Your bike's waistline
Written by: Dan Empfield
Date: Thu Sep 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.
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.
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.
Articles related to this one
Tour de Bike Fit
Read this first, if "tri bike fit" is new to you. It's a guide to the dozens of articles in the bike fit section. The articles are grouped, and we offer our advice about the order in which they should be read. 12.30.08
Read this first, if "tri bike fit" is new to you. It's a guide to the dozens of articles in the bike fit section. The articles are grouped, and we offer our advice about the order in which they should be read. 12.30.08
Bike geometry
I ride a 59cm bike or, if it's a road race bike (as opposed to a tri bike) I'll ride a 60cm bike. If it's a Litespeed tri bike, I'll ride 57cm, and so it goes. How are these bikes measured, and why do I ride different sizes depending on the manufacturer? 8.06.02
I ride a 59cm bike or, if it's a road race bike (as opposed to a tri bike) I'll ride a 60cm bike. If it's a Litespeed tri bike, I'll ride 57cm, and so it goes. How are these bikes measured, and why do I ride different sizes depending on the manufacturer? 8.06.02
F.I.S.T. Axioms
This second in a series of articles describing our tri bike fit system lays out the axioms on which our F.I.S.T. system relies. 9.13.07
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Comments
FIST bike fit system
........
Reviewed by: chris brands, Apr 10 2009 11:42AM
Very nice article about the FIST system. A lot of comment are generally true. Now the thing for bike fit is how to take all the statical data into an dynamic fit. From an biomechanical point of view is is sometime hard to fit an triatlete in an good postition who is very poor in his gluteus flexebility. On the other hand an triatlete who is poor is his hamstring flexibility could be easily fit in an good postion. My experience is that is also has to to with the specification of the body of the triatlete. Specially when he has an history of injuries. I loved the story but a see a lot more variables in the system. By the way i am doing dynamical bike fitting in the Netherlands, Europe.
missing link?
Reviewed by: mike, Apr 15 2008 9:31AM
is there a link missing on "here" at the end of the second to last paragraph?
