Zenith Aircraft Builders and Flyers
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Query for the group: Zenith Rubber Donut nose gear suspension
Query directed to all who have the system installed on their airplane....
I had a chance to do some taxing and flying today after having recently installed the system on my STOL CH750 . The system is nice and smooth on the ground and notably quieter (as in silent) than the steel spring bungee I had. Inflight was like having power steering on the rudder pedals, smooth and very little pressure for any required rudder deflections. HOWEVER...during ground taxi, I get into a mainly continuous "bob" up and down at any taxi speed faster than a slow crawl if I am on anything but smooth pavement. Any bump self initiates the event, which does not increase in amplitude or frequency, but is surely annoying until I step on the brake and slow down to a crawl. My only thought is that there is to much stored energy in the compressed state of the rubber donuts. My spacing on initial setup resulted in about 1/4" distance between the lower bearing and the steering tubes - seems within suggested guidelines. I am considering reducing the compression a bit and seeing what happens with that effort. In the meanwhile I'm soliciting ideas or thoughts from the forum please. Thanks so much for reading and any response.
Cheers
Dave & Sky Turtle
Replies to This Discussion
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Permalink Reply by John Austin on
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I've not noticed any of this "bob" at all with my donut suspension. Actually, I think you need to consider increasing the compression/pre-load rather than reducing it. The way the system works is that the donuts are progressively "squished" (sorry for the technical terms! Ha!) as the gear compresses. This results in the donuts' height decreasing and spreading slightly laterally, decreasing the i.d. of the donut and progressively gripping the nose gear strut tighter with the resultant gradual slowing or resisting the compression. Then, once max travel for that event has occurred, the stored energy tends to expand the donut's height and increase the i.d., progressively releasing the "grip" on the nose gear strut. As I've said before, to me the effect results in the suspension "feel" you get with a hydraulic shock - smooth and compliant but not bouncy.
I'm not an engineer, but it would seem reducing the compression/preload would let the stack "bounce" more before the resisting action of the donuts begins. After several flight cycles, my steering arms were closer to a 1/2" above the lower bearing block. I'd try adding another 1/4" of preload to your present setup and see if that eliminates the "bob."
John
N750A
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Permalink Reply by David J. Beaulieu on
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Thanks John, I've got a few hours today to see if I can resolve the "bob",,,will post results.
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O.K. managed to get up to the hangar today and compress the donuts approx another 8mm. Didn't crank and do a taxi check but the front end seems a bit less bouncy as I load and unload weight on the nose gear. Tomorrow is suppose to be a good flying day and will have a chance to evaluate then.
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John, the extra compression seems to have done the trick. Thanks for taking the time to reply and help. I made a reference mark on one of the the side gussets for future use when I have to decompress the donuts for greasing or other maintenance. The extra bit of compression made the "bob" go away, slightly increased rudder pressure for ground steering, and had no adverse effect on flight rudder pressure requirements. Sky Turtle and I are smiling!
Cheers!
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Permalink Reply by Brian Tollefson on
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Dave,
You seem to be about 1 step ahead of me on all the upgrades i'm considering on my zenith. I upgraded to the steel spring a few months back and it works great other that the noise. Is it worth switching over to the donut system? Sounds like that will get rid of the noise. Will it be durable...how long do you expect the rubber donuts to last?
thanks,
bt
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Brian, I wish I could give you something more definitive...I have no clue how durable the donuts are. I strong suspect they will be good for the life of the airplane nose gear. To me, it was worth the switch just to get rid of the noise associated with the steel bungee. It also seems to be smoother overall with any taxi or landing events.
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Brian, I wish I could give you something more definitive...I have no clue how durable the donuts are. I strong suspect they will be good for the life of the airplane nose gear.
Purely speculation on my part, but I think it's likely you're correct! The donut system has been used for decades on Mooneys and those are much more heavily loaded than the Zenith system. It's not uncommon for Mooney flyers to finally get around to replacing the donuts after 20 years or more! It seems the prime factor in the donuts losing compression is the amount of time they're under compression and how much weight is on them - also, extremes in temp are not a good thing, either. The good news is that a "failure" is not anything catastrophic, they just gradually get harder and firmer. Obviously, this is somewhat of an "apples and oranges" comparison as it is not the same donuts, but it's all we have!
After a reasonable number of break-in cycles, it might be interesting to measure the stack height or make a reference mark as Dave did and follow this annually - with my very light Jab 3300 on the nose and the moderate temps of my region, I'm betting there won't be much change during the rest of my flying years. Fingers crossed!
John
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