We all know bikes should be designed based on the type of riding – road, mountain, cyclocross, touring, cruising, to name a few. The basic premise behind custom frames is that bikes should also be designed to ergonomically fit the rider. We see this premise in play with stock frames too – they typically come in small, medium, large, extra large, and some come with women’s specific geometry modifications. Custom frames start there, but take it much further, tweaking angles, lengths, weight, and diameter for a completely tailored ride.

So just how different do these custom frames turn out compared to standard stock frames? We talked to Jon Cariveau at Moots. We figured he should know – Moots has been building custom frames for 30 years, and since 1991, also produces a stock line.

Cariveau said 20-percent of their business is custom frames. If you were to extrapolate that number to the industry, it's fair to say that stock sizing fits approximately 80-percent of cyclists. The other 20-percent have physical dimensions too big or too small for most frames; physical ailments or limitations that prevent them from riding stock frames; a specific cycling style, such as spinning or mashing, that they want to capitalize on through their frame construction; or are just unapologetic gear heads who crave the experience of working with a design engineer, and have the ability to put their money there their mouth is. We won’t be covering this last category, since it’s a personal preference more than a necessity. As for the others, we asked Cariveau to give us the lowdown. Here’s what we found out:

Anyone over 6-feet 4-inches tall, or fewer than 5-feet, is going to fall outside the range for a stock frame. That one is obvious. But here’s one that’s not so obvious. Body proportion can also put you outside of the range, even if you’re of regular height. Consider two 6-foot tall men, one has long legs (a 34 and one-quarter inch inseam), and the other has short legs (a 31-inch inseam). Remember they are the same height, so what else must be out of proportion? Torso length. Compared to the average 6-foot man, Lanky will have a shorter torso and Stubby will have a longer torso. Despite the same height, these two men require completely different tube lengths. For example, to compensate for his short torso, Lanky will need a shorter top tube coupled with a longer head tube to optimally reach his handlebars. But by lengthening his top tube, his handlebars come up too high compared to his saddle (the handlebars should typically have some drop). Suddenly it’s an entire frame geometry manipulation; custom frames to the rescue.

The second biggest reason for custom frames is also physical – a bad back, or inflexible hamstrings can make biking painful. A custom build helps mitigate the pain, and for some people, makes biking an option when it otherwise would not be. Cariveau said tight hammies and hips are the two biggies. You’re not going to see these folks with their torsos parallel to the ground on their road or time trial bikes. They simply can’t stretch themselves out that way. A custom frame with a little more height built into the front of the frame comes in handy for those who just can’t handle too much tilt between the saddle and the handlebars.

Physical therapist Erin Dunham added that neck pain is another common complaint on the bike. The lower a person has to get to reach his handlebars, the more he has to cock his neck back to see the road before him. “Bringing a rider more upright will tuck the chin into a more natural position and reduce neck irritation,” she said. A custom frame finds the balance between putting a cyclist too upright and putting him in constant, uncomfortable flexion – be it the back, hamstrings, or neck.

A third reason for indulging in a custom frame is often overlooked: riding style. People who fall on either end of the pedaling spectrum, spinners and grinders, can actually improve performance by using a frame with corresponding tube diameters. If you’re a grinder, you need a stiff frame, which is accomplished through oversized tube diameters with thicker walls. A frame built that way would ride too harsh for a spinner, or as Cariveau put it, “would beat them to death.” Smaller tube diameters with thinner walls are the key for those folks, a build that would be far too soft and mushy for the mashers.

If none of these reasons apply to you, and you’re still dreaming of that custom frame (which will cost about $650 more than a stock frame, on average), then perhaps you’re a bona-fide gear head. Not necessarily a bad thing, we just thought you should know.