
Originally Posted by
MartianDestiny
You will want to check your stem vs. the bar width at the stem as there are two sizes. Oversized (31.8mm, I believe) and standard. You'll either need to get bars to match your stem or plan on changing the stem as well.
This would be the clamp size, but not the width.
I just copied this from wikipedia because I'm lazy. The one thing I would add is that sometimes bars (especially those with shallow drops) are flared (a la randonneur style) so the width of the drops is greater than the width at the hoods. Personally, I prefer to know the width at the hoods because that is where I spend 90% of my time.
From wikipedia --
Width
Drop bars come in a variety of widths from 36 cm to 50 cm. Usually a rider will pick a bar that approximately matches their shoulder width so that their arms can be approximately parallel. The width is measured at the end of the drop section but the exact method varies from manufacturer to manufacturer. Some measure from outside edge to outside edge (e.g. Deda, ITM, TTT) whereas others measure from center to center (e.g. Cinelli, Profile Design, Ritchey, Salsa).[1] The figure returned by measuring outside to outside tends to be 2 cm greater than measuring center to center for the same handlebar.
Clamp diameter
Care is needed when choosing a handlebar to match a stem, or vice-versa, as there are several standards. The ISO standard for the clamping area of a handlebar is 25.4 mm (1"), which is used on the majority of mountain bikes and many Japanese-made road handlebars. However, the Italian unofficial standard is 26.0 mm, which is the most common clamp size for road bars. There are also intermediate sizes such as 25.8 mm to try and achieve compatibility with either an ISO or Italian stem, and the old Cinelli-specific size of 26.4 mm. In practice, many modern stems with removable faceplates are quite accommodating of slight differences in handlebar clamp size, but the older type of stem with a single pinch bolt must be accurately matched. In the days of quill stems, a road stem was clearly identifiable from its "7" shape, but nowadays it can be hard to tell the difference between a "road" (26.0 mm) and "MTB" (25.4 mm) stem. Manufacturers frequently omit the clamp size from advertising or packaging.
A new standard is an oversize 31.8 mm or 31.7 mm (1.25") clamp for both MTB and road bars. This is rapidly taking over from the previous mix of sizes on sports oriented bicycles, although other accessories that mount near the stem also need to be oversized to fit (some brackets are adjustable). Standard brake levers can be used as it is only the central section that is oversized. Shims are available to fit a 31.8mm stem to either a 25.4mm or 26.0mm bar, so many new models of stems are oversize-only.