Seat Tube Length is as good as any place to start. A while back ago, there was a basic formula established to come up with size of bike a person needed. You measure your actual inseam, shoes off, preferably with an inseam device or the lay mans approach with a book shoved up into your crotch against a wall. Centimeters x .657 = center to center (of frame) size, gave you your bike size, i.e; 52cm. This was back when frames were pretty much horizontal and the seat tube-top tube intersections were measured at the center of that junction from the center of the bottom bracket. Now with everything sloping, mix that with companies taking their own approach to measuring seat tube length, top of the top tube at the seat tube joint, and some measuring to top of the seat tube or seat mast (and that extending above the junction 1cm to more than 6cm), it raises all kinds of confusion in interpretation of what some one needs, from one brand to the next and even within the brand. Point: seat tube starting point is only that, you may need to go up or down in size. So, we now look at stand over height. Obviously, once you have shoes on, this will give you a lift, but even though it is necessary to be able to comfortably stand over the top tube, at some location along it's plane, it is still not as important as your upper body's ability to properly spread out, it's height or angle relative to the legs and hip position, and the weight applied onto your hands at the bars.
Stack and Reach, a newer concept in determining or rather measuring the front end height and reach. This is a decent way to look at someone's needs from bike frame to bike frame. You must check, again, on the type of headset the frame uses, as this can take away 2.5cm or add (approximately). Stack is based off measuring the height of the top of the head tube (at the center of it) from the Bottom Bracket. Reach is based off measuring the distance (horizontally) from the center of the BB to the center of the head tube at the top of it. I have to add that the spacers on the steerer tube (the fork's shaft that comes up through the head tube) along with the angle/length of the stem help or subtract from your particular needs. Another way to look at this is in the Head Tube length and the Top Tube length or Effective Top Tube length. Either way, these two measurement are, in my opinion, the most important for proper fitting than just the seat tube. As you will find, from model to model, brand to brand, these numbers will vary a lot. What is right for you is partially determined by a professional fitter, the current bike you are riding (assuming you know what is right and wrong with it), and possibly your test rides on various geometry styles.
Top Tube length and more importantly Effective Top Tube are another key area of concern and evaluation when considering the correct overall size. Since most bikes slope (top tube from head tube down to the seat tube), and at varying angles of slope from brand to brand, model to model, and along the various sizes, Effective is a more accurate number to look at. In fact, many only list Effective vs Actual. For instance, the taller the head tube, and the greater the slope downward, and angles on the Seat and Head Tubes, this number could be shorter or longer than you need. Effective is measured from the top of the Head Tube at the center-horizontal to the ground-back to the center of the Seat Post center. Other factors that can help or detract from these numbers being in the right/wrong direction is Seat Post offset, saddle position (up/down, and fore/aft), then the Stem length and Handlebar dimensions.
Head Tube Angle affects reach, handling (quicker or slower), weight distribution, and even toe overlap with the front wheel. For the most part, this is pretty dialed in now a days from size to size.
Fork Rake also has impact on steering responsiveness. Measured as the amount the fork center line varies from the center of the dropout. This is usually predetermined from the factories, complete bikes, and frame sets. So, changing that is not an option. Most range from 4.3 cm to 4.7 cm (the overall is 4.0-5.2). The lower the number, the quicker the response of steering, essentially. However, many other factors impact this, like; stem length, weight distribution, chain stay length. Another consideration we use in determining fork rake when building a custom frame or a frame up build, is toe overlap and front end stability. For the most part, this is not something you need to consider too much but, wanted to explain it's impact.
