I’m not quite satisfied with the bungee struts. I want to sleeve the lower strut where the bungees attach. I’ll try to get some 7/8 tomorrow, and redo them. I was thinking about how much pull there is going to be on those nubs, and how it would give, if it did. I think what would happen is that it would kink the lower bungee strut tube, pinching it tight onto the inner guide rod. Putting a doubling sleeve on it would make it a lot stronger and less likely that could ever happen.
Just about got my bungee struts done. I got half the stop plates ground to shape, then had to come in and warm up. It currently has 8” of vertical wheel travel, with the slightest bit of positive camber, and 3.5” of strut extension. I’m tempted to give it just a little more camber. If I increase the camber slightly, and flip the strut bolts, I’d probably see 10” of travel. The bungees I bought are the 1080 HD CW, rated at 900 lb. I’m going to try them on and see if it has any spring. The regular 1080, rated at 750 lb. is probably more appropriate. As I understand it, the rating is the straight pull strength, so doubling it over, like on a Cub, or as I plan to do, would double it. So, 3,600 pounds per side, with two bungees doubled over. Because of the mechanical advantage of the suspension, that would equal 3,150 lb. (7,200 x .4375). At 3 G’s, my 850 gross plane would push down 2,550. With the regular 1080 bungees, the theoretical pull is 2,650. That sounds like the right number. 1080 CW. I used 3/4 and 5/8 .058 for my bungee struts. I welded in 1/2” (3/8” i.d.) tubing for the bushings. The short end, I plug welded the 5/8 inside the 3/4, so the seam would be smooth where the pieces meet, not welded. The inner extends 10” inside the lower, leaving over 6” inside to hold the angle and guide it back in straight, when it’s fully extended. I might weld a plug in and make a rubber stop, so it doesn’t slam steel to steel when it’s pulled to the stop.
Sorry it didn’t work out right the first time with your gear, Buckchop. It sounds like Randy is a good guy, so I’m sure he’ll get it squared away. 3/4” ride-in with an empty plane seems like the springs are light. With 5” of spring, and geometry similar to mine, you’ll have about 10” of upward wheel travel. I was figuring on 6”. I think the bungees safely stretch to double their length, so I could probably have more. I’d say there would be bigger problems at play if I’m landing where I need more than 6” of suspension travel. Motocross bikes have 10”-12”. I was waiting to see your post when you got it going, then I saw the J3 gear for sale and decided to give it a shot, myself. I’ve read most, if not all, of the the posts about wide gear. Springs will be cheaper than bungees. Four new bungees ran me $200. Ow. That was half as much as the 4130 steel for the whole gear assembly, with lots left over. I don’t think there’s much difference between a spring bottoming out versus the bungee suspension bottoming out against a limit cable or shock. There just needs to be enough resistance, and travel, in the first place. And try not to drop the plane like a bag of bricks. I like the wider A-arms that I have. They give the plane more leverage to not get twisted up. My biggest concern with the whole project is the weight. I know it will be a bit heavier, at least a few pounds. I’d like to get a pair of smaller, lighter, more manageable tires.
It took me all day to finish one A arm, after I already had the leg amputated and the new one tacked in place. There was a lot of on and off to check the fit, and I spent a lot of time scratching away at the rear leg to make clearance for the lift strut bolt. I finally cut out a 7/8” wide rectangle, ground it down a ways, and welded in a piece of .050”. I’ll strap the back of it, too, to reinforce that weak point. It’s right where it goes from the sleeved part, to the thinner walled, smaller diameter, original leg. These original J3 legs were 1-1/4” front and 1-1/8” rear. The back was .049” thickness, and I couldn’t check the front. Both had been repaired, with an inner sleeve in the front, and an outer on the rear. I made my new hind leg with .058” 1-1/4. I’m trying to keep it light, but I’m still leaning toward over-building it. If starting from scratch, I’d use the lighter stuff. .058” fits snugly over the next 1/8” size down. I used 3/4” .049” for the cross bar at the top of the A-arm. The V is .058”. The other leg should go quicker.
