A quick note on bicycle sizing: Bicycle sizing sight-unseen is a black art. A bit of voodoo if you will.
The best way to be sure about bicycle fit is to ride it and find it comfortable – an uncomfortable bike will become nothing but a dust collector. There is some math, however, that can guide you to the right size.
The first thing you have to do is measure your proper medical inseam, otherwise known as your Pubic Bone Height (PBH). Your PBH is the height of your actual pubic bone, measured in centimeters, while standing barefoot on the flat ground. This is not your pants inseam – you must measure this accurately. For a complete guide to finding your PBH, please see the Rivendell Bicycle Works website.
Once you have this number, you can easily determine which bikes are too big, which are too small, and which fall into your approximate comfort range.
Measuring The Bike – city bikes, road bikes, comfort bikes, etc:
Enter your measured PBH in cm:
Standover Height – This is the height of the top tube (cross bar) of the bike above the ground. I post this number for most of our used bikes, and most manufacturers include this number in their published frame geometry charts. Don’t pick a bike with a standover greater than your PBH, as you’ll “nut yourself” when you dismount. Most of the time you’ll want a few centimeters of clearance there. For step-through (aka girls’ style) frames, the measured standover is not a good indicator. I’ll often quote an “effective” standover, giving a measurement of where the top tube would be if it existed – this is just to help compare ballpark sizes.
Saddle Height – Take your PBH in cm and subtract 10.5. This gives an ideal saddle height, give or take a half centimeter in either direction. (Thanks again to Rivendell for the method and numbers). For a sight-unseen purchase, you can use the seat tube length measurement from the manufacturer’s specs in order to determine if your saddle height can be comfortably accommodated on any given bike. Most manufacturers quote seat tube length as the distance from the centre of the bottom bracket (the point about which the pedals rotate) to the top edge of the top tube. (Note, however, that some measure to the very top of the seat tube where it sticks out of the frame, and some measure to the centerline of the top tube). WHat you need to determine from the charts, or from a measurement on the bike, is the distance from the centre of the bottom bracket to the top of the actual top tube. You should pick a bike with a seat tube length that allows for a few centimeters of exposed seat post, plus the height of the saddle itself. Subtracting 15cm from your ideal saddle height should put you in the right ballpark. You can adjust this number if you want to use a super low profile racing seat, or a double sprung 20cm tall Brooks B190.
Top Tube Length, Seat Tube Angle, Bottom Bracket Height, and all those other numbers on the geometry charts – For most day to day utility cyclists, these numbers don’t matter much. They most certainly affect the type of ride the bike will provide, but unless top performance is your number one priority, don’t worry too much about them. I will mention, however, that a lower seat tube angle and a longer head tube length generally signal a more relaxed comfort-oriented geometry. So you can keep those in the back of your mind.
BMX bikes, Mountain Bikes, Tallbikes, Recumbents, etc – The above information is for standard road/city/hybrid/utility bikes. Downhill mountain bikes, full syuspension bikes, BMX and other outliers can not be sized this way. And remember, in the end it’s about the ride – if it’s not comfortable, you won’t ride it. So a nice long test ride is a vital part of the process.Subscribe to blog posts via email or via RSS.