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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Nice write up John, thanks for the evaluation!   Dave

Yes, nice info and photos! Any word on price?

The new system is in-stock and available for shipment - contact Zenith for pricing.

John

From the flyer they handed-out this weekend, $359 for the kit, $116 if you want them to modify your existing nose strut.

Thanks.

Any info when this might be available for the CH801? I totaled my last 801 from a failed bungee. Even with frequent inspection and replacement I flinch every time I set down.

When Jon Croke did the video interview with Roger at AirVenture, he didn't mention the new suspension was available for the  801.  Perhaps Carl or someone else who attended the Open Hangar weekend and saw one of the flyers about the new system can tell us if the system is available for the 801?

Out of curiosity, Sam, what's the o.d. of the nose gear strut for the 801?  The 750 STOL, Cruzer, and 650 all use 2" struts and thus can all use the same kit.

John

Sam, what do you mean “totaled” it? Was your 801 completely destroyed? If so that’s awful, sorry that happened. 

The flyer only mentions the 750 & 650. A builder asked about the 601 and they were "pretty sure" it would work. No mention either in print or during the presentation of 801 availability. As John noted, if the strut was the same OD, I could see where it might work, assuming the bungee cord was similar to the 750/650. Just speculating, I could see where if the strut was taller that you might need another donut or 2, but that wouldn't be rocket science to source.

According to the information sent to me by Zenith, the 801 front gear tube is 2" OD x .083" Wall.  If it is the same thing that they are using on the 750, no wonder that 801 owners have had problems with cracking or (as in my case) collapse of the tube. Since the nose gear load on the 801 is a lot higher, it probably would require not more rubber donuts but donuts with a higher spring rate (lbf / inch). The empty weight on my FLG is 402 lbs. Put 2 -250 lbs guys up front and it probably jumps to  650 or more. The other concern I have is whether or not the reinforcement at the top is sufficient for the higher loads.  But either would be easily surmountable. 

If I am going to pull the engine and mount to do a mod like this, and cut up my gear tube, I think that I will go one step further and modify the front steering so that the rudder is not connected to the nose gear in flight (like a Cessna) and the connecting rods from the rudders to the steering do not jump up and down so viciously on round ground. 

Thanks for the info. 

Not to be a sour grape but I think as a retrofit the Viking replacement looks to be more cost effective. When I initially saw posts about this I was under the impression that the Zenith solution was going to be significantly cheaper than the Viking. Considering that you have to replace/modify the existing strut, the small cost difference probably doesn't justify it.

You do have to modify the strut, but it just takes a few minutes to chop off the bungee hangar tubes and weld the resultant holes shut - I think anyone with average welding skills could easily do it.  The welds are non-structural - all you're trying to accomplish is to provide a smooth surface for the donuts.  I don't have any welding skills at all, but I do have a metal fabrication shop nearby that gets all my business.  I took the strut by and they modified it same-day and best of all, "no charge!" When they do that, I always put a tip in their break room jar for coffee and donuts - the edible kind, not the nose gear kind!  ;>)

The price Carl quoted is a modest savings vs the steel alternative and I wonder what the weight savings, if any, vs the steel bungee is?  Has anyone weighed the complete (including hardware, etc) steel bungee installation?  I "think" I've heard 3.5-4 lbs, and I don't know if that is net of the removed bungee or not (few oz for the bungee - the transverse pin that the bungee wraps around is retained for the steel system but removed for the donut system).  So, it may be the weights are near identical or perhaps the steel bungee is marginally heavier?

Small savings in money and weight may not be significant, but as we all painfully know from building airplanes, they do add up!  :>)

I think an important factor for each builder to consider on retro-fit installations will be the ease of installation of each of the two systems which will be determined by the amount of room they have between engine and firewall and perhaps other components mounted on the firewall adjacent to the firewall gussets and also any potential obstructions above the nose gear strut.  The Zenith donut system has no additional hardware mounted above the top of the nose gear strut, but, as I understand it, the steel system has additional hardware in this area which sometimes interferes with engine mount tubes, etc. - however, I "think" in most cases modifications can be made to accommodate the steel system. 

Especially in retro-fit situations, I think each system will have its pros and cons when the unique requirements of each engine and mount (affecting working space for installation) and arrangement of firewall components are considered.

As Loren always says, "Nice to have alternatives!"

John

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