Technical Session 2009

Tires

by Cam Finnigan
November 18, 2009

This year's Technical Session was hosted once again by Volvo of North Vancouver. The subject was tires and was presented by Jason Leber. Jason was a former Volvo Master Tech, and has worked for Yokohama tires. He covered many topics including tire sizing, tire ratings and tire safety.

Introduction

Attendance was unusually low at this year's tech session, owing possibly to the heavy rain, wind, deep puddles, power outages, crazy traffic and various other factors that we have seen in November. For those attending, the trip was certainly well worth it. The Club is thankful to hosts Volvo of North Vancouver and Jason Leber for the presentation.

Jason is not only well known within the club for presenting or assisting in technical sessions all the way back to the mid 1990s, but has had a wealth of experience in the local automotive scene both as a Master Volvo Technician, a technical rep for Yokohama Tires Canada, and as an educator with BCIT and Volkswagen Canada.

There were probably people wondering what there could possibly be to talk about regarding tires. For those in attendance, we soon realized that there is a whole lot to talk about regarding tires, and two hours really only just scratches the surface.

Ratings

The first topic was that of size and marking. There is a wealth of information embossed on the sidewall of the tire. This information not only tells about the physical dimensions of the tire, but tells about the construction and rating. The example tire that Jason showed us was marked “195/65R15 91R”. This code breaks down as follows:

Not all tires are marked with an aspect ratio. All performance tires will show the figure, but less expensive and perhaps older tires may not. For tires that are not marked with an aspect ratio, the default is 82%. This would certainly be true for vintage tires.

The load rating is based on the volume of air contained in the tire as it is the air that holds the vehicle up and not the tire itself.

Speed rating of a tire will give some indication of how well a tire may resist failure due to the tortures of high performance driving. See the associated table for a relation between the speed rating letter and the associated maximum rated speed.

To ensure that people are not tempted to overdrive their tires with their high performance automobile, tire shops will not install tires of a lower speed rating than what the vehicle was originally specified for. If, for example, your car was born with “V” rated tires, and you want to buy and install “H” rated tires, the tire shop is supposed to turn you down. The interesting point about this is that winter tires are typically not available in high speed ratings, the rationale being that you shouldn't be driving as fast.

In addition to the size and rating markings, a DOT “Department of Transport” marking can be found on all tires. This molding tag will indicate the tire plant, location, and date of manufacture. Typically, the last four digits are in the form of week of the year and year.

The standard rating for tires is the UTQG, or Uniform Tire Quality Grading. This standard rates three aspects of tire performance: tread wear, temperature, and traction. The subject tire that we looked at was rated 460, A and A, respectively. Tread wear is a linear scale of how many kilometres a tire can be expected to travel before the tread wears down to a set point. An average performance tire is likely to score in the range of 60 to 320, all season 400 to 600 and ultra high life 600 and above. The scale factor is considered top secret, but rumoured that it might be something like 200 km per unit. Interestingly enough, winter tires normally do not come with wear ratings. Temperature is an indication of how hot the wire can be operated at speed before being subject to catastrophic failure. Traction is the traction result of a straight-line braking test on wet pavement.

Winter tires are now marked with the mountain and snowflake logo.

The traditional M+S really does not necessarily mean a good winter tire.

Construction, Damage, Pressure and Loading

Tire construction is composed of layers of fabric or fibre of materials such as rayon, polyester, aramid (a strong, heat-resistant synthetic fibre), steel, nylon and others. Each material has its own benefits and costs. Different materials can also result in a different feeling of how the tire reacts with the road.

Jason told us that most damage caused to tires is due directly to incorrect inflation pressure, mostly under inflation. What pressure should you run your tires at? There is really no set answer. It will always be marked on modern cars, either on the door inner or in the owner's manual. You should never run your tires at a pressure less than those markings. In addition to the vehicle recommendations, the tire will be marked with maximum pressure, cold, on the side of the tire. This is the highest permitted inflation pressure. The tire we had indicated 44 psi maximum.

