ZTTO Mountain Bike Bicycle Front Derailleur Clamp Adapter Shim 34.9mm to 31.8mm. Front derailleur adapter, conversion ring, reducing sleeve, shim, 34.9 to 31.8. 1 Front Derailleur Shim. The real color of the item may be slightly different from the pictures shown on website caused by many factors such as brightness of your monitor and light brightness.