Online Community of Zenith Builders and Flyers
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.
(Disclaimer: No business or financial affiliation with Zenith Aircraft.)
I seriously doubt that the rubber donuts (less the weight of the removed pieces) is much- if any -heavier than the steel bungee. I do not particularly care for the steel bungee design for several reasons: you still have the steel cables to corrode, twist and eventually fray, It provides no better (possibly worse) rebound damping than the regular bungee. You still have the issue of not being able to control the preload very well.
If you don't like the rubber donuts, for around $100 or less you can buy a steel "coil over" spring with precisely the correct spring rate and length,the necessary 2" split collars and flat spacers at tractor supply, and the plastic tube that keeps the spring from rubbing the strut tube. Lighter than any of the alternatives. But like the steel bungee, there is no rebound damping. They make "coil over springs" that are designed to slip over a 2" OD shock absorber body. Any of the on line performance car parts places can sell you one.
I have been investigating the source and price for the rubber donuts. All the other parts you can do by yourself.
Oh and by the way any of the options is cheaper than the boost in insurance premiums and the loss you take when you total your plane from a failed bungee as I did. This winter one or the other of the options is going on my CH801.
"I do not particularly care for the steel bungee design for several reasons: you still have the steel cables to corrode, twist and eventually fray..."
Sam, I installed the steel bungee in my 601XL a couple of years ago and am really pleased with it. I was able to get the preload adjustment just right the first time I adjusted it (maybe just lucky?) and there are no steel cables associated with the installation, so I'm curious about your comment.
I've nothing against the new Zenith rubber donut strut, but have not experienced any rebound with the Viking steel bungee. YMMV.
I think you're correct, Stan - the first steel bungee (prototype) had cables and not straps.
You didn't happen to weigh yours, did you? Just curious as to the weight.
I did, John, but it's recorded in my aircraft log for the W&B change, and the log is in my hangar. All the steel bungee parts weighed just over 3 pounds as I recall. I'll get the actual weight next time I'm at the airport (probably later this week).
John, I removed the rubber bungee (0.33 lbs @-12.15" arm) and installed the steel bungee (3.50 lbs @-12.15" arm).
Thanks for the information. Once installed, looks like both systems are about 3 lbs heavier than the original bungee system. As I said, that's a small price for eliminating the single-point failure potential and regular bungee changes.
I just received my package from zenith and am thinking about installation. Drawings says, "weld a cap over the hole[s]". That suggests a "bump" on the side of the gear tube. Will that cause a problem? I assume that since the tube is .065 thick, .032 or .04 would think enough to cover the hole?
Should read weld a cap "in" the holes, and then grind smooth. You're right, too can't have a bump here.
Nice write up of the modification.
I could only squeak-in a brief test-flight after installing the donut system because of the weather. Finally, we had great weather in our area today and I got to much more thoroughly check-out the new system. I've found that even the steering effort for taxi is greatly reduced - likely due to the lack of torsional resistance vs the bungee. Of course, the rudder effort was again very light. Try as I might, I simply could not feel the "notch" in the lower bearing block. The controls are well-harmonized and now about all you have to do is "think" about moving the rudder and it moves. At the same time, once it is centered, it seemed to stay in-place and not wander. I also let "George" (my 2-axis autopilot) fly the plane and the rudder remained stable.
I flew to KCSV (Crossville, TN) which has a nice long runway. My plane has a modified tail (HT fences and elevator tip extensions) which lets me easily lower and raise the nose at-will on the roll-out after landing. I was able to replicate Roger's demonstration in the Zenith video where he repeatedly bounced the nose up and down on roll-out. This was entirely controllable and predictable - there was no excessive rebound and it felt more like a hydraulically-dampened shock. I also noticed the system seemed quieter.
While the system is not hydraulic, the donuts do provide a dampened suspension: as they are compressed vertically, they are squished horizontally between the aluminum spacers, progressively increasing the pressure of the donut (and sliding resistance) against the nose strut. As the compression is released, the donut's "grip" on the strut progressively releases. All of this results in a dampened response to compression and rebound.
Of course, this a new product and it will take some time to assess durability and retention of elasticity of the donuts, etc. However, similar systems have been used in other aircraft with good longevity and certainly there is no possibility of single-point failure as the bungee has.