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Dual Bungee Nose Gear Suspension
A popular, recurrent topic in the Forums centers around the Zenith nose gear bungee suspension design. As I always say, Chris Heintz is the master of simplicity and lightness and you can't get much lighter than his bungee system! As with all designs, there are some inherent compromises:
- The bungee is a single-point failure system.
- Bungees seem to be unpredictable as to durability and failure, and deteriorate more rapidly with exposure to heat and oil.
- Bungee replacement requires disassembly/removal of the nose gear.
- The single bungee imparts some torque resistance to the nose gear turning.
A popular alternative is the "steel bungee," which does not require periodic replacement, is unlikely to break, and induces no additional resistance to steering inputs. The only disadvantages are weight (I've seen 3.5 lbs quoted?) and expense.
An alternative bungee design may seem strange coming from me, since I have the Zenith single bungee system and it works perfectly! My rudder is silky-smooth and I and other pilots I've asked to fly it cannot detect the centering notch in the bearing support. However, I have had one bungee break (it was a non-event - had 9" prop clearance after breaking) and since some do have bigger problems, I was determined to see if a new design could offer some improvements while keeping weight and costs down.
Here's a sketch I came up with:
I've already thought of some improvements to this design:
- Make the nylon bearing a slip-fit on the nose gear leg, but over-size the steel rotating collar's hole so it does not touch the nose gear leg (the steel rotating collar will be shaped similar to a 2-bolt exhaust manifold flange). The nylon could be flush riveted to the collar or even simpler, be shaped the same as the steel collar and utilize the same bolt holes as the steel collar to secure them together in alignment.
- Rather that suspend the bungees from "hooks," which might concentrate too much pressure/too sharp a bend on the bungee, weld up a tube with a tang on the end (such as is currently used on the back of the nose gear leg) to hook the upper end of the bungee on. The tube could be welded to a flat metal strap with a bolt welded to the top of the strap to allow the bolt to pass through the rotating collar and allow removal and adjustable pre-load:
While still using bungees, there are some advantages to this design:
- Minimal weight increase, minimal cost increase.
- No single point failure. If one bungee breaks, half the support is still there vs. none!
- Allows easy bungee removal/replacement without removal of the nose gear, encouraging routine periodic replacement
- Tensioning and pre-load are accomplished with a nut on the bungee hook bolt (and locked-in with a jam nut). Negates the need for special tools to stretch and hook the bungee in place and also doesn't require a lot of room between the engine and firewall to accomplish this.
- I talked to SBC Industries about the design and was assured the two smaller bungees (which, for the same amount of suspension travel, will stretch roughly proportionally the same as the single bungee) would perform similarly to the single bungee. SBC said my design "was much better" since I could adjust the pre-load and tweak the suspension as necessary! He also liked the fact that the bungees are working in a straight line and there are no torsional loads! He also said the smaller #9044 bungees are conservatively rated and 2 would be perfect substitutes for the single #1280HD.
Some unresolved questions/problems
- I think there is only about 1" clearance on each side of the nose gear leg - this would mean the "hook" for the bungee would have to be kept to minimal dimensions to avoid interference with the support alongside the nose gear leg. Thankfully, the smaller bungees are also smaller in diameter, which helps. Also, I'm assuming the two bungees would more-or-less keep the rotating collar aligned - slight fore and aft movements would be OK, but one would have to ensure there is no possibility of jamming or locking the nose gear leg if one bungee breaks - probably unlikely with the nylon bearing preventing the steel collar from touching the nose gear leg.
- I show the bearing support welded immediately above the present bungee hook tubes. Likely one would have to put the two bungees in a test rig with a scale and stretch them to determine the pre-load length and to establish the bearing support placement. This same rig could also be used to determine the load at maximum suspension travel to ensure the test ratings are not exceeded
This is merely an experimental and speculative design at this point. I'm thinking this might be an interesting winter project. I'd be interested in comments, constructive criticism and suggestions as I refine the design!
Fire away!
John
N750A
Replies to This Discussion
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Permalink Reply by Timothy D. McCall on
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John
I like this idea.
Not seeing much of a downside here.
Tim
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Permalink Reply by John Austin on
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The toughest thing is designing a "hook" with a threaded rod for the top of the bungee. I went over to the hangar yesterday and measured the width of the nose gear support that the nose gear leg runs vertically in. It is 3.90" wide and the gear leg is 2" diameter. That only leaves 0.95" of clearance on each side of the gear leg and there are a few rivet tails protruding that might be close. (I suppose those could be drilled out and the tails placed on the outside or even better, flush riveted on the inner face of the support). It is complex in that the flanged face of the support is running at a slight angle to the nose gear leg so the clearance varies. The absolute minimum clearance is 0.75" between the possibly intruding rivet tails and the leg. So, all that is to say the "hook" needs to be as narrow as possible (the bungee is 9/16" diam) to avoid clearance issues. I think I can get it down to 0.75" width which would work and offending rivets could be reversed or flush riveted.
I think it is do-able, but it is going to be tight!
John
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Permalink Reply by Chris Hoare on
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Looks good John will keep an eye out for this very interested as do not like the single bungee and was looking at the steel one before first flight
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Permalink Reply by Jon Croke on
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Nice, John!
Now that you have a straight shot between two points - how about replacing the bungee with a permanent, metal coil spring on each side? It would hook onto the pin at the bottom and hooks at the top. Super inexpensive, available in any tension, easy to unhook.. and no need for replacement (heat/age wont bother).
Just the first thought that popped into my head when I saw your diagram.
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It was much easier for me to relate to bungees, especially after talking to the manufacturer and being reassured I was in the ball park comparing the two smaller bungees to the larger single one. ( I had already picked the #9044 and the manufacturer independently recommended the same one!) I have no way to prove it, but I have a gut feeling that the two bungees will probably last much longer than a single bungee since they would "share" the stresses and there is almost no torsional or twisting forces on the loops. At worst, it could turn out that it cost twice as much (two bungees instead of one) but last more than twice as long? (Yeah, I know - wishful thinking! Ha!) However, like I said, loss of one bungee shouldn't be as dramatic and it will be far easier to replace them!
One of my first ideas was to use steel springs and as you say, it would be inexpensive and permanent. However, I started looking at readily-available spring specs and got the impression that I probably couldn't get one in a small enough diameter to fit in the space between the gear leg and the support. However, I may be wrong about that because quite simply, I don't know much about springs!
If someone who knows something about springs can come up with one that is perhaps 4-6 " long and will allow about 3" travel and support a resting load of approximately 100+ lbs per spring and do all that with a diameter of approximately .75 inches, then that would be ideal.
John
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Thanks for the link, Ken! I looked through all their stock springs and several other vendor's, too. I couldn't find any extension springs anywhere near the load requirements and extension distance required for the nose gear suspension and keep the o.d. less than an inch and the overall length 6" or so. I guess that's why the "steel bungee" has a single, large diameter compression spring!
While the bungees have their drawbacks in that they aren't permanent and require occasional replacement, they do have excellent suspension characteristics for such a compact package. I think the very minimal up-front costs - a few dollars of steel, nylon, and buying 2 bungees instead of 1 - coupled with ease of replacement, and "possibly" much longer life (just a hopeful guess! Ha!) would make the design a significant improvement over the present design. To be fair, welding costs could vary from free if you DIY to several $ if you hire it out (I'd have to - I can't weld very well!).
John
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