Seat Tube Angle is an area that is wide ranging from brand to brand, size to size. This is a particularly interesting area of design for me personally. The basic reason or determination of it is leg length and femur length. Many stats state that the shorter the legs, the shorter the femurs, and you bring the Seat Tube Angle forward (toward the head tube). Male vs Female: Female's usually have longer legs (inseam) than men for any given height. Shorter men and men in general have slightly shorter legs but, it's mostly in the femur. Mix this with the smaller bike frame's make up or geometry dilemma and what you generally find is the smaller bikes have a steeper Seat Tube Angle-i.e; 74-75 degrees. This is one way to also get you closer to the handlebars, assuming you are shorter everywhere else. This is where my personal problem with many stock bikes exists. Bare with me for a second, as my main point is to show how the bell curve and stock geometry bikes do not work for everyone. I am short legged compared to my 5' 8" height, so I get more of my height in my torso length. But, to be specific, my femurs are long, which means in order to get my knees over the pedal spindle or behind, with a 74 degree Seat Tube Angle I have to either use extreme set back seat posts-more than 3 cm, or go custom. Once I get the seat back, then the reach tends to be far too long, add to this that I have been battling an extremely bad back over the last 15 years and I can no longer bend forward as much, so my reach and height need to be even closer. I'm not embarrassed to say that Trek's WSD (Women Specific Design) tends to be more correct for me (stock geometry) than the given 52 cm men's frames. However, I am a lover of Titanium frames and I get to spec my own custom geometry and this has been my preferred approach for my last three road bikes. That all said, I have seen a good trend in bringing that Seat Tube Angle back-slack, and shortening the reach and raising the height. For most, including the WSD, this has been very good.
Chain Stay length is set, mostly, for quicker acceleration (so shorter in length), and for tire size allotment. This length does not vary much on most of the road bikes, sport or race. However, on taller front end bikes (sport rd-Trek H3), touring bikes, and similar, these lengths do get longer. The reason is by extending these out it helps to push weight back into the front end-to stabilize the front end at high speeds. See, when more of your weight bias is on the back wheel you need to balance the weight bias for safe handling. This is something we do with our Steelhead Bicycle frames and I did with my own custom frame.
Now, the last item of note is Bottom Bracket Drop (how much drop from the horizontal line from axle height), some only state Bottom Bracket Height (from ground). The lower this number is on Drop, the higher it is from the ground, thus raising your center of gravity but, allowing for better pedal clearance to the ground, a consideration for criterium type racing or pedaling through corners. The higher the number for Height is also doing the same thing. The greater the Drop, or the lower Height number, sets your body lower, improving stability (common on touring bikes), minimizing in toe overlap for shorter/smaller frames, and helps to increase stand over height, especially beneficial when mixed with tall head tube bike frames. I personally designed a really low Bottom Bracket Height for the above mentioned reasons and I love the feel.
So, there are the basics and for the most part, the most important aspects to understand in bike geometry as it relates to road bicycles. I must point out that we are in an age of a lot of sub-category bikes, special use road bikes, relaxed, racing, commuting, etc. So, most of my points do apply to all of them but things like wheelbase, fork rakes, chain stay lengths, will vary quite a bit.
Some people through the years have come up with really quirky "rule of thumbs", or ways to quickly determine proper fit or if a bike is set right. One of them is when you are sitting on the bike and hands on the bars, and you look down at the front hub, the bars should hide the front hub. This is so far off the beaten path that I actually get angry about it. This is not even pointing out the importance of proper saddle position, proper bar reach and height set first, and leaves no directive of where your hands should be on the bars, not to mention bent elbow (which is correct) or straight. So, needless to say (but, I will), this is just the start of why this sends a really bad concept into some one who does not know any better. Add to this, varying morphological variations, long neck, long arms, bars up high for bad backs, so on and so on. Then, you look at the varying frame types, head tube angle, fork rake, fork offset, wheel size, etc. I posed this question to 7 different bike manufacturer's, to see if they built bikes off this notion, and not a one of them subscribed or gave any truth to this adage. There are many, many more but, I really do not wish to waste my time or yours on giving any of them and credulity. So, let's move on......
Now that Bike manufacturers's are both listening to bike fitters and learning the importance of bike fit and it's impact on geometry, we are all in a good spot with options for comfort, safe handling, and a bike that can do more than one thing well. Sort of like having your cake and eating it too. I am especially happy with the gains made for women! For years, they have been around 50% of the buyers but, heavily ignored in frame geometry, size ranges, saddles, bars, shoes, shifters, and having high end options. We have seen huge gains in options for the ladies. The bike makers are slow to change, slow to respond, but we are in a good place in the cycling industry. Things should only keep getting better, especially because sales growth have not slowed for about the last 11 years. Now, if custom can be had at no extra charge, quickly, and in any material.......then I will be very happy!
In my next post I will talk about the facts, the pros and cons, and opinions on the different frame materials.
Happy Riding!
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