Online Community of Zenith Builders and Flyers
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.
(Disclaimer: No business or financial affiliation with Zenith Aircraft.)
Sam, I had a nose gear weight of 244# with a total empty weight of 852# with my STOL 750
DO NOT UNDER ANY CIRCUMSTANCE COOMMUNICATE WITH ANY FAA PERSON WITHOUT A LAWYER ,
I LOST MY CERTIFICATE FOR ALMOST A YEAR BECAUSE OF NOT HAVING ONE PRESENT ,! ! Tom
I also found out the hard way , NEVER TALK TO FAA ABOUT PROBLEM WITHOUT ATTORNEY PRESENT !
Tom Hankamp builder RV4, now building 701
Has anybody thought about machining out the hole in the collar so that a Nordlock washer can be used? Probably would be safer than loctite. We could then set it at a predetermined torque and not worry so much about having one collar loosen and back out.
I fitted my Cruzer with this mod and have been extremely happy with it. Much better overall design.
If I was that worried about a collar loosening, I'd just stick with the 2 collars provided and not worry. I removed one collar because they weigh a half-pound apiece and Roger said one is OK. I just don't see how a loctite'd set-screw is going anywhere and am quite comfortable with one collar. I did, however, put some torque-seal on the lower edge of the collar and have periodically inspected it for cracking or popping off, but it hasn't moved one bit!
Nice review of the product. Good job on the installation write up.
Thanks for the write up and pictures. Just finished installing mine. Have not even installed the wings yet. I really like the new setup. During removal I determined the bungee pin was rubbing on the back of the strut. Job was not hard at all. Your pictures help illustrate the less than stellar but adequate instructions.
I got a 701 and the first bungee worked great and lasted forever but then the replacement was a hair long so the strut wouldn't stay seated in the vee blocks and wandered around with resulting annoying random yaw. Got tired of that and called Roger and ordered the rubber donuts and spacers. Had the basics for the mod in mind and fairly confident we could make it work.
ZAC put the load where it was originally designed to go but on the 701 I couldn't figure a way to do that without radical surgery cause the channel behind the strut is narrower so decided to try taking the load up to the mount. ( I got a Rotax 912 ULS and the ZAC mount). I got it done working off and on for about a week. It was a slow go and a PITA; having to work between the engine and firewall. Had to do the welding in that restricted space(I only use acetylene) so had to make all kinda preps for that...wet towels packed tightly around and sheetmetal shields here and there. I've done a lotta that kinda stuff for decades and never burned the plane down but never have looked forward to it either. I was too stubborn to take the muffler off but if had it to do over I would remove it for sure. I've got several hours on it now and so far it's been very good. My strip is rough and it's handled that with no problems. Feels good on my roughest bumps. Like it's sposed to. Airborne it flies better than it ever has. It's better that it was when the bungee was at it's best. The rudder feels good...it won't self center from an intentional yaw but reacts naturally to light opposite rudder for the rollout. I'm happy with it so far but time will tell what we really got. And the engineers may cringe because of all kinda obvious engineering nonos...I'm not worried about that; I'll just watch it close. But I never recommend my shadetree stuff; in fact I urge others to avoid my redneck engineering and stick with what the pros recommend. ZAC knows their stuff and I'm just an experimenter that learns by doing. Don'y copy my stuff.
a few pics
This is what I ended up with. Angles welded to the upper mount leg and triangulated. It's stiff and I believe will be ok over the long haul, we'll see. I'm gonna add a few more pics after this posts successfully.
Here's a few more pics
Had to give the donuts something to push against so added 2 delrin bearing blocks to the plans single one and then a flat 125 AL plate against that for the stack of donuts to ride on below the xmember flange. The donuts wouldn't fit between the main FW crossmember flange and the NG strut. The flat plate is wider than the angles welded to the engine mount so puts all the load on those angles and on the mount. The channel behind the NG strut and the main FW xmember are pretty much unloaded. Added a strap reinforcement to the xmember and that butted up against the main angles so whatever load was carried by the flange is mostly transferred to the mount thru the angles. I'm running 8 of the donuts
The spacers interference fit with the main channel behind the NG strut so had to relieve those. Time will tell if they rotate and rub the flange of the channel. If they do will trim the flanges back and rivet an outside angle to the remaining flange. I ended up running very little if any preload cause the rudder wanted to start feeling a little heavy. Mostly I just snugged the donuts up against the stop, tightened the clamp and gave it a try. On mine that doesn't seem to be critical. I don't have a backup for the clamp slipping other than the strut will hut the mount after about 4" of up travel. I got 10" of prop clearance so should be ok.
I got 146# on the NW when empty...you guys running anything heavier than a 912 may(probably) have an entirely different situation. Your call as always.
Forum Moderator's Note:
At Joe Spencer's request, he would like to bring this mod to the attention of the 701 builders and flyers. I've started a new topic "701 Nose Gear Puck "Donut" Suspension" in the 701 Forum. Please continue Joe's discussion there for questions and comments relevant to the 701.
For questions/comments relevant to the 750 and 601/650 series, please continue the discussion in this thread.
Zenith.aero Forum Moderator