Wednesday, November 29, 2023

# Selecting alternate or replacement tires for large, heavy trailers

RV Tire Safety
with RV tire expert Roger Marble

I found a thread on a forum for folks who own large, heavy 5th wheel trailers. This info would apply to non-fivers too.

While I understand the concern for the tire dimensions, that is NOT the most important specification.

Number one is to ensure any replacement tire is capable of supporting the load you are placing on your tires plus a margin, soÂ  you need to first confirm your actual tire loading.

Ideally, you would get on a scale, with the RV loaded to the heaviest you ever expect to travel with, and learn the actual load on each tire, as there are very few RVs with the load split evenly axle to axle or side to side.

HERE is a worksheet you can use.Â  You will have to do some hunting around as you can’t get individual loading on most truck stop or CAT scales. You will need to find a local building supply, feed or grain dealer,Â  gravel pit or possibly cement delivery company.

Lacking that, you could use a truck scale but to be safe you need to apply some math to estimate the load unbalances.

First, assume a split of 52/48% between axles, or with a three-axle trailer assume one axle is supporting 35% of the total. Then assume a 53/47% split on the heavier axle for side-to-side loading. Yes, some RVs have been measured with individual position scales and found a 1,000# unbalance.

So with the measured or calculated heaviest loaded tire, and the dimensions checked, you are ready to shop for tires.

You need to realize that ST tires have a higher load capacity than LT-type tires. This is because the load formula for ST-type tires is based on a max speed of 65 mph even if the “handling rating” speed symbol suggests differently. So you can’t just use the numbers when comparing tire sizes, as an ST235/75R16 carries a significantly different load than an LT235/75R16Â  even with the same Load Range (D or E or F, etc.).

You can then consult the Load & Inflation tables for the tires under consideration. The good news is that with the exception of Michelin, 99+% of the tires out there follow the same table info so you can use Bridgestone or Goodyear, etc., for LT, and Maxxis or Goodyear for ST-type tires. You can look at different tables HERE if you want.

When selecting a tire you need to get the tire capacity at least 15% greater than for your measured or calculated tire load. This allows for sway, load shift due to road crown, and wind-side load to the tires you are buying.

After you do the above, THEN you can confirm tire dimensions knowing the load capacity needed.

On my blog, I cover why you should run the inflation number molded on the tire sidewall (lower the Interply Shear) and why you should always run a TPMS along with other info on Interply Shear and the effect of temperature on tire pressure.Â  You might even subscribe.

Hope this helps.

Read more from Roger Marble on his blog at RVtiresafety.net.

Â ##RVT854

Roger Marblehttp://www.RVTireSafety.net
Retired Tire Design and Forensic Engineer w/50+ years of experience. Currently has Class-C RV. Previous Truck Camper, Winny Brave, Class-C & 23'TT. Also towed race car w/ 23' open trailer and in 26' Closed trailer. While racing he set lap records at 6 different tracks racing from Lime Rock CT to Riverside CA and Daytona to Mosport Canada. Gives RV and Genealogy Seminars for FMCA across the USA. Taught vehicle handling to local Police Depts

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bob kelly (@guest_32576)
5 years ago

Hi Roger,
On your article above, what constitutes a “large,heavy trailer?
I have a 9500lb GVWR 5th wheel, is that large,heavy? I was thinking about getting the Maxxis 8008 ST tires, it appears that they would work for my rig.