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.  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 (recommended by Roger). 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.)

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Replies to This Discussion

My kit showed up a couple days ago. I see no identification on the donuts.

Ditto - no i.d. marks or numbers on mine.

John

Hi John,

Just wondering if you received installation instructions with your puck install system.  My kit arrive with a detailed drawing but no written directions.

  James

James,

I believe John is out of the country on vacation until 02/20, so his reply may be delayed. John's 3rd paragraph above says he received step by step instructions. I would call Zenith and ask them to fax or email the instructions.

Jim

James,

As of a month ago there were no installation instructions.  I called and talked to Katlin.  She said to call and talk to Roger .  I never did.  The only question I had was the “pre-load” that John mentioned in this post.  With normal weight on the nose, I think you want the rudder attach tube to ride just above the bearing.  Then when in the air, they will ride on the bearing and self center.

     If you call Roger and get a different or better answer, please let us know 

Jeff

I wonder how the manufacturer can manage their quality process without a means of lot or batch identification?

An instruction sheet was included with my kit - Roger should be able to get you one. However, the drawing is pretty self-explanatory. I'd post my kit instructions except I'm out of the country for 2 more weeks. My first post basically follows the instructions step by step.
John

Could any of you please tell me how to get in contact with this Roger person who seems to have all of the answers. I haven't a clue who he is or how I can contact him by phone or mail. 

Thanks!

Stat at Zenith in Mexico, MO.  Don’t know number but sure it is on web site.  Ask anyone who answer to speak to Roger.

Roger works at the Zenith factory.  Call Zenith and ask for Roger:  573-581-9000 Monday to Friday 7 AM - 5 PM Central

I ordered a donut kit for my 801.  I received it, but have not installed it yet.

It's been my experience that Roger is usually readily available on the phone early in the a.m. between 7 and 8 a.m.
John
Thanks for the help running down Roger. He said that they an sell me the donuts and washers individually for my CH810, not as part of a kit. He mentioned that I may need some additional donuts for the extra weight (I agree). So that I can get an idea as to how much additional load I will need to carry, can any of you share with me what the empty weight on the front landing gear of your CH 750 was when you did your W&B check? I think I will start off with using the ratio of the 750 FLG weight to the CH 801 FLG weight to know how many additional donuts to order.


Again- thanks all for your help.

PS - I just made my goal of operating into or out of 500 airports last week on a trip back from Florida. Now stands at 506 airports in the log books, probably 100-150 of them in the CH 801. (it is just so easy and tempting to drop into a new airport on a trip in an 801. When I changed the oil and noted the hours I saw that I have about 575 hours flying the 2 CH 801s that I have owned.


If I had the rubber donuts instead of bungee in my first CH 801 all of those hours would have been in the same plane.

PS - A caution to all of you. The FAA's confidential information isn't. You may recall that I was falsely accused of hitting a power line last Sept, only to be finally exonerated many months later. Well 2 weeks ago, I got a notice from my insurer that my insurance was cancelled because of that accident that never happened, that the FAA said they never released any data about (but they did, it showed up on the internet 2 days after). I eventually got it sorted out and got my insurance back. But beware, when dealing with the FAA you are guilty until proven to be innocent by any shadow of a doubts (as if 3 witnesses: one a minister and one a cop plus a video of the other plane hitting the power line was not enough.) Can you believe that they actually ran tests to ensure that I hadn't made a fake video? And this was after they had the tail number & NASA report from the plane that did hit it? Despite the so called confidentiality they leak information - true or false - like a sieve. Another point to be aware of. If you do ask for your own records (you have to file a freedom of information request), the data that they will release may be months behind the data that they actually have on file, and will only be a synopsis from their own data base. No actual documents will be made available. I am buying the AOPA legal services plan.

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