Sometimes people use their own terms when asking a question. Some RV forums have people answering questions using incorrect terms, which can mislead or confuse others. This is just a “failure to communicate,” which doesn’t help anyone.
Here are some common terms regarding tires with their correct definition or example of proper use.
Load Range: A letter code—E, F, G, H, etc.—that establishes the inflation and maximum load capacity for a given tire size. (This replaces the old, out-of-date “ply rating” term.)
GAWR: Gross axle weight rating is the maximum design load capacity on an axle. You should never exceed that number.
GVWR: Gross vehicle weight rating is the maximum design weight of a vehicle. You should never exceed that number. Note that the GVWR may be lower than the mathematical sum of the front GAWR plus the rear GAWR.
Tire load capacity: This is the number of pounds a tire can support at a specified inflation. The Load & Inflation tables establish the various steps in load capacity associated with a level of inflation. Sometimes people use the term “tire weight,” which is incorrect. A tire itself may weigh 39 lbs., but it may be able to support 2,800 lbs. when properly inflated, according to the Load & Inflation tables.
CIP or cold inflation pressure: This is the measured psi (pound per square inch) in a tire when the tire has not been warmed from running or from being in direct sunlight for the previous 2 to 3 hours. Technically, we are always talking about the cold inflation pressure whenever we are discussing tire “inflation pressure”.
Hot inflation pressure: Sometimes we may want to discuss the pressure in a tire when it has been warmed from running or from being in sunlight. If this is the case, the term “hot inflation pressure” should be used to be sure we all understand the condition of the tire being discussed. You should not be bleeding down the inflation of a hot tire except under special, specific conditions.
TRA or Tire & Rim Association: This is the U.S. group that published the Load & Inflation tables for tires made for use in the USA. There are similar organizations with similar tables in Europe (ETRTO) and Asia (JATMA). The numbers of pounds (Kg) and the associated psi (Kpa) are many times the same or could be slightly different. The difference is due to using SI or metric dimensions, and rounding and conversions between units. It is legal to sell tires in the U.S. that were designed to ETRTO or JATMA standards. However, the tires must still have the symbol “DOT” and the individual tire serial code molded into the tire sidewall. And they must still be certified by the tire manufacturer to be capable of meeting all the appropriate DOT regulations.
DOT or U.S. Department of Transportation: The regulatory agency that specifies the performance requirements for tires intended for use on public roads.
“DOT Certified”: This incorrect phrase is many times used by those who do not understand the regulatory process. The DOT does not certify any tire. It is the tire manufacturer that is responsible for “certifying” that all tires they sell are capable of meeting the DOT Federal Regulations.
FMVSS or Federal Motor Vehicle Safety Standards. You can read Subtitle B of these standards HERE.
Tandem axle: This is found on many trailers where each axle has one tire on each end of each axle. Here is some information on tandem versus single axle trailers, and on dual versus tandem tires.
Tire loads: Tire loads are ALWAYS based on the load on tires on a specific position, i.e., RF, LR, RR or LR, etc. Front tires are “single”. Rear tires on most motorhomes, mounted side by side, are “duals”. The tables show load capacity for individual tires in the “single” position or in the “dual” position. You need to pay attention, as some companies publish their own tables and show axle loads or axle end loads, which can lead to confusion.
Tires on one end of an axle do not “know” what the load is on the other end of the same axle. RV companies, however, assume axle loads are perfectly balanced end to end at 50%/50%. This allows the RV company to select the smallest (lowest cost?) tires capable of supporting 50% of the published GAWR. But if one end of an axle loads that tire to 55% or 60% of the GAWR, we would be looking at a tire overload situation.
In trailer applications with two or more axles, there may be some load transfer between the axles such that the ends on the right side may share some of the side-to-side unbalance. But I know of no case where all the side-to-side unbalance is shared equally among all the tires on one side of a trailer.
I hope this helps your understanding of tire terminology.
Roger Marble
RELATED
- Tire terminology: Basic tire construction, Part 1
- Tire terminology: Basic tire construction, Part 2
- Tire terminology: Basic tire construction, Part 3
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