Why the second axle? Do you really think it is necessary or just better?
Not completely necessary, but that depends on how the trailer is loaded. With no motor, empty, in the conguration wherin it can be towed by the legendary Ford Taurus, the trailer capacity is probably adequate. I've determined that with the 50HP motor, two batteries, some water, some fuel and all the other stuff I normally carry in my boat, the trailer is always overloaded by somehwere over 200 to as much as 800 lbs. I towed my boat to Florida and back (3000 miles total) for two weeks on the water. With the single axle in the heat of the summer, with empty tanks and as much extra stuff as I could fit in the tow vehicle to avoid overloading the trailer. At highway speeds of 65-70 mph, the bias tires got alarmingly hot, and one of them with less than 1000 miles on it lost a big chunk of the tread. No idea how, only know that it happened. Plus, the trailer behavior in crosswinds, when being passed by large trucks, etcetera was unacceptable to me.
My solution was to add a second axle and four wheel disc brakes. Since then I have towed my trailer to Florida and back two additional times, and you simply cannot believe the improvement until you've done it both ways.
Others have simply upgraded disc brakes, or upgraded to larger wheels and tires (that doesn't answer the overloaded axle question); a few have changed out for a higher capacity axle with wheels and tires. Any of these provides an added margin from the stock trailer configuration. Everybody has their own comfort lever for these things, and for those who have done them it may be enough. I consider them all indequate to the way I use my trailer. After towing with a Ford Explorer for over two years, when it came time to replace the Explorer I upgraded to a Ford Expedition for the same reason. Like the stock trailer, the Explorer worked, just not very well.
I have two "bad" things to say about the second axle. 1) There is no doubt that when making sharp turns, forward or reverse, the tires scrub and the trailer suspension loads up heavily. Knowing that the tires and suspension are loaded to just over half their rated capacity, I'll live with it. 2) In our country full of toll roads, it costs me more to travel the distances. I estimate maybe $40 extra each way to Florida.
I am a former tyre man and I beg to differ with your point on mixing tyres. They will track differently and you may notice the trailer preferring to wander to one side or the other when motoring.
I'm not mixing tires side to side. I have two radials on the rear axle, two bias on the front axle. The tires may behave differently on the different axles, in fact, I'm quite sure they do; at speed the bias tires still get a lot hotter than the radials. But on a trailer, even if the axles track differently, it's transparent to the behavior of the trailer. The trailer tracks straight as an arrow under all conditions. If/when I lose one of my bias tires, I'll replace both with radials. The spare will still be a bias; and I'll consider it for emergency use only.
I've done considerable research and I know all about tire construction, sizes, load and ply ratings and the different capacity rating systems, etcetera. There is in fact considerable disagreement among the "experts" concerning stiffer versus more flexible sidewalls for trailer tires, so I won't argue it here.
Light truck tires are not the way to go for this trailer. The highest capacity tire of any type you can buy for a 14" rim is the 215-75D14 ST (special trailer) which came standard on the Mac trailer for this year; 1870 lbs capacity each. There are no 14" LT tires rated for the same capacity. The only way to get adequate or better tire capacity for this trailer from any other tire, of any type or construction, is to upgrade to 15" rims. If your trailer came originally equipped with the 205-14 tires (earlier 26X trailers had these, 1760 pounds each) you can get a modest tire capacity increase by upgrading to the 215s.
One warning about the 215s: In the U.S. at least, you absolutely cannot find one at a tire store. BTDT. They will order one form a warehouse which can be in from 1-3 days, but that doesn't help at all when you're on the road trying to get somewhere. If you have these tires and no spare, you're asking for trouble.