As my tyres wear out, they definitely have less grip in wet conditions, but on dry roads their grip seems to get better with age. Is there a reason for that or am I kidding myself? EM
Technically, so-called "slick" tyres (with no tread at all!) can give better grip than treaded tyres…but only on clean and dry tarmac! They will give significantly less grip if the road surface is even slightly loose or soft or wet.
Before looking at why “slicks” are the fastest option for racing cars if conditions are suitable, let’s be clear: for ordinary cars and general motoring, the right treads (the newer the better) are essential for good all-conditions grip, and there are several risks and no benefits in using tyres that are well worn or nearly bald.
Racing slicks have potentially better grip because 100 percent of the footprint of their running surface is in contact with the road. With treaded tyres, the area of surface-to-surface contact with the road is reduced (and can be nearly halved) because only the tops of the tread lugs or bands – not the grooves between them - touch the tarmac.
Further, although racing slicks have no tread lugs they do have other differences – perhaps most significantly very much softer rubber compounds which “cling” more closely to the tiny irregularities in a “smooth” tarmac surface – so tightly that some of the rubber stays stuck to the track so they blister with heat and "wear out" in the space of 50 kms!
Car tyres use compounds that are designed to last for 50,000 kms! Clearly, they are a lot less sticky (and do not suffer nor harness anything like the forces of a racing car).
For ordinary motoring the question is not tread or slick, but what “pattern” of tread will best serve your personal motoring conditions.
There are dozens of variations to suit all needs, but they boil down to three basic options: street tyres, off-road tyres, or a mixed-purpose pattern somewhere between those two (commonly branded “All-terrain” or similar).
Street tyres have almost continuous bands of relatively shallow tread, with fine diagonal slits in the tread surface to help channel water sideways.
This pattern makes less noise because there are fewer and smoother lateral “edges”, lasts longer because there is a continuous running surface (the treads are a continuous band, not tall and bendy four-sided lumps) and is more stable at higher speeds because there is more surface to surface contact area and less tread squirm.
For all those reasons they also give more grip on wet tarmac, so long as the grooves are deep enough to channel the water away fast enough that the tread bands stay in contact with the tarmac.
At speed, tyres need to channel several litres of water per second (!) to prevent aquaplaning. But street tyres have generally lighter casing construction not best-suited to percussion from rocks/potholes, nor for repelling thorns, and are prone to tread “clogging” on muddy surfaces that turns them into…slicks.
Off-road patterns have treads with chunky stand-alone lugs with steep vertical surfaces in all directions designed to “bite” into soft or slippery ground; provide dramatically more grip in those conditions, and resist clogging by having very wide channels shaped to throw mud sideways. But they make more noise, wear out faster, and offer less traction at high speed on tarmac (especially if wet).
Based on the pros and cons of the street/off-road options, you can take your pick from many mixed-use tyre patterns ranging from slightly chunkier street tyres to slightly less extreme off-road or something in between.
On the tread depths of well-worn tyres, the legal minimum varies around the world at circa 2mm, and some tyres brands have a coloured marker embedded in their treads to warn when the limit is approached.
Before you consider keeping a tyre beyond that limit, bear in mind that this is the “minimum” allowed, not a recommended guideline.
So, unless your motoring is always at very moderate speeds on very mild road conditions, the sensible approach is how long “before” the limit is reached you should consider replacing the tyres, not how far “beyond” the limit you should risk.
Even if the tyre is still strictly “legal”, at anywhere near the limit it is more prone to punctures and the casing is probably less robust.
The amount of extra use you might eke out of a nearly bald tyre will save you next-to-nothing in overall long-term running costs, and expose you to higher risks of tyre failure, aquaplaning etc. On safety-critical items, save yourself the worry. Better safe (even by a thousand kilometres) than sorry.
