The derailleur hanger is the foundation for the modern drive train to work properly. With modern bikes getting as many as 12 cogs in the space that we use to put 8 it's never been more important to make sure the hanger is perfectly straight. Sadly many of the current tools just aren't capable of achieving this kind of precision. That's why we went through the extra steps to make this tool half precision measuring instrument and half pry bar. We hold tolerances as tight as a quarter of a thousandth of an inch (.0063mm) in some critical dimensions of this tool. We also gave you the ability to lock in the feeler so you don't have to second guess your measurement after bumping it on the tire. The top half will rotate out of the way of fender stays and frame tubes. The feeler also stores inside the top half of the tool, very handy for the traveling mechanic. The T bolt extends past the tool for access to those problematic hangers found on some modern full suspension bikes. It's fairly compact at just under 11" ( 28cm). It's also re-buildable. While ideally your HAG should be sent back for a rebuild, it is possible to do them in the field. Compatible with the three most common wheel sizes, 26, 27.5/650B and 700/29) This will be the last hanger tool you'll ever have to buy.