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thoughts on frame sizing....(4 posts)

thoughts on frame sizing....got2ryd
Jun 4, 2001 5:14 AM
i went with a friend to shop for a new road bike yesterday. the sales person took measurements and decided that she needs a bike with a 50 cm top tube. her torso is longer than her inseam. with this information she test rode one model and i noticed she was really leaning far foward. afterward she complained of being uncomfortable with all that weight on her hands. this could be corrected with a more upright stem, but personally i have never heard of this method of sizing. i have always found a bike by sizing the seat tube. she has only started shopping so she has many more shops and models to check out. any information on this topic is greatly appreciated. oh, and i read the article by sheldon brown on sizing. he too mentions top tube sizing. thanks!
Initial thoughts on frame sizing....PsyDoc
Jun 4, 2001 8:40 AM
There are a couple of reasons she may have felt uncomfortable. First, she may not yet have good flexibility that has a tendency to alter your position on the bike. Second, the stem on the bike she test rode may have been too long. Do you know what size it was? If it was a 110mm (11cm) stem, then ask the shop if they will swap out the stem and then let her go on another test ride. Sizing by the seat tube is one of several important measurements. The reason the seat tube measurement is emphasized is that it relates to standover height and that is important from a safety standpoint. Some folks advocate that the smallest standover height one should have is their inseam measurement minus 1" (or 2.54cm). For example, if you friend has a 74cm inseam, then she should not consider bikes with a standover height of 71.46cm or greater. Again, this is just one of many viewpoints on the subjects.

Here is my standard reply concerning frame sizing that also includes several good resources on the topic. This information should be used as a good starting point for frame sizing. Frames differ not only in their size, but also in the angles that are used which may influence your choice of frame size.

A typical solution is to take your inseam measurement from your pubic bone to the floor in bare feet about 6 inches apart. Place a book with a 1 1/2" or so binding between your legs and pull up until you cannot pull up any further. In other words, you want to measure to the pubic bone so pull up hard. Make sure the book is level. Have a friend measure from the floor to the top of the book binding.

Then, take that number and convert it to "cm" by multiplying it by 2.54. Next, take the converted measurement and multiply by .67. That will give you a good estimate of the frame size (measured center of bottom bracket to the top of the top tube; commonly referred to as c-t) you would need to get. But, someone commented that this could put riders over 6-feet on a frame that is too small (c-t) and they may be best served by subtracting 27-28cm from their inseam length for a c-t frame size.

In order to find the frame size you would need from a manufacture that measures frame size from the center of bottom bracket to the center of the top tube (commonly referred to as c-c), you would multiply your inseam by .65 or you can subtract 1 - 1.5cm from the c-t value.

I found that the more I knew about frame sizing, the more confused I became. Here are a few links on sizing information that will benefit you.

Note: click on "Ergobike: Competition Bicycle Size/ Proportions Analysis" and at the bottom of the new page that opens, you can choose your inseam measurement. The program then calculates all the other measurements that are "average" for someone with your inseam. You will get a plethora of information back.
Having said all that....cory
Jun 4, 2001 10:44 AM
That's all good advice, or at least as much of it as I could follow is good advice. One of the things people often forget when they're sizing bikes, though, is that a lot of our formulas and rules were devised by and for racers, who have different needs, abilities and priorities than normal people. Before you choose a frame, I'd recommend you read Grant Petersen's stuff on bike fit in the Rivendell catalog or on the Web site,
I've been riding for 30 years, pretty much following the standard rules about top tube, stem, the relationship between handlebars and saddle, etc. Four months ago I bought an Atlantis from Rivendell, and since I was buying Grant's bike I decided to follow his guidelines in setting it up.
I was instantly more comfortable than I've ever been on a bicycle, and I've owned a lot of them. I'm riding longer and farther, and I don't go noticeably slower. Even if I did, though, I'm not racing the thing. I'd rather be able to ride comfortably for 50 miles than go 1 mph faster for 25.
Initial thoughts on frame sizing....notes_clp
Jun 4, 2001 10:44 AM
PsyDoc: Thanks for the great links! I just bought a Lemond and I am a little worred that the top tube is to long for me, so I look forward to working through some of those sites.