Cycling bibs offer comfort if you're putting in long hours in the saddle. Rather than wearing a two-piece combo with shorts that have a tight waistband, ride down to reveal parts unknown and a chamois that doesn't stay put, bibs offer comfort and stability. The benefits of bibs, which look like regular padded bicycle shorts but expanded into overalls, can only be experienced if you have the proper fit. Once you put them on, slide a regular cycling jersey over the top and you're ready to ride.
When you try on a cycling bib and stand up, the straps that cross the shoulder should feel a little tight. Bicycling Hub says this is advisable because when you get into a riding position, the straps loosen a bit. You don't want a bib with straps that are too loose and slide off your shoulders as you ride. When you try on bib shorts, crouch into a riding position or ask to get on a trainer in the store so you can be sure you've got a proper fit.
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Sizing and Comfort
Bib shorts are fitted according to your height, but keep in mind that everyone's torso is a different length. You'll have to try on several pairs to find the brand and size that feels most comfortable to you. The straps shouldn't feel like they are digging into your shoulders, and the elastic around the thighs shouldn't ride up. Consider the position and thickness of the short's padding, or chamois, too. A bulky, bunchy chamois will cause discomfort and chafing on long rides, but one that is too thin may not offer enough protection.