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Monday, July 7, 2014

Is it OK to Replace Bias tires with radials

My 25' 1985 trailer has a plate on the left front street side that indicates 7.00 X 15 and 45 psi. D range, I think.
Right now, there are Roadkings LT on it. I inherited the Airstreem from my aerospace engineer Dad and know based on 5+ years it's time to replace them So, what manufacture would you recommend and what pressure? GVWR is 6,800#, axles are 3,200lbs. I believe the original tires were bias.
It really isn't difficult to find a new radial tire for your RV but there are a few basic steps needed.

You gave the GVWR and Axle rating but made no mention of the actual load on each tire. There are a number of posts showing that the load is seldom split evenly between axles or evenly side to side.
There are worksheets available to help you calculate the individual loads if you can't find a location with individual scales (this is difficult to do so use the worksheet). Here are 2 sites with forms you can use.
Bridgestone Commercial Truck Tires
RV Trailer Weight Calculators - Towing Capacity - Ratings

OK Lets assume you get or calculate the individual tire loads. For this example I will use the following numbers 
LF 1,550      RF 1,390
LR 1,500      RR 1,440

I would recommend a 15% "Safety Factor" or "Reserve Load" when looking at the tire loading.
1,550 + 15% = 1,782#

I also recommend an additional "Safety Factor" of +10% on the air pressure
You need to find a tire capable of carrying at least 1,782# at its Max pressure -10psi

The +15% is a safety factor for possibly different loading in the future
and -10 psi is to allow you to set a pressure and not need to worry about changes in ambient day to day.

Now since we are talking a multi axle trailer you need to run the max pressure in the tire all the time. This has been covered in other posts in this tire blog but it has to do with the unique side loading for trailers.

Consult the tire Load/Inflation tables and tire specification sheets to find a tire that meets the above load & inflation capability and fits your trailer for width and OD.

One final consideration. Years ago when there were still many bias tires in the market the wheels were not always designed for radial tire loading. You should probably also either confirm your wheels are approved for radial tires or also get some new wheels.

It's not hard to do this but you do need to spend a little time doing the research correctly.

PM me if you need help

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