Maintaining max tire pressure not best in all instances
Posted Jul 15, 2010 By Brian Turner
Email
Print
Tweet ThisEMC Lifestyle - "Hi Brian,(In a recent letter (June 17) concerning tire pressure by "Ian", I think I likely am the person referred to for giving the advice on tire pressure (to keep it set at the maximum spec printed on the tire sidewall rather than what's listed in the owner's manual). I worked (now retired) as a paramedic for 35 years and for the past 30 years have been teaching vehicle dynamics at my skids school.
We recommend max tire pressure at all times to assist the tire staying on the rim during emergency driving situations. As a result of physics and weight transfer during emergencies, tires are almost always pushed beyond their load capacity causing disastrous results when the tire pulls off the rim or actually rotating on the rim during panic braking due to excessive forces and weight transfer. I have experimented with and experienced both. (
Softer sidewalls result in less steering response....during sliding or skidding lateral forces cause the tire to roll pulling the tread off the road surface and eventually pulling the tire off the rim. (Softer tires wear quicker due to heat build-up from the sidewalls flexing excessively, require more gas consumption to rotate and result in less traction as the tread is not held squarely on the road surface. Softer tires will also "over-inflate" quicker due to the increase in internal air pressure caused by heat from the flexing sidewall. (
Max air pressure is "not" over-inflation, I have a collection of my old top quality tires alongside my skid pad that are never run at less than Max pressure, they are worn down to the steel cords in some cases, every one is worn as smooth as a table top and perfectly even from shoulder to shoulder. (
As for the "more harsh/rougher ride" concept, your comfort is mostly governed by the stiffness of the shocks and the thickness of the foam in your seat, not the air in your tires. Most people cannot tell a difference of ten pounds in their tires by the ride, but that same soft tire could end up killing you during a skid if the tire comes off the rim.(We compare air pressure to an insurance policy, hopefully you never have to require it, but when it's life or death you have to be ready and prepared."((
Bob
While running tires at their maximum pressure is fine for rubber treads that never leave a skid track or race course and are constantly pushed to the limit, when it comes to everyday driving by ordinary commuters, Bob's advice on tire pressure leaves something to be desired and I enlisted the help of the world's foremost tire manufacturer, Michelin, to provide an impartial and helpful response. The technical team at Michelin North America's South Carolina office was kind enough to review Bob's letter and supply the following information.
"The maximum inflation pressure as well as the maximum allowed load of modern passenger car tires are determined by international standardizing bodies in the tire industry (Tire and Rim Association, European Tire and Rim Technical Organization, Japan Automobile Tire Manufacturers Association, etc). The specifications for maximum load and maximum pressure are based on tire size and are listed in the published handbooks of the standards' organizations. These values are used to determine the testing conditions for certification by the pertinent regulatory agencies - National Highway Transportation Agency and Transport Canada for example. Maximum loads and maximum pressures are not in any way related to the specific vehicle on which the tire is used. Recall that the same size tire can be used on several different vehicles with different loads and different recommended pressures. ((
When vehicle manufacturers select tire sizes for use on their cars the maximum nominal load exerted by the vehicle cannot exceed a specified percentage of the maximum load of the tire size (again, a requirement established by the pertinent regulatory agencies - NHTSA or TC). This is to account for the increased dynamic loading conditions encountered by the vehicle in use (load transfer on cornering, braking or accelerating). This is what determines the minimum size tire required for a vehicle.
The actual tire size, and recommended inflation pressure are selected by the vehicle manufacturer after exhaustive testing looking at all aspects of performance - comfort, handling, fuel economy and accident avoidance. Before deviating from a vehicle manufacturer's pressure recommendation, consumers should verify the acceptability of that change with the vehicle manufacturer. Consumers should also ask about the impact of changing inflation pressures on vehicle warranties. Only the vehicle manufacturer has the capability to perform all the testing required to make this determination."
Saturday July 17 is All British Car Day at Ottawa's Britannia Beach Park. Entry fee for vehicles is $20 (and that gets you lunch for two) and admission is free for spectators. The show runs from 10 a.m. to 4 p.m. and more than 180 vehicles are expected.
Follow a lovely country drive to the Athens Lions Club's hosting of its 31st Annual Farmersville Exhibition (or Steam Fair as it's more commonly known) at the Athens Fairgrounds from Friday evening (July 16) to Sunday the 18. A reasonable admission fee applies, but there will be a full field of activities with music, vintage tractor pulls, stationary engine demonstrations, craft fairs, and the ever popular Shriners' Breakfast, just to name a few highlights. This is a hit for young and old alike.
If you have any questions, opinions, or stories on anything automotive please drop me a line, (By email to emc@perfprint.ca listing 'Question for the Car Counselor' on the subject line or by post to Record News Communications, 5 Lorne St., P.O. Box 158, Smiths Falls, Ont. K7A 4T1).
Yours in service,
Brian Turner
blog comments powered by Disqus
