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Thursday, January 25, 2018

Another question on why Trailer tires fail more often than tires on motorized vehicles.

On a thread on an RV trailer forum Peter made this statement
"At highway speeds interply shear is not as much of an issue, as I understand things."
I posted this reply:
Speed itself is not the issue that causes Interply Shear in Radial tires. Two things happen to radials as they are driven.
One is the tread (and belts) must flatten out when the tire contacts the road. This results in the steel cords moving relative to each other. This is a shear force.
Second is that when the contact patch or "footprint" is forced to turn a corner there is some slipage between the direction of travel that aligns with the center of rotation and the actual direction of travel.

On motorized vehicles the front tires have a slip angle and the side forces are what actually results in the vehicle turning. But of you were to project the center of rotation toward the center of the driving radius you will find that those centers are close together This is due to the "Ackerman" designed into the front end alignment.
Yokohama has this nice graphic.

This image has been resized. Click this bar to view the full image. The original image is sized %1%2.

Multi axle trailers however have two axles and 4 tires, with no tire rotating around a centerline that points to the center of the turn radius.

Here is what happens to trailer tires.

These tires are forced to higher than normal "Slip Angle" through any turn, Not just the extreme tight turns when backing into a parking space.
Damage to tire structure is cumulative and while a small turn imparts less shear than t tight ture even small forces can do damage on a molecular level.
Here are some links to various articles on Interply Shear.

Duals on a large truck slip on the pavement when forced through tight turns and with inflations over 100 psi they do not deforn as much as trailer tires with lower levels of inflation.

Increased inflation will lower the interply shear. It will never lower it to the level seen on a motorized vehicle Only Passive Steer axles on trailers similar to wat is seen on the back end of cement trucks can lower the shear too but I don't see any RV trailer company offering that expensive alternative. Afterall they know you can't make a warranty claim on failed tires on the RV company as belt failures from Interply Shear is a long term proposition.

Friday, January 19, 2018

Snow chains in RV application

Most of us only deal with ice in our drinks or as used to get our beer cold but some folks may find themselves wanting? to drive in areas where roads get snow and ice covered and even in areas where the use of chains is required.
The question is; What does the RV owner do when it comes to using chains in the dual application?

If we choose to travel in such areas, it is important to have the correct type and size of "chains" for our application or we may end up doing damage to both our tires and the side of our coach.
There are a variety of types of chain set-ups and even some alternatives such as "cables". I cannot address the legality of the alternate styles in all areas other than to offer that if there is an advisory that chains must be used and you get stuck and do not have chains on the unit you may face some fines and other penalties so you need to confirm the details of the requirement in your area before spending your money or before traveling out and about on snow and ice covered roads.

If you do not have duals your choice is to be sure you select the equipment correct for your size tire.

Duals have a few options. There is going to be a difference in cost, weight and ease of install so you need to do some research.
There are "Triple Rail" as seen in this video. There are some "cable" type systems as seen here, and some newer designs as seen HERE .

I have not been involved in any direct comparisons so cannot offer any advice on relative performance. For that, you may need to do some additional investigation on the internet or even talk with some over the road drivers. Looking at the different videos it appears to me that some designs may be more durable than others.

I do know that driving on dry roads with chains can cause some serious tire damage so only you can properly evaluate what system would be best for your situation.

Friday, January 12, 2018

Why do trailer tires fail more often than tow vehicle tires?

Here is the question posted on a travel trailer forum.
Why so many trailer tire failures and so few, if any, tow vehicle tire failures?

This post could be considered Part 2 of my post of last week. 

The primary reason IMO is staring everyone in the face and the information is molded onto the tire sidewall so you do not need to be an Einstein, but almost everyone chooses to ignore the science so they can save a few bucks.

Fact: Tire load capacity is a function of air volume (tire size) and inflation pressure. This is well established and is confirmed by every Load & Inflation chart printed. Increase size or increase pressure and you increase load.

