The first step to successful winter dressing is choosing the right base layer. These garments perform two important tasks: they sit close to the skin, transporting moisture away from the body to prevent chilling and they're also designed to trap air to help keep you warm.
An ideal base layer should sit unnoticed under your winter mid-layer and work in conjunction with the rest of your layering system. A lightweight base layer is an extremely versatile garment and should be standard wear in any cyclist's wardrobe.
If you're an active, sweaty rider go for a base layer with good moisture management. If you like to ride at a more sedate pace, then warmth and the thicker insulating base layers are more suitable. Of course the best solution is to find a compromise between the two.
How base layers work
The fabric is designed to suck sweat off your skin and then transport it to the outer edge of the cloth (this is what 'wicking' is). Some fabrics do it by capillary action – ie. the inside end of the fibre is thinner than the outside end).
Others do it with a water-repelling hydrophobic fibre or coating that the water just steams straight through. Meanwhile, wool soaks up sweat like cotton, but – thanks to cunning sheep science – it doesn't lose any thermal properties, so although you're damp, you'll still be warm.
What to look for in a base layer
Short sleeves are more versatile and you can add arm warmers if extra warmth is need. If you get cold easily then go for long sleeves for more warmth. Long sleeves should go all the way to the wrist and should be a tight fit to prevent them rolling, bunching, or billowing.
Always check for Raglan sleeves. These reach all the way to the neckline, covering the shoulder. The seam sits under the arm and to the side of the chest and back. This is much better than a single seam located on top of the shoulder where it can be restrictive and rub against the skin. Raglan sleeves also give a better fit and a greater degree of freedom.
It's important to move moisture quickly away from the skin to stay warm and comfortable. If you sweat heavily or are a high-activity cyclist go for a high-moisture movement fabric over an insulating one, and add to your mid-layers to provide extra warmth when required.
Most fabrics are woven so they have a four-way stretch. This allows them to hug the body's natural shape for the best performance.
Base layers do get smelly, so any material that delays the build-up of bacteria is a bonus. Merino naturally resists odours – wool fibres are smooth so there's nowhere for bacteria to hide and fester.
Unfortunately, the rougher fibres of synthetic base layers are great for breeding bacteria. Some tops use a microbacterial treatment that fights them off for a bit, but within a few rides all synthetic tops will have a certain nasal ambience.
Other synthetic tops use activated carbon treated fabric to hold in the odours and release them during the wash and dry cycle. Most base layers are machine washable at 40°C and some even higher. Take note of the drying instructions because some can't be tumble dried.
High necks are ideal when it's really cold but can get hot when it's not. A good compromise is a soft, snug-fitting crew neck that will sit against the neck of the mid-layer. Thicker, thermal base layers may feature a zip but they also adds bulk.
Base layers provide warmth by trapping air next to the skin. This effect is due to the weave, the fabric fibres or a mixture of both. The colder the conditions, the thicker the base layer, it's as simple as that.
How warm you want your base layers to be obviously depends on how warm you naturally are, and how warm you want to stay. Some folk can ride in sub-zero conditions with just a thin base layer and a shell, while others need three inches of fleece just to go out in autumn.
The less the better and any present should be flat-stitched for improved comfort and increased strength. Most seams go under the arm and either side of the shoulder to the neck. Some fabrics use contour mapping. By changing the weave of the fabric the structure can map to the body's shape without the need for numerous panels and seams.
Should be long in the back to keep you covered in the saddle, but should also stay in place.