Summer and winter tyres: changing, storing, replacing

The visible difference between summer and winter tyres is the tread pattern; the effective one is the rubber compound. Rubber is elastic when warm and goes hard when cold. Hard means the tread no longer conforms to the microscopic roughness of the asphalt — and that small-scale interlocking is a large part of what we call grip.

A winter compound is built to stay elastic at low temperatures. The price shows in summer: the same compound goes too soft in heat, the tread blocks squirm, the dry braking distance grows and the tyre wears fast. It is the same mechanism with the sign reversed.

The tread pattern does the second job. The wide grooves clear water; the fine sipes in the blocks open as the tyre rolls and key into snow — snow grips best on snow. That is why a winter tyre needs block depth in order to work at all, not merely to satisfy a minimum requirement.

The familiar rule of thumb about months is a memory aid, not a criterion. The criterion is the temperature at which you actually drive — in the morning, not at midday, and where the route runs, not where you live. The crossover sits in the single digits above freezing and depends on the specific compound; every manufacturer places it a little differently.

Two drivers with the same car can therefore both be right at different moments. Someone commuting across flat country with the car in a garage changes later than someone who crosses a pass in the morning or parks in a field where frost forms overnight.

Separate from that is the legal question: whether, from when and with which marking winter tyres are required is set by each country for itself, sometimes with a fixed period, sometimes according to conditions, sometimes only on signed routes. The valid text is held by that country's road authority. The temperature argument above applies regardless — it says what makes physical sense, not what is prescribed.

An all-season tyre is exactly what it sounds like: one compound and one pattern meant to do two jobs at once. That works well in the middle and less well at the edges. At mild temperatures on a wet road it is close to a summer tyre; in real heat on a motorway and in real snow on a gradient it is a long way from either specialist.

So the sum works out for a particular kind of driver: a flat region, a modest annual mileage, no regular winter trip to the mountains, nowhere to store a second set of wheels. It does not work out when a lot of summer motorway and a real winter fall in the same year — then you pay for the compromise twice, once in wear and once in stopping distance.

Which marking an all-season tyre must carry to count as winter-capable in a given country is once again a national question, not a technical one. Technically, all that holds is this: the marking records a test that was passed, not how the tyre still works in snow at the end of its life.

Before a wheel comes off, mark where it was — front left, rear right. Chalk on the inside of the rim, or a sticker. Without that mark, after two changes the set is four similar wheels with no history, and every statement about wear patterns is lost.

The wheel bolt torque is in the vehicle documents and differs between models. It is set with a torque wrench, not by feel, and expressly not with an impact gun as the final step. Too tight is as much a mistake as too loose: an over-stretched bolt does not hold better, and a warped brake disc is a common consequence.

Two things get overlooked. First the seat of the wheel bolt: taper, ball or flat with a washer — the shape must match the rim, and a wrong bolt looks correctly seated and still does not hold. Second the hub face: rust and dirt between hub and rim bed in after a few kilometres and the wheel is then loose. The mating face gets cleaned before the wheel goes on.

Rubber ages through three things: heat, light and ozone. So the right storage place is cool, dark and dry — and far from anything that produces ozone. That means brushed electric motors, a welder, an old fridge in the basement. A balcony in the sun is the worst place imaginable; an unheated cellar is a good one.

Wash the wheels before storing them, and not out of tidiness. Brake dust holds salt and moisture against the rim for months, and stones in the grooves work their way further in. Then label each wheel — position and date — and set the pressure to the figure the manufacturer gives for storage; somewhat above running pressure is usual, so the sidewall stays unloaded.

How they lie depends on whether the tyre is on a rim. Complete wheels are stacked flat or hung by the rim — the rim carries the load, the sidewall does not. Tyres without rims stand upright and get turned a quarter turn occasionally; stacked, they deform, because nothing holds them in shape.

A tyre has three independent ends, and reaching any one of them is enough. The first is tread depth. The legal minimum is national law, published by the road authority concerned; it is the line below which a tyre is not permitted, not the point at which it is still good. On a winter tyre the practical end comes considerably earlier, because keying into snow depends on block depth.

The second end is damage, and here there is no negotiation. A bulge in the sidewall means the carcass has broken inside — the tyre is replaced, not repaired, however much tread is on it. The same goes for cuts reaching the cord, and for a puncture in the shoulder or sidewall; only a hole in the tread area is repairable, and only from the inside, after demounting and visual inspection.

The third end is age. The compound loses plasticiser over the years, and you can see it: fine cracks in the base of the grooves and on the sidewall, a dull, hard surface. A tyre with plenty of tread and cracks is used up, however new it looks. How to read the date of manufacture on the sidewall, and what manufacturers and trade bodies publish about it, is covered in the piece on the DOT code.

A worn tyre is a record. If both shoulders are more worn than the centre, it ran chronically underinflated — the tread then bows inwards and carries on its edges. If only the centre has gone, the pressure was too high. Both are the owner's errors, not the chassis's.

If instead only one side is worn, usually the inner one, the axle geometry is out: camber or toe has shifted, often after a kerb strike or through worn bearings and bushes. Replace only the tyre in that case and you have bought the same wear pattern a second time. The alignment belongs with the tyre purchase, not after it.

Two further patterns are worth a look. Feathering — blocks higher on one edge than the other, rough to the hand in one direction — points to toe, and is audible as noise before it is visible. Scalloped patches around the circumference, often hollows a hand's breadth apart, point to a tired damper or a wheel bearing; there the tyre is the symptom, not the cause.

At a viewing the tyres say more about the previous owner than the state of the interior. Four tyres of one brand, from one production period, evenly worn, plus a second set in storage — that is a car on which someone did the obvious things. Four different makes with four different depths are also an answer.

If you are selling, put the tyres into the listing text, with details that can be checked: size, make, measured tread depth per axle, whether a second set is included and whether it is on rims. This is one of the few items a buyer can measure without a workshop — which means an honest statement here earns trust for the rest of the listing.