Determining your best Frame Size
The first step in choosing a road bike is determining your best frame size. Frames typically come in 2cm (~1") increments and are measured from the middle of the bottle bracket axle to the point where the cross tube meets the seat tube (the tube the seat post sticks into).Frame sizes are usually given as a Seat Tube measure (A in the diagram above) from the centre of the bottom bracket shell to the centre or top of the horizontal tube (so either Center to Center (C-C) or Center to Top (C-T) I measure everything C-C so my 55cm frame would be equivalent to someone else's 56 or 57cm frame if they were measuring to the top of the Top Tube.)
Frame size depends not really on height but on leg length, specifically your inseam measurement or Pelvic Bone Height PBH (See Rivendell Articles section below). Two people can be the same height but have different upper and lower body proportions.
You can pay a specialist $200 to measure you up and do a custom bike fitting. Or, you can Do It Yourself.
I usually propose three methods for determining frame size: Quick and Easy, Complicated and Moderate.
Frame size depends not really on height but on leg length, specifically your inseam measurement or Pelvic Bone Height PBH (See Rivendell Articles section below). Two people can be the same height but have different upper and lower body proportions.
You can pay a specialist $200 to measure you up and do a custom bike fitting. Or, you can Do It Yourself.
I usually propose three methods for determining frame size: Quick and Easy, Complicated and Moderate.
1. Quick and Easy DIY method to get your Inseam/PBH
To get your inseam/PBH: 1. Stand back to a wall with your bare feet slightly apart. 2. Stick a rigid T-square (a thin hard cover book works well) between your legs and jam it up as high as you can, til it hits bone. 3. Keeping the T-square in place, step away from the wall and mark this spot on the wall. 4. Repeat 2 or 3 times to get a good average. This is your inseam. Most bike dimensions are in metric, so to convert your Imperial (inches) multiply by 2.54x. Briefly, your ideal frame size is about 67% of this measurement (this is known as the "Lemond System" (developed by champion rider Greg Lemond). Slightly bigger (add 1-2cm) for a casual recreational riding frame (so you'll feel less hunched over.) Saddle Height Arguably, saddle height (from the middle of the bottom bracket spindle to the top of the saddle) is more important than actual frame size. I ride folding bikes which have weird pint sized frames that do not resemble any normal bike frame. To compensate for the compact frame size, they have crazy long seat tubes. But as long as my seat is set to the right height (82cm) and my handle bars are at the right height (though handlebar height is less important that saddle/seat heigh as the legs and knees are doing all the real work) I am comfortable. Lemond advocates a Saddle Height of 0.883x your inseam or PBH. I for instance have an inseam of 93cm and Lemond's system recommends a frame of about 62cm and a saddle height of 82cm. There are however alternative views on Frame Sizing. Recently there seems to have been a move to more compact frame sizes, down 2-4 cm (and would have me on a 58 or 60cm frame.) |
2. the ZInn Fit CalculatorOnly a little more complicated, I also like the Zinn system for bike sizing. It only requires three body measurements and spits out a fairly complete sizing suggestion.
https://zinncycles.com/measurements/ 3. The Jenson USA fit systemI like this bike company's FIT SYSTEM
4. Competitive CyclistThis very thorough complete sizing system found on the Fit Calculator at the Competitive Cyclist website. It walks you through measuring all your body parts - inseam, femur, arms, etc. -- and spits out a complete 12-item list of frame dimensions. Handy to have for fine tuning but a simple PBH or Inseam measurement is the starting point.
the problem is knowing which of the three suggested setups is best for your style of riding. 5. Road Test (Trial and Error)Once you have your inseam the next step is to get on a bike and try a few setups until you find something that feels comfortable. I generally advise people to hop up on a bike, brace themselves with one hand (against a telephone pole or a garbage can) put your HEEL on a pedal and rotate it back til fully extended. Sitting on the saddle you should just be able to lock and unlock your knee. No Lock - the saddle is likely to low. Won't unlock -- too high. Adjust accordingly.
But the proof is in the pudding. Even a brief test ride will tell you "Too Big", "Too Small" or "Just RIght". You will know the fit is right (or at least close - there is always some fiddling required once you are in the right ballpark fit-wise like a shorter stem, moving the seat forwards or back on the rails, adjusting the seat height...). An advantage of buying from TOCycles is that if after a few weeks to riding (for a novice rider, it will take several rides to just get used to the shifting systems) you don't feel the fit is right, bring it back for an exchange or buy-back (up to 10 days you get 100% of your money back. Longer? We'll talk. But we'd only keep 10% max for a month's use, 25% for an entire season. |
