Your bike’s waistline

September 13, 2007

You'll want to follow the articles in succession, starting with Intro to the F.I.S.T. Method:

1. F.I.S.T. axioms
2. F.I.S.T. protocol
3. Measuring conventions
4. Seat height
5. Cockpit length
6. Hip angle
7. Armrest drop
8. Tools of the trade
9. Your bike's "waistline" <– You are here
10. Translating fit specs to bike specs

You should've read our article on Tools of the trade prior to this if you're reading in order. Our series begins here.

It's important to understand that how a bike fits is half the battle. How it handles is the other half. A lot of companies find themselves behind the eight ball on this second geometric theme. I've developed my own nomenclature in which I divvy up a bike frame's specs between fit-specific and handling-specific agendas. Most of the fit-specific specs occur at the level of the bike's top tube, and the handling-specific stuff happens around the bottom bracket — hence above and below the waist.

How a bike frame fits is determined by where it's saddle is positioned relative to the bottom bracket, how long the bike's top tube, and how tall or short the head tube (normalized for bottom bracket drop, wheelsize, and whether the headset is integrated or not). That's it. Nothing else on the bike matters as regards fit.

But, there are certainly are other geometric parameters. There is steering geometry, for example. This is determined by a frame's head angle and fork offset. This has nothing to do with how the bike fits, but it certainly impacts a bike's handling.

Messing with the steering geometry impacts another important element of the bike's handing, its weight displacement. By shallowing the bike's head angle and adding fork offset, you can add front/center without much changing the bike's steering characteristics. Adding front/center (BB to front wheel axle) corrects a steep seat angle's weight displacement, and causes it to corner better. But that might come at a price, if you add so much front/center that the bike's wheelbase becomes unreasonably long. (The front/center issue on timed race bikes is discussed at some length here.)

Other issues that impact handling include chain stay length and bottom bracket drop. There are fairly narrow windows inside which a bike can be built that maximizes its handling and allows for a good fit. For example, a road bike in my size (I'm 6'2") might have a wheelbase of 100cm, maybe 102cm. A tri bike is going to have a longer front/center, hence a longer wheelbase, perhaps 105cm. Once the wheelbase is longer than that, the front and rear wheels start to work independently of each other, like surfing a longboard instead of a more nimble shortboard. Draw the wheelbase shorter than 105cm and you risk making a bike with too much weight over the front wheel.