Winter tyres on cars certainly work...in the right conditions. However, that' doesn't mean a winter tyre "works" on bicycles.
The story is a complicated one, but I'll try and lay it out clearly.
First you have to understand that car and bicycle tyres are not in any way similar. I've already lined a thread that explains the reasons why. The primary take-away there is that tread on bicycle tyres intended for use on tarmac roads is unnecessary and counter productive. It is pure psychology.
Car tyres come in three seasonal categories namely:
a) Summer tyres (for very hot climates where the minimum temperature is very seldom below 7degrees C). These are not for the UK.
b) All-season tyres where the maximum temperatures are in the twenties and minimums in the minimums just below freezing. These can be used all year round in the UK.
c) Winter tyres where the minum temperatures can get way below zero. These have to be swapped out for all-season tyres in the summer.
The difference between a) and b) is purely rubber compound. Manufacturers may well adorn them with different patterns of tread but that's not what makes the difference. The difference is purely that the all-season tyre has a compound that doesn't harden as much when the temperature gets really low. This ensures that it still has reasonable grip when the temperature drops. The difference has nothing to do with the way it mechanically deals with snow or water or frost or whatever. It is purely softness.
Winter tyres, category c) above, are different. They have a still-softer compound that maintains grip in even lower temperatures but are also slightly narrower (for the same fit) and have different tread pattern that includes many smaller cuts. These cuts are not grooves but thin, narrow cuts which cannot transport water. These cuts, called sipes, are located inside blocks of thread which have wide grooves around them for transporting water. In other words, they look like summer tyres but have small cuts inside the tread blocks.
Transporting water is not their function. Their function is to squirm under pressure. Squirming does two things: It allows the tyre tread blocks to change shape and shed snow or mud (helped by rotational speed of the wheel) and, it creates extra friction inside the tyre (i.e. increases rolling resistance) so that the tyre can run warmer than other tyres. That's right, the tread design can determine the running temperature of the tyre. The thinking is that a warmer tyre is softer and offers more grip. This extra heat is accentuated under heavy braking or cornering because that's when the tyre squirms the most and needs grip the most.
Let's get to grips with grip.
Tyre grip can be classed into two categories.
a) The physical-chemical effect of Van der Waals forces.
b) Purely mechanical effect of tread pattern.
Physical-Chemical Grip. Passenger tyres rely 100% on this. This means that any grip is generated by small di-pole attractions between the molecules of the road and the molecules of the tyre. THERE IS NO MECHANICAL INTERLOCKING INVOLVED. Let me repeat that: There is no mechanical interlocking involved. In other words, the tread shape has nothing to do with this type of grip. There is no digging in, no hooking onto rough bits, no reliance on surface texture. All that's required is that the tyre molecules are squeezed up against the road surface molecules as closely as possible so that Van der Waal's can switch on. Wikipedia or your old chemical textbooks can do a better job at explaining Van der Waals weak forces than I can. Go read up on it. In a nutshell, it works like a very, very, very small magnetic force that only switches on on close (extremely close) proximity. That's why there's more grip when you push hard downwards on your cars or shoes. You are increasing the Van der Waal's. Van der Waal's does not, as you know by now, rely on shapes or blocks or penetration or roughness. You can demonstrate this for yourself by taking a pencil eraser and pulling it across your perfectly smooth bathroom mirror. If you push harder, the eraser is more difficult to drag along. If you push hard enough, Van der Waal's is so strong that it rips bits of plastic from the eraser. Try it. Please.
This is how a passenger car stays on the road - Meneer* Van der Waal keeps it on the road, allows it to accelerate, brake and corner.
Winter tyres have softer rubber that allows the tyres to make better contact with the road and switch on more of these tiny Van der Waal magnets. Why don't we use winter tyres all the time? Because they are too soft and perform poorly in comparison to other tyres. They only outperform other tyres when it is cold, at all other times they create longer braking distances. There is no advantage to using them in the UK because it simply isn't cold enough for All Season tyres to start performing worse than a Winter tyre would.
Van der Waal's forces cannot be measured using dimension and is expressed purely as a co-efficient, known as the co-efficient of friction. It is the "strength" of the friction, if you like. A hot, semi-molten dragster tyre has a co-efficient of friction of 2 and an ice skate on ice, something like 0.2 (these things can be looked up in tables).
B) Mechanical effect of tread pattern. This applies to surfaces which are softer than the tyre and we rely on the tyre to make an imprint in this surface so that it can compact the soft material and make it a bit "stronger" to offer a mechanical interlock between the tyre and the surface - typically compacted mud or sand or snow. Passenger cars don't rely on this mode of friction and SUVs/ATVs have tyres which do rely on this. They perform poorly on hard surfaces because the squirm and "walk" around corners causing understeer, poor braking, noise and high rolling resistance. If you've ever had the opportunity to fit a mountainbike with aggressive knobblies intended for mud, such as Michelin WildGrippers (aka WildSlippers) and taken a few turns on tarmac, you'll know what it feels like to have understeer on a bicycle on good surface. The weak little lugs bend and flex, causing very poor steering. However, in mud they work better than a smooth tyre intended for tarmac. I've explained the mechanism above, and further above explained why the flexible lugs shed compacted material stuck in the tyres. It is important that these tyres don't hold onto the mud or snow but start each revolution with a clean slate. Otherwise it is mud-on-mud and they can't dig in.
This type of lugged tyre cannot work on ice. That's because the ice is harder than the tyre. Lugged tyres rely on soft substrate so that it can make an impression, compact and take advantage of the little Lego-interface it has created. On ice, such as black ice or white ice or whatever colour solid, hard ice you'd like to throw at them, none of these two types of grip works. Ice has very little Van der Waal's attraction to other materials and thus has a low co-efficient of friction. It is no use trying to apply different types of rubber or different tread patterns to the problem, nothing will grip on ice, period.
There is a subset of the grip described in this section (mechanical interlocking) and that is tyres with metal studs. This is basically a way of making the tyre harder than the substrate. Sharp little studs penetrate the ice and creates grip that way.
Back to bicycle winter tyres.
A bicycle winter tyre (without studs) is a fallacy. It is a clever bit of psychology designed to make you buy special tyres that will solve a perceived condition.
a) Bicycle tyres don't need grooves to shed water.
b) Tread pattern doesn't matter unless we're talking deep, narrow flexible lugs that can penetrate the soft substrate.
c) Sipes don't matter because bicycle tyres don't generate enough heat to change compound hardness.
d) A very soft tyre designed for winter bicycle use could offer some extra grip on cold, hard, non-iced surfaces, but then it would have to be smooth and very soft because sipes would give you understeer as dangerous as ice itself.
I have no doubt that companies like Swalbe understand all this. After all, they employ engineers qualified in tribology, as well as polymer chemists. However, these Bicycle Winter Tyres were designed in the marketing boardroom, not the laboratory.
* Meneer means Mister/Sir in Dutch. Van der Waal was a Dutchman.
