Hi everyone.
I recently bought a new set of tires for my car - the old ones were ~3 years old, and worn down to the wear bars. I noticed a significant improvement in the handling of my car, but a coworker stated that it was simply in my head. I figured that he was wrong - compare an old eraser to a new eraser, and you can see how much rubber degrades over time. We had a pretty long discussion, and there were two points where I felt I didn’t have enough information.
The first point was how much do grip do tires lose as they age? It is important to note that this conversion was about dry weather performance only, not wet. The tread depth would of course have a significant impact on wet weather performance.
The second point was the difference between summer only and all-season tires. Would there be a lot of difference in the grip of summer tires vs. “equivalent” all-season tires?
I looked around on google, and there wasn’t a lot on what I was looking for - I found stuff like tires getting flat spots when stored, and Ford’s recommendation to replace spare tires after 6 years or so, and stuff on the steel belts inside rusting, but nothing that provided any real indication of tire degradation.
I changed from worn kumho ecasta 712s(High performance summer tires) to new Kumho Ecasta ASX’s (High performance all seasons.). Handling went to CRAP, I seriously thought I was going to slip off the road. However I was told that there was a hundred mile break in period for the Kumho ASX’s(something about homogenization of the rubber afer repeating heating). After that hundred or so miles handling improved dramatically, however it’s definitely not as good as when I had ecasta 712’s.
Mind you the 712’s and ASX’s have different tread patterns and are possibly(probably) made of different rubber.
I find that new tires actually handle worse than old ones, because the thick layer of rubber on new ones allows the tread to squish around more. Worn-out tires feel more direct and give a crisper response to steering input.
When you ask about the handling and performance of aging/worn tires, there are three major factors, contact patch area, lateral flexibility, and degradation of the tire compound. A new set of performance tires that has been shaved (i.e., much of the tread removed to increase contact patch area and reduce lateral flexibility) will usually be faster both in acceleration and around a race course than an otherwise identical new set with factory tread depth.
However, as tires age naturally, they may become less sticky due to degradation of the rubber itself, so a 2yr old worn set with 20,000 miles will perform worse than the shaved tire despite roughly equivalent tread depth.
Assuming we’re talking summer performance tires, generally, the lower the treadlife number, the “stickier” the tire. In the real world there are lots of factors that affect tire performance, and they are not easily quantified.
The difference between mud and snow (AKA M+S, all season) and “summer only” tires is in the tread design and rubber compound used. Tires that bear the M+S designation are usually more robustly lugged, use firmer rubber compounds, and have at least 25% open area (according to C&D). Most tires are all season (bear the M+S designation), meaning that they have some mud and snow capability. Winter tires with the “snowflake” logo are even more capable under these conditions and have passed more stringent traction test under snowy/icy conditions. If it is not obvious from their price and the sizes in which they are offered, high performance, or “summer only” tires can usually be identified by their lower treadwear ratings.
The unscientific conclusion I’ve reached due to nothing besides message board posts on a half dozen different boards is that the rubber is probably substantially less grippy and soft-riding past year 5 than before.
That being said, if you are in Mexico, it might be shorter, and if you’re in Siberia and do nothing but short trips, it may be longer.