Soft flask

A soft flask is a collapsible plastic bottle used by runners and cyclists to carry sports drinks or water; it shrinks as it empties, reducing sloshing and bulk.

What makes soft flasks useful

Soft flasks are typically made from thermoplastic polyurethane (TPU) or similar flexible materials. They range in size from about 250 milliliters to 750 milliliters and are designed to fit in vest pockets or waist belts. Unlike rigid bottles, a soft flask collapses as you drink, so the remaining liquid doesn’t slosh around. This makes them popular for trail running and racing, where minimizing bounce is important. Most models have a wide mouth for easy filling and a leak‑resistant cap with a bite valve or straw for sipping on the move. Some flasks include removable straws that allow you to drink without removing the bottle from your pocket. Because they compress, they can be stashed in a pocket when empty, saving space. Soft flasks are not insulated, so the temperature of your sports drink will gradually rise; to keep drinks cool, some athletes freeze a half‑full flask the night before. Thorough cleaning after each use is essential to prevent mold growth inside the narrow neck and bite valve. Mixing a fresh sports drink or electrolyte solution at the right dilution helps avoid sticky residue that can be hard to wash out.

Using soft flasks effectively

Before using a soft flask, rinse it with warm water to remove factory residues. When filling, avoid carbonated beverages that can expand and leak. Squeeze the flask to remove excess air, screw the cap on securely and test for leaks before placing it in your vest or belt. During a race or training session, rotate flasks so you always know which one contains a carbohydrate-rich drink versus plain water. Practice drinking from the bite valve while moving to find a comfortable cadence. After each use, wash the flask with mild soap and allow it to dry completely; storing it in a freezer between uses can inhibit bacterial growth.

Soft flasks provide a lightweight, low‑bounce way to carry sports drinks, making them ideal for endurance athletes who prefer to keep hydration within easy reach.

Related Terms: Hydration pack, Bidon, Dilution ratio, Custom blend, On-course fluids