What "Size" are your tires? Part 1 of 3

 

What size tire is it?          This is Part one of a three part series:

That seems to be an early response whenever you ask a question about tires. Whether you ask about price, load capacity, or inflation, the first reply may be “What size is it?”  The answer you give should not be “It’s a 22.5”  or  “it’s a 225-R-15”. These are just partial answers and indicate to many that you don’t really understand all you should about your tires.

The reason you need to provide the complete tire description information is simply because there are so many possible replies and starting to narrow down the possibilities by properly identifying the tire is just the first step in learning the details so the person that is offering the help can provide the correct answer you seek and not just a wild guess. Knowing your complete tire description can be confusing and sometimes it seems as though we tire engineers and government agencies have conspired to make things difficult. What you need to remember is that there are three basic features that must be established first. Tire type, tire physical dimensions and tire strength.

This article covers a number of different type tires. Please do not skip over any part as the knowledge provided here builds on previous covered information.

Let’s start off with TYPE. This is usually a function of application. For most tires, there is a letter code as the first part of a “Complete” identification of a tire. For most tires sold in the US the code for tire type is either a “P” for Passenger, “LT” for Light Truck, “ST” for Special Trailer, or no letter for commercial or heavy duty. Tire engineers sometimes call these commercial sizes “TBR” type, which is short for Truck Bus Radial. If you are reading this article, most likely you have a Recreational Vehicle or RV of some sort so the use and application of tires on the various type of RV will be our focus. There are of course many other type tires. OTR for Off the Road or AG for agricultural or AT for All-Terrain or M for Motorcycle and others, but we will not cover those and will focus on the type, size and strength tires used in various RV application.

First, we will cover “P” type tires. Most of us own or have owned some form of passenger car that came with P-type tires. Older and smaller trailers may also come with "P" type tires mounted by the manufacturer. When our car required replacement tires, we seldom needed to think much about the proper size nomenclature, as it was the responsibility of the tire dealer to confirm the appropriate type and size tire that was needed.  Our car tires would probably be identified as a P195/75R15 94S or similar combination of letters and numbers. The P as you now know indicates Passenger car application. 195 is the width in mm.  Not the tread width but the maximum width. 75 is a ratio of the tire height from the wheel to the tread as a percentage of the tire width. “R” stands for radial. Since there are very few non-radial “D” or “Diagonal” construction tires, we don’t need to go down the road of old tire construction. The "15" is the wheel size. Finally, there is the “Service Description” which is a combination of “Load Index” number, 94 in our example and finally the “S” is the Speed Symbol. In the US, the speed symbol is really just an indication of the level of handling capability or steering response with increased handling potential as we move from Q to R, then S followed by T, U, H, V, W, Y and finally Z. Unlike Europe where you are required to replace tires with the same speed symbol, we in the US have the option of changing the rating but should expect the steering response to get slower if we go to a lower symbol. The final bit of information concerning the use of P type tires in RV application, the load capacity of a P type tire must be reduced by dividing by 1.10 when using a P type on a truck, trailer or “Multi-Purpose Vehicle” (SUV or Station Wagon) according to tire industry design standards.

We will continue with ST a LT and other type tires in Part 2 and 3.

Another example of how I arrive at tire inflation recommendation for Motorhomes.

Got this request and supporting information:

Can you help me find the proper tire pressure for the following?

Winnebago Class C motorhome on 2018 MB Sprinter chassis
Dual rear wheels, Tires are LT215/85R16 115/112Q  Load range E  (Continentals)

GVWR = 11030
GAWRF = 4410
GAWRR = 7721
Door placard says 61psi, front and rear.

Actual weight as loaded, including driver & passenger

Front = 4200
Rear  = 6600
Total = 10800

According to the Michelin load chart for this size tire, the front about matches my placard at 61psi

However, if my rear with duals is 6600, should I set the rear pressure closer to 50? 
At 61psi the rear impact harshness seems high over road cracks and expansion joints.

Would love to hear your comments as I’m thinking about lowering my rear pressure some.  Trying to improve ride, but not hurt tire performance.
 

