This article along with many others can be found at http://www.roadbikerider.com/clothing/how-choose-cycling-shorts
You should choose road-cycling shorts based on the quality of materials and construction. But also crucial is how well they conform to your unique anatomy. Sometimes a relatively inexpensive pair may work better for you than a high-zoot model.
Shorts, like saddles, are tough to recommend because of differences in butts, crotches, seats and riding positions. Every rider has to try on shorts, buy the model/size that fits snugly but comfortably, then hope for the best on the bike. It's hit or miss, and some luck is involved. Just as with saddles, there is no universal answer.
That said, here are guidelines that'll point you toward better choices.
- Price. Generally, the more expensive the shorts, the higher the quality. Avoid cheap shorts because the material and construction may be substandard. They may be sewn from only 4 or 6 pieces ("panels"), which won't give you the best anatomical fit. The padded liner ("chamois") may not be large enough, soft enough or sewn without irritating seams. Cheap shorts aren't as durable, either, so in the long run they really aren't a bargain. When touring and washing shorts by hand, wringing can break threads and blow out seams if the manufacturer cut corners on quality.
- Panels. The more the better. Usually, 8-panel shorts conform to your body better than those made from fewer pieces. Better manufacturers use flat-seam stitching so additional panels won't result in abrasion or other discomforts.
- Liner. Crotch liners are synthetic nowadays (not real chamois leather). That's a good thing because the material can't dry, crack and cause more irritation than it prevents. A large, smooth, absorbent, one-piece, moderately padded liner has the best chance of feeling comfortable. Liners that have seams, grooves, distinct sections and/or a waffle-like texture may work fine for you -- or maybe not. There's no way of knowing for sure before riding. Beware of thick padding, which can bunch and chafe. Also problematic are gel inserts. Because they're in plastic compartments, moisture transfer can be blocked, causing excessive dampness and skin irritation.
- Leg length. This goes up and down like hem lengths in the fashion world. Long, so-called "Belgian" shorts will be in style for a while, putting the legs just above the knee. Then the pendulum swings the other way. Short shorts, like those marketed for spinning classes, are favored by riders who want to avoid tan lines that show when wearing casual shorts. But they shouldn't be so short that the nose of the saddle rubs on bare skin.
- Waist length. Proper cycling shorts are cut high in back to keep skin covered in the bent-over riding position. Likewise, they are low in front so you can bend forward without restriction. The front shouldn't be so low, though, that it's below your hip bones with nothing to help hold it up.
- Waist band. The elastic should be wide enough that it doesn't feel like a cord around your middle. Some manufacturers add a drawstring. Just elastic is fine. Just a drawstring is not. If that's the only thing keeping shorts in place, you'll feel restricted in certain positions or when breathing deeply.
- Leg grippers. Nothing is more frustrating than shorts that ride up and let material bunch in the crotch. Check the leg grippers to be sure they're wide, made of "sticky" rubber-like material and securely sewn in. The legs should feel comfortably snug, not tight.
- Stretch. Most shorts are made of a stretchy fabric generically called spandex. They're easy to pull on and don't feel like you're wearing a 19th century corset. On the other hand, you may come across shorts with fabric that purposely resists stretching. The idea is to provide help to your pedal stroke. The fabric "stores" kinetic energy on the rear part of the stroke and releases it when you push down. This concept is also used in competition suits for weight lifters. I'm not aware of any studies that prove a benefit for cyclists.
- Bibs. Shorts with built-in shoulder straps can't sag. They keep the chamois snug against the crotch to limit movement and irritation. For men, this prevents the chance of things moving out of place when pedaling out of the saddle. However, the high front makes it difficult for guys to urinate. (Some prefer to roll up one leg instead of contorting to pull down the front.) Women usually prefer shorts without bibs so they don't have to remove their jersey to take what cycling commentator Phil Liggett calls a "natural break." Bib shorts are more expensive than standard shorts.
- Size. It's best to try on shorts before buying them. Sizing varies among manufacturers. Fred is 5-foot-10 and just under 160 pounds, but wears size XL in some shorts while M is too big in others. Some U.S. manufacturers have noticed the "plumping of America" and cut their clothing bigger. It's risky to buy shorts by mailorder unless you're replacing a model and size you've worn before.
- Overallfit. In general, snugger is better. You don't want any uncomfortable restriction, but you do want the shorts to stay exactly in place. Remember that properly designed cycling shorts will look a bit baggy in the butt when you're standing in front of the dressing room mirror. Then crouch forward into the riding position and watch them mold to your body.