Thankfully, you can wash the liners inside a motorcycle helmet because, well you know.
After a while you probably wouldn't have the stomach to put your head inside your helmet much less manage the smell. And if you did, expect to transfer whatever stench developed inside the helmet to your hair and scalp which only serves to alert others of your presence.
So wash the liner regularly.
Think of the helmet liner like an article of clothing. It needs washing and depending on the weather and the degree of sweating it might need a washing after only one ride. Most riders though, depending on length of ride and time of year can probably get away with washing every three rides or so.
But conduct a sniff test if you have any doubt.
Helmet liners come mostly as high-density foam lined with a moisture wicking cloth. These liners secure inside the helmet and act as a buffer between your head and the hard shell of the helmet. Without a liner, you would find wearing a helmet quite uncomfortable. Liners also help mold your head to the helmet preventing any movement and securing the helmet to your head once strapped. Other liners wrap around your head like a beanie that work as added insulation and warmth for cold days.
Motorcycle Helmet Liner
Wearing anything so closely fitted to your head will cause you to sweat even in winter months. Airflow through your helmet provides additional cooling and helps dry sweat but even in the cold you probably need to wash the liner after several rides.
Washing a Helmet Liner
Before you do anything check the label for washing instructions. Most likely you can toss it in the wash with your regular clothes but double check first. All this to say you can wash most, if not all, helmet liners in the washing machine. However, because of the garment's size, hand-washing then letting it air dry works just as well and helps the liner last longer. It takes just a few minutes and something we recommend.
Beanie Style Helmet Liner
A washing machine filled with clothes, even front loaders, can stretch and even tear the liner and the spin cycle puts it through the wringer, quite literally. The padded elements can also misshape. Soaking the liner in a bucket with laundry soap, rinsing, squeezing the excess water out and air drying helps your helmet liner last much longer and prevents the foam padding from bunching up. If you use the washing machine then forego the dryer and let air dry. The nylon or microfleece beanie type liners hold up pretty well in the washing machine and dryer - though if you wear both the inner liner and the beanie type just wash them together and air dry. Those who ride regularly often have two or more liners for the purpose of always having a fresh one available.
Lastly, with the liner removed, clean the inside of the helmet using a helmet spray or similar cleaner/deodorizer.