the nose gear bearing - is that what you mean? Otherwise, I'd look for a cable fitting or rudder catching on something.
All I can say about the puck system in this regard is that mine turns so smoothly it feels like it has ball bearings - I can't detect the nose gear bearing notch at all.
John
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that it does not move freely when airborne. Smooth as glass on the ground. Of course maybe there is enough tire friction to overcome that I don't notice the "catchiness" on the ground. It does have the benefit in that it is almost like having trim capability in that it tends to stay where I put it if close to centered.
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Have you tried reducing the pre-tension of the spring? Sounds like it's bearing-down on the bearing notch more than is necessary?
John
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No. On first full power application the stud poked a hole in canopy and I think I cut it off flush with the nut. :-(
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Permalink Reply by Carl Orton on
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Comes up as 404....
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Permalink Reply by Loren Warner on
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I tried to share a u-tube made by Viking showing weight difference between the Viking Spring and Donut System; Not working and for some reason and I'm unable to delete. Anyway, the Viking Spring is slightly lighter.
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