Tuesday, September 25, 2007

TIRE PRESSURE - THE INSIDE STORY

Performance factors

A tire is a pneumatic system, which supports a vehicle's load. It does this by using a compressed gas (usually air) inside to create tension in the carcass. It is important to realize that a tire carcass has a high-tension strength, but has little or no compression strength. It is the air pressure that creates tension in the carcass and allows the tire to function as a load-carrying device. That's why inflation is so important.

The tire does not support the load-the air pressure does. The manufacturer's ratings for the maximum load and inflation pressure are critical tire design elements. If not observed, the handling and performance of your motorcycle will be greatly affected.

Two points to remember:

  1. A tire must transmit handling (acceleration, braking, cornering) to the road.
  2. A tire also acts as a spring between the rim and the road. This spring characteristic is very important to the vehicle's ride.

Under/Over inflation

Under inflated tires can result in imprecise cornering, reduce ability to support the load (traction), higher running temperatures, irregular tread wear at the edge of the contact patch, fatigue cracking, overstressing and splitting in the sidewall.

Special Note: If splitting happens on the inside of the tire and is hidden from your sight, then it can spread and eventually result in a blister and blow out of the sidewall. This sudden deflation may cause loss of control with serious consequences.

Over inflating tires does not increase load carrying capacity, but will result in a hard ride through the transmitting of shock loads to the suspension, reduces the a tire’s ability to withstand road impacts, and cause accelerated tire wear in the center of the contact patch.

Temperature Effects

Air pressure is affected by temperature. The air under pressure in a tire is no exception. Typically, an inflation pressure can change by 1 psi for every 10 degrees Fahrenheit of temperature change. Higher temperature means increased pressure.

For example, if a tire is inflated to 35 psi on an 80-degree day in July, it could have an inflation pressure of 23 psi on a 20-degree day 6 months later in January. This represents a normal loss of 6 psi over the six months and an additional loss of 6 psi due to the 60-degree temperature change. At 23 psi, this tire is severely under-inflated.

If the rear tire is inflated to 40 psi on a summer morning with a temp of 60 and a high of 90-degrees--tire pressures can reach around 44 psi possibly exceeding the tire’s maximum rating.

Checking Tire Pressure
How does one go about checking tire Pressure? Several factors determine recommended tire pressure: Weight, ambient temperature, road conditions, and comfort (handling) Each factor must be considered when adjusting tire pressure.

Check cold tire pressure frequently with a good quality gauge that holds a reading, and always before extended trips.

Check your pressures daily when you are on a trip and remember that every 1 psi of air lost is approximately equivalent to losing 60 or 70 pounds of load carrying capacity.

Heavier loads-dual riding and/or luggage

For high-speed, fully loaded or dual-riding touring motorcycle applications, inflate front tires to maximum recommended by vehicle manufacturer for OEM fitment and rear tires to maximum load inflation pressure on sidewall. Notice that that front tire is inflated to the motorcycle manufacturer recommendation, not the sidewall. This is the optimal pressure for best braking performance under the load conditions tested by the manufacturer.

Another rule-of-thumb is to increase both front and rear tire pressure by 2 psi for every 100 lb. (up to maximum motorcycle load capacity stated in the owner’s manual.) All increases to tire pressures are usually within 4 psi of motorcycle manufacturer recommendation for an unladen motorcycle but never exceed maximum pressure stated on tire.

Making life simple

Check the tire pressure in the morning before a ride. If the ambient temperature is between 50 to 60 degrees; set the psi to motorcycle manufacturers specs for OEM tires. Any variance in conditions usually takes care of its self after that with the exception of extreme load changes and adjustments for rain, snow, dirt, or grass.

What if my tires are not OEM?

Call the Tire manufacturer and say the following:

Hi my name is [that would be you]. I recently bought your [Model] tires for my [make, model, year] and was wondering what the recommended inflation is with an ambient temperature of 50-degrees Fahrenheit and [ x pounds—that’s your weight] of load.

After he/she recovers from the shock of your remarkable intelligence, you will have your baseline for making adjustments to tire pressure.

Thanks to Dunlap and Metzeler for providing technical data.

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