What effect does frame geometry have?

0scar

What effect does frame geometry have on riding? I've heard that a flatter top tube offers a more aggressive riding position than a sloping one (or is that the other way round?) and that clearance is important. But what does any of this mean and what do the different angles do?

And isn't it more about the relative heights of the seat and bars?

Thanks!

cyberknight

pasted from wki .........

Frame geometry

The length of the tubes, and the angles at which they are attached define a frame geometry. In comparing different frame geometries, designers often compare the seat tube angle, head tube angle, (virtual) top tube length, and seat tube length. To complete the specification of a bicycle for use, the rider adjusts the relative positions of the saddle, pedals and handlebars:

* saddle height, the distance from the center of the bottom bracket to the point of reference on top of the middle of the saddle.
* reach, the distance from the saddle to the handlebar.
* drop, the vertical distance between the reference at the top of the saddle to the handlebar.
* setback, the horizontal distance between the front of the saddle and the center of the bottom bracket.
* standover height, the height of the top tube above the ground.[4]
* toe overlap, the amount that the feet can interfere with steering the front wheel.[5]

The geometry of the frame depends on the intended use. For instance, a road bicycle will place the handlebars in a lower and further position relative to the saddle giving a more crouched riding position; whereas a utility bicycle emphasizes comfort and has higher handlebars resulting in an upright riding position.

Frame geometry also affects handling characteristics. For more information, see the Bicycle and motorcycle geometry and the Bicycle and motorcycle dynamics articles.

[edit] Frame size
Commonly used measurements

Frame size was traditionally measured from the center of the bottom bracket to the top of the seat tube. Typical "medium" sizes are 54 or 56 cm (approximately 21.2 or 22 inches) for a European men's racing bicycle or 46 cm (about 18.5 inches) for a men's mountain bicycle. The wider range of frame geometries that are now made have given rise to different ways of measuring frame size; see the discussion by Sheldon Brown. Touring frames tend to be longer, while racing frames are more compact.

[edit] Road and triathlon bicycles

A road racing bicycle is designed for efficient power transfer at minimum weight and drag. Broadly speaking, the road bicycle geometry is categorized as either a traditional geometry with a horizontal top tube, or a compact geometry with a sloping top tube.

Traditional geometry road frames are often associated with more comfort and greater stability, and tend to have a longer wheelbase which contribute to these two aspects. Compact geometry road frames have a lower center of gravity and tend to have a shorter wheelbase and smaller rear triangle, which give the bike quicker handling. Compact geometry also allows the top of the head tube to be above the top of the seat tube, decreasing standover height, and thus increasing standover clearance and lowering the center of gravity. Opinion is divided on the riding merits of the compact frame, but several manufacturers claim that a reduced range of sizes can fit most riders, and that it is easier to build a frame without a perfectly level top tube.

Road bicycles for racing tend to have a steeper seat tube angle, measured from the horizontal plane. This positions the rider aerodynamically and arguably in a stronger stroking position. The trade-off is comfort. Touring and comfort bicycles tend to have more slack seat tube angle traditionally. This positions the rider more on the sit bones and takes weight off of the wrists, arms, neck and, for men, improves circulation to the urinary and reproductive areas. With slacker angle, designers lengthen the chainstay so that the center of gravity (that would otherwise be farther to the back over the wheel) is more ideally repositioned over the middle of the bike frame. The longer wheelbase contributes to effective shock absorption. In modern mass manufactured touring and comfort bikes, the seat tube angle is negligibly slacker, perhaps because of the need to otherwise reset welding jigs in automated processes and thus increase manufacturing costs, and thus do not provide the comfort of traditionally made or custom made frames which do have noticeably slacker seat tube angles.

Road racing bicycles that are used in UCI sanctioned races are governed by UCI regulations, which state among other things that the frame must consist of two triangles. Hence the designs that lack a seat tube or top tube are not allowed.
A cyclist riding a time-trial bicycle with aerodynamic wheels and aero bars

Triathlon or time trial specific frames rotate the rider forward around the axis of the bottom bracket of the bicycle as compared to the standard road bicycle frame. The reason for this is to put the rider in an even lower, more aerodynamic position. While handling and stability is reduced, these bicycles are designed to be ridden in environments with less group riding aspects. These frames tend to have steep seat tube angles and low head tubes, and shorter wheelbase for the correct reach from the saddle to the handlebar. Additionally, since they are not goverened by the UCI, some triathlon bicycles, such as the Zipp 2001, Cheetah and Softride have non traditional frame layouts, which can produce better aerodynamic

cyberknight

part 2 .............
Wheelbase

Wheelbase is the horizontal distance between the centers (or the ground contact points) of the front and rear wheels. Wheelbase is a function of rear frame length, steering axis angle, and fork offset. It is similar to the term wheelbase used for automobiles and trains.

Wheelbase has a major influence on the longitudinal stability of a bike, along with the height of the center of mass of the combined bike and rider. Short bikes are much more likely to perform wheelies and stoppies.

[edit] Steering axis angle
Telescopic forks on a BMW motorcycle reveal the head angle or rake
Example of a chopper with an unusually large rake

The steering axis angle, also called caster angle, is the angle that the steering axis makes with the horizontal or vertical, depending on convention. The steering axis is the axis about which the steering mechanism (fork, handlebars, front wheel, etc.) pivots. The steering axis angle usually matches the angle of the head tube.

In bicycles, the steering axis angle is called the head angle and is measured clock-wise from the horizontal when viewed from the right side. A 90° head angle would be vertical. For example, Lemond[1] offers:

* a 2007 Filmore, designed for the track, with a head angle that varies from 72.5° to 74° depending on frame size
* a 2006 Tete de Course, designed for road racing, with a head angle that varies from 71.25° to 74°, depending on frame size.