Do you have a picture of how the shock is mounted on your suspension? I’ve been considering different ways of mounting a shock. I could go without it, but I’d like the damper if I can find a reasonable way to do it.
I’m considering using small shock absorbers. It would be nice to have some damping. My shock struts are 30-3/4 center to center, which makes some Cub setups a bolt on affair. I saw one pair of used ones for $1,700, which is more than my entire suspension. There’s a Super Cub gear, complete with dampers and bungees, for $800 on Craigslist. I’ve been looking at mountain bike and mini bike shocks. I could make that work. I need 3” of shock travel to allow 6” of vertical travel of the wheels.
Nothing to it, right? My plan is to put a couple gussets on the cabane V attach brackets, then I’ll be done with that. I stuck the bushings in the shock struts and used them to locate the gear temporarily. I’ll cut them apart and make the bungee mounts later. I got the left hind quarter tacked in place. That was fairly straight forward. The old J3 legs I got, had 1-1/8 rear legs that had been spliced with 1-1/4. I cut it off at the weld and sleeved it again with 1-1/4, cheating the angle to make it slightly wider to line up with the Kitfox mounts. I could have fit the cabane V mount inside the landing gear mount bracket, but the bolt has better shear strength if the gear is tight to the brackets, so I put the cabane mount on the outside, and I’ll lengthen the bushings to fit the mounts. The hardest part is taking off the mill slag without a sand blaster. I have over an hour at the bench grinder, taking it off with the wire wheel.
Making some progress on the gear. I’ve spent much more time looking at it and researching than I have actually working on it. Another hour in the morning and I’ll be done with the V. Next, on to the shock struts, then I’ll perform the surgery on the back legs. I’ll have to be a little creative to make it clear the lift strut bolt, but I can make it work. A few observations. The cabane gear is, by function, a falling rate suspension. That is, the spring will affect the pull of the gear less as it travels farther upward. This is because the gear legs will move up more than they move out, the more they travel upward, from 45 to 90 degrees, but the shock struts are counteracting the outward movement. At less than 45 degrees, the tires will move outwards more more than upwards. Lowering the center of the V will help counteract that falling rate effect. I decided to use bungees for my gear. I went with the two bungees of the lighter J3 Cub, which might still be on the stiff side for this, but they’re readily available. For the cost of buying the gear, I got all the materials to build it, the J3 legs with 1-1/4” axles, plus a set of Goodyear 26x11-4 Airwheels on the old 4” wheels with expander brakes, and a few sets of McCauley 6” wheels and brakes. The guy I got the Airwheels from, pointed out that the little wheels and drum brakes are actually lighter than the Cleveland wheels and brakes that many people have switched to. They do look good on it, too.
The attached article documents the changes needed to qualify a J3 Cub for 1220 gross weight. It has a lot of information on the landing gear that is useful for Kitfox landing gear conversions to cabane style gear.
I can make that work, God willing. It would gain about four inches in height, and eighteen inches in track width. About nine inches out on each side. I’ve been toying with the idea of using the rear-most attach bracket at the lift strut attach point to hinge the hind leg. It almost reaches it now, without modification. Having a longer stance makes it less likely to twist. I might have to beef up the structure back there, but it should be pretty decent, since the lift strut would put a similar amount of strain on it. Don’t mock my mock-up. I had to work around the tire, you know. I’m not making it knock-kneed, or moving the gear back a foot.
Looks like I got my work cut out for me. I drew up a cabane gear and picked up a pile of 4130 and a pair of J3 Cub legs. Kind of figured I’d cannibalize the old gear, seeing it was a proven design with 1-1/4” axles all ready to go. With some fairly minor modifications to the legs, and my own cabane V and shock struts, I should have a decent set of wide gear that won’t fold up on me. The gear legs weren’t too pricey, at $150. I bought enough 1-1/4” to fab my own legs, if I need to go that route. Fun times ahead.
I have an original, right wing, six gallon aluminum tank that has a minor leak. It was repaired, but looks like it was cold lapped... I wouldn’t recommend its use as a fuel tank, but I like the wing storage idea. If someone wants it for that, it’s available. I have two big tanks to install, so I won’t be using it for anything.