Bicycle frame geometry

Bicycle frame geometry

Updated: 16/11/2018.

This post explains the term: “bicycle frame geometry”. Frame geometry consists of important attributes like fork trail, rake, wheelbase etc. All those will be defined, explained, along with how they affect bicycle’s handling and ride characteristics.

Contents:
1. Bicycle frame
2. Steering – front part of the frame
…2.1. Head tube angle
…2.2. Fork rake
…2.3. Fork trail
…2.4. Steering recapitulation
3. Chainstay length
4. Wheelbase
5. Bottom bracket drop (BB drop)
6. Seat tube angle
7. Seat tube length
8. Head tube length

 

1. Bicycle frame

Frame is the spine and the heart of a bicycle. All other parts can easily be swapped, but the frame is what determines what combination of parts a bicycle can accept, what kind of use bicycle is best for and what seating positions can be achieved on a bicycle.

For example, frame choice determines what kind of brakes a bicycle can accept (disc, v-brake, or road bike brakes), what the maximum tyre width is, whether mudguards and racks can be mounted etc. This is easy to tell when choosing a frame – what kind of mounts it has. Also, producers mark these things in prospects (e.g. “disc brake frame” etc.).

Another thing that frame choice influences is seating position, comfort and bike handling. However, it is a lot less straight forward (than disc and mudguard mounts) and this post will explain how geometry influences ride and fitting characteristics.

When it comes to bicycle frame geometry, good producers usually provide a table with lengths and angles. For example:

Frame geometry All the important dimensions and angles are noted.
Frame geometry
All the important dimensions and angles are noted.

As it is shown in the table above, geometry differs (more or less) for each frame size, even with the same model, of the same producer. Frame geometry is especially important if carrying loads on the front is planned (e.g. front panniers), since steering is usually calculated with expected front load in mind.

Text will give common values for a typical touring and a typical road bike frame. Both frames sized for an effective top tube length of 57 cm.

Effective top tube length is explained in this article: How to choose a correct frame size

2. Steering – front part of the frame

Front part of the bicycle, along with the fork, has three important factors:

  • Head tube angle
  • Fork rake (it will be explained)
  • Fork trail (also 🙂  )

2.1. Head tube angle

Angle is measured compared to the horizontal, with both wheels of planned size (usually the same size). In mechanics literature, this is called “rake”, while in cycling literature, “rake” is used for referring to fork offset (explained in chapter 2.2.).

Head tube angle
Head tube angle
  • The closer head tube angle is to vertical (90 degrees), the quicker the steering. However, if it is too vertical, bicycle will be twitchy, unstable.
  • The sharper the angle, the more effort it takes to steer the bicycle. But slack head tube angle also improves stability.

Touring bikes that carry a lot of weight take advantage of more stable, 71 degree head tube angles. Road bikes that favour quick responsive steering and are not loaded usually settle for 73 degrees.

2.2. Fork rake

Distance for which the front wheel dropout is moved forward from the head tube axis. Forks are usually angled forward a bit, or have a curve to the front near the bottom. In mechanics literature, this is called “fork offset”, while “rake” is used to refer to what is called “head tube angle” in cycling literature – there are many such misnomers in cycling literature and jargon.

Fork rake. Distance of the front wheel dropout from an (imaginary) axis going from the middle of the head tube.
Fork rake.
Distance of the front wheel dropout from an (imaginary) axis going from the middle of the head tube.
  • Increasing the rake makes steering faster
  • Decreasing it makes steering slower

Rake influences distance of the front wheel from the pedals and wheelbase length. It also affects steering but by increasing/decreasing trail, not directly, which will be explained.

Touring bikes often have a lot of rake, in order to increase wheelbase as well as to avoid toe overlap.

Typical values are 45 mm for touring and 40 mm for road bicycles.

Still, since rake is in conjunction with trail (and head tube angle), touring bikes end up having slower steering compared to road bikes, in spite of having more rake. Explanation follows now:

2.3. Fork trail

Fork trail is a result of head tube angle, fork rake and front wheel diameter. Front wheel diameter increase, also increases trail, while other two factors have reverse influence, e.g. bigger rake results in smaller trail.

