The Hope Rear Thru Axle is a lightweight and versatile addition to any mountain bike setup, designed for UDH compatibility, ensuring a smooth connection between your drivetrain and wheels. Its slim profile and durable construction make it an excellent choice for both casual and serious riders. The "size options" refer to the adjustable design, allowing for optimal fit and clearance in various mounting positions on your bike. With its quick-release system, you can quickly and easily attach and remove it during rides or maintenance tasks. The Hope Rear Thru Axle's combination of performance, durability, and versatility makes it a must-have for any mountain biking enthusiast.