Bigger wheels? Heavier axles?

[Bill]

In another thread, Denny_A wrote:

I'm not familiar with newer models with 15" wheels and higher axle ratings, but I believe some models still have 3500 lb. limited axles

I have long been a proponent of a stronger axle on the 2619 and 2720-series TMs, and I promised to get one on my new 2720SL.
Turns out I was wrong about that! Greg, an engineer at the TM plant, convinced me of the error of my ways.

The weight of a fully loaded 2619 or 2720 TM is not far short of 3500 pounds, depending of course on the model and the way you pack it. The axle is rated 3500 pounds. "So why not go to a 5000 pound axle, and gain some margin?" I asked myself.

Bad idea. Why? With a 3000-3300 pound trailer on a 3500-pound axle, the ride is "cushy", as it was designed to be. Put the same trailer on a 5000 pound axle, and the ride is extremely harsh because the suspension is almost rigid - it isn't loaded anywhere near its design point. And with a harsh rigid ride, things shake apart in a hurry. Most of us have experienced a few screws that shake out of place from road vibration, or a stove grate on the floor, or a Fantastic-fan vent cover that pops open a bit under vibration. It would be much worse with a trailer with no suspension at all (unsprung). This is basically what you would get with a 5000-pound axle loaded to only 3000 pounds. So I am convinced. I will stay with a 3500-pound axle.

With regard to larger tires, I am less convinced. The 2619 and 2720 TMs come with Goodyear Marathon ST215/75R14 14" tires, which are rated 1870 pounds at 50 psi. For 2 tires, the total is 3740 pounds, which is more than the axle rating. So it's a good combination, right? Well, yes and no. The problem I see is that the rating of the 14" tires is critically dependent on proper inflation pressure. The tires may be rated 1870 pounds at 50 psi, but at 45 psi they drop to 1790 pounds, and at 40 psi, only 1660 pounds.

The next size up in the Marathon line is the ST225/75R15 load range C. It is rated for 2150 pounds at 50 psi, and 1880 pounds at 40 psi. To me this would be a better choice. I think this is the tire used on the 3023 and 3124s.

(Just to complete the discussion, there is also a load range D tire in the same physical size, rated a whopping 2540 pounds at 65 psi. You wouldn't want to run it at 65 psi, though, because it is hard as a rock, and would shake the trailer just like a hard axle does.)

So why not a 15" tire, to provide a bit more margin and a lot less sensitivity to exact inflation pressure. Greg at TM - if you are there, I'd like to hear from you. And from any member of this board who has ideas or knowledge - or best of all, EXPERIENCE!

All numbers taken from the Goodyear web site. There was along thread on this topic a couple years back, but I don't recall that we reached any conclusions.

Bill