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Frame Size?(2 posts)

Frame Size?Convert
Feb 6, 2002 1:38 PM
What size road bike would I need? I'm interested in buying a bike from the classifieds or from e-bay, but not sure what frame size to get.
My Mtn. bike is a 18" Jamis. I am 5'8" but have long arms. The 18" frame fits me just right.

Thank you in advance for your advice.

re: Frame Size?PsyDoc
Feb 6, 2002 1:42 PM
This is a standard reply as questions of frame sizing often present themselves on this board. 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.

Once you have a good starting point, then you really need to go out and ride a few different sizes to see which one fits you most comfortably. I have a 33 1/4" inseam (or 84.45 cm). I used to ride a 56 cm (c-t) Merlin Extra-Light. I now ride a 57 (c-t) Seven and a 19" Specialized mountain bike. I chose the 19" over the 17" because the 19" just felt more comfortable.

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. BUT, keep in mind that this analysis is for a "Competition Bicycle Size" and the size you feel more comfortable on may be a size larger or smaller.
Chris Huber advocates finding your frame by taking your inseam measurement x .66 for
your center-to-center frame size. You would need to add about 1.5 - 1.8cm to this value
to get your center-to-top frame size.