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Old 03-15-2015, 05:57 PM   #43
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Originally Posted by dznf0g View Post
So if you want 15% reserve on a p rated tire on a trailer, you shouldn't load beyond 75% of max sidewall stated load at max sidewall pressure?

I'm not the tire guy - but I think it's actually 76.5%.

If you had say a 100# capacity p-rated tire you'd only be able to use 90# (per DOT standard to de-rate them 10% for trailer application). Capri's 85% of the 90# would be 76.5# or a combined reduction of 23.5%.

So to apply BOTH the DOT standard AND Capri's 15% headroom recommendation for the Michelin LTX P235 75R15 that would equate to 1670# per tire (2183*.765) or just shy of 6700# for a double axle trailer. Mine weighs in at 5880# fully loaded (with FW tank full) for camping so I feel comfortable I've got even more headroom than needed (due to DOT) and recommended (by Capri).
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Old 03-15-2015, 06:33 PM   #44
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Originally Posted by SteveSueMac View Post
I'm not the tire guy - but I think it's actually 76.5%.

If you had say a 100# capacity p-rated tire you'd only be able to use 90# (per DOT standard to de-rate them 10% for trailer application). Capri's 85% of the 90# would be 76.5# or a combined reduction of 23.5%.

So to apply BOTH the DOT standard AND Capri's 15% headroom recommendation for the Michelin LTX P235 75R15 that would equate to 1670# per tire (2183*.765) or just shy of 6700# for a double axle trailer. Mine weighs in at 5880# fully loaded (with FW tank full) for camping so I feel comfortable I've got even more headroom than needed (due to DOT) and recommended (by Capri).
Using those numbers I get 2050# per tire for the 16" Michelin (2680*.9*.85). That's 8200# total.

So even 16" tires aren't going to cut it for a newer 30' trailer (8800 GVWR)?

With the stock 15" Goodyear Marathons I get 2159 per tire (2540*.85, not derated 10%). That's 8636# total.

So, if I'm doing this right, using the 15% safety factor means the Airstream stock tires aren't sufficient.

I'm all for safety but maybe 15% is too much?
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Old 03-15-2015, 07:01 PM   #45
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The 16s are LT, not P rated so the 10% de-rating wouldn't apply. The next 15% is the recommendation of a tire engineer in the forums, not a requirement.

I would imagine everyone's mileage will vary on this. But actually weighing the trailer loaded for camping is key. You may find the 16s work for you or you may find you want to do something different. Lots of options.
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Old 03-15-2015, 07:23 PM   #46
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Originally Posted by SteveSueMac View Post
I'm not the tire guy - but I think it's actually 76.5%.

If you had say a 100# capacity p-rated tire you'd only be able to use 90# (per DOT standard to de-rate them 10% for trailer application). Capri's 85% of the 90# would be 76.5# or a combined reduction of 23.5%.

So to apply BOTH the DOT standard AND Capri's 15% headroom recommendation for the Michelin LTX P235 75R15 that would equate to 1670# per tire (2183*.765) or just shy of 6700# for a double axle trailer. Mine weighs in at 5880# fully loaded (with FW tank full) for camping so I feel comfortable I've got even more headroom than needed (due to DOT) and recommended (by Capri).
Yup, didn't do the math...just off the cuff, quick and dirty.
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Old 03-15-2015, 07:25 PM   #47
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Using those numbers I get 2050# per tire for the 16" Michelin (2680*.9*.85). That's 8200# total.

So even 16" tires aren't going to cut it for a newer 30' trailer (8800 GVWR)?

With the stock 15" Goodyear Marathons I get 2159 per tire (2540*.85, not derated 10%). That's 8636# total.

So, if I'm doing this right, using the 15% safety factor means the Airstream stock tires aren't sufficient.

I'm all for safety but maybe 15% is too much?
Again that is and engineering recommendation for "over spec". I wonder if there is any margin already built into the original sidewall specs on tires....I know there is on GVWR....etc.
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Old 03-15-2015, 08:43 PM   #48
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What happened to the day when we threw the wife, kids, dog in the station wagon and hooped up the tent trailer and went camping and enjoyed the week end?
Now we have to also consider the type of hitch, the weight on each wheel and if the trailer is loaded or unloaded. We also check the weight rating on each tire and where they are made.
We check the shocks on the tv and tt, how much downward pressure on the A frame and the list goes on and on.
We also check for different meals and how they are cooked; how about a hot dog and beans?
Camping is getting so technical that one almost wants to stay home or go to a motel.
Well,too much rambling.
LET'S GO AIRSTREAMING.
mike
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Old 03-16-2015, 06:07 AM   #49
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Using those numbers I get 2050# per tire for the 16" Michelin (2680*.9*.85). That's 8200# total.

So even 16" tires aren't going to cut it for a newer 30' trailer (8800 GVWR)?

With the stock 15" Goodyear Marathons I get 2159 per tire (2540*.85, not derated 10%). That's 8636# total.

So, if I'm doing this right, using the 15% safety factor means the Airstream stock tires aren't sufficient.

I'm all for safety but maybe 15% is too much?
Scott,

You derated the LT tires and they do NOT need to be derated. Only P type tires need to be derated.
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Old 03-16-2015, 06:09 AM   #50
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Again that is and engineering recommendation for "over spec". I wonder if there is any margin already built into the original sidewall specs on tires....I know there is on GVWR....etc.
Yes, but both sides of the equation need to be done - design and utilization.
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Old 03-16-2015, 11:33 AM   #51
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Scott,

You derated the LT tires and they do NOT need to be derated. Only P type tires need to be derated.
Thanks for the clarification!

That would move things to 2278 for the 16" LT tires, or 9112 total. Even more than the 15" stock tires.
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