iannic:
My brother sent me this (originally a post on a BMW forum he's on) from one of the mods there who knows a thing or two about tyres:
"As requested, an attempt to give some general information and guidance
on winter tyres, as these can play a vital role to get your rwd BMW
safely through a proper winter. Apologies for the length, but with the
amount of recent winter tyre posts, it might be beneficial.
It has been said a few times before, but winter tyres are not just about
snow, but also about temperature. The rubber mixture used on summer
tyres hardens out from + 7 degr. C and loses more and more grip, the
colder it gets. So even on a dry or just wet cold winter road they can
make the difference of stopping in time or hitting something. Obviously
even more so, the more wintery the conditions get with snow, slush etc.
As a simple example, you don’t go for a winter walk in flip flops and
shorts, but look for best traction shoes and other winter accessories
for safety, comfort and also performance. The requirements of a car are
similar, although not a direct comparison.
The rubber mixture of a winter tyre contains much more silica, which
keeps them softer and more grippy, even down to negative temperatures.
The softer compound also helps when flexibility is required to grant
snow grip. They also have multiple fine sipes in each tread block, which
(in combination with the softer rubber) keep opening and closing during
use, hence allowing to better adapt to he road surface and also
constantly creating little edges, which also account for the better snow
and slush grip.
They also improve the aquaplaning scenario in a straight line. Lateral
aquaplaning is not improved, due to the carcass construction of a winter
tyre.
Now to a few standard statements for winter tyres
- In these conditions any winter tyre is better and more safe than a
summer tyre
- The narrower, the better for improved snow and slush cutting
abilities
- Use the smallest, convenient wheel dia possible, resulting in a higher
profile tyre for more flexible rubber on the wheels
- Tyre pressures are usually run at the higher end of the manufacturer
recommendation for the size
Usually the well known big brands are amongst the best winter tyre
manufacturers, too. (Usually)
Winter tyres are no wonder weapon on roads covered in ice and snow, but
coupled with reasonable winter driving techniques get you much further,
safer and easier.
The smallest setup you can fit, is usually defined by the size of your
front brakes. Some can do with 15”, others need 16” or 17” as a minimum
wheel size to clear the brakes. In terms of tyre size, the smallest
setup mentioned on your tyre pressure sticker in the door frame or the
handbook should be perfectly fine for winter use.
Generally, most winter tyres have a lower speed rating, so don’t worry
if it is lower than that of your summer tyres.
Mine usually go on from some time in November until early or mid March,
obviously depending on weather conditions.
Although the same legal limit counts for winter tyres, they will be past
their best when going below 4mm tread depth and (as other tyres, too)
should not be used anymore or replaced, when older than 10 years
latest.
Cost:
If you do not intend to keep changing the tyres on your existing wheels
for whatever reason (size etc.), the best option is to find a set of
suitable second hand wheels, you can keep the winter tyres on, so you
only swap the full wheel set twice a year. Really, the only additional
cost is the second set of used wheels. The initial tyre outlay will
amortise over the next 3-4 years, as your summer tyres last longer with
not being used over the winter months. Also the most tiny prang that can
be avoided with winter tyres already offsets the outlay."
