The Viking Steel Bungee was developed from customer requests:
"I have a flying CH-701 at present and was always wondering how to improve the bungee on the nose wheel strut. I have added a collar to the strut to keep it from total collapse but am leery of the keeping the prop out of the dirt if the bungee "shits the bed". I noticed your picture on the Zenith Builders site and backed through all the available pictures plus the ones you show on your site. I am curious about more detailed pictures showing how your system works - and what modifications are necessary to install your system. Right now the bungee holds the nose strut down against a aluminum "L" bracket which causes friction in the rudder system. Also torquing the bungee as the rudder move adds to the system stiffness - ala' the 701 rudder that doesn't want to center. I couldn't tell from your pictures how your system works thus my email. Thanks in advance for you response."
Very good questions and you show a clear understanding of the limitations of the original system.
Basically, the steel bungee works in reverse of the original rubber bungee, yet the suspension load is transferred to the original cross tube. The Preloading of the spring is handled internal to the spring, using an adjustable rod internal to the spring, providing no force to the airframe while in flight. The ramp effect of the original plastic material has been milled away and the steering arms now ride on a flat surface.
This is the bottom spring saddle. It supports the "Steel Bungee" spring by resting on the internal cross tube of the nose wheel strut. This is the same tube previously used to support the rubber bungee externally. A rod run from the center of this support, (bottom spring support saddle) through the spring and through the upper support saddle. The rod has a threaded section allowing for spring preload to be adjusted. The preload is about 1" in a CH-750 for a firm, yet nice ride. By introducing the preload directly to the spring, no force is transmitted to the bottom plastic ramp blocks of the original system, leaving the rudder pedals free to move.
Two 4130 steel side straps with welded on threaded studs are used to transfer the entire suspension load back down to the original load bearing cross tube. These are also adjustable and are used to remove up/down play in the strut, after spring preload has been set. This allow for no play and also no load against the air-frame, other than during ground operations.
This is the top spring saddle. The center hole is for spring preload adjustment. The two outer holes support the weight of the airplane nose through the 4130 side straps.
This is an optional step:
Mill away as shown on the plastic block if you would like less friction during flight.
Start by removing plastic cap on top of the nose wheel strut. (On the 701, carefully drill out the top cap with a 1 3/4 hole saw and using a burr, remove any remaining edge to prevent the spring from catching)
Use the top spring saddle to locate the position of 2 x 21/64 (9mm) holes. Use a 1 3/4 hole saw with some layers of tape to make a pilot for centering while marking. Drill holes. (CH-701 use holes closer together and top saddle will have 2 sets of holes. Outer holes will be for everything but the 701)
You will be able to install the bottom spring saddle and it's rod as shown if the sheet metal overhanging the top of the firewall is not in the way. Our prototype airplane has this trimmed back slightly. If not possible, drill a small hole for the rod to go up through during installation, then the cowl will cover it later.
Coil up the spring liner and install.
Install the top polymer washer into the top saddle
Drill a couple of holes about 1/2" (13mm) inboard of the existing holes for cotter pins that will prevent inboard movement of the side straps, once installed.
Install side straps in between vertical sheet metal firewall ribs (nose wheel enclosure) and through the drilled holes. Orientation is such that threaded sections are parallel with the firewall after installation.
Install a couple of washers and the coupling nuts. Tighten equally left and right until about 1/4 (6mm) left of thread. Install safety wire to prevent nut from backing up. Be sure to feed the center rod into the top saddle as you tighten the side straps. Use additional washers if you later want the suspension tighter.
It is not necessary to put a nut on the center rod. But if you add the felt washer / large area washer and tighten nut just until the side straps START to get loose, this is the point where the load has been removed from the bottom plastic saddle. It makes it easier to turn the nose wheel.
So, to adjust anything:
- Bring the nose wheel off the ground
- First set the tightness of the suspension with the side nuts.
- Then tighten the center rod just enough to feel the load being removed from the side straps
To re-adjust (Start over):
- Remove center nut
- Set the tension you want on the spring by adding or subtracting washers / adjusting side nuts. Bounce suspension until you like the feel.
- Install center nut and tighten until side straps start to loosen. (Nose wheel off the ground}
We will post more info as we get feedback from customers so please write and send pictures
The system is now so popular Viking is stocking 25 sets at any given time