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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Loose they are 3/4" thick.

Thanks for the guidance John! I installed the retrofit puck system in my CH750 STOL ready for ground and flight test. The installation went relatively smooth following the instructions and I had plenty of space to work between engine (UL Power 350IS) and firewall. The instructions do not provide any guidance on what constitutes proper tensioning but I think I have it right as I tensioned twice and squeezed the pucks up as best I could. I have attached a few picks on where the gear strut sits relative to the upper and lower bearings.

Cheers,

            Dave

Attachments:

Your measurement of the puck stack looks OK to me.  I, too, have a light engine (Jabiru 3300) and my stack height was exactly 200 mm. and the system performs well and feels compliant when hitting bumps.

John

Hi again John, did some ground and flight testing with the new puck system and I must admit I'm not 100% impressed. The steering on the ground is stiffer than it was with the bungee. (Maybe this will take some time to work itself). In the air it is almost to loose and free...there is no self centering of the nosewheel as the lower pins are above the lower bearing angles. Almost seems like there should be another puck (#11) to allow the lower bearing to work properly. Everything is well lubed so I don't think that is an issue. I have tensioned per previous reply and pics above so I'm not sure where to go next.

Cheers,

Dave

First report like this I've heard of, David - everyone else seems to universally report much easier steering and lower rudder pedal pressure required.  In flight, the steering pins should be resting on the lower bearing block. (Push the tail down and get the nosewheel off the ground and it should fall into the lower bearing block and self-center with the pins in the angles.)  On the ground, the pins should lift up out of the angles when the nosewheel is bearing the full weight of the nose.  Only thing I can think of is did you clean the strut of any paint, powder coat, etc. where the pucks slide and also in the area that slides through the bearing block?  If that doesn't work, perhaps a consult with Roger at Zenith might help - if he comes up with a solution, let us know!

John

I have installed the pucks on my 701 and they work fantastic smooth steering on the ground and in the air best thing I have done to it 

Geoff

Has anyone confirmed the shore hardness of the pucks as received from Zenith?  I've seen guesses around 25-30 Shore and that they were softer than what people expected them to be.

JR

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