Trail length lets us know how quickly a bike steers. The other factors mostly just influence steering through affecting trail – indirectly.

  • Less trail results in faster steering
  • More trail results in slower steering
Both bicycles in the picture have the same head tube angles and wheel diameters. The fork on the left has less bend, that is smaller rake. This results in left bike having more trail.
Both bicycles in the picture have the same head tube angles and wheel diameters.
The fork on the left has less bend, that is smaller rake.
This results in left bike having more trail (and slower steering).

Touring bicycles have more trail, in order to slow the steering and make heavily loaded bicycle more stable on a fast descend for example. The problem with having more (or too much) trail is difficulty to keep a straight line when riding at slow speed. Also, it can cause bike to steer from side to side when pedalling while standing. Heavy front load on touring bikes often compensates for this.

Typical trail values are around 65 mm for touring and 55 mm for road bikes.

2.4. Steering recapitulation

Slacker head angle and more trail results in slower steering. Practical for carrying loads on a bike, while not very convenient for a light bicycle.

Less trail with steeper head angle results in quicker steering. Good for light racing bicycle, inconvenient for carrying load.

Somewhere in between those extremes falls the steerer geometry of most cyclocross and hybrid bicycles.

 

3. Chainstay length

Chainstay length
Chainstay length

Chainstay length influences 3 things:

  1. Steering – longer chainstay increases wheelbase length and makes a bicycle more stable, but also makes steering (and maneuvering) slower.
  2. Comfort – longer chainstay also increases comfort. Short chainstay gives an effect of sitting directly on top of the rear wheel, while a long one makes rider sit half way between two wheels, making the ride more comfortable.
  3. Luggage carrying capacity – with a longer chainstay, panniers mounted on the rear rack will be further from the pedals. Very important for big footed riders.

Typical values are 46 cm for touring and 41 cm for road bikes.

4. Wheelbase

Longer wheelbase makes bike more stable, more comfortable, but makes steering slower.

Touring bikes have slack head angle, long fork rake, and long chainstay which gives them long wheelbase. Road bikes have it all the opposite way.  🙂

Wheelbase
Wheelbase

The influence of wheelbase on steering is almost as big as the fork trail and angle.

Typical values are 1060 mm for touring bikes and 995 for road bikes

 

5. Bottom bracket drop (BB drop)

Bottom bracket drop (relative to the wheels axes)
Bottom bracket drop
(relative to the wheels axes)

Because saddle height is set relative to the pedals, lower bottom bracket results in the saddle being placed lower (for the same rider height), which results in a lower centre of gravity.

Lower centre of gravity is always good – it gives better stability and maneuverability. However, since bicycle is leaned into turns, this matters a lot less than for cars.

What does matter is ground clearance. Frames built for off road riding often have lower BB drop (hence a higher bottom bracket) to prevent it from catching bumps and rocks. Also, when pedalling in a turn, a bicycle is less likely to strike the ground with a pedal if the BB is higher.

For city riding, lower BB enables the rider to put a foot down wile seated and waiting at a traffic light.

All other things being equal (wheelbase, tube material and thickness), a smaller BB drop (therefore higher BB) makes shorter chainstay, shorter seat tube and down tube. This results in a lighter and stiffer frame. 

Values differ, depending on the frame purpose, but some middle, often used value is around 7 cm.

 

6. Seat tube angle

This depends mostly on frame size, since optimal pedalling position does not change that much between bikes.

Seat tube angle - Measured from horizontal line
Seat tube angle
– Measured from horizontal line

For frames “sized” 57 cm, usual angle is around 73 degrees.

 

7. Seat tube length

Often stated as frame “size”. One of the least important factors in frame geometry. Value is important only to shorter riders to know whether they can stand over the frame.

 

8. Head tube length

If a frame is meant for a more upright riding position, head tube is longer. Touring bikes often have around 20 cm, while road bikes go around 15 cm. Head tube length changes with frame size.

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