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Old 04-16-2007, 08:26 PM   #15
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In '81 weren't they still specifying 7.00x15 tires? I didn't think they switched to the 225/75R15 metric sizing until later. The old 7.00x15 tire size was a hair over 30" in diameter. Virtually impossible to find now days (in a radial anyway). It's the reason some of us are looking at the switch to 16" rims; so we can get back to the tire diameter and ride height the trailer was designed for.

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Old 04-16-2007, 08:42 PM   #16
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Some Math

You can figure out the theoretical height using the tire's description:

The new tires are metric rated, such as 225/70-15

What this means is that the tire's section width is 225mm, the tire's wall height is 70% of the section width (which would be 70% of 225mm), and the rim diameter is 15"

So if you do the math, the overall height of the tire (neglecting "squish") would be:

Height = { (225mm/25.4)*(0.7)*(2) } + 15

Height = 27.4"


You divide the mm by 25.4 to get it in inches. You have twice the wall height because you have rubber both above and below the wheel.

So, you can use this formula to calculate the height of any tire.

A 235/75-15 would thus be 235/25.4*.75*2 + 15 = 28.9"

Conversely, the other format commonly used for truck tires would be like a 31x10.50, where the height is 31" and the section width is 10.50".

I'm afraid I don't know the older rating system. A 7.00x15 I'm assuming the section width was 7" and the rim 15", but I don't know what the wall height is.

Best of luck,
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Old 04-16-2007, 10:37 PM   #17
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Quote:
Originally Posted by Pacerized
Sorry, I'm also wondering if I should buy class C, or D rated tires for a 25' duel axle trailer that weights 4700 loaded, and 3600 dry. Thanks: Charlie
Hi Charlie:

I don't have a copy of the 1981 Airstream travel trailer catalog, but I do have the 1980 Airstream travel trailer catalog which includes a 25' Excella II, so I'll give you the tire data for it, which might (or might not) apply to your 1981 version of that trailer.

All 1980 Excella II trailers (25', 28' and 31' lengths) had as the standard base model four bias belted black wall tires in load range C tires on automotive style steel wheels with wheel covers as standard (no tire or wheel size is listed).

Optional tires for the 1980 Excella II and International trailers were either:

four 700-15 LT (Light Truck) load range C bias belted white wall tires, or
four 700-15 [Michelin model] XCA load range C steel radial black wall tires.

on optional forged aluminum wheels (no wheel width listed, but it would have a 15" diameter). Hope this data helps.
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Old 04-16-2007, 11:40 PM   #18
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Thanks for looking that up Weewind:

It surprises me that LT tires were optional since I've read on here to stay away from anything other then trailer tires ST. I guess the LT tires come in a 6 ply to be load range C. I do have the optional aluminum rims, but I wouldn't think that would matter so much. I guess I should stick to the C's since that's what came on it. I don't want more vibration in the trailer then I should have. The 4 ply passenger tires I have on it now actually ride very well. I never get any sway while I'm driving. Being lower in profile might make up for the softer side walls.


Charlie
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Old 04-17-2007, 02:38 PM   #19
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Just moving out of a pass. tire may give you some height. Not sure what size pass tire you have on it now - you didn't say.

And your hitch set up, .... not sure because you are not specific, is the ball on a removeable square tube? If so you could buy a drop down ball assy and adjust for a small amount.

Any tires you buy for your trailer will have to be twice as tall as the amount needed. IE if the amount you need is 1" than the replacement tire would have to be 2" taller in diameter than the existing tires. The amount of tire above the center of the wheel does not lift the trailer any.

And because the trailer is 26 years old the axles may have settled as mentioned above. Having the axles inspected as you state above is a wise move.

So several factor are at play. The pass tires. Possible axle settleing or lowering of the trailer. And a drop down ball assy may all be factor.

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Old 04-17-2007, 04:57 PM   #20
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Tire Dimentions

Pacerized
Go to the web site of the company that manufactured the tires on your trailer now.
There you will find a "Specifications" Page that will give you the Maximum sidewall width and the Outside Diameter of your tires.
With that information you can determine what different tire you want to go to by looking up the tire you are interested in.
If you need help with this tell me what you want to know and I'll try and find it for you.
You said that the tires on the trailer are P205. What are the other numbers (P205/70R15 or P205/75R15?)
It can be confusing.
Beginner.
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Old 04-18-2007, 09:38 AM   #21
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I have the airstream at the dealer now, and I'm not certain of the tire width #, I would guess it's a 205 X65, or X70. The hitch I have came with the airstream, but not the receiver. It has the load equalizer bars, and the ball is not on an adjustable track it's welded. To adjust the ball, I'd have to get a new hitch assembly with the slots for the load equalizer bars. That would run $290 at the dealer. I only went up an on my TV tires by little less then an inch, and it road fairly level last year, so I don't think I need to gain that much. Since I have to get new tires anyway, and they have to be taller then what I have now, I think I'll wait to see how level it is with my existing hitch. The tires have to help some. I may be able to get a better deal online if I need the hitch. It would seem that the tallest trailer tire I can get is the 235 X 75 in 15 inch, and that's still lower in profile then what most likely came on the unit new, so I'm leaning towards going that way. If I do this would it be best to go with D's for additional sidewall support, or would D's make the ride noticeably rougher?
Thanks for all the input.
Charlie
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Old 04-18-2007, 12:32 PM   #22
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D's or C's --- I would make that decision based on load. Lilely I would use the same load range that came with the unit.

With that said, I would weigh the trailer, add weight for full water tanks and waste tanks, propane and add some more for gear that isn't on the trailer that I take with me. (OR load the gear in as if I were going camping and then weigh the trailer) And finally add 10% to that figure.

Then divide that weight # by the # of tires. This is the amount each tire would be asked to handle. Find a tire load rating that is just over that final number. Be it C or D.

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Old 04-19-2007, 08:01 PM   #23
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Thanks for all the advise from everyone. I ended up having them put 225 75 15's in load range C on it. My 25' Excella is one of the lighter 25' models made at only 3600 dry, so I think D's might have made the ride a little too stiff. We did gain about an inch over the 205's that the PO had on it. I stopped by today to look at the Rock Guard that the dealer had ordered, but they'll have to customize it to make it fit. Due to this they have to keep the trailer another week, and I won't be able to tell if the tires were enough to level the trailer until then.
Charlie
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