Base layers are there to keep you comfortable while cycling. They do this in two ways: by trapping a layer of air next to your body to keep you warm, and by transporting sweat away from your skin to keep you dry.
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If you want to get technical about it, a base layer forms the bottom of a multi-layer clothing system designed to manage the climate differential between the inside and outside of your garments. As such, you're likely to wear them on all but the very hottest days.
How do base layers work?
Some base layers wick moisture away from the skin through capillary action (the fabric’s inner face has lots of tiny voids to make it porous), while others do it using hydrophilic coatings that work to actively draw moisture through the garment.
What are base layers made from?
What to look for in a base layer
Warmth (but not too much)
Base layers provide warmth by trapping air next to the skin. This effect is due to the weave design, the fabric fibres used, or a mixture of both. The colder the conditions, the thicker the base layer you use – it's as simple as that.
How warm you want your base layer 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.
Long, short or no sleeves?
Assuming you want sleeves, the first thing to consider is how long you want them to be. You probably won’t want a long-sleeved base layer if all your jerseys have short sleeves. Short-sleeved base layers will work with long- and short-sleeved jerseys but you may find the sleeves bunch up around your shoulders, especially underneath some of today’s closely tailored tops.
Whether you prefer short or long sleeves, you’re better off going for ones that use a raglan design. Raglan sleeves extend over the shoulder and all the way up to the collar. The design provides more freedom of movement for your arms and uses a diagonal seam that runs from your armpit to your collarbone, rather than circling your shoulder.
The other option is to go sleeveless. If you can’t bear to go without a base layer on even the hottest days, a sleeveless, string-vest style garment might be the way to go.
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.
As few seams and zips as possible
As a general rule, you want as few seams and zips as possible. The reason for that is simple: a base layer is worn next to your skin, so you’ll want to avoid anything that might catch, nip, rub or irritate as much as possible.
That said, you can’t avoid seams entirely so look for garments with seams in places that won’t interfere with your movements. Seams that are flatlock stitched rather than coverstitched are less likely to irritate skin, as the joint between fabric panels is level rather than lipped, so they’re the ones to seek out.
Some fabrics use contour mapping, which changes the weave of the fabric so that the structure can map to the body's shape without the need for numerous panels and seams.
Some base layers have high collars but those that do are geared more towards winter use, and may be too warm to wear in the summer months. A lighter base layer with a lower collar is more suitable for warmer weather and can be worn with a high-collared jersey/jacket and neck warmer when the mercury drops.
The bottom of the base layer is just as important as the top and it’s worthwhile looking for one that extends down a little further than the typical jersey. A base layer with a longer body provides more coverage but also more material to overlap, so you’re less likely to be exposed if your jersey rucks up or shorts slide down.