trailer tires and tow speeds

[I'mdonewiththis forum]

Not all ST tires are rated at 65 MPH anymore. The tires on my '17 are speed rated at 75 MPH as stated on the sidewall of them.

[jollyrogr]

13 hours ago, Heavymetal said:

We run LT tires also speed limited to 106 are there about. I normally run at 68 but will run 75 on a very good dry no traffic freeway. I run 8 tires and on a hot day the tire temps are around 118 degrees and I check them at every stop. We have a little over 24k miles towing so far this year and no telling how many in total and have had zero tire problems.

Sounds like good results.  What tire specifically do you run?

[Hot Rod]

I'm late to the thread, so I need to catch up.

Converting the 6 lug trailer with 15" to 17.5".

Tire sizes vs. calculated diameters: 
225/75/15    28.3" tall
215/75/17.5 30.2" tall  (smallest 17.5" size)
I say "calculated" as the mfg actual size may vary slightly and they round off to the nearest metric size to stamp on the size, and the actual height may be a tenth or two shorter or taller.  Refer to the specific mfg website for their actual size.  Another poster had an accurate comment on how to calculate height and width from the metric size.

So the overall diameter is 2" larger on the 17.5 tire.  This means the spacing between the tires will be reduced by 2".  So one poster mentioned 4" current spacing, this would reduce it to 2" which is more than adequate.  On my trailer it was only about an inch and a half and these stout tires will never "grow" enough at speed to rub together.  The 2" diameter increase means 1" increase in radius, so your tire will be 1" closer to everything else.  As in the top, front, and rear of the wheelwells on the trailer, so you need to make sure you have the extra room there.  I would jack up an axle, or roll one tire up onto a block, to get it to max compression and make sure you have more than an inch clearance with the current tires to insure no rubbing when loaded and the suspension compresses on bumps.

Width will not be an issue unless your current wheels are unusual width/offset (rare).  The 17.5" wheel will be wider than your 15" wheel, but the 215 tire is narrower at the sidewall, and the stiff 17.5 tires will not bulge, flex, and roll like the 15" tires did, and will need less static clearance at the sidewalls as compared to the 15's.  If you give me your current tire size and width and backspace of your existing wheels I can calculate for you the difference in sidewall clearance (if any).

IF your current 6 lug axles are the 6000# variety (as opposed to 5200#), again in most cases, the brakes, hubs, and bearings are identical to a 7000# 8 lug axle with the exception of the 8 lug vs. 6 lug studs for the wheels.  All you need to buy is the hub/drum assembly with 8 lugs.

You will need the stud piloted 17.5" wheels for use with a standard 7000/7200# axle.  Those have the correct oversized hub bore and conical seat lugs to work correctly with your axle.  The other type of wheel is the hub piloted wheel which is for use on 8000# and heavier axles, and are hubcentric and use 5/8" flat washer lugs and will NOT work with the smaller axles.  The stud piloted wheels will be rated at 3500#, but that is fine for the trailer weights in this discussion, the rating is limited by the stud piloted configuration and the same wheel machined for hub piloted configuration carries a 6050# rating.  Both versions have a 130psi rating.

Alcoa link:https://www.arconic.com/alcoawheels/north-america/en/products/product.asp?cat_id=814&prod_id=4096

I did this conversion on our old triaxle gooseneck living quarters race trailer, with the Alcoa wheels and Cooper RM170 225/75/17.5 tires.  I too used to carry two 15" spares, and changed WAY too many of them.  It seemed like it was a rare trip that I did not blow a tire.  I know for sure I was going through about 6-8 tires a season.  We were loaded right at the max capacity of the 15's, and as you know they will not hold up at that point.  After switching to the 17.5's, the spare never touched the ground for 2 1/2 seasons and still looked like brand new when we sold the trailer.  The tire savings over time can more than pay for the conversion if you are putting on a lot of miles.  Not to mention lost time, inconvenience, and danger changing a tire on the side of the road. And peace of mind.  Also I would have to keep the speeds to 60mph or so on the 15's to keep the temperatures down, with the 17.5's I could run as fast as i wanted and the temps would barely change.  There is a lot to be said for having a large margin of over rating of the tires vs. the actual load on a trailer.

In my case my trailer only had 5200# axles and I could not swap the hubs (at least in my case).  But I got lucky and found an older sprint car trailer with 7200# axles, swapped the axles with my trailer (not a big job, 4 bolts per axle) and resold the sprint trailer with my 6 lug axles and tires for the same money.

I will heartily endorse the conversion to 17.5 to anybody that asks.

[jollyrogr]

Good post Hot Rod. My axles are 5200# so I'm thinking the most cost effective upgrade is to get 16" wheels and some steel cased LT rated tires. 

[Hot Rod]

Just be careful measuring your clearances beforehand on the 16" conversion.

225/75/15     28.3" tall
215/75/17.5  30.2"
235/80/16     30.8"    (goodyear marathon and similar st tires)
235/85/16     31.7"    (goodyear G614 and LT tires)

That was one of the reasons I decided on 17.5 when I was agonizing over 16" vs. 17.5" to upgrade from my 15".  I would have had just barely enough room for 235/80/16 but was not happy with any of the ST tire choices.  And just did not have enough room for a 235/85/16.  Both between the tires, and that particular race car trailer had axles that were indexed to sit pretty low and the top to wheelwell clearance would not have worked either.

[jollyrogr]

LT225/75R16 is 29.3" tall and is the direction I'm leaning.