In motorcycles, the steering axis angle is called the rake and is measured counter-clock-wise from the vertical when viewed from the right side. A 0° rake would be vertical. For example, Moto Guzzi[2] offers:

* a 2007 Breva V 1100 with a rake of 25°30’ (25 degrees and 30 minutes)
* a 2007 Nevada Classic 750 with a rake of 27.5° (27.5 degrees)

[edit] Fork offset

The fork offset is the perpendicular distance from the steering axis to the center of the front wheel.

In bicycles, fork offset is also called fork rake. Virtually all road racing bicycle forks have an offset of 43-45mm due to the almost-standard frame geometry and 700c wheels, so racing forks are widely interchangeable.

The terms "rake" and "offset" became confused when bicyclists misunderstood[citation needed] the reason for the curl at the fork end, shown in the adjoining diagram, believing its purpose was shock absorption.[citation needed] With the bare fork in hand, rake is undefined, although one might believe the term refers to similarity with a leaf rake whose tines curl in a similar manner. The term "rakish angle" means steep, rather than that the fork has a curl at its end. Today, some fork blades are straight, having their offset introduced by an angled fork crown.

Required rake angle arose from early times when lightweight bicycles suffered fork failures from road shock.[citation needed] Most fatigue failures of forks result in a fork blade breaking at the rear edge of the fork crown from repeated vertical road shocks. Before most roads were paved, fork rake had a lower angle so the fork would be loaded in axially on rougher surfaces. As most roads became paved, bicycles forks were made steeper, which also gave lighter steering.

In motorcycles with telescopic fork tubes, fork offset can be implemented by either an offset in the triple tree, adding a rake angle (usually measured in degrees from 0) to the fork tubes as they mount into the triple tree, or a combination of the two.[3] Other, less-common motorcycle forks, such as trailing link or leading link forks, can implement offset by the length of link arms.

[edit] Fork length

The length of a fork is measured parallel to the steer tube from the lower fork crown bearing to the axle center.[4]

[edit] Trail

Trail, or caster, is the horizontal distance from where the steering axis intersects the ground to where the front wheel touches the ground. The measurement is considered positive if the front wheel ground contact point is behind (towards the rear of the bike) the steering axis intersection with the ground.

Trail is often cited as an important determinant of bicycle handling characteristics [1], and is sometimes listed in bicycle manufacturers' geometry data, although Wilson and Papodopoulos argue that mechanical trail may be a more important and informative variable.

Trail is a function of head angle, fork offset or rake, and wheel size. Their relationship can be described by this formula:[5]

\it{Trail} = \frac{R_w \cos(A_h) - O_f}{\sin(A_h)}

where Rw wheel radius, Ah is the head angle measured clock-wise from the horizontal and Of is the fork offset or rake. Trail can be increased by increasing the wheel size, decreasing or slackening the head angle, or decreasing the fork rake or offset. Trail decreases as head angle increases (becomes steeper), as fork offset increases, or as wheel diameter decreases.

Motorcyclists tend to speak of trail in relation to rake angle. The larger the rake angle the larger the trail. Note that, on a bicycle, as rake angle increases, head angle decreases.

Trail can vary as the bike leans or steers. In the case of traditional geometry, trail decreases (and wheelbase increases if measuring distance between ground contact points and not hubs) as the bike leans and steers in the direction of the lean.[6] Trail can also vary as the suspension activates, in response to braking for example. As telescopic forks compress due to load transfer during braking, the trail and the wheelbase both decrease.[7] At least one motorcycle, the MotoCzysz C1, has a fork with adjustable trail, from 89 mm to 101 mm.[8]

[edit] Mechanical trail

Mechanical trail is the perpendicular distance between the steering axis and the point of contact between the front wheel and the ground. It may also be referred to as normal trail.[6]

Although the scientific understanding of bicycle steering remains incomplete,[9] mechanical trail is certainly one of the most important variables in determining the handling characteristics of a bicycle. A higher mechanical trail is known to make a bicycle easier to ride "no hands" and thus more subjectively stable, but skilled and alert riders may have more path control if the mechanical trail is lower.[10]

[edit] Modifications

Forks may be modified or replaced, thereby altering the geometry of the bike.

[edit] Changing fork length

Increasing the length of the fork, for example by switch from rigid to suspension, raises the front of the bike and decreases the head angle. [4]

A rule of thumb is a 10mm change in fork length gives a half degree change in the head angle.

[edit] Changing fork offset

Increasing the offset of a fork reduces the trail, and if performed on an existing fork without lengthening the blades, shortens the fork. [

cyberknight

and if it makes any sense let me know :shock:

cjcp

0scar wrote:

What effect does frame geometry have on riding? I've heard that a flatter top tube offers a more aggressive riding position than a sloping one (or is that the other way round?) and that clearance is important. But what does any of this mean and what do the different angles do?

And isn't it more about the relative heights of the seat and bars?


Thanks!

A more relaxed geometry is intended to offer more comfort over longer distances than more racing specific or crit bikes. For example, a longer head tube, so you have a more upright position. But, you can still tinker with the effective geometry by fitting an angled stem or extra spacers (if the latter is an option) or fitting a longer stem so you're more stretched.

To sort of contradict the above, my commuter has a longer top tube and stem than my racing bike, which I've kept more set up for sportives. The stretched position on the commuter means that it's more aggressive but can't do as many miles before the lower back starts to hurt.

cyberknight

i know this may not be relevant but found this bike fit spreadsheet whilst poking about



http://www.incidentalcyclist.com/wordpress/wp-content/uploads/2008/04/biomechanicalcyclingv6english.xls