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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 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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Permalink Reply by John Austin on
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Try this link for the video: https://www.youtube.com/watch?v=B2kX293xmD4
However, in the comparison, both steel collars are weighed in the puck system - you only need both to pre-tension the pucks and then one can be removed. They weigh 8 oz. each. Also, the steel tube crossbar that retained the lower ends of the original bungee is removed in the puck system and retained with the steel bungee system. So, accepting the weights in the video, an actual puck installation would be about 6 oz or so lighter than the steel bungee system.
John
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Thanks John
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Permalink Reply by Fred Williams on
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I’ve had both the Viking spring and now have the Zenith puck system on my Cruzer. They both get away from the failure point of the bungee system. I like the puck system better. It is quieter and I think smoother.
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Permalink Reply by charlie rosenzweig on
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Fred,
Always a help to hear from someone who has actually tried both. Thanks,
Charlie
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Permalink Reply by Michel Ruf on
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John,
when there is the engine weight on the donuts, how much do the lower studs (where the steering rods are attached to) raise out the notch of the bearing in your installation?
I tried to tight the suspension but as I did not grind the donuts they sit quite tight on the gear leg. Made it a little more difficult to tight. So when I got the engine weight on the donuts the studs raised a little out the lower bearing. What has the nice effect that rudder movement on ground is quite easy. But I guess if it's to much there might be strokes on the upper bearing of the donut system when touching the ground on landings.
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When I pre-loaded the donuts, I compressed the stack about 10 mm. My Jab 3300 weighs about 178 lbs with all accessories and I have 218 lbs on the nose gear strut according to my W&B. With that pre-load and weight, the steering rod arms ride about 5 mm above the lower bearing (go back to my original post in this thread and look at the last picture - that was taken with weight on the strut).
I also thought the donuts were rather tight on the strut, so I did attempt to slightly increase their i.d. by sanding with an oscillating spindle sander, but I don't think I really removed very much material at all. Be sure to lubricate the donuts with silicone grease.
The end result was steering/rudder movement on the ground is very light and smooth, as you noted, and it seems to be much more supple or smooth over bumps. In the air, it feels like the rudder is on ball bearings and pedal effort is very light with no "notchiness" or catches.
John
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Thank you John for - as always - great Information and Help
Michel
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Permalink Reply by Greg Priest on
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Excellent write up John you filled in the lack of information on my Zenith Cruzer plans etc....You saved my bacon and now to de rivet the top area and install and bolt the overlooked part # L3-1. Thanks to Kit Hoare for pointing out my mistake and directing me to your excellent step by step article with photos.
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Permalink Reply by Gary Welch on
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John: I'll be reinstalling my strut in the next few days and noticed there is no mention of a torque value for the machine screws on the shaft collar. Or did I miss it?
I've gotten very anal about torquing everything on my plane, including screws...
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As I recall, I don't think there was a torque value given in the instructions, but I likely used the TFAR method: "That Feels About Right!" You could look up a torque value for a similar screw on a chart, but what I found was if those screws were anywhere near tight, they were already snugging the collar around the strut and locking it in place. Roger did mention that it's perfectly OK to remove the second collar (which I did) after pre-tensioning the donuts (those collars weight 8 oz each!), but use Locktite on the remaining collar's screws. I also put a drop of Cross Check Torque Seal paint paste on the bottom of the collar where it intersects the strut. That way, I can look up through my lower cowl opening and verify the collar has not shifted or slipped. It hasn't budged a bit in about 80 hrs and many landings.
John
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