If we all drove around on tires with LT level sizing and inflation there would be almost no failures other than those caused by road hazard or punctures.

People want softer ride, better fuel economy and lower cost so alternatives were introduced into the market.

LT tires can be considered the standard for tires designed to carry load at highway speeds.

P type tires have a higher load capacity than identical size LT tire but if you check the actual load on most car tires, you will see they have 15% to 25% or more "Reserve Load" capacity (more capacity than needed). Inflation is set for cars based not on load but ride, handling and fuel economy. You also know that P tires must have their load capacity de-rated when P tires are used in truck, SUV or trailer application. When P type tires are run with zero reserve load you will probably get more failures. (See Ford Explorer crashes.)

ST tires have a higher claimed load capacity than LT tires and TT companies use the smallest (lowest cost) tire that can, most of the time, meet the specification for load.

How then is it that ST tires are rated to carry more load?
The load formula for ST tires is based on a 65 mph Max operation speed and an expected shorter life. The formula was developed back when bias tires that wore out at less than 15,000 miles were the norm

Since 1990 when ST tires were still "Alpha" sized ( H78-15ST) the load formula has not been changes as far as I can determine.

The science indicates that if you want tire durability on your trailer as you get on your truck maybe you need to consider running the same type tires i.e. LT type and of course that means you also need to use LT Load & Inflation limits.

Here is some info I posted Sept 14 2011

"Here are some facts from the Tire & Rim Association industry standards book 

P235/75R15 105S (Standard Load -35 psi @ max load)
2028# 35 psi 112mph on a Passenger car
1844# 35 psi 112mph on a SUV or P/U or trailer (no Dual)

LT235/75R15 LRC
1512#single 1377# Dual 50 psi 85mph

ST235/75R15 LRC
2340#single 2040# Dual 50 psi 65mph

NOTE I did not mention the actual load on a tire but only the specified max. Weight data indicates many TT have one or more tire in overload.

Friday, January 5, 2018

ST type tire max speed

Originally Posted by LI Pets 
The 65mph is not really correct it depends on load and air pressure.

They can go to 80mph.
The above is from an online forum.

As I have pointed out previously:

The load formula that populates the Load/Inflation table, used by the tire industry for ST type tires is based on the assumption that 65 mph would be the MAXIMUM operation speed. If the operation speed was to be between 65 and 75 then users were to use a 10 psi higher value for their measured load.

Example ST235/75R15 with 2,030# measured load needs to run 50 psi minimum cold not 40 psi cold.

If running 75mph to 85 the Load capacity number must be reduced by 10% in addition to the +10 psi adjustment.

Example ST235/75R15 would be rated to carry (2,030 x.9) or 1,827# at 50 psi.

I know this adjustment process seems backward but that is the way tire loading is calculated.

Now I am sure some will say that the new ST tires come with speed ratings faster than 65. IMO many of these ratings were applied just to avoid import taxes. I know of no magic rubber that somehow gives an ST235/75R15 LR-C the ability to support 2,340# at 87 mph with 50 psi in it while an identical sized LT235/75R15 LR-C is only able to support 1,985#.

Before you say "Ya but the tire companies probably made big improvements in the new ST tires", I would ask what makes you think the tire companies would not want to be able to offer better load capacity to their LT tires?

Load capability is basically the tire air volume x air pressure, with adjustments for speed and expected service. So if you have a P/U pulling a TT the "service" would be the same and the speed would be the same so how can the tire with "ST" on the sidewall carry more load at the same speed?


Now you are more than welcome to believe in magic or marketing claims but IMO using the load /infl tables without doing the adjustments will probably result in an increased likelihood of belt separation. So when you have a failure please do not post something here along the lines of  "I just had a blowout. I didn't hit anything and always check my air".  Tire failures usually occur because of cumulative internal structural damage from heat and time. The excess heat comes from the combination of speed/inflation/load.