Geometry
Bicycle frame geometry effects both a bicycle fits and the way it handles.
In many cases you can get a bike that to “fit” by simply changing the stem and seat post position but in the extreme the results can produce a bike that does not handle well. For example, I could ride a super small frame and simply get an extra long seat post and a very high stem with a lot of extension. A bike that is too small but then adjusted to fit will be fine for simply getting a workout but this bike will not handle and respond to rider inputs very well.
The way a bike handles is influenced by just about all the aspects of geometry. The head tube angle and fork rake give the bike the basic personality. Aspects like the amount of weight supported by the front and rear wheels and the overall wheel base length are also very important.
When you get around to defining the fit and geometry of a bicycle you can distill the problem down to a few key measurements.
The following notes refer to the measurements in the above drawing.
470mm fork length
Fork length reflects how much potential suspension travel the fork can support. This is driven by the fork choice but it will have a big impact on the design of the frame. When working with a suspension fork I design the bike using the length of the for with the suspension sag in effect.
45mm fork offset — basic steering input (part 1)
The fork offset is a key metric because it has a large impact on how the bike will handle. This is also called rake or tail and is the measurement of how far forward a fork places the axle vs. the theoretical line that extends from the head tube. Many people do not think much about offset because it is established by the fork manufacturers and in particular with suspension forks it is really not something you can change.
For a 26er fork offset is fairly static at around 39mm.
For a 29er, fork offset is not yet “standard”. Rock Shock has 29er forks with 38mm, 46mm, and 51mm of offset. Fox has 29er forks with 44mm and 51mm offset. The Trek and Fisher OE versions of the Fox Forks run 51mm offset while the standard Fox is still using 44mm.
I think the new 51mm offset forks is a good idea but time will tell if they become standard. For right now the most conservative approach is to design 29er frames around 45mm fork offset.
71mm head tube angle — basic steering input (part 2)
Head tube angle is very important. To really understand head tube angle you need to think about it only after you have established the fork offset. A frame designed around a 51mm fork offset should have a different head tube angle then one build for a 38mm fork.
Head tube angle should ideally also be related to frame size, center of gravity, and the type of handling the rider is looking for.
105mm head tube length
Head tube length has an impact on the frame stiffness because it controls part of the size of the front triangle. The head tube length also as an impact on what stem you will need and what you will need terms of spacers and riser bars.
601mm top tube length
The top tube length is the theoretical distance of a level top tube. Many bikes have top tubes that are angled but we still use the measurement of the theoretical level distance. The length is related to the rider size and impacts the length of stem that will be needed and the amount of weight the front wheel will carry.
430mm Seat tube length
The seat tube length has an impact on stand over height and overall stiffness for the frame.
74mm Seat tube angle
The seat tube angle has an impact on how far back the rider will be seated and in turn will influence the mechanical of pedaling. You can also run a steeper seat tube angle and then use a set back seat post to gain a little more rear tire clearance.
440mm Chainstay length
The length of the chainstays have a impact on the stiffness of the rear end and on the weight distribution between front and rear wheels.
48mm Bottom bracket drop
The amount of drop for the bottom bracket is the primary control for the pedal clearance and stability. With a mountain bike, you want your bottom bracket to be high enough to avoid pedal strikes but low enough that the bike is stable and easy to control.
The Bottom bracket on a road bike will be lower then a mountain bike. With a road bike you need to have a high enough bottom bracket that you can corner while pedaling with the risk of pedal strikes.
742mm Seat Height
The seat height on a bike has a very large impact on the dynamics of pedaling.
716mm Rider Compartment
Rider Compartment or Cockpit by default is the horizontal distance from the center of the saddle to the center of the handlebar.
32mm Handle Bar drop
Handlebar drop by default is the vertical distance from the center of the saddle to the center of the handlebar.
