structure of rear wheel to support cargo trailer Some years ago I used a rear carrier with a bundle buggy lashed on to carry stuff around.  I had a problem with wheel wobble.  I thought a rebuild as 4 cross (instead of std 3 cross) might help.  The guy in the shop where I bought the spokes noticed a cracked rim (actually both front &amp; back).  After I rebuilt the back wheel 4 cross with a new rim, the wobble was gone.  Don't know if the 4 cross fixed it, as I don't know how long the rim was cracked.  

I am thinking of building a cargo trailer, and am wondering if anyone has any knowledge on whether rebuilding a wheel to 4 cross for the additional stability it may bring (the old bike is long gone) is worth the effort for short trips with 50-60 lb of freight.  the trailer would be attached to the rear axle, not the seat post.
I would not bother. But then, I would not bother with 4 cross in the first place. Spoke count matters more - a 24 spoke racing wheel is just not designed to be used for load carrying. What I think is important is (in order) 32 or more spokes, a properly built wheel, a decent rim, and a decent hub. You can do a lot just with a properly built wheel because if the spoken tension is nice and even even a relatively low-quality rim will not be stressed past its limits.

I've carried a lot of heavier things and built a few load bikes and trailers over the years. Especially with a trailer, 30kg is not a particularly heavy load. For the rider, loads under half your body weight should be easy for anyone who rides regularly. Your main issue is likely to be lateral forces on the rear wheel, and the way to reduce those is riding style. I would leave buying a better rear wheel until after you have problems with your current one. Largely because you probably won't have problems with it. Unless it's a low spoke count wheel, in which case I suggest getting a second hand steel frame MTB and never using your racing bike for lowing a trailer if you can avoid it.

Also, FWIW, I found towing a trailer with 30kg of stuff on it quite hard on a fixie. If you're going to do that, at least make sure you have both front and rear brakes because stopping suddenly on a fixie is hard enough as it is, adding a trailer just makes it worse.