Modern silica-based tyres don't heat-cycle and degrade like old rubber-black tyres. Lots of common tyre ideas were outdated since 1990s, but haven't faded away. One comparison test is to overcome traction limits of old tyres. Then installed new tyres of exactly same model and go back out and compare lap-times. Also tracking data over time is useful tool.
BT-45 on my commuter bike with date-code 2009 finally wore out in 2020 with 16K-miles. They were already 9-yrs old when I got bike. With many trackdays tossed in when my racebike was down for maintenance. Last sessions on them were just a bit faster than 3 years earlier.
Alpha-13-SP on by track bike went for 49 track-days over 2-yrs. They were already 5-yrs old when I got them. My last trackday on them setting PB times at Laguna Seca. Overheated them plenty of times before then too!
Many, many sets of Q3+ for free from ex-AFM MotoAm racer kids. I'd get over 45 trackdays on them with close to 1000 heat-cycles. Last laps on them usually faster than starting ones.
EXHIBITS
BT-45 that came on my commuter, 9-yrs old at time I picked up bike. By 16K-miles and 11-yrs old, they had some minor sidewall cracking, but still gripped just fine. Predictable limit behavior with gentle catchable slides.
Alpha-13SP, MotoAm specific version, slightly softer than regular Alpha-13
These were found at back of trailer when they cleaned it out. Was 5-yrs old when I spooned them on
By time I wore them out 2-yrs later, they had over 5000 laps and about 1000 heat-cycles in them. Set fastest PB lap @ Laguna Seca on them at very end of their life.
Overheated front-tyre plenty of times in +100F heat of summer @ Thunderhill. After cooling off, they came back and gripped just fine.
GP-A Pro D207, before I got into racing, I did about 10-yrs of trackdays on my '05 RR. Got these 6-yr old tyres from AFM racer. Oil-slick look on surface is caused by heating-up tyre but not really riding very fast to wear it. Commonly seen with newbies using tyre-warmers on sticky-tyres, but they ride at beginner pace. Very dangerous because sticky-tyres may have high-grip limit, but extremely sudden breakaway behavior with no warning when you go over limit. This guy would've been better off with more intermediate street-tyre, like Q2 at the time. After my 1st session, no more oil-slick and warmers not really needed.
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That's another aspect of modern silica-based tyres, they actually warm up quickly and not need warmers. Michelin has nice trackday slick that doesn't need warmers, similar to Q4. It really only helps save 1 warm-up lap for racers who can take warmers off and get onto track right away. For trackday folks, with all sitting around waiting for track to clear, tyres will have cooled off to pre-warmer temps anyway. First couple laps should be easy to warm up tyres anyway. After which time, tyres will be up to temp regardless if they had warmers on before getting on track.
I think it's better to gauge
tyre's condition rather than age as predictor of grip & safety. You can get old tyre that's in great unused shape, or brand-new one that's been improperly stored at too low and too high temps and end up with horrible performance. A durometer was helpful with old carbon-black tyres, but not that useful on newer silica-based tyres because compound doesn't dry out and harden with age.
As with any tyre, you'll want to take it easy at 1st, especially for winter-riding. Warm them up with brisk straight-line acceleration and hard braking. Then after couple miles, you can add some cornering. And take it up to limits of cornering gradually. This technique will let you find safe limit of any tyre, regardless of age or condition. Only difference will be speed when they start sliding.
Most crashes are
caused by mistakes 1st, such as line-selections through corner. And
then secondary result of that is overcoming tyre's limit by panic braking or overcorrecting steering/leaning. It's not tyre's fault. Very few people ride fast enough to overcome traction-limits of dry-rotted cracked rubber anyway.
