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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.)
Replies to This Discussion
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Permalink Reply by John Austin on
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I'm into my annual this week and while I was inspecting the new donut suspension, I applied some torque-check paint on the bottom of the collar:
Originally, 2 collars are used to adjust the preload, but the bottom one can be removed if desired to save weight (they weigh about 8 oz. each). Roger suggested that if you only use one collar to apply Loctite to the screws that clamp it to the nose gear strut. I thought it might be a good idea to also apply some torque-check paint - that way, if the collar ever shifted or slipped, the paint blob should crack or get pushed-off and it'll be obvious some unwanted movement of the collar has occurred.
John
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Permalink Reply by Ken Ryan on
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John, how would you compare rough field performance for the bungee vs the doughnuts? Also, where is the video you referred to of Roger bouncing the nose? Thanks.
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The donut suspension seems to be just as compliant handling rough bumps as the bungee did. It actually "feels better" since it is quieter and smoother with the dampening that occurs with compression and release.
I found the video link - the demonstration of the donut nosegear starts about 7+ minutes into the video:
https://www.youtube.com/watch?v=0MAWHfXhLPs
I was able to easily replicate the demonstration Roger performed. No practical purpose, but I believe it does demonstrate the controlled rebound and predictability of the new system. However, it is fun to do! ;>)
John
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Permalink Reply by Chris Hoare on
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Got my new nose donut system installed very happy with the way it feels and the ease of instalment.
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Hi Chris,
Was your installation a retrofit or installed during the build prior to engine installation? Which engine do you have?
John
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I was ready for weight and balance plane is finished but has not flown yet weighing this week so was pushing to get it installed before hand , first flight soon I hope .
had to remove oil tank and battery to get access but found it straight forward to install I have a rotax 912 ,I got the holes welded up by my local engnearing shop as I did not want to send back to zenith because I am located in Australia.
The feel of nose wheel now is a lot smoother very happy never liked the bungge.
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Permalink Reply by Gary Welch on
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My bungee/nose gear install has not been quite right since first flight. At first it was a bit of a annoyance while learning to fly the 601. After almost 300 hours it has become a serious aggravation.
I understand that the main problem in my case is likely some part of the install that I have not been able to figure out. I have greased the heck out of it, loosened the block nuts and replaced the original bungee.
Yet the nose gear will never properly center in flight. As soon as a make a turn or heading change it will not return to center - obvious by the slightly off-center ball. Then I have to "kick the ball" a few times and finally it stays centered - until my next course correction.
In addition, I have a lower back problem and the stiffness of the rudder ends up hurting my back after an extended flight.
I fly every other weekend a 340 mile round trip and this problem has gotten to the point where I don't want to deal with it any more.
I was looking closely at the donut install and it appears that I would have to remove my engine (an O-235) - and half of my firewall components in order to be able to install the donut system.
The problem portion is that upper stop. I don't see any way I can get in there to drill out the 16 holes in firewall gusset and then be able to insert the upper stop and bolt it into place without removing the engine, battery box and my fuel pump system.
This is kind of a bummer because I was looking forward to installing this on my plane.
I guess it's the steel bungee for me, which does look like I can install it without major demo of the firewall forward.
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I agree, Gary! Looks to me it'll be difficult to install anything back there - including just changing the original bungee! However, that's what I pointed out - in retrofit situations, I think donut vs steel bungee will be dictated by ease of installation. Doesn't help at all, but that's the most crowded firewall I've ever seen! :>(
OTOH, the Jabiru is one of the lightest engines used in the STOL 750, so consequently the mount is quite extended from the firewall for W&B reasons - I had oodles of room, so it was a very easy retrofit for me. As I recall, the aft portion of my engine is approximately 12-14" forward of the firewall.
FWIW, I used one of those 90 degree aircraft drills - I think it is a "Dotco" brand - to get in there and drill-out my rivets and up-size the holes. They are high-dollar and commercial production quality, but Ebay usually has used ones that are still serviceable for reasonable prices. However, it almost looks to me that in your situation, it might be easier to unbolt the engine mount from the firewall and swing it forward to get reasonable access to the central firewall.
If you go the steel route, please document it on the forum to help others!
Good luck!
John
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Permalink Reply by Joe Hopwood on
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Thanks for the excellent report on this product John. I replaced bungee with steel spring option before installing engine. I may not have done it exactly correct as my rudder is still a little stiff in the air. Actually stiff enough that with a little pressure as needed it functions much like rudder trim would. So if ball a little off in cruise, I step carefully until centered and it stays centered as it would if I had and used rudder trim. I believe I would have given serious consideration to the donut mod were it available.
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this is exactly the problem i was hoping the steel bungee would fix.
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Permalink Reply by Loren Warner on
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Joe,
curious, have you tried different adjustments of the Viking Spring?
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No, not really! I adjusted when first completed the plane so that the airplane set level. It seems any adjustment away from that would either raise or lower the nose and not sure I wish to do that. I do only have 15 flights so perhaps it will loosen as it wears in a little. Also, I have not lubricated the plastic pad that the cross bar sets on since installation a few years ago. I will probably try some grease. Looks like John's has plenty of some white lubricant.
I actually like it the way it is since I don't have rudder trim. Of course the ball may only be off a little due to the rudder being a little stiff and slip stream not completely centering on its on.
It does feel very good on the ground and big difference between ground and airborne is weight being off the plastic pad with weight on nose gear.
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