Ya' see, in order to generate the force needed to turn a vehicle, the tires have to generate a "Slip Angle" - the difference between the direction the tire is pointing and the direction it is actually going.

The image to the right shows what happens to a tire during cornering.

And, of course, the greater the slip angle, the greater the cornering force, but also the faster the wear.

"Spirited" driving isn't good for tire wear rates.

That also means that vehicle alignment - especially toe - affects how rapidly tires wear.

Not only does too much toe cause rapid tire wear, it also interacts with other aligment parameters, such as camber, Ackermann (various spellings!), and inflation pressure to aggravate the situation. One could think of toe as a multiplier for conditions that cause bad tire wear.

UTQG Treadwear Test

This is a US government required test for all passenger car tires, except for winter, deep tread (not sure what that means), dedicated spare tires, and tires whose production levels are below 15,000 per year. That also means that LT tires and other tires not designed for passenger cars (like ST tires) also are not required to be tested.

The treadwear rating is a number that is supposed to be displayed on the sidewall of the tire along with the other 2 UTQG ratings.

The Test:

• 400 mile course on public highways in west Texas at normal speeds for 7,200 miles.
  • There are actually 3 separate loops that total 400 miles.
  • There is a break-in period of 800 miles (2 laps around the 400 mile course) followed by an additional 6,400 miles (8 laps) where the actual wear rate is measured.
    • For reference, this would be 8% of an 80,000 mile tire.
• The government doesn't conduct the test. The tire manufacturer (or importer) does.

The comparison is to the SRTT (Standard Reference Test Tire) for EVERY test, so you can compare between brands.

The government can require the tire manufacturer to show the documentation from the test.

The Bad News:

• There is a lot of variability in the test. A quick calculation of a comparsion between course monitoring tires yields a 16% difference, high to low. On an 800 treadwear rating tire, that's (rounded off ) 120 points!
• The actual measurement miles of the test is pretty small (6,400 miles).
• There doesn't appear to be any provision for dealing with rain and snow.
  • I suppose the course monitoring tire is supposed to take care of that, but I know that tread compounds don't always respond consistently among themselves.
• The rating can't be overstated, but it can be understated.
  • It is not unusual for a tire manufacturer to adjust a secondary tire line downward to make the first line tires look better.

The Science:

More rubber on the road = slower wear rate. The image to the right is an eggagerated example of how this could work.

It is my understanding that the reason this is so is that with more rubber in contact with the road surface, the more force is generated during cornering, so less slip angle for a given cornering force. The average person wouldn't notice the difference, but the tire does.

The larger the tread elements, the better the wear rate.

Notice in the photo to the left, how the individual tread elements are different sizes? The tire on the left is going to wear better than the tire on the right - all other things being equal.

UNFORTUNATELY, all things aren't usually equal. In particular, the tread rubber compound is likely different - and tread compound can have a HUGE!! difference.

Rolling Resistance, Traction, and Tread Wear?

There is a technology triangle involving Rolling Resistance, Traction (especially wet traction), and Treadwear. This would be the time to explain about the chemistry of tread rubber compounding - but I'm just not knowledgeable enough to do that. But I can say this:

There appears to be "families" of tread compounds, where the ingredients are varied to produce different levels for each of those 3 properties. Visually, it's like placing a dot somewhere inside that triangle and getting differing levels of performance - all in relation to each other.

But the rubber chemists are a talented bunch and they are constantly figuring out how to change that triangle.

To the left is an illustration of how that might take place. The green triangle is what happens (exaggerated) when Silica and Silane are used in a tire. That means there is a whole new familiy of tread compounds.

That's the principle behind tires labled LRR (Low Rolling Resistance) or some other related verbage. It doesn't mean that the rolling resistance is actually low. It means that the rolling resistance is better compared to tires with similar wear and traction levels.

Put a different way, LRR is a relative, not an absolute, term.

Treadwear Warranties:

A warranty is not a guarantee. A warranty is about what happens if the product doesn't perform as indicated - in this case, that the tire is worn out before the indicated mileage.

There are always provisions that affect the warranty:

• The warranty usually applies to replacement tires, not to OE tires (tires that come on new vehicles) - even if there is a warranty in the replacement market.
• The warranty usually applies to a set of 4 tires. Sets of 2 or individual tires aren't covered.
  • A set of 3 where one of the originals was replaced would be somewhat problematic, but there is an opportunity for negotiation.
• The warranty only applies to the original purchaser of the tires. In other words, the warranty is not transferable. Typically that means the sales invoice needs to be presented. Without that, the warranty claim is usually denied.
• The tires have to be down to 2/32nds tread depth remaining.
  One can usually negotiate if ALL the tires are below 4/32nds.
• The tires have to have been rotated on a regular basis.
  • That means that tires have to have about the same tread depth remaining.
  • Staggered fitments (different sizes front to rear, where front to rear rotation is impossible, usually results in the warranty mileage being reduced to 50%.
• The tires have to be evenly worn. (no irregular or uneven wear.)
• There is usually a time limit - 72 months seems common.
• The replacement tires are prorated - meaning that the replacement tires will cost less based on how much short the projected mileage is - plus it is the projected mileage (to 2/32nds) that is used, not the actual mileage if the tires have more than 2/32nds.
• The replacement tires have to be by the same manufacturer.
  • A really sharp dealer will be able to handle the replacement into another brand because the tire manufacturer usually doesn't require a receipt for the new tires to be returned with the warrantied tires.

The net effect is that very few mileage warranties are accepted. Unfortunate, but true. That is why the mileage being warranted is usually quite large. The cost to the manufacturer is soooo very little, but the dealers love them - and so the marketing folks love them, too!

Barry's Tire Tech - Main Page