Tires: Trailer tire ratings, tandem axle, tongue weight

Question
QUESTION: What tire (size, weight and speed ratings) would you advise for the following:
    Enclosed equipment trailer
    Length, overall - 29 feet
    Loaded weight - 15,000 lbs. total
    Wheel size - 16" x 6.5"
    Tandem Axle and spring rating:
        4,300 lbs. per wheel          
        8,600 lbs. per axle   
        17,200 lbs. total
    Tow vehicle - 55,000 lb motorcoach, 500 hp    
        Detroit diesel, capable of 70-75 mph speed
        limits

Thanks, Loyd

ANSWER: Loyd,

Let me walk you through the process I would use.  But you need to be aware that I am a tire engineer and not a vehicle engineer, which means I uderstand the ins and outs of tires, but how tires fit into the vehicle picture is on the edges of my experience, where a vehicle engineer would have experience selecting and sizing tires for specific applications.  But here goes:

The GVW of the vehicle is 15,000 #.  The tires could experience that entire load less the tongue weight - which I will assume will be 10%- on the low end to make sure I have enough load capacity in the tires.

That means the tires will have to carry 13,500# - or 3,375 # each - assuming the load is evenly distributed - a bad assumption.

So to comensate for that assumption, and to account for load shifting and other stuff, I want a 20% reserve capacity in each tire.  So I want a tire with a 4,050 # capacity that fits on a 16 X 6.5" rim - Well.....There aren't any!!!!

There is an ST235/85R16 LR E that lists a 3640 # capacity and, interestingly, there is an LT235/85R16 LR E that lists a 3042 #.  I suspect the real difference in these tires would be the built in amount of "reserve" and not a difference in construction.

I also note there is an LT235/85R16 LR F which uses 110 psi with a 3415 # load capacity.

So working this backwards - and I am assuming this is an existing vehicle - it appears that the vehicle engineer selected tires with minimal reserve capacity (and I am being kind here!) - and my experience says this is going to result in a higher level of tire failures that the public is willing to accept.  I assume he did that based in the fact that the vehicle would not be disabled if one tire failed - and there is minimal risk to people.  Frankly I find this appalling!!

I guess the bottom line here is that I would advise that you need to change rims so you can use tires that have more capacity - either wider rims or larger diameter.  Since I din't know what your space limitations are, I can't take this any further.

---------- FOLLOW-UP ----------

QUESTION: I presently have the following new tires on the trailer:
    Goodyear G614 RST
    LT 235/85R16    126/123L
    Load range G    
    14 ply rating
    30.1 inches diameter
    3,750 lbs at 110 psi per tire
    15,000 lbs total capacity - 4 wheels
    10% tongue weight leaves 13,500 lbs on tires, 3,375 ea.
    Load is balanced and secured.
    Fender space is limited for wider or taller tires.
    Taller tires would require spacers between spring
         hangers and frame to raise fenders.  
    I can't find any speed restrictions for these tires.

Can you advise of any speed restrictions on these tires?

Can you advise the wheel and tire size if I raise the trailer without increasing the width since it is already 8.5 ft. maximum?

Thanks,

Loyd  

Answer
Loyd,

If the tires say they are 126/123L, that means they have a Load Index of 126 for single application and 123 for dual application and a speed rating of L = 120 kph (75 mph).

As far as what tire would work if you raise the trailer:  This is where my experience as a tire engineer comes up short.  I think using a larger capacity tire is also going to require a larger capacity rim (that part is fairly obvious), but that would also mean the bolt pattern - and therefore that hub / axle assembly - would also need to be changed.

If I have this right, the next step up in tire size is 225/70R19.5 - 31.9" diameter - and you'll want a Load Range G (3970 # at 110 psi.)