By Sheila Ascroft
How do you know what size of bike you need? Some bikes are measured in inches, others in centimetres. Why? Mountain bikes evolved in Californian the early ‘70s and so use the American-favoured inches. Road bikes on the other hand have a long European tradition and are always measured in centimetres. Most of the other bikes designed for the road—racing, touring, recreation, some hybrids and cyclo-cross—also use the metric system.
Measurements for bike sizes can be from the centre of the bottom bracket to the centre of the top tube or to the top of the top tube or to the top of the seat tube. And some top tubes are not horizontal but sloping. Where is the bottom bracket you ask? It is where the bike frame tubes join at the bottom with the chain stay tubes. It’s the circle in the diagram below.
Wait it gets worse. Mountain bikes have 26-inch diameter wheels (except for the 29 inch wheeled bikes created by Gary Fisher), while most road bikes have 700 cm (about 27.5-in.) so a mountain bike will seem shorter. Remember too that the height of the bottom bracket from the ground may also vary; it is higher on a mountain bike to provide extra clearance for getting over logs and lower on touring bikes to provide better stability. Yep, sizing can be confusing.
A1 – Seat tube length (centre-to top) A2 – Seat tube length (centre to centre) B - Top tube length (centre to centre) C – Stem length (centre to centre)
Your height can provide a very idea of what size frame you might need, but a lot depends on how long your legs are. Two people may be the same height, but one may have longer legs than the other has and require different size frame. For example, I’m 5’4″ (162 cm) with short legs and prefer a mountain bike in the 14-15 inch range, while a friend who’s 5’3″ rides a 16 inch. In road bikes, I have ridden a 48 cm, but when I stood over the top tube, my crotch was touching it. I now happily ride a 44 cm (measured centre to centre) even though the manufacturer’s label on the frame has it as a 43 cm. Confusing eh?
What is more important to know is your standover height. You need to have at least an inch (2.54 cm) of space between your private parts and the top tube. On a mountain bike, it’s better to have 3-4 inches (7 to 10 cm) of clearance, especially if you plan to do the rough trail stuff.
To find your standover height, you need to measure your inseam or the distance from your crotch to the floor in sock feet. The simplest way is to have a friend help you. Stand close to a wall, feet about 6 in. (15 cm) apart and look straight ahead. Place a book to simulate the bike saddle between your legs and pull up firmly against your crotch. (One end of the book should be touching the wall.) Make sure the book is level then have your friend measure from the book spine to the floor. My inseam is 73.5 cm (29 in.).
For a road frame that is measured—centre to top tube centre (c-c), the general guideline is to multiply your inseam (in centimetres) by .65. For me, 73.5 x .65 = 47.7 cm frame. If I measure my frame centre to centre, it’s 45 cm. According to this formula my current bike should be too small, but it isn’t. Remember this is only a guideline; we are all built slightly different.
For a mountain bike, try a frame that is 4 to 5 inches (10-12 cm) smaller than your road frame size. For example, if you ride a 55 cm c-t road frame, look for a 17-18 inch c-t frame.
The following chart taken from Canadian Cyclist’s 2000 Buyer’s Guide provides a starting point for bike sizing. (This chart was originally adapted from Canadian national coaching certification program.)
|Inseam||Mtn frame size||Road frame size|
You can find other bike fitting tips at www.coloradocyclist.com/BikeFit/index.cfm.
I’ve been cycling for 20-some years and writing about it for the last 10. My articles have been published in newspapers and magazines — and now on the women’s cycling website! I’m a member of the Ottawa Bicycle Club and the Canadian Kilometer Achiever Program. www.sheilaascroft.com