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

[fcatwo]

We've never had a flat tire to deal with but I have the wheels pulled and the bearings professionally packed each year. The stock tires can barely be removed without opening the TM and I doubt larger tires would clear unless the top is opened. We have the 2" lift. Not a big deal and offered for info only. YMMV

[YWORRYDOG]

That is good news, that you can get the wheel off with the TM closed!!!!
I never tried it that way, as the owners book, says to open it. I cringed thinking of an emergancy on the side of the road way off level and having to open it.

The 14" wheels will clear (with some wiggling) as I had two blowouts one camping trip with the Carlyle tires and had them both replaced without opening the camper. If I'm not mistaken, the service person let some air out on one of the replacements to make it easier, since he had a compressor on his truck to refill afterwards.

I wonder if buying the 5000 axle and using the 15 inch tires at less pressure would be both safe and have a cushioning factor. It would seem that on those trips that you had to travel with full tanks, extra propane, and batteries that you could then bring the air pressure back up and the extra weight would weigh down the heavier axle safely.

[Bill]

Originally Posted by Hondo

I wonder if buying the 5000 axle and using the 15 inch tires at less pressure would be both safe and have a cushioning factor.

Hondo -

I am far from an expert, but I don't think so. At 50 psi, the 15-inch tires would be no softer (and perhaps a bit harder?) than the 14-inch tires at 50 psi. Add to that the harsh stiffness of a 5000-pound axle, and it would still shake everything pretty badly.

Bill

Bill, you are a wise man. Your comments about the heavier axle leading to a rougher ride are mostly correct, but I still think it may be a good idea.

In July of 2005, DB (Dear Boyfriend) and I ordered a 2720SD with the heavier axle and 15" tires. And two lift kits. We were looking for both extra clearance and a tougher trailer that could handle a bit more weight and abuse. We live and camp in the west, and roads are rough out here. We picked up the trailer and immediately took it on a 16-day trip. I have to say, we loved the extra ground clearance and at least four times we were forced into off-road situations due to road construction, an accident, and other things where we had to turn around almost in the ditch. If we had not had that extra ground clearance, we would have snapped off the sewer pipes. So I am in favor of the heavier axle, but here's my problem/question:

When we picked up the trailer, the sticker on the side said to inflate the tires to 50 psi, and that's where they were when we got it from the dealership and they stayed that way for our maiden voyage and two other trips. We had spent 32 nights in the rig and logged hundreds of miles on it and were very pleased. We had a few loose screws and issues with things getting bounced around, but nothing major.

Then I got to thinking (always a bad idea) and realized that the sticker on the side of our rig said we had 14" rims and we don't. So I contacted the factory and told them that my sticker was incorrect because I had a different axle and tire, I knew the max pressure for the tire was 65 psi, but I wanted to know the recommended pressure given that I had a 2720SD. They did not answer the question, other than sending me a new sticker that said 65 psi, so perhaps they did not pay attention to my unique situation. Anyway, I then assumed that I was supposed to inflate the tires to 65 psi like the sticker said.

Last week we inflated the tires "properly" and holy smoke - when we opened up the trailer after our first dirt-road experience there were screws all over the floor, the oven door had come apart, and trim pieces had come off the cabinets. The roads that we had traveled were no worse than others we had taken in the past, so I conclude that the heavier axle in itself makes for a somewhat rougher ride, but it is the harder tires that really shake the **** out of your rig.

Now I need some advice. I'd like to bring the tires back down to 50 psi like they were before. Do you think this is safe? The tire is a ST225/75R15 and from what Bill said in his post, I could go really low and still be rated for the weight of the rig. But is there more to it than that?

If I could stay with softer tires I would remain very pleased with my decision to get the heavier axle because quite frankly, I don't see how we could operate without that extra clearance. We stay on the pavement or well-graded gravel almost all of the time, but there are always brief moments when we are in a situation where we need some clearance, and I'm sure glad that we have it. I can't remember how much extra clearance we obtained by going with the heavier axle and larger tires - we could have used three lift kits and the standard axle, perhaps, but it's too late for that now. It's also nice not to have to worry about exceeding the weight limit.

