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Old 09-06-2012, 01:34 PM   #1
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Did u have to make a hitch adjustment after moving from 14 to 15 wheels?

I am looking to do the upgrade this week and wondering if increasing wheel diameter 2 inches that you had to make hitch height or weight distribution adjustments? Anyone have experience with this? We gave a 23 fb 2008....
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Old 09-06-2012, 01:47 PM   #2
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Most certainly. Increasing the wheel diameter will increase the height, therefore throwing off the levelness compared to the old set up and nessitating an increase in the tongue height. Once the TH increases there will be a necessary adjustment in the hitch height. As for the WD, not really since you will only change the height and not the load.

I have the same trailer and considered going to a 15" tire & rims.

What are you going to switch to?

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Old 09-06-2012, 01:48 PM   #3
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Originally Posted by bonginator View Post
I am looking to do the upgrade this week and wondering if increasing wheel diameter 2 inches that you had to make hitch height or weight distribution adjustments? Anyone have experience with this? We gave a 23 fb 2008....
Er, 15-14=1
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Old 09-06-2012, 01:51 PM   #4
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I have not done the change of which you speak, but in theory, you would need to raise the ball height the same amount your raise the trailer. i.e., if going from 14" tires/wheels to 15", the increase of height at the frame would be 1/2" (increase in radius), and so to compensate, you would need to raise the hitch ball 1/2".

This is assuming the tire cross section remains the same, but it would be better to measure the overall diameter change of the tire/wheel combo, and divide by two.
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Old 09-06-2012, 02:05 PM   #5
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I have had discount tire spec out new 15 inch rims with Michelin LTX M/S2 P235 75 R 15 tires on it.

Their tire calculator indicates that moving to this configuration will make the following increase:

Sidewall height:.59 inches
Width: .79
Height/diameter: 2.18
Circumference: 6.86

So believe this then yields a 2.18/2 increase on height at minimum.

They speced this out with American eagle 058 15 inch 0 offset rims with the bolt pattern formula 23 fb.

We are going with the GYMs due to some time constraints with dealing with hitch adjustments and will go to 15s next time around. Our 08 has 2007 GYMs from china on it now and we have put over 5k on it since we bought it.

It appears the 15s can work on our 23 fb if we had the time to deal with hitch adjustments. Price difference was approx $850 more to do the full 15 inch upgrade with 5 tires versus just replacing 5GYM tires
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Old 09-06-2012, 02:06 PM   #6
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Correcting n on my last PST. Circumference increases by 6.85.
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Old 09-06-2012, 02:23 PM   #7
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Different tires of the same nominal size have slightly different diameters, mostly based on the depth of the tread. You can check the manufacturers' websites for diameters of the specific tire you are interested in vs. the diameter of the OEM Marathons.

I did so when we switched from 15" Load Range D Marathons to 16" Load Range E Michelin LTX tires. The LTX had a deeper tread, but the difference was only several 16th's. So the difference was one inch plus a few 16th's, but the height change is half that. It is hard for me to understand how the difference between a nominally 14" tire to a 15" tire could be much more than 1" even considering different tread depth.

You may have to re-adjust the hitch. It could be the increase in height of the trailer will be so small no adjustment may be necessary. The only way to tell is to change wheels and tires and see if the trailer is still level or as close to it as is possible. Adjusting the hitch should be enough without increasing the ball height.

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Old 09-06-2012, 02:36 PM   #8
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Looking at my old 15" rims and tires vs. my new 16" rims and tires, there is about a 3/4 inch deference. Plus the old tires had less than 1/2 the original tread. So it's about 1/2 inch difference is size. 2 + inches of difference is not possible unless you really increase the the size a lot.

1/2 - 1 inch in height change is probably not even noticeable. It will change as tires wear or what psi you run in them.

I wouldn't worry about it till you get them on.
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Old 09-06-2012, 02:45 PM   #9
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Quote:
Originally Posted by bonginator View Post
I have had discount tire spec out new 15 inch rims with Michelin LTX M/S2 P235 75 R 15 tires on it.

Their tire calculator indicates that moving to this configuration will make the following increase:

Sidewall height:.59 inches
Width: .79
Height/diameter: 2.18
Circumference: 6.86

So believe this then yields a 2.18/2 increase on height at minimum.

They speced this out with American eagle 058 15 inch 0 offset rims with the bolt pattern formula 23 fb.

We are going with the GYMs due to some time constraints with dealing with hitch adjustments and will go to 15s next time around. Our 08 has 2007 GYMs from china on it now and we have put over 5k on it since we bought it.

