Replacing 14" Marathons with 15" Marathons

[Bill]

As noted in the “More Marathon Failures” thread, I completed the retrofit of my 2720SL with larger tires. The OEM tires, of course, are 14” Goodyear Marathons. Thanks to Bobby at West Monroe Tire, I had a set of new ST225/75R15 15” Load Range D Marathons to try out.

So here is the story. With the wheel well covers (fender skirts) removed, it was plain to see that there was plenty of clearance forward of the tire, aft of the tire, and beside the tire near the frame. A larger tire would have no fit problems there. In addition, my TM has the factory lift kit, so there was plenty of clearance above the tire. If you don’t have a lift kit, I simply don’t know if there is enough clearance to accommodate the suspension travel as you go over bumps in the road. Perhaps others can comment. The only question was the clearance between the tire and the fender skirt. Would the slightly wider tire (only 3/8 of an inch wider) rub on the skirt? The local tire guy was worried about that, and didn’t want to do the conversion. I finally told him that we would do one tire. If it worked, we would continue – if it didn’t, I would eat it. He agreed.

The OEM wheel is white-painted steel, so I had the tire guy order 3 more of the same. The original 14-inch wheel is 6 inches wide at the tire bead (called 14x6), so I had him order 15x6 replacements. The new Marathons can be installed either on 6-inch wheels or 7-inch wheels, but bearing in mind the trouble experienced by Rocky Mtn Ray, I opted to stay with the 6-inch width. The original wheels had 5 lugs on what is known as a 4.5 inch bolt pattern, so I made sure he ordered new wheels with the same pattern. This ensured that the new wheels would install directly onto the existing hubs. This “5 on 4.5” is apparently standard, so there was no problem here. The old wheels had zero offset (defined in the discussions in the original thread). I thought about going with a slight offset, but since zero is standard, I went that way with the new wheels, too.

When the wheels came in, I examined the inside of each one (the part that will be covered by the tire). The manufacturing info is stamped into the steel. Among other things, I saw Made in China, a recent date code, and Max Load 2600 Pounds. This last is important. The TM’s OEM wheels have a load capacity of 1850 pounds or so, and it is important not to go lower. Apparently there are some lightweight wheels out there with a capacity of only 1500 pounds. Don’t get involved with these!

My cost for each wheel was $35. I had the tire guy remove the original rubber valve stems and replace them with all-metal stems. Once each tire was mounted, I had them balanced (they took a LOT of weight!). We popped the new wheel/tire onto the left side of the TM. It fit perfectly, so we did the other one.

As expected, the slightly-wider tire did come closer to the fender skirt. As the first one to try the retrofit, and the undesignated guinea pig, Rocky Mtn Ray took a pretty drastic approach to fixing this situation. I didn’t want to go that far. Over the past couple weeks, I’ve discussed different approaches to this issue with TMO member MtnGuy (Chap), and we developed a couple pretty good fixes.

After talking with Chap, I brought my TM home to study the problem. The basic problem turns out to be that the skirt isn’t flat, it is BOWED IN! If it were flat across the opening, it wouldn't rub. So WHY is it bowed in? I've made a sketch (attached) that ought to help visualizing it. (You'll have to zoom in a bit - it is a pretty busy sketch.)

The curved edge of the skirt is folded back on itself, so the skirt is thicker on the edge than it is, say, 1 inch inward from the edge. The skirt's attachment screws are about an inch inboard from the edge, so when you tighten the attachment screws, it warps the skirt. The formerly-flat part of the skirt bows inward toward the wheel. The solution is to put a spacer (a couple washers) between the back side of the skirt and the TM wall, and run the attachment screw through this spacer. The right size spacer will bring the skirt back toward flat, as shown in the sketch. But not entirely flat.

When the skirts were installed at the factory, they got warped inward, and they have now acquired a permanent set. Even if you take them completely off the TM, they don't come all the way back to flat. Ray tried to bring them back with his heat gun, but it didn't overcome the permanent set. An alternative would be to install a slightly over-thick spacer. This actually forces the skirt to bow OUTWARD just a bit. A bit of outward bowing will also improve the seal between the skirt and the lower shell of the closed TM, reducing dust infiltration.

So in conclusion, replacement of the OEM 14” Marathons with new 15” Marathons went without a hitch. Thanks to Bobby and Chap for making it happen. Now we’ll see if that is a good long-term solution to the failure problem.

Excellent post and diagram, Bill. I'm sure that will help others.

I have had this question asked, and did not know the answer. Did you have any problems putting this wider/larger tire in the spare tire rack?

[fcatwo]

Good move Bill and thanks for sharing the process. I got close to doing the same but lost my nerve in the end. Will you use the full 65psi -- and do you still sit level without adjusting your hitch?

I wonder if you'd gone with your initial thought of getting an offset rim if there would have been the problem with the skirt. Maybe then it would have hit on the inside . . . Thanks for all the info.

[Bill]

Did you have any problems putting this wider/larger tire in the spare tire rack?

I haven't even tried. I fully expect to have problems. On my TM, the spare tire rack isn't tall enough even to accept the 14" tire gracefully. In order to get the rack up on the ledge, I have to lift it with so much force (i.e., using my bottle jack or a Wonder Bar that I carry) that the rack tubes are bent. There is no way in the world that it is going to accept a tire that is even thicker. I'm going to have to modify the rack, probably as suggested by Eric Larson in http://www.trailmanorowners.com/foru...ead.php?t=5828

Will you use the full 65psi?

Probably not. Even at 50 psi (same as the 14" tires) the load rating is improved from 1870 to 2150 pounds. I may run at 55 psi (2270 pounds), but I'm afraid that at 65 psi, the tire would be so hard it would shake the TM apart.

