Well, I'm a little late today! But, hey! This class is free, right? Okay, so last time I disscussed the importance of geometry and how it relates to the way a rider fits on the bicycle in a general sense. Let's take a closer look!
Probably without much argument, most folks would agree that the most important thing to get right in regards to "fit" is the relationship between the saddle and the bottom bracket/ pedals. This would make sense since it is the primary interface between the engine and the chassis. Kind of like the primary drive on a motorcycle, come to think of it! Anyway, the human body has been found to be most efficient on a bicycle, in regards to power output, when the legs of the cyclist can be nearly extended straight at the six o'clock position in the crank arms rotation. Of course, there is the posistioning of the foot on the pedal, which I won't get dragged into here, but it is important. Along with proper leg extension, the relationship between the riders hips and the center of the bottom bracket shell is of paramount concern, as well. Usually, the riders hips are posistioned somewhat behind the bottom bracket: more over the rear wheel than directly above the crank assembly. Just exactly where one sits is determined by the seat tube. ( in a traditionally designed double triangle type framework) The typical seat tube runs from the upper portion of the bottom bracket shell to the junction of the top tube and seat stays. Most seat tubes have a rearward lean to accomodate the proper posistioning of the hips to the crank assembly. The amount of lean is most often referred to as the seat tube angle which is referenced to in degrees, like the degrees of a circle. Geometry, get it?
It seems that when it comes to production hardtail mountain bike frames the seat tube angle that has been arrived at as being "ideal" for the "average" cyclist is 74-73 degrees from an imaginary horizontal line drawn through the center of the bottom bracket shell. Of course, if you use a seatpost with setback, you are actually somewhat slacker than that average angle. (You always knew you were a slacker, now didn't you?) You can obviously affect that even more by how you posistion the saddle on it's rails, and there is also the non-setback post for those who like the hips over the cranks even more. I suppose you could even turn your set back style seatpost around so the setback becomes a setforward, but then you are getting into multi-sport territory, and I ain't gunna go thar!
If you really do not go for the "one size fits most" philosophy of the production bike, you will find that the custom builder will go into great detail to find the proper seat tube angle for your needs. Sometimes it will fall within the the established "norms", but alot of times it won't. This is one of the reasons that a respected custom builder can make a bike feel "just right" without a lot of twiddling on the part of the cyclist. Either that, or you can buy five different seat posts trying to get the right setback! Get your hips in slightly the wrong place, and your power output suffers. Obviously, the length of the seat tube is somewhat important, as it affects the leg extension issue, but also helps determine setback to some degree, and is intimately tied into your top tube length, which we haven't even gotten to yet!
So, we can see how the proper seat tube length and angle can be important to proper posistioning of the engine. Next installment will deal with the effects that all this can have on handling. I think this is quite enough for you all to chew on over the holiday weekend, so I'll be back again with another class on Sunday evening. Till then!