I'd like to know if covers work well for motorcycle in cold weather, for example -20C. If not, is there an alternative option, assuming garage is not available?
If your bike is sitting for long periods in extreme cold the cover will not help with cold exposure. Wind chill has no effect on inanimate objects. The cover design and material are the biggest factors determining whether or not it is suitable for long term use. As long as the humidity level is fairly low (as it would be in -20 degree temps) the cover won't promote rust. The issues with long term cover use are that if the cover is not a tight fit the wind will blow the cover against the finish and abrade the paint. Another potential issue is that moisture can become trapped under the cover and actually promote corrosion issues. It is important that the cover be of a breathable material. That means that moisture won't penetrate the outside but can evaporate from the inside similar to the way Gortex works.