Tires: TT tires, st205 75r15, light truck tires

Question
QUESTION: I read the article in the current TL mag on using light truck tires, but now I have a question. I have 235-70X15 tires from my ranger. They appear to be the same in diameter as the 205-75-15 on the trailer. 4 ply tread, 2 ply sidewall 1900 lb load rating @ 35psi. The trailer tires are 4ply tread, 2 ply sidewall 1836 rated @ 50 psi.

Now the question will the 235's work on a 5000lb TT ok? Even if they are P rated they still come in at 1730 lb load rating, the trailer is a 27ft Dutchman so basic weight is around 4000lb should be a load of 6K when loaded.

The tires are Bridgestone Duelers and do not have a P, Xl or any other type of information other than size and the info I posted above. I mounted 2 to check for fit and ther is no sidewall buldge evident as some have indicated when P rated tires are used.

I just cant get a straight answer. My daughters husband and my son play in tournement softball and often the games go late into the evening or last all day with the kids in the sun/cold as appropriate. Trailer is not for hauling xcountry just to the local tournement sites so the new baby will not have to be kept out side in the elements.

Thanks hank


ANSWER: Hank,

It looks like you have done a lot of good background work on this subject, but there are still a few gaps.

First, a 235/70R15 requires a rim width between 6" and 8", where an ST205/75R15 requires a rim width between 5" and 6 1/2".  So if the rims are 5" or 5 1/2", this is not going to work.

Plus, you will be exchanging a tire with a load carrying capacity of 1836# for one with 1730# load capacity.  This would be a loss of 400#.   This is, of course, the wrong direction.

And one little tidbit.  The US tire standardizing organization (the Tire and Rim Association) is the only one that recognizes the use of the letter "P" in front of the tire size - and is also the only one that recognizes that Passenger Car (PC) tires are sometimes used on Light Trucks and Trailers - so they are the only ones who have specified the 10% derating for those applications.

Nevertheless, I think it should apply whether the "P" is there or not.

I am also of the opinion that trailer manufacturers not only under report the actual weights of their vehicles, but haven't gained the experience from the Ford / Firestone situation a few years back  - that tires need to be over specified.

So I am of the opinion that a 6K# GVW trailer ought to have 4 1568# load capacity tires on it.

So, provided there is enough room under the fenders (Don't forget that the top of the tires deflects when cornering, so there has to be clearance deep inside the fender), it appears this is an OK swap.

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

QUESTION: Thank you for the fast reply, as to the width of the wheels I am not sure they are the OEM wheels so I guess they should have the info someplace.  Now a follow up, what type of wheels can be used?  Friends have gone to aftermarket truck aluminums but I have also heard that they can also cause problems is this a truism or just another tale?

I dont think standard steel truck wheels are sufficient as they have do not have the welds that trailer wheels do and are cheaper in construction.

Would AM aluminums be of sufficient strength for this application if needed?  and last question is 40 PSI within the realm of reality for this application vs the 35 psi on the sidewalls as I have read that you can go over on pst max if there is evidence of overheating in the tire?

Thanks  again

Answer
Hank,

I'm sorry, but I can't tell you about the fitment of wheels.  My expertise is in the rubber parts, but here's what I do know.

There are different bolt patterns for wheels - different number of lugs and different bolt circles.  Obviously you can count the number of lugs, but measuring the bolt circle diameter is a bit tricky.  The following link will be somewhat helpful, but you probably need to get some local expertise - like at an RV center.

http://en.wikipedia.org/wiki/Wheel_sizing

But you also need to consider the offset - the location of the mounting surface relative to the centerline of the rim.  Sometimes this is specified on the wheel itself, sometimes not.  Usually it is given in terms of mm - and if it is specified on the rim, it might look like this "ET40", where 40 is the offset in mm.

What about the width?  This is the most likely thing to be marked on the wheel.  It will look like this:  "6.5JJ" where 6.5 = 6 1/2" and JJ is the type of rim flange.  Don't worry about the rim flange code (the "JJ").  There are only so many and for this type of application, it will either be J or JJ or K - and it just doesn't matter which in this case.

What type of wheels?  You need to have a wheel that will carry the load.  This may or may not be stamped on the wheel.  BTW, most of these marking will be on the backside and you have to take the wheel off to see what's there.

I don't think it matters what the wheel is made of - it's the quality of the wheel that is important.  Wheels have been made from steel, aluminum, magnesium, titanium, plastic, etc., and they all do very well if they are manufactured correctly.

But as some have said, there are problems with aftermarket wheels.  but it stems from the quality, not the material.  

But contrary to what you've stated, trailer wheels tend to be lower quality.  After all, an out of round wheel won't be felt by the driver or passengers - they are in the tow vehicle, not in the trailer when the vehicle is rolling down the highway.

And 40 psi for a passenger car tires in this application would be good.

I suggest you find an RV specialist and work with him on this.