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Does anybody know the mathmatical formula for sizing bikes..(11 posts)

Does anybody know the mathmatical formula for sizing bikes..bike4tennis
Feb 10, 2003 9:27 AM
based on your own body geoemtry?
re: Does anybody know the mathmatical formula for sizing bikes..jtolleson
Feb 10, 2003 9:31 AM
Several good sizing sites on the internet, including
www.wrenchscience.com
WS is a good place to start...here's a sizing formulabent_spoke
Feb 10, 2003 10:36 AM
This formula has been attributed to Lemond (maybe others)

For center to center (c-t-c or cc) frame:
frame size = .65 X inseam measurement in centimeters

For center to center (c-t-t or ct) frame:
frame size = .67 X inseam measurement in centimeters

You take the inseam measurement from the base of your crotch, where you tail bone rests on the seat to the floor.
Convert inches to centimeters by mulitplying by 2.54cm/in & apply formula. Wrenchscience also helps you determine saddle height & more importantly your reach to the handle bars based on flexibility & other measurements(torso).
thanks very helpful, need morebike4tennis
Feb 10, 2003 11:05 AM
Ok i have me sizes so how do i use and compare my sizes to a bike
thanks very helpful, need moremotta
Feb 10, 2003 12:18 PM
Multiplying your inseam by .883 will give you saddle height(that is phonetically HITE, it is ht not th at the end).
Multipyling inseam by .67 will give you frame size Ctr/top.
Your TT will be determined by the frame you buy unless you are going custom. Reach will tell you what size stem you need.
Best I can figure on Wrench Science the reach calculation is
(Torso + arm length)/2 + 4cm
you get torso length by subtracting your sternum notch measurement from your inseam.
So if I have a 150cm sternum height, an 87cm inseam and a 66cm arm length my reach would be 68.5cm reach. Your reach is your top tube plus your stem length. So with a 58cm frame with 56.5cm TT I would run a 120mm stem to achieve the 68.5cm reach and my saddle height would be 77cm as measured from the center of the BB to the top of the saddle.
thanks very helpful, need morejtolleson
Feb 10, 2003 12:26 PM
I assume you are asking how to measure the bike? Most measurements are of "seat tube length" ... those numbers you see like 52/54/56, etc.

You measure from the center of the crank bolt, straight up the seat tube. Different manufacturers measure differently. Some go all the way up the seat tube to the MIDDLE of the top tube (that horizontal cross bar). Others measure to the point where the TOP of that top tube meets the seat tube (and a few measure all the way to the top of the seat tube collar... not often). These variations can result in differences of 1-3 cm.

Another decent proxy is using your cycling inseam (floor to snug up into crotch) and comparing it to the STANDOVER figures for the bike. Allow about an inch of standover.

Of course, these kinds of measurements emphasize only HEIGHT of the frame. The reach to the bars can actually be a more significant issue for many riders, especially for those with shorter torso. For example, if I bought exclusively based on my inseam (which is 81.75 cm) I'd undoubtedly buy a bike that had too long of a reach for my comfort (I am more leg than torso).

If you want to post your measurements here, you might be able to get some general guidance. It is only general, because everyone feels different. Your flexibility can be a factor, for example.
re: Does anybody know the mathmatical formula for sizing bikes..ryder1
Feb 10, 2003 10:55 AM
Here's one:

http://www.bsn.com/cycling/ergobike.html
algebraic formula_rt_
Feb 10, 2003 12:36 PM
rider + [(LBS/distance) + (Bikes/price range)]
______________________________________________ = proper bike for your size
(test rides X bike sizes available)(good shop advice)

rt
By George she's got it! LOL nmLen J
Feb 10, 2003 2:19 PM
Be careful...Uprwstsdr
Feb 10, 2003 2:35 PM
I have a 32" inseam (81.2cm) if I multiply by .67 I get 54.4. However, as I am 6'2", a 54cm frame would be too small. I ride a 58cm frame. I never thought of my legs as short, but apparently compared to others on this board they are (at least for my height). Now, I admit I have not entered all my measurements into a size calculator, so perhaps given my long torso a larger frame would be recommended. My point is do not rely soley on calculators, get on some bikes and see how they fit.
agreedbianchi boy
Feb 10, 2003 7:42 PM
Most of the fit formulas are geared toward cyclists who can or prefer to ride with an aggressive position -- that is, with a large drop from the saddle to the handlebar. If you not are not comfortable riding that way, these formulas will recommend too small a frame. According to the wrench science and colorado cyclist formulas, I should ride a 54-55 c-c frame, but I know from experience this is way too small for me. I prefer to ride with about a 1" drop from the saddle to handlebar, so to get that position I need a 56 or 57 frame. With the threadless forks/stems being put on virtually all new bikes, it is hard to raise handlebars sufficiently on frame that runs small.