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Friday, July 26, 2019

Max Load - Max Inflation & Cold Inflation confusion continues

From an RV Forum "My understanding is that the number on the side of the tire is not a recommendation. Rather it is the maximum cold temperature that the tire is rated for. You should never run above that number. Without actual weights, I would run the recommendation from the motorhome manufacturer on the placard near the driver's seat, or a little higher, but never over the sidewall number."



The information on the tire sidewall is a statement of the maximum load capacity and the cold inflation required to provide that load capacity.

Different tires have the word "max" placed in different parts of the phrase which leads to the confusion.

Here is a post on passenger tires.

Here is a post on when the Minimum inflation is the Maximum.

Note: A a statement of Max Load "At" stated inflation such as "Max load 3,000 lbs at 80 psi" is leaving off the implied "minimum" before the 80 psi.

Also introducing the word "cold" seems to confuse some.

Maybe this will help:


Tire inflation should only be set when the tire is at ambient temperature and has not been warmed by being in sunlight or driven on for the prior two hours.

A tire may be loaded to the number of pounds stated on the sidewall, only when the tire has been inflated, when cold, to the stated pressure on the tire sidewall.


The problem is that these phrases don't fit in the same place as current "Max Load 3,000 lbs at Cold inflation 80 psi"



Friday, July 19, 2019

Lug nut torque. A critical SAFETY item.

Those that have read their owner's manual, may recall seeing a mention of the need to check the lug nut torque. In this case, we are talking about how "tight" the nuts should be to ensure the wheels stay on our vehicles. Others may have a warning label on the inside of their RV. This is from my Class-C Coachmen.

Note: Class-A and RV with 10 bolt wheels probably require a service truck and torque in the 450 Ft Lb range, so this info may not apply to you as you will need significantly different tools.






First the simple answer:
- Look up the specification in the Owner's Manual.

Here is a sample page from Heartland RV. Note they tell you how often to check and how tight to make the lug ut and even the pattern for the sequence of tightening the lug-nuts.


- With your torque wrench follow the instructions for both frequencies and how tight the lug nut should be.

- What to do if you don't know what a torque wrench is? Watch this video

- What to do if you don't own a torque wrench?
There are a bunch of videos that compare wrenches HERE   But you can get an OK wrench like THIS for $20  or a better quality one like this for $100.   You do not need to buy the most expensive wrench as hopefully you will be only using the wrench a couple times after a tire or wheel was changed on your RV. Or maybe a couple times a year per your Owner's Manual instruction  For infrequent use I have found Harbor Freight non-digital adequate. Just be sure to get a 1/2" as your lug nut torque spec is probably in the 75 to 130 Ft-Lb range. Don't get a wrench that is right at your spec as it will probably have a short life and may not be accurate at 100% of its rating.


Now some background:
Do not lubricate the stud or lug nut with oil. This can affect the torque reading and could lead to stripped threads in lug nuts ($) or broken wheel studs ($$$).

This web site gives a technical background on why not to lubricate lug nuts.


Some Technical Info for those that want to know more:

Torquing a bolt is to get a certain amount of stretch out of the bolt. If torquing a bolt to 80 Ft-Lib, it is technically more accurate to say "When you torque this bolt to 80 ft-lbs, it will cause it to stretch by .0030 inches". In critical applications such as race engines, many engine builders use certified bolts and actually measure the stretch of the bolts rather than using a torque wrench.

Think of the bolt like a spring. When you torque it, you're stretching it so that it exerts force on what you're holding together. This video goes deep into bolt stretch vs torque.

When I was driving my race car, confirming the torque of the lug nuts was MANDATORY before each track session. With this as a standard practice, I never had a wheel come lose in 30 years of racing.

##RVT906




Friday, July 12, 2019

Why inflate Motorhome tires differently than Trailer tires?

Found the following in a thread on an RV owner's forum. This came after there were comments about the advantages of inflating trailer tires to the tire sidewall inflation number but inflating Motorhome tires based on the measured load on the tires.

 "Such a hard concept for most to understand."

As an actual tire design engineer, not just someone that has used a lot of tires or bought or sold a lot of tires, I feel I might have a slightly better understanding of the science behind why tires fail.
I try and make the information easy to understand but I find that many simply refuse to accept the fact that my 40 years experience that includes thousands of failed tire "autopsies" might qualify me to give sound advice.
If you simply look at the experience of three groups of tire users. Excluding punctures or pothold impact breaks.
1. Regular motor vehicles. People get about 40 to 50,000 miles before the tires "wear out"  less than 1% experience tire failures.
2. Class-A and Class-C motorhome users. Many only drive 5 to 8,000 miles a year. It is recommended that starting at 5 years of age, tires be professionally inspected. This does not mean a simple walk around to look at the tread depth but close inspection with good lighting. Maybe even using a pit to allow the inner sidewalls to be inspected. Annual ispections thereafter are recommended and replacement at 10 year tire age "no matter how good a tire looks" This group also has a low structural failure rate not tracable to air leak or impact.

3. RV Trailer users "Towables". Based on numerous reports of higher structural failures i.e. belt/tread separations, and some strange patterns left in lose gravel where a trailer was turned 180 degrees I had some computer simulations run and the numbers provided an explination for the "why" towables have a mush worse structural failure rate. The forces inside the tire structure are significantly higher (+24%) in trailer application (i.e. towables) than in motor vehicle applications This force is identified as Interply Shear and it shows up as trying to separate the top steel belt from the bottom steel belt in radial tires.

While it would be possible for RV Trailer companies to make design changes to trailer suspension to allow for "passive steering" as seen on large cement trucks with a tag axle, I doubt they would go to the expense simply to extend tire life.

While lowering the actual load on a tire in trailer service can lower the Interply Shear force I doubt that it is possible to lower the load by 40 to 50%. One thing trailer owners can do to lower this force is to increase the cold inflation to the tire sidewall "max" Sorry to say you can not reduce the Interply Shear to zero as this is the nature of radial tires.

Friday, July 5, 2019

Modern radial tire assembly -Worldwide

A very good 10 min video from a number of different companies on how modern radials tires are "built," how some of the material is processed, tires are cured, handled in the warehouse and even how some retreads are made.

There isn't a tire plant worker in the world (China, Europe, or the US) that would not recognize what the equipment is doing.

Yes, the machines are painted different colors and the robotic handling of the tires is done with different equipment but the end result is almost identical.

This uniformity in the process is why, I have so much difficulty in accepting the idea that because a tire is built in one country, it is automatically more likely to fail than when it is made in another country.