My Reply:

Looking at your GAWR and scale readings tells me you are not overloaded. You are relatively close on the front and only have a 5% margin at 210#. The Rears are better off with a 14% margin. Given that I have seen some Class-C RV have 3% to 4% side to side unbalance, I would be more comfortable if you could reduce the actual load on the front tires until you can confirm actual "4-Corner" weights at a building supply scale or gravel pit, or some other location where you can get the individual tire loading.

Your weights in the rear are a bit lighter than I normally see as on my 24' Class-C which has scale tire loading of 1,900# & 2,100 on F  (4,000 axle)
and  3,550 &3,850 on R  (7,400 axle)

But back to your situation

I confirmed that the weight chart from Michelin is same as general industry standard. Sometimes Michelin has different numbers than the rest of the industry so unless we are running Michelin tires I tend to not even look at the Michelin charts. I also prefer to deal with individual tire loading and some Michelin charts are axle loads.

4,200 x 51% = 2142# and consulting the chart I find 60 psi as the minimum. I Also always suggest +10% inflation margin to learn the cold set pressure which gives 66psi

6,600 x 50% gives 3,300# 51% gives 1683 and the chart suggests 45 psi as the minimum.  Adding my 10% yields 50 psi.

I add this inflation margin to avoid the need to adjust my tire pressure whenever the Ambient Temperature changes. I don't adjust tire pressure till the 10% drops to 5% inflation margin.


BUT the 51%/49% side to side split is conservative and I am not comfortable suggesting lower than Certification Label inflation without knowing actual tire loading. So I suggest:
66 Front and no less than 61 psi on the rears.

Ride and harshness are really a function of chassis design, springs and shocks. Using the tires to "improve ride" by lowering pressure without all the numbers, may result in shorter tire life.

I trust that this is clear enough such that others can do the calculations on their own. 

 

##RVT1018

Are you qualified to drive an RV if you don't know how to check the air in your tires?

 Quick post on the need to know how to properly maintain your tire inflation.  Another personal opinion piece from this Tire Design Engineer.

Read this question on an Facebook page

Where do you take your motor home to get air pressure in tires checked?

There were a number of comments questioning if the person asking the question should even be driving. Some offered that they could get air at a truck stop. Others felt the person asking the question might not be sufficiently trained in the safe operation of a large Motorhome.

I offered the following.

Safe operation of your RV, and car or truck includes having the tires properly inflated ALL THE TIME. A tire failure is just not an expensive inconvenience but your vehicle on the side of the road is a safety hazard for both you and every vehicle that passes your disabled vehicle. More than half of all tire failures can be traced to their operation with insufficient air pressure. 

In my opinion every RV, 45' Motorhome to 8' trailer should have a Tire Pressure Monitor System or TPMS that have been properly programmed and checked for operation at least once a year. Since 2005 almost all cars have been equipped with this critical safety device as mandated by DOT. 

I can only guess why DOT decided to exclude RVs from this safety requirement. Was there push-back from the RV industry and the importers of lower cost ST type tires? As there might have been when improved qualification tests were implemented on Passenger and Light Truck tires in 2002 when ST type tires were excluded from the requirement to pass more demanding testing? IMO there is no excuse for not having a TPMS on your RV.

While it is true that personal injuries due to tire failures in RV application are minuscule compared to the number on cars and trucks, they are not zero. It does appear that reducing personal injuries are the driving force behind DOT safety requirements and with so few injuries occurring that involve tire failures on RVs there is probably little or no pressure on DOT to improve the safety of tires in RV application (ST type).

 If you check the TPMS each morning you should notice any loss of air pressure and can adjust the tire pressure while at the campground where you don't have a car passing 6' away at 70 mph. No tools are required to install a TPMS. No special mechanical knowledge is needed to program the system either other than being able to read the instructions. 

Claiming that you check your air pressure at each rest stop isn't good enough unless you can let us know how you check the pressure while driving 50 mph down the highway, or guarantee with 100% certainty that no valve core will ever leak after a tire had its pressure checked.

 

##RVT1017