Because people range so much in size!When I first got into riding seriously, I got the longest cranks I could (180mm), because it made sense to me that with my 6’6” height I should have the biggest frame, widest handlebar, longest stem and longest crank available. That logic after all held true with clothing, beds, cars, etc., so why not with bikes?Later, when I got my first really nice racing bike, a Masi equipped with 177.5mm cranks, I noticed that when I switched those cranks to 180mm, I immediately started dropping the guys I’d been climbing evenly with. The next year (1980), when I was first on US National Cycling Team and was having my bike fit checked, Edward Borysewicz (“Eddie B.”), the US head coach at the time, told me I needed considerably longer cranks yet. My quest for cranks longer than 180mm began then and never stopped until I could offer proportional-length cranks for tall (and short) people.Little kids’ bikes have small wheels and short cranks as well as small frames, stems and handlebars because it works best that way. A small child is so inefficient as a rider that he or she cannot get the bike going if it is not close to optimally efficient for them in terms of sizing. Remember that, besides being new to balancing on a bike, a kid’s bike is much heavier relative to their weight than your is to your weight.
As the child grows, kids’ bikes available to them have increasingly longer cranks and larger wheels, as well as bigger frames, stems and handlebars.
Given that, doesn’t it seem a bit strange that when we become adults our bikes all have the same wheel size and essentially the same crank length? Are we all suddenly optimally suited to the same crank length and wheel size? Nobody questions that there needs to be a wide range of frame sizes, stem lengths, and handlebar widths to fit everybody. However, you can count on your riding buddies and bike shop salespeople questioning it if you want to ride a crank outside of the given 5mm range from 170-175mm.
Many of the standards in the bike industry are based on traditions that started pre-WWII (even pre-WWI) when people, at least in first-world countries, were smaller on average than they are now. And it also stands to reason that the last thing crank manufacturers, distributors, and bike shops want is the expense in tooling and inventory of as many crank sizes as there are bike frame sizes (or shoe sizes!).
