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Need help on frame size(3 posts)

Need help on frame sizemgolebiewski
Mar 24, 2003 6:38 AM
I am looking to buy a new road bike and via suggestions here am looking used on e-bay and need some help with sizing. Unfortunatley I have a cheap Schwinn that came in Small Medium and Large and to boot is a compact frame.

I am about 5'6"? Roughly what range of sizes should I be looking at?
re: Need help on frame sizeFredrico
Mar 24, 2003 1:28 PM
Hey, 5'6,"

You'll be looking for a 52 cm. frame with horizontal top tube, or smaller if the frame has a sloping top tube.

A good way to be in the ballpark is to stand barefoot against a door sill, or other suitable upright, and measure the distance from the floor to a book as if you are sitting on it, like on a saddle. Square the book against the upright, mark the height where you're sitting, measure it and thats your inseam. Frame size = Inseam x .65 for a small racing frame or .66 for a larger touring frame.

As frame top tubes are proportional, a frame with a 52 cm. top tube would be close to the same size as one with a 52 cm. seat tube. If you are short legged but long bodied, you would go with the larger frame to make enough room for upper body reach, or vice versa if long legged and short bodied. Frames with sloping top tubes are often sized by top tube length, which would more or less fit the formula above for seat tubes.

There are numerous bike sizing charts and programs on manufacturers' and retailers' websites, many of which are complicated, obscure, and downright mystifying. I have successfully ridden traditional horizontal top tubed frames from 53-56 cm. with equal comfort. Reach can be fine tuned by altering stem lengths and tilt angles, saddle height by adjusting seatposts.

A larger frame generally distributes road shocks over a wider area and is therefore more comfortable than a smaller frame, which will generally feel stiffer. Frame makers spend a great deal to time designing out these differences, however, and the differences aren't apparent in the smaller frame sizes which you'll be dealing with, anyway. This is just to help in deciding whether to err on the large size or on the small.

There's also an old sizing dictum, now stated by Rivendell Bicycles, that placing the handlebars about the height of the saddle or only 1-2" lower, provides the most comfortable riding position FOR BEGINNERS, casual riders who don't live on their bikes, or anyone except racers or triatheletes who want to be really aerodynamic. So if you can't get the bars almost as high as the saddle, the frame is going to be too small. This is a mistake made by many beginners who get sized by racer types in the bike shops. Inseam x .884 = saddle height. That should help also.

Good luck. It can take several years of riding and experimentation with positioning to settle on an optimum fit. Everyone ends up slightly different.
For the third time todayKerry
Mar 24, 2003 5:43 PM

For adjusting the fit of the bike, there are roughly five starting points:

1. Seat height (top of saddle to center of pedal axle) at 108-110% of inseam.
2. Saddle parallel to ground.
3. Saddle fore/aft adjusted so that a plumb bob from the bony protrusion just below the kneecap passes through the pedal axle when the cranks are horizontal. This is known as KOPS (Knee Over Pedal Spindle)
4. Front hub axle obscured by the handlebars when riding in your "regular" position (drops, hoods, or tops).
5. Top of handlebars 1 to 4.5+ inches below the top of the saddle depending on your flexibility and size.

These are all starting points for "average" proportioned people, and many folks like to move away from these starting points as they learn what makes them more comfortable, powerful, or efficient. You want to get the fit of the frame as close as you can, then do minor adjustments with the stem, seat post, saddle position, etc.

A lot of this is personal comfort, and we all tend to adapt to a given position over time. For example, a given stem length may be right for you, but it may feel long at first. I use the "handle bar obscures the front hub" rule for my fit, but others claim better position (for them) with the hub in front of or behind the bar. I'm 6' tall and ride with 11.5 cm drop from saddle to bar, probably more than most people would like but fine for me. Some are suggesting zero drop from saddle to bars - it's about comfort, efficiency, and aerodynamics. The ERGOBIKE calculator is pretty good, but it is not infallible. I would suggest riding some miles (over 100 total, and over 500 would be better) and see if you adapt to the position. There are no hard and fast rules, just general guidelines, when it comes to these things.