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Friday, January 17, 2020

LT vs ST for RV trailers?

It's no longer 1970 with a National 55 mph speed limit with ST tires being introduced i.e. "pushed" by a large tire co, as an an alternative to the real LT truck tires of the day. I have been told that one of the "selling points" for this then-new type of tire was that "no one would ever pull their 15' trailer faster than 55 with their bumper hitch."
Well, times have changed. We now have '1-Ton" Diesel Pickups that can pull a 35' trailer up the side of a mountain and run 75+ all day long. Trailers now come with two bathrooms, residential refrigerators, and two AC units multiple TVs and other heavy equipment that we never considered possible in 1975.
What hasn't changed is the fundamental science that a tire's load capacity is still basically   "Load = Air Volume x Air Pressure".  In fact, the actual load formula still used for current ST type tires is identical to the one used in 1970 when the 65 mph speed limit for ST type tires was in the industry standards. I can find no mention of alternate materials delivering increased load capacity in those standards. Yes materials have improved and radials are better than bias tires but basically the only benefits all these "improvements" delivered is longer life and better fuel economy and tread wear. Some construction features such as the addition of Nylon cap strips or full cap ply have allowed an increased resistance to the heat from higher speeds but I haven't seen any increase in load capacity in either Passenger or LT type tires over the past 40+ years.
As I have posted in this blog, "There is no free lunch". If there is, why haven't tire companies increased the load capacity of P and LT type tires if all these "improvements" are available to tire engineers?
What "feature" is in ST type tires that gives then the +10% to +20% more load capacity over an LT tire of the same physical size?
IMO there is no reason why RVs could not be supplied with LT tires other than it would increase the cost of the tires to the RV company.
Remember it is the RV Co that is responsible for selecting the tires they provide. We all see numerous posts from some who are running truck 17.5 size or LT 16" tires with improved durability so I see no reason to believe that the entire market could not benefit from a switch from ST to LT type tires.
One thing is that the LT tires that have to meet the new DOT standards (FMVSS 139) that were introduced in 2002 are probably more durable than the ST tires that are still only required to meet the standards on the 1970's.

I am not saying that some of the newer ST tires with  newer construction are not significantly better than the same size tire from the 90's but IMO there is still a limit.

Friday, January 10, 2020

"I never hit a pothole"

The title for this post is a direct quote made by many people who have suffered some tire failure. If you think about this claim for a moment and then think about the road conditions we all see in our day to day driving experience one has to wonder just where these people are driving.

In our life as drivers, I am sure that all of us have hit some objects other than simple 1" deep pot hole. Some objects could include bricks, lumber, railroad tracks, curbs, and puncturing or cutting objects such as nails or screws.

Here are some examples of tire damage I discovered in my work. The first three were each submitted with a written claim that the tire was "defective" so I can only assume the driver made little or no effort to inspect the tire or wheel for evidence of what might really have happened.
The last tire, with the wooden stake through the tread was submitted with a claim that the tire was defective because it was "making noise". IMO the noise would have been from the wood hitting the pavement at speed.

I have no expectation of changing the minds of those who want to make a claim of never hitting a curb or pothole or another object. I do want to relate the findings I recently discovered in a technical paper on tire forensics and impact damage.

In the paper, the reference studies involved a process of obtaining a couple dozen both new and used tires P type, LT type and sizes appropriate on Some Class-A RVs. The rim diameters ranged from 14" to 19.5". Tires came from 8 different tire companies. Each tire was inspected and no externally visible signs of damage were found.  Each tire was run on the appropriate DOT laboratory tests to confirm the tire was in good condition. Each tire was then mounted and hit with a heavy pendulum to simulate hitting some object or pothole on the road using a machine like this from STL company

 The tires were again visually inspected and any evidence recorded. Finally, every tire was run on a drum durability test under a load until failure occurred.

Failure Rate  100%
Miles to failure for new tires   1,826 mi  to  41,400 (avg. 11,586)
Miles to failure for used tires  5  mi      to    7,458   (avg. 2,092)

Can anyone here list every object they knowingly or unknowingly hit over the past 2,000 miles?

I know that I have posted a number of times that tires do not always fail at the instant of impact or damage. Clearly, these above studies confirm these observations.

Some reference material for those interested:
Price, V & Follen,G (2019). Road Hazard Impacts: Their Influence on Radial Passenger Tires and the Forensic Signs They Leave Behind
Bolden, G. C., Smith, J. M., & Flood, T. R. (2001). Impact Simulations in the Lab. Tire Technology International.
Bolden, G. C., Smith, J. M., & Flood, T. R. (2005). Impact simulations - what happens when a tire/wheel impacts a road hazard. Tire Technology International, 44.
Bolden, G. C., Smith, J. M., & Flood, T. R. (2006). Structural Impact Damage Under Varying Laboratory Conditions. Tire Technology International, 10.
Gent, A. N., & Walter, J. D. (2005). The Pneumatic Tire. Washington: National Highway Traffic Safety Administration, U.S. Department of Transportation.
Giapponi, T. R. (2008). Tire Forensic Investiagation Analyzing Tire Failure. Warrendale: SAE International.
McClain, C. P., & DiTallo, M. A. (2001). Tire Examination After Motor Vehicle Collisions. In K. Baker, Traffic Accident Collision Investigation (9 ed.). Evanston, Il: Northwestern University Center for Public Safety.
Tire Industry Association. (2005). Passenger & Light Truck Tire Conditions Manual.