Originally Posted by sch
Be aware of the completely different sizing of frames depending on whether they have a top tube that is parallel to the road or slants down from the front to the rear of the bike. Slant top tubes are more forgiving of sizing errors or put another way fit a larger range of riders per size.
Steve makes a valid point when it comes to compact frames with a sloping top tube. I posted something similar to this a year and a half ago, so I hope those who have read it before will forgive because I feel it may be helpful to those new to the sport.
After measuring and studying hundreds if not thousands of customers measurements over the years as a custom frame builder. I came to the conclusion that although human bodies are all different; they do follow certain rules of nature.
Choosing frame size on inseam alone does not work across the board because for example it is common for a 6 foot man to have an inseam as short as 30 inches; and you cant put a 6 foot man on a 51 cm. frame which is what his inseam suggests.
Tall people are not scaled up models of short people. Most of the height difference is in the legs; body length differs by a lesser proportion. If you have long legs then you also have long arms. Short legs; short arms. This makes sense since most animals are four legged; why should we be any different?
Leg length is a combination of the inside leg measurement plus the length of the foot. Length of the foot is important because when pedaling the toe is pointed downward at the bottom of the pedal stroke; so the foot becomes an extension of the leg.
People, who have a short inside leg measurement for their height, generally have longer feet. (Bigger shoe size.) It is as if they were designed as a much taller person, but their heel got turned further up their leg; making a short leg, long foot.
Imagine two people both six feet tall standing side by side; one has a 34 inch inside leg, the other a 32 inch leg measurement. Because they are the same height it follows the one with the shorter leg has a body 2 inches longer; he also has longer feet and shorter arms than the other guy. They can both fit on the same size frame, (59 cm. center to top i.e. 57 center to center.)
They will both have close to the same seat height, because the short leg guy has a longer foot so his seat needs to go higher than his inseam would suggest. They can also use the same top tube length and handlebar stem, because one has short body long arms; the other long body, short arms; making their reach the same. Minor adjustments in seat height and stem length may be called for.
It has occurred to me that with these compact frames on the market now and only available in small, medium, and large; customers are only ball parking frame size anyway. It has long been my opinion that frame size is linked to the overall height of the rider more than any other measurement because of the rules of nature I have just spoken of.
I have formulated this based on my own frame sizing chart. If you are 5 3 to 5 5 frame size equals Height divide by 3.3. For people 5 6 to 5 10 frame size = Height divide by 3.2 and if you are 5 11 to 6 4 frame size = Height divide by 3.1
An example would be someone 6 2 = 74 divide this by 3.1 = 23.87 in. (61 cm. measured center to top. i.e. 59cm. center to center. A person 5 7 = 67 divide this by 3.2 = 20.93 in. (53cm. center to top. i.e. 51cm. center to center.) The easiest way to convert from inches to centimeters is to get a tape measure with both metric and inches on and simply read across. If you dont have a tape measure the formula is inches x 2.54
The above gives you a frame size for a traditional frame with a level top tube. I have heard it said that with a compact frame, reach, which is a measurement from the seat post to the handlebars (C to C horizontally), is more important than seat tube length; I am inclined to agree. Reach would be estimated frame size center to top plus 10cm. Example frame size 56cm. = reach 66cm. This does not mean everyone uses a 10cm. stem because the effective top tube length may shorter that 56cm. so the stem length needs to be correspondently longer.
If you want something more accurate go to the chart on my website.
The chart was derived not by any mathematical formula but by records of custom frames built over many years. Most people find it pretty accurate. What I have put forward here is an attempt to come up with a simple formula that comes close to this. If you are an experienced rider dont change your position based on this alone. It is intended as a place to start for a newcomer to the sport.