It appears the 15s can work on our 23 fb if we had the time to deal with hitch adjustments. Price difference was approx $850 more to do the full 15 inch upgrade with 5 tires versus just replacing 5GYM tires

Assuming your figures are correct the height of the axle will increase by 1.09 inches . I would suggest using your jack now to lower the front of the tongue 1 inch from level and see what it looks like. The problem is that the drop down bars I have dealt with adjust in 1.5 inch increments. So assuming that you were in the best position to begin with, you will have a choice of 1 inch low on the tongue or 1/2 in high. Therefor I'm thinking that you may well be able to get to level by just using a little more weigh distribution. I have not made any adustment in my hitch going from 15" to 16 " wheels and tires.

Ken
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Old 09-06-2012, 04:48 PM   #10
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A I have not made any adustment in my hitch going from 15" to 16 " wheels and tires.

Ken
Nor have I. The difference was so small it was not worth re-adjusting the hitch. One of things I wonder about is whether it is possible to find someplace that is actually flat. Small differences in elevation from the front of the tow vehicle to the trailer wheels can be impossible to see. I could look for a 40' level but I don't think I can find one. Parking lots are advised if you can find a level one, but most are not level so runoff can go toward the edges to drains or surrounding grass. In the Rockies, level is something you hear about somewhere else.

When I adjusted my hitch, I drove the truck into the shop where the concrete floor is pretty close to level, but the trailer doesn't fit through the door. So I built up wood scraps outside to be level with the shop floor and pulled the trailer onto them. Since I don't have a 25' level, this was dicey and I used an 8' piece of straight aluminum (how much deflection was there?) and a 4' level to get the wood to the proper height. Was this process accurate? Was the level accurate? No, but was it within reasonable expectations? Maybe.

My leach field for our septic system is pretty flat, but it is not perfectly level—I checked it as it would have been easier than what I ended up doing. I couldn't think of any parking lot anywhere nearby that was level and often parking lots are full of cars anyway. You don't actually need a level surface, but you do need one where the entire length of the trailer and tow vehicle are on the same grade. Then when you use a level, you have to make sure you have the bubble in the same place for both truck and trailer—on a slight grade, that is very hard. Making it even harder is that there is no where on the truck that is perfectly parallel to the tires—and I found the one side of the truck is a little lower than the other side. I ignored that. I assumed the truck was level in the shop and measured the wheel wheels as I went through the adjustment process. But did I measure at the exact same spot in the wheel wells each time? Probably not, though I could have put a paint spot on the paint—I wasn't going to do that. The trailer is easier to check for level—put a level on the belt line assuming it is parallel to the ground. I hope it is, but I had no way to check that.

My point is that when adjusting the hitch, lots of small errors creep into the process. As time goes on and tires wear, more changes. With lots of miles, Airstream axles change their position and the trailer gets closer to the ground, further increasing (or decreasing) error.

So I did the best I could. It was a lot better than the dealer did and the trailer towed better. There are several places to adjust an Equalizer, each one affecting the others. I experimented with each until I got the trailer level and the truck close—less than one inch difference between front and back wheel wells. A difference in height of the trailer axles of 1/2" to 3/4" may not make a difference that can be adjusted. It could make the trailer just as off level in the opposite direction as it was beforehand and thus no meaningful change.

I don't mean to say this is not important. But unless you have a testing lab with well calibrated tools, you'll be off a bit more often than not. It is possible the trailer was slightly lower than level at the tongue and will become slightly higher. That would be good luck. Bad luck would be if you were too high before and become even higher.

One more point—does a change in the height of the axles translate to the exact same change at the tongue? It seems so, but is that affected by the distance from the axles to the tongue? This may require a knowledge of geometry that I have had 55 years to forget. And are both axles mounted perfectly so they are each the same distance from the ground? We assume these things are correct, but I didn't expect the truck to be lower on one side than the other either (the shop floor could be off level, but I think I checked that).

There are too many variables to predict this. About the only thing you can be sure of beforehand is to make sure the new tires will fit in the wheel wells and then see what happens.

A lot of words, but better than what I was doing, loading up the trailer. I guess I'd better get back to work.

Gene
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Old 09-06-2012, 05:28 PM   #11
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Gene,

Wow, that must have been the most diligent effort to find a level place on a oblate spheroid since the days of Columbus. I commend you.
As we squids say ,"bravo zulu."

Take a look at my profile picture. Everything looks level. I am sure that must be one the few truly level places on earth.
Ken
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Old 09-06-2012, 05:45 PM   #12
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IF the glass stays on the counter, it's level
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Old 09-06-2012, 07:37 PM   #13
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Thanks, guys. Makes sense. I ended up going with another set of GYMs given my time constraints and now that I know what I need/want for 15" tires/wheels, will do that next time when I have time to deal with any adjustments. Appreciate all of the advice. This has been quite the learning experience!
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Old 09-07-2012, 08:52 AM   #14
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Thanks, guys. Makes sense. I ended up going with another set of GYMs given my time constraints and now that I know what I need/want for 15" tires/wheels, will do that next time when I have time to deal with any adjustments. Appreciate all of the advice. This has been quite the learning experience!
Bong', we're on the level.

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