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Friday, December 8, 2017

4-Corner weights and a "Meaningful" measurement.

I was following a forum thread on using closed state scales to learn your "4 corner weights" This is possible in WA & OR and some other states. 
Some commenters felt the practice was not wise as the area around the platform scale might not be perfectly flat. I posted my thoughts on the topic.

Not sure if I understand the concern over the accuracy of using a large platform scale to try and get a handle on your side to side loading.

I have a blog post on "Measurable vs Meaningful" that some might want to review.

When setting the inflation based on the published Load & Inflation tables, you should always "err on the high side." This means if your measured weight were 2,005# you should go UP to the first weight that exceeded 2,005# and maybe even higher.

Let's assume the table for your tire gave 90 psi at 1,900#  and 95psi at 2,210#. I would recommend you select 95psi as your MINIMUM inflation.  But what would you do if the chart for your tire showed an inflation for 2,010# or 2,005# or 2,000#? Would you cut things so fine as to select the inflation for these loads?  I hope not. What might happen if 50 minutes after getting the tire loads you top off the fuel and buy some groceries? You are now most certainly over the measured 2,005#. Are you gonna run back to the scales?  I wouldn't, and I would not need to as I have a built-in cushion of load capacity over the minimum needed to support my actual load.

We haven't addressed the question of your tire pressure gauge accuracy. Unless you have access to ISO Certified laboratory gauges as I do, you probably need to assume that your gauge is off by at least one increment reading of the pressure scale. For some truck type stick gauges that means 5 psi. What is your cushion that covers your gauge accuracy?

My "cushion" results from a few actions.
1. I always go up in the charts to the next inflation level and if my measured load is within 100# of a level, I will go up TWO levels of inflation which would be 10 psi.
2. I also add 10% to the selected table inflation and again round up to the next "0" or "x5" pressure

The additional 10% means you can avoid having to chase the Cold Inflation level every morning when the temperature drops and pressure drops a few psi.

When I checked my "4 corner weights" on the state scale was the area beside the platform exactly level? No, it wasn't, but it looks close enough that even if my weight was off by 100# my other adjustments would more than compensate.


Friday, December 1, 2017

Are you smarter than your tire salesperson? With Michelin LTX you may need to be.

As an RV owner, I believe that in most cases it is important for us to know as much as possible about our tires. Sometimes this means knowing more about our tires than the average salesperson.

Here is a real-life example:

An owner of an older Travel Trailer posted
"Please correct me if I'm wrong... The Michelin Defender LTX M/S 235/75R15 is a true LT tire and does not need to be derated?

Shopping for tires on my Airstream 2017 23FB. I'd love to just go with the 16" SenDel S02/Michelin combo but not sure if the clearance is there for them."

Just before the above post it had been pointed out  the correct designation is P235/75R15 XL
The "XL" is the tip-off as that stands for "Extra Load" which is only found in Passenger type tires.

LT tires have Load Range C, D, and E
P-type have Standard Load (no special marking)  and XL which is lower in inflation than an LT-C.

Your tire dealer should have made clear the type tire they are talking about.  Sometimes RV owners need to know more than the salesman if you want to get the tire you need or want.

I had also previously posted on that thread:
"Be sure you understand if your LTX tires are "LT" type or "P" type. If P you need to calculate the actual tire capacity when used on Trailer, SUV or PU-truck.

The adjustment is 
(Load molded on tire)/1.10 = Load capacity on the RV or truck or SUV application.

No load capacity adjustment is needed for LT type tires if placed on an RV, truck or SUV.
The LT in "LTX" does not make a tire an actual "LT type tire."

I don't know if the marketing folks at Michelin realized the confusion they were spreading when they came up with the name "LTX" and put that designation on BOTH LT type and P-Type tires. I would not be surprised if there aren't a good number of RV owners who think they have an LT tire when what they were sold was actually a Passenger type tire.