[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.