What is the Shimano 34.9mm clamp band adapter for?

This is Shimano's large 34.9mm clamp band adapter for front derailleurs, allowing a braze-on style front derailleur to be mounted on frames with a 34.9mm round seat tube. Many high-performance carbon and aluminum frames use 34.9mm seat tubes, and without this adapter a braze-on front derailleur cannot be installed. The adapter clamps securely around the seat tube and provides the correct braze-on mounting tab geometry for Shimano front derailleurs. A small but essential fitting for builds that require this seat tube diameter.

For 34.9mm seat tubes. Compatible with Shimano braze-on style front derailleurs. Verify your frame's seat tube diameter and derailleur mount type before ordering.