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Yet another frame size question, but it's TOP TUBE!!!(3 posts)

Yet another frame size question, but it's TOP TUBE!!!Ginz
Dec 12, 2001 7:56 PM
I pretty much have the hang of choosing a frame size based on formulas other theories. But, what method should I use to assess the appropriateness of the top tube and how do I select a stem length/angle? Is this done by "eye", based on feel and experience, or is there some guideline based on arm length, etc.?

re: TOP TUBE!!!guido
Dec 13, 2001 1:24 PM
There's even a school that says top tube length is the first thing to consider when buying a bike. This comes from slanted top tube mountain bikes and now more and more road bikes, where you can't measure optimal size by standover height, and the saddle height is widely adjustable on a long seatpost.

Bike designers make reach to the handlebars proportional to saddle height, obviously because larger people have larger upper bodies (reach) as well as legs (saddle height). So if you get the saddle height right, then you can dial-in reach to the handlebars with a shorter or longer stem.

The other way, by reach first then raise or lower the saddle for leg length, is adjusting body position around the stem and not the crank, which is stupid because the legs are doing all the work, sitting on the saddle and turning the crank around, and must be more finely tuned to work efficiently and avoid injuries.

The upper body does nothing except breathe and steer, so can get comfortable within a wider range of dimensions. If you raise the seat, then your stem will be lower, and you'll be hunkered down with more weight on the handlebars and front wheel. If you lower the saddle, the stem is higher, you're sitting more upright, and more bodyweight is on the saddle, less on the bars. You want about 60% on the saddle, 40% on the bars, so you can maintain good traction on front and rear wheels, post over bumps, keep your center of gravity over the crank, and generally control the bike.

Your handlebars are within a few centimeters of optimal if they obscure your view of the front hub, "when your hands are on the hoods." Beyond that, there's alot of mumbo-jumbo. The upside is that they probably all end up with the same results. A well fitted bike will be a comfortable bike that you can work with all day.
lots more to it....C-40
Dec 13, 2001 3:46 PM
There's a lot more to adjusting fit than can be adequately be covered in a couple of paragraphs. Check out, or for lengthy discussions and considerable disagreement.

The frame's seat tube angle (STA) must be considered along with the top tube length to be meaningful. On a midsize frame like the 55cm that I ride, a 54.3cm top tube with a 74 degree (STA) will fit the same a 55.5cm top tube with a 73 degree STA. The only difference is the position of the saddle on the seat post needed to produce the same knee-over-pedal position.

One functional test for reach to the bars is knee to elbow clearance when riding in the drops. If your knees and elbows hit when riding in the drops, the stem and/or top tube should be longer, at least by racer's standards. Recreational riders who seldom ride in the drops, might get along fine with a significant amount of knee to elbow overlap. If you must use an excessively long or short stem to achieve the desired reach, the top tube length & seat tube angle combination is not optimal.

Bar height can vary widely depending on whether the fit is intended to be more upright for touring/recreational riding or low for fast riding/racing.