Maximum load ratings are normally given for 36 PSI. However, running them at 44 PSI will give less rolling resistance and better fuel economy. Air pressure changes about 2 PSI for every 10 degrees C. Jason suggested the best bet is to start with higher pressure and monitor the tread carefully for wear using a tread depth gauge. You should gauge the tire across the tread at several places around the circumference. If the tread shows uneven wear across the tread, then the pressure should be adjusted accordingly. Winter tires normally give better grip if they are on the softer side.

Inspection for Damage

It is important that a tire be inspected periodically and any debris lodged in the tread be removed. If rocks or other debris is left there long enough, objects can begin to wear into the surface of the tire, a situation known as stone-drilling. If left long enough, this could eventually expose the core of the tire. Inspect the sidewall for curb rash, abrasion caused by driving the tires up against curbs. Watch for bulges, especially goose-eggs. These are areas when air has leaked through an inner layer causing separation or embolism inside the outer layer. This is a sure sign of some type of inner damage.

If the tire happens to be off the wheel, an internal inspection can be performed by rolling the tire along a smooth floor, leaning slightly over to one side and pressing the tire down against the surface. This flattens out an otherwise curved tire surface and should disclose any cracks that might adversely affect the tire.

Tread Depth and Wear

On a new tire, tread depth will normally be in the range of 10/32 to 13/32 [7.9 to 10.3 mm]. You will find the tread bar in the bottom of a tread grove. When the tread is worn even with the bar, the tire is considered completely used up and should be replaced. When the tread is measured, do so at five places across the width of the tread, repeated several places around the tire.

It is common for the tire tread to develop into a sawtooth shape in the direction of travel. This is a sure indication of direction of rotation.

If at any time steel is seen protruding from the tire, the tire should be replaced. The beads should be carefully inspected to make certain that they have not been damaged in installation or removal. If any chunks are missing from the bead area, the tire should be replaced. Damage can occur in the shop if the bead are not properly lubricated before removing them from or installing them onto a wheel.

If a tire ever sustains a puncture it can be repaired with a plug, a patch or, better still, a plug and patch. This combination will not only keep air from leaking out, but will also keep water from getting into the core of the tire.

Advantages/disadvantage of Tire Types

Winter

• Good traction on snow and ice and the ability to shed water.
• Siping – those tiny squiggles cut into the tread.
• Now tires have complicated sipes for ice traction. Snow traction is derived from larger blocks.
• Tires now feature carbon which acts as a sponge to water. This provides grip from 0 down to -6 degrees.

All Season

• Satisfactory traction in snow, water and dry.
• Good wear characteristic.
• Circular groves are added to help channel water.

Summer

• Excellent grip in dry conditions.
• Acceptable shedding of water.
• Block width is commonly varied around the circumference of the tire to help break the harmonic pattern and reduce the noise generated.

How Many Winter Tires?

How many winter tires should you have on your vehicle, two or four? With AWD, your need a full set of matched tires for sure.

If you have only two winter tires, should they be installed on the front or the back of your vehicle? With two winter tires on the back, if the vehicle is driven through a slippery corner at the limit of traction, it will tend to understeer and normally go off the road in a straight fashion. Most people don't know what to do in a spin, and that can cause a roll.

Nitrogen Inflation

Traditionally, people have been inflating their tires with compressed air for a century. A recent popular trend is to use nitrogen for inflation. There can be a slight increase in cost, so is it worth the money and the inconvenience of finding pure nitrogen? What exactly are the advantages of using nitrogen. Air is 80% nitrogen and 20% oxygen, so what is the down-side? Air normally also contains a certain amount of water vapour and that vapour has a tendency to change the pressure to a greater degree over temperature. Eliminating that water content is supposed to help the tire maintain more constant pressure. An additional advantage is that nitrogen has larger molecules, so leakage rate is slower. This explains why your helium balloon loses pressure in a day. Green valve caps signify a tire filled with nitrogen. N2 runs cooler. Jason strongly recommends checking tire pressure frequently , which is far more important than running nitrogen.

Balancing

Colour marks on the sidewall. Yellow dot should be aligned to the valve stem, it's the heavy part of the tire. Red is the stiff spot and should go to the low spot on the wheel, if there is one. Road force balance balanced the tire with the equivalent road force applied to the tire. If the rotating assembly is balanced correctly, there should be maximum one weight per side. Two weights at different locations on one side can always be reduced to a single weight.