On another note, this is my first post, but I have been lurking here for a year and found this forum to be incredibly helpful. Thanks to all of you for your thorough information. We love our TrailManor and plan to attend the Jubilee this summer.

I'm not a tire loading expert, but I am an engineer, and having read Bill's excellent treatise at the beginning of this thread and consulted the Goodyear website I think you will be just fine reducing pressure to 50psi as long as your TM's weight is close to "average" and for the following reasons:

> Setting tire pressure for maximum trailer load I believe means setting the pressure based on what the maximum load of the trailer you're towing will be, as opposed to the maximum trailer load that the tire will take (and therefore the maximum pressure for the tire).
> The consensus of this excellent forum seems to be that 3500 lbs. is a good estimate to use for a 2720SL or SD loaded with typical options and for a camping trip. It is also the consensus that 50psi is the correct pressure for this trailer on standard 14" tires.
> This falls in line with the tire loading tables from the goodyear website. From the tables, 50psi is the tire pressure for ST215/75R14 tires with a trailer load of 3740lbs (as Bill also states), which leaves some margin for the case where some pressure is lost before the next top up with air. From the same tables the maximum load at 50psi for your ST215/75R15 tires is 4300 lbs, so you have even more margin before the tires are overloaded.

From the above logic, you might even get away with 40psi (3760lbs trailer load from the tables), but if 50psi is working for you on the rough roads, then I would stick with this especially since your axle is probably heavier than standard.

So what have we learned? Probably that the upgraded axle and tires can indeed allow you to really load your TM down with stuff as long as you adjust tire pressure accordingly. But if you do, you'd better stick to the smooth roads.

Bill, if I'm off base, then please correct me.

Paul

Thanks for the advice, Paul. I especially like it because it is what I wanted to hear. When I picked up the trailer at the dealership, I assumed that the tires were at 50 psi because that's what the incorrect sticker said, but maybe they actually knew all about this issue. Or not.

Keep in mind that this is my first TM so I have nothing to compare it to when I say that our original 50 psi experience was not very rough. All I can tell you is that the contents of the trailer and most of the screws were in fine shape at the end of each trip and there were no problems.

Originally Posted by pbuck1

So what have we learned? Probably that the upgraded axle and tires can indeed allow you to really load your TM down with stuff as long as you adjust tire pressure accordingly. But if you do, you'd better stick to the smooth roads.

I'm not so sure about this conclusion, based on what Bill learned from the factory. Maybe my problem is that I am not loaded down with stuff but have the tires pumped all the way up. If I actually had enough weight on the trailer to justify the use of 65 psi tire pressure, it may not have been such a bad ride and maybe one can use it on rough roads fully loaded. Bill made it sound like the problem is a mismatch between light trailer weight and heavy-duty axle/tires.

You make a good point about my axle weighing more than those on other 2720s. One of these days we really should weigh the trailer at the beginning of a trip when we are fully loaded and also once when we are almost empty - those weights would be good pieces of information to have when deciding on a tire pressure.

Just took a trip from Denver to Houston with my 3124 KS.. 1100 mile trip one way.
Got my first flat when I entered the campground in Houston.
The tires are difficult to remove without taking off the skirting. Remove 2 square head screws and lift the skirting up.
The spare was very easy to pull out. I like the design, but found out the spare was not balanced by the factory.

The Houston and Dallas freeway traffic scares the hell out of me.. Better know where you are going --
I would rather be on a 4-wheel drive trail.

P.S. My tires look low at 50 psi.. Looks much better at 60-65 psi where they are supposed to be.
4-runner runs much better at sea level than at 7000 ft. pulling the TM. "the suprise is how much better"..