General Cycling Discussion - Formula for bike sizing?
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05-04-10, 11:57 PM
Is there a formula for bike sizing using various body measurements to obtain an ideal, or close to ideal bike size? I guess that would have to include frame size, crank length, etc.
First, bear in mind that bikes are generally sized by the seat tube for leg length. But people have different leg-to-torso proportions, and torso length relates to top tube length.
If you get it wrong, it's easy to adjust for leg length--raise or lower the saddle. You need a welder to change the top tube (carbon fiber frames excepted).
(For completeness, yes, you can make small adjustments with longer or shorter stems, but go too far and you'll adversely affect fore-aft weight distribution and handling.)
That said, the Competitive Cyclist Fit Calculator (http://www.competitivecyclist.com/za/CCY?PAGE=FIT_CALCULATOR_INTRO) is a place to start.
05-05-10, 09:16 AM
There's been a ton of stuff written about this, and it's debated endlessly on every cycling Web site. Competitive Cyclist is a good place to start, though in my experience it tends to put you (well, me) on a frame that's a size or so too small. I had better luck following Grant Petersen's recommendations at Rivendell (www.rivbike.com), though he only promises that they'll work for his bikes. I'm not convinced the current trend toward smaller (and supposedly lighter and stiffer) frames is a good idea for recreational riders, especially geezers like me. I rode for years on 62cm and even a few 60cm frames because that's what the formulas said I needed (and it's also the largest size most shops stock). Rivendell's guide put me on a 65. I was nervous about going that big, but it was instantly more comfortable than a bike i'd had for years and really liked.
On the other hand, I'm in my 60s and just ride around. If you want to go fast, maybe not.
05-05-10, 01:49 PM
Yes there is but it is not perfect but for normal people, normal being a mesomorph build with proportional legs, arms, torso, the calculation of .883 X PBH = SH where PBH is your pubic bone height in bare socks, SH is the center of your saddle height in line with the seat tube. This formula is pretty darn accurate with some adjustment made for shoes, paddles, foot size and your particular body. Again, for a level top tube bike, the seat tube would then be backed out that gives you approx 1.5 to 2 inches stand over in stocking feet. Whatever frame size (seat tube length that is) then get a bike with a similar top tube length and a 100cm stem, however, obviously, there are huge variations in body types and so IMO, the bike that fits you is one that has a top tube that is comfortable with a reasonable stem length of 90 to 120cm and to which you can get leg extension per the equation above.
The "Rivendale" fit sounds a lot like the "French" fit described in that often given web link. A too big frame with high bars and a vertical position on the bike. I find this the least comfortable positions. I have one of my four ride-able bikes with the Eddy fit and the others are Competitive fit or what I guess we might call, Competive Plus size because I still have the bar tops low-er than the Eddy fit but not extremely low but in one of my bikes. No expert for sure here, just what I have seen on myself and a few others.
LOL, back before mtb seat posts, with a Campy post, I was told that the frame was right when the seat post was pulled to the NO GO line and your leg extension was correct.
05-07-10, 02:27 AM
Thanks. I'll check out the Fit Calculator tonight and get all my measurements. I forgot to mention that I commute and do longer rides for fun. No racing.
Lots of fit articles here:
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