Buying 16" wheels and LT tires.. right choice?

[dznf0g]

Originally Posted by CapriRacer

Perhaps this will help.

When tires are designed, the starting point is the tire load tables. These tables not only standardize the load/inflation/dimension issues for vehicle manufacturers - so they can chose a tire and have confidence that putting another brand on isn't going to cause an overload situation - but also for the tire manufacturers to design tires.

To develop these tables, the service conditions are also considered.

Taking a Passenger car type tire as an example, I think it is easy to see what "Passenger Car Service" means - highway speeds, highway surfaces, softly sprung, low inflation pressure, etc. But in order to make a P type tire suitable for Light Truck usage (which is defined as application on a pickup, van, or a trailer), the load table gets modified - and rather than publish a whole new table, the instructions are to derate the tire by 10%. When that is done, the tire has a new "standard".

By contrast, the load tables for LT tires are specifically developed for Light Truck Service, so the load table is directly applicable.

By further contrast, ST tires have an additional restriction that LT tires don't have - 65 mph speed limitation - and that results in a different load table. If one were to compare the load tables, one would find the loads on the ST tires to be higher than for the same size LT tire.

Now let's move onto reserve load. This is the application of the idea that it is better to use equipment much higher rated than the situation calls for.

For example, if a trailer is going to need 5,000# axles, it would be better to use 6,000# axles. What normally happens is this sort of thinking is done by the design engineers and is invisible to the end user.

In the same way, a tire ought to be specified such that it isn't overloaded, and some distance away from being overloaded. That's where that 15% comes in. If things were working as they should, one wouldn't even be aware that this was taking place. One would use the tire size and the inflation pressure specified on the vehicle tire placard. Passenger cars is a good example.

Unfortunately, things don't always work as they should. Some trailer manufacturers didn't do a good job of sizing tires and in order to make up for that, you have to do what should have been done before you bought the trailer - and that is size the tires to fit the job.

I had a professor who taught engineering design, and he said "Over-design / Under-utilize!" That is what the 15% reserve capacity is all about - good engineering practice.

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?

[JimGolden]

I run six of the Maxxis UE-168 Bravo tires in 235/75-15Q D Load rating (2337lbs capacity per tire) on my 34 footer and have had great success with them for the past 25,000 miles.

Maxxis used to make a UE-168 Bravo in 225/70-15 that was E rated. It looks like they've discontinued that one.

I stumbled onto Maxxis by chance, but can say nothing bad about them. I am getting due, age wise, for a new set.

I'm a huge Michelin fan. But, if I can stick with a good 15" and not have to buy six new wheels, I will probably go that route. I see another set of these UE-168's in my future.

http://www.maxxis.com/catalog/tire-2...-ue-168%28n%29

Cheers,

[SteveSueMac]

Originally Posted by dznf0g

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).

[SCOTTinNJ]

Originally Posted by SteveSueMac

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?

[SteveSueMac]

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.

[dznf0g]

Originally Posted by SteveSueMac

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.

[dznf0g]

Originally Posted by SCOTTinNJ

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.

[Airtandem]

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

[CapriRacer]

Originally Posted by SCOTTinNJ

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.

[CapriRacer]

Originally Posted by dznf0g

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.

[SCOTTinNJ]

Originally Posted by CapriRacer

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.