Firewall Forward

November 23, 2012

Setting the Nose Wheel's Breakout Force - 3.0 Hours

I have just one more thing to do with this nose wheel assembly before putting the airplane on her gear. I never did take care of this, back when I was putting it all together. The plans call for tightening the big nut under the wheel, which compresses a pair of concave washers. As this happens, it increases the amount of force required to move the nose wheel from side to side. Up until now, mine has been very loose and moves freely. You don't want it installed this way. The proper amount of force is required to keep the nose wheel from being too squirrelly and unstable, leading to wheel shimmy. So I wanted to get this set, and the hole drilled through the gear leg for the cotter pin. I suppose it could be done after mounting it to the airplane, but this just seems better to me.

So I got busy and rigged up a way to hold the motor mount with the gear leg in place, so I could do this task. Here's what I came up with.

I put the motor mount/gear leg assembly on my work bench, with the gear leg hanging down over the side. This was the perfect position to work with. I temporarily screwed some wood blocks to the work bench surface, and then I ran some long wood screws through the motor mount, with a scrap piece of 1/8" aluminum in place to hold the motor mount securely in place to the workbench.

I used my yellow ratchet-strap to hold the motor mount down on the back side, so when I put pressure on the nose wheel, it wouldn't topple forward.

Next, I unbolted the wheel and took it off. I didn't want to have to do this, but the plans call for pulling from the point where the axle bolt goes through the nose wheel fork, so I didn't see that I had a choice. If you look carefully, I made a simple wire hook out of a piece of coathanger, and put it in the hole where the axle bolt goes. This is all detailed in the drawing. This is to pull on and measure the breakout force. Sitting on the floor in the picture below, you can see a digital fishing scale that I borrowed from my son Luke, and the special nut wrench that I made a while back. You get this nut wrench as a kit from Vans and rivet it together yourself.

Here you can see how it works. You tighten up the big nut and compress the washers, then hook up the fish scale and pull, watching to see how much force is required to cause the wheel fork to "break out" and move.

I spent a lot of time on this, gradually tightening up the big nut and measuring. This is far from an exact science. It's very hard to pull consistently and smooth enough to get a precise reading. The gauge is going all over the place. Nevertheless, I pulled and tightened, measured it one way and then pulled it the other way. It's supposed to take 22 lbs. of force to move the fork. That seems like a lot, but I read lots of comments online. If it's not tight enough, you can get shimmy on the nose wheel. and potentially some other problems as well. The 22 lbs. seemed like a lot, but a good number of people recommended even more. When I finally had it about as good as I think I'm going to be able to get it, I drilled the hole through the gear leg for the cotter pin that will hold it in place. I did a lot of reading about this, and a lot of RV owners find that they need to adjust this after you start flying, or make further adjustments down the road a while after your plane is new and things break in. If that's the case, I'll be able to pull the cotter pin, rotate the big nut a little at a time to a new surface, and then put a new cotter piin back in place. So I put the wheel back on. This is now ready to go on the plane!

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