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Old 08-18-2007, 08:59 AM   #1
Bill
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Default Replacing 14" Marathons with 15" Marathons

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.
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Old 08-18-2007, 09:26 AM   #2
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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?
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Old 08-18-2007, 10:15 AM   #3
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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?
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Old 08-18-2007, 11:32 AM   #4
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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.
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Old 08-18-2007, 11:42 AM   #5
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Quote:
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

Quote:
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.

Quote:
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.

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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.

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Old 08-18-2007, 01:10 PM   #6
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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.
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Old 08-18-2007, 01:55 PM   #7
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Leon -

Yes, the original screws are long enough, although it wouldn't be a problem to put in longer screws if needed.

The outer clearance depends on how much you de-bow the fender skirt, of course, but for me it is now about 3/4". This is about what it was before I made the change.

And Freedom, you are right, it does not give me license to increase the loaded weight of the TM. The axle was (and still is) the limiting factor, but as we noted earlier, we haven't had any reports of axle failure. So as Jim says, all I hope to accomplish by making the change is to reduce the rate of tire failures.

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Old 08-18-2007, 06:31 PM   #8
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Bill, on the fender skirt, a thin strip of metal. 1/8"x1/2"x1"(available at ace hardware and probably Home Depot) laid in from end to end, behind the skirt, just might take the bow out of it. It would call for a couple of longer screws unless the metal did not go any farther than each end of the wheel well. Then it could be attached to just the skirt with 2-3 short screws. I don't have the same problem that you had because my TM is a 3023(2004) and it came with the 15" wheels and tires which, so far, have not given me any grief. Time will tell on the tires though, as we are about to head up to Vermont and points in between( we are in Ga.) We keep the trailer in the garage with the tires parked on pads of carpet. So far. they look good-no sidewall cracks, etc......Jim
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Old 08-18-2007, 08:48 PM   #9
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Jim -

I think you are right. This is one of the approaches that Chap and I talked about, and I think he actually did it, with some success. I expected him to post his experience, but he has been off-line for a while. Chap? You out there?

Bill
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Old 08-19-2007, 09:17 AM   #10
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Quote:
Originally Posted by Bill View Post
Jim -

I think you are right. This is one of the approaches that Chap and I talked about, and I think he actually did it, with some success. I expected him to post his experience, but he has been off-line for a while. Chap? You out there?

Bill
I am just getting back online....my puter was acting up yesterday.

Like Bill said, we have been PMn back and forth the last couple of weeks or so.

Jim has come up with a similar idea that I used. I used 1"x 1/8" aluminum flatbar riveted behind the fender skirt, right under the dimple. I put washers over both rivet ends (especially needed on the plastic fender skirt). I made sure that the rivets were well to the ends of the flatbar (outside of the tire area), and installed from the outside so the "long" part of the rivet is on the inside of the fender skirt (so it would not rub against the top wall seal). I cut my flatbar to fit over most of the fender skirt, but cut it short enought to fit inside of the fender well. BUT......If you leave it a little long, it may provide the spacer that Bill has suggested.....probably owners need to see how there fender wells are made, and tweak/file/cut according to their setup. My screws are on the outer edge of the skirt (~ 1/4", vs. Bill's 1"), and the fender well is tapered in some, so a little longer flatbar on mine could help pull the skirt out.....does that make sense??? Also, since 1"x1/8" flatbar is not very rigid, I had to bend the aluminum to counter act the bend in the fender skirt, and might have to continue that in the future as things settle in.

My missus added: Bill's fix addressed the root of the problem, and mine fixes the result of the problem.

And to add about those 1500 Lb. load limit wheels......not knowing initially, I was the 1 that bought a set of those. I bought them from my local RV dealer, who stated that is what they use on everything....a little scary. I PMd wmtire and Bill, and they both relayed their concerns with these. The wheels were now the weak link in my setup, and I just didn't like that. So I turned around, and ordered a set of 2600 lb load limit wheels, and will get them installed next week. A little wasted money, but a lot more peace of mind.

I hope to post a couple of pictures of my fender skirt fix in the next couple of days.....the missus has the digital camera at work.

Chap
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