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.)

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Replies to This Discussion

Permalink Reply by Paul Bonasera on March 20, 2025 at 8:27pm

I'm still rocking the Bungee on my STOL 750 with a Lycoming engine. Works great and I did change it once, not to hard to do so I think I will stay with it. Just my 2 Cents.

Permalink Reply by Harley Myler on April 21, 2025 at 8:16am

That ship has sailed for me.

(disclosure: I cut it, was so brittle nothing moved)

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Permalink Reply by Amer Karim on April 22, 2025 at 2:38am

Nice write up John,

My simple questions to everyone are a bit rhetorical: 

1. How much excursion do you get from the new donut setup? Can you taxi on a rough trip with a cup of coffee in your hands or are you bouncing around inside?

2. How much energy do they absorb and dissipate into the suspension system? Or how much less energy is going into the airframe?

Based on my experience, the best nose suspension system has to have enough excursion to allow for rough strips while mainting adequate prop clearance, and it has to have a damper in it, that can covert the kinetic energy of the impacts into heat energy dissipated by the damper. Gas shock dampers can be 50% to 80% efficient, so they reduce the force transmitted into the airframe and your body by a similar margin.

I do all my flying in Kenya and over the years I have built different airframes, Kitfoxes, RV10's , and I'm now building a Cruizer, all with similar donut rubber shock suspension systems. I found the donuts to be less than impressive to say the least. When piled up and stuck in a hydrulic press, and cycled at several tonnes of pressure there was minimal excursion and they didnt even get warm suggesting that there was minimal energy absorbtion by them.

Our strips in Kenya are much rougher than what you have in the US and Europe, especially in the game parks and exotic places that we like to fly into. I found that despite doing my utmost to protect the nose gear during take offs and landings, I had problems... cracks in the metal, bent bolts.. you name it. So I decided to modify all the nose suspensions and change them to spring and damper systems, with great success, and now take offs and landings are a pleasure, the suspension moves up and down as it should, rather than me bouncing around in the cockpit, and I am no longer worried about nose gear failure or excessive forces going into the airframe, no cracks or bent bolts anymore.   

Anyway, my concerns are that if the energy absorbtion of the donuts is less than the old bungee then there is an increased risk of cracking developing at the mount and in the firewall attachment areas, and also that the airframe is being subjected to shocks and impacts. Given the bungee had issues as well and was a potential single point of failure.

I have therefore modified the nose suspension for my Cruzer build to include a spring and gas shock in place of the rubbers, while still allowing the use of the nose steering system. I have also strengthened the nylon block mount, trimmed the nylon block a little bit to allow easier rudder pedal movement, and have preloaded the shock to what I estimate my final nose weight will be, that can be adjusted later once the engine and prop are on. The excursion of the shock is 4 inches, 2 inches to allow for engine weight on the ground (static preload) and 2 inches for take off and landing movement and potential oops and hard impacts on landing (dynamic excursuion). It was a simple procedure and only took a couple of hours to modify and fit. If it can absorb even 50% of the energy on taxe off and landing then I think my airframe and I are better off compared to bungees and donuts.

I'll keep you informed of my results and tests on the system in due course when I get the plane finished and flying.

Kindest regards 

Amer

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Permalink Reply by Harley Myler on April 22, 2025 at 7:05am

Interesting take on the subject. When I first encountered the bungee system I was a bit taken back because it is unbelievably crude, but very inexpensive and it does the job given the size of the aircraft (speaking CH650 here).

When i saw the puck/disc/donut/biscuit system upgrade I was elated because my main experience is with Mooney aircraft. After Mooney abandoned torsion bars (main gear) and gas shock, maybe spring NG, they went to pucks and never looked back (talking vintage M20e, but I believe the Acclaims had them too). The M20e had 11 Nord pucks, 3 in the NG and 4 in each main. They needed to be replaced when they got brittle and there were checks each annual that mainly involved how thick they were when jacked up IIRC.

I only replaced mine once, well, my A&P replaced them because it was no small task. Landing was noticeably improved afterwards. You may find the following discussion interesting.

To be fair, the Mooney gear, especially the NG, is extremely complicated and in comparison with the Zenith, quite heavy as one might imagine. One thing the conversion does do for the Z is to greatly improve ground steering and the failure modes (aside from structural issues) are practically non-existent.

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Permalink Reply by John Austin on April 22, 2025 at 7:22am

The Zenith donut system has now been around for several years and I'm not aware of structural/airframe problems being an issue.  One thing to remember is that the Zenith 701 and 750 series function more like a tailwheel aircraft with little stress on the nosegear - at takeoff, a few seconds of throttle and the nose gear is off the runway before any high speed impacts with surface imperfections could happen.  Similarly, when landing, the main gear take the brunt of the impact and again, the speed is so slow that when the nose gear comes down, any impact with holes or bumps is at relatively slow speeds. (You might argue an exception for the extreme STOL as in contests, but again, as far as I know, the Zenith donut nose gear system and airframe are holding up well in these extreme conditions.

I know there have been some attempts at hydraulic/pneumatic systems but usually there has been a weight penalty.  Jon Humberd is presently developing a system and I believe he said there wouldn't be a weight penalty - it'll be interesting to follow both his and Amer's projects.

John

Permalink Reply by Harley Myler on April 22, 2025 at 7:34am

"I'm not aware of structural/airframe problems being an issue."

They are not pathologic, see attached.

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Permalink Reply by John Austin on April 22, 2025 at 7:58am

I was addressing the donut system - this report concerns a 30 year old plane and I assume didn't have a donut system for the majority of those years, if at all.

John

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