Do you still sit level without adjusting your hitch?

I haven't checked it, but I don't think the extra half inch of trailer height will be enough of a change to get me excited. After all, the hitch adjusts in 1-inch increments.

I wonder if you'd gone with your initial thought of getting an offset rim if there would have been the problem with the skirt.

That certainly would have helped the situation, since the tire is not centered in the well. But the tire guy insisted that "they don't make trailer wheels with an offset", so I decided not to fight him. He was enough of a pain before that. Believe me, most tire guys are NOT what you see in WMTire.

Bill

[Bill]

WMTire asked

Did you have any problems putting this wider/larger tire in the spare tire rack?

and Bill replied

I haven't even tried. I fully expect to have problems ... I'm going to have to modify the rack.

I should also have said that until I get the rack modified, I will continue to carry the 14" spare. If one of the 15" tires fails, I can see no reason not to put a 14" spare on that side. Sure, the TM will be just a bit tilted (1/2 inch higher on one side than the other), but until I can reach a tire store and replace the failed tire, I don't think it's a big deal.

Bill

The sprung weight didn't change, just the unsprung weight. I don't think the new tires will affect the total weight by more than a few pounds. Unless you're referring to the thought that "Hey now that I have bigger tires, I can carry more junk!" Now that could be a problem with some people, but I don't think it would be with Bill. He seems to be pretty level headed - to me any way! I think this was all in an effort to cure an already existing problem.

Originally Posted by Freedom

The sprung weight didn't change, just the unsprung weight. I don't think the new tires will affect the total weight by more than a few pounds. Unless you're referring to the thought that "Hey now that I have bigger tires, I can carry more junk!" Now that could be a problem with some people, but I don't think it would be with Bill. He seems to be pretty level headed - to me any way! I think this was all in an effort to cure an already existing problem.

Where would you carry more "junk"? LOL

[fourteener]

Bill / Dave / Bobby,

First off, THANKS for all of your help, you are all outstanding!

I just noticed under the "More Marathon Failures" post, Andromeda (David) just posted that he has heard Goodyear is releasing a ST205/75R14 Class D, although it may be backordered currently. Can anyone verify that for me (come on wmtire, help me, help me :-).

Secondly, I was getting ready to make this upgrade this next weekend, and thought I best raise my camper and see if I have the required clearances for the 15" wheels and tires. My '07 2720 Trailmanor appears to have some challenges. First the aluminum blocks in-between the axle and frame looks to be factory installed, but I don't seem to have the 3" of clearance that is recommended for the upgrade. I only have between 2-2.5" of clearance at the top of the tire to the black plastic wheel well. Also the aluminum block that I thought was the 'lift kit' is only 1.5" thick. See pictures below. Please note that the camper is on the axle in the photo, not jacked up, so in these pictures the axle is under load.

Can you tell me, if I am to order the lift kit, does it come with the 4 longer bolts and the two blocks required to do the work? Also, do I leave the aluminum blocks in place and simply add the lift kits?

Again, if Goodyear is coming out with D rated 14" Marathons, I would like to hold off and save myself a bunch of un-needed expenses and time (ie hunting down tires and rims, installing a lift kit, retro-fitting the spare tire rack, etc, etc.) that I could be accomplishing other work.

Any update is much appreciated.

Thanks Again,
Mitch

[ShrimpBurrito]

Originally Posted by fourteener

I only have between 2-2.5" of clearance at the top of the tire to the black plastic wheel well.

That may be enough. You don't say what tires you have, but a Goodyear Marathon 205/75R14 is 26.1" in diameter and has a load capacity of 1740 lbs (http://www.tirerack.com/tires/tires....wall=Blackwall. My 225/75R15 Carlisle tires (not my ideal choice in brand, but as I said, this was an unplanned upgrade in the middle of a trip) are 28.3" in diameter and have a load capacity of 2830 lbs - they are load rating "E" (http://www.carlisletransportationpro...wables/tires43). So they are 2.2 inches larger in diameter, but of course, only half that extra size is measured on each side of the tire. That means that if you mounted that tire, you'll have 1"-1.5" of clearance at the top. Have the tire shop add the lift kit from the factory if you don't want to do it, and that gives you an extra 1", and you are back up to the clearance you have now (I am nearly certain the lift kit I got was 2.5" tall). Then you're done. Piece of cake, no more tire worries.

Originally Posted by fourteener

Can you tell me, if I am to order the lift kit, does it come with the 4 longer bolts and the two blocks required to do the work?

Yes. Or at least mine did.

Originally Posted by fourteener

Also, do I leave the aluminum blocks in place and simply add the lift kits?

I suppose you probably can, but I wouldn't if the lift kit alone gave you the necessary clearance. Why?

Originally Posted by fourteener

Again, if Goodyear is coming out with D rated 14" Marathons, I would like to hold off and save myself a bunch of un-needed expenses and time (ie hunting down tires and rims, installing a lift kit, retro-fitting the spare tire rack, etc, etc.) that I could be accomplishing other work.

Don't overthink this -- it's not hard to do, and it shouldn't take much time. The rims are nothing special - any decent tire dealer should either have them in stock, or be able to get them on short notice. I did the upgrade in the midst of a cross-country trip -- which you don't want to do -- but it simply involved me sitting in the parking lot for 4 hours. Most of that time was me being paranoid about measurements like you are now, making posts to this forum and waiting for responses, etc.

If you can close to the same weight rating of a 15" wheel with a 14" wheel, than go for it. But my guess is you can't. If I were you, having been down this road and having tried the "I want to try to get by with 14 inch wheels" approach, I would put in the extra effort and do it right the first time. But that's just me.

Dave