Bicycle and motorcycle geometry is the collection of key measurements (lengths and angles) that define a particular bike configuration. Primary among these arewheelbase, steering axis angle, fork offset, and trail. These parameters have a major influence on how abike handles.^[1]

Thewheelbase is thehorizontal 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 termwheelbase used for automobiles and trains.

Wheelbase has a major influence on thelongitudinal stability of a bike,^{[clarification needed]} along with the height of thecenter of mass of the combined bike and rider. Short bikes are much more suitable for performingwheelies andstoppies.

The steering axis angle is calledcaster angle when measured from vertical axis orhead angle when measured from horizontal axis. Thesteering axis is theaxis about which the steering mechanism (fork, handlebars, front wheel, etc.) pivots. The steering axis angle usually matches the angle of thehead tube.

Inbicycles, the steering axis angle is measured from the horizontal and called thehead angle; a 90° head angle would be vertical. For example, Lemond^[2] 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.

Due to front fork suspension, modernmountain bikes—as opposed toroad bikes—tend to have slacker head tube angles, generally around 70°, although they can be as low as 62° (depending on frame geometry setting).^[3]

At least one manufacturer, Cane Creek, offers an after-marketthreadless headset that enables changing the head angle.^[4]

Inmotorcycles, the steering axis angle is measured from the vertical and called thecaster angle,rake angle, or justrake;^[5] a 0° rake is therefore vertical. For example, Moto Guzzi^[6] offers:

• a 2007 Breva V 1100 with a rake of 25°30′ (25.5 degrees)
• a 2007 Nevada Classic 750 with a rake of 27.5°

Thefork offset is theperpendicular distance from the steering axis to the center of the front wheel.

Inbicycles, fork offset is also calledfork rake. Road racingbicycle forks have an offset of 40–50 mm (1.6–2.0 in).^[7]

The offset may be implemented by curving the forks, adding a perpendicular tab at their lower ends, offsetting the fork blade sockets of the fork crown ahead of the steerer, or by mounting the forks into the crown at an angle to the steer tube. The development of forks with curves is attributed toGeorge Singer.^[8]

Inmotorcycles with telescopicfork tubes, fork offset can be implemented by either anoffset in thetriple tree, adding atriple tree rake^[9] (usually measured in degrees from 0) to the fork tubes as they mount into the triple tree, or a combination of the two.^[10] Other, less-commonmotorcycle forks, such as trailing link or leading link forks, can implement offset by the length of link arms.

The length of a fork is measured parallel to the steer tube from the lower fork crown bearing to the axle center.^[11]

Trail is thehorizontal distance from where the front wheel touches the ground to where thesteering axis intersects the ground. The measurement is consideredpositive if the front wheel ground contact point is behind (towards the rear of the bike) the steering axis intersection with the ground. Most bikes have positive trail, though a few, such as thetwo-mass-skate bicycle and the Python Lowracer, have negative trail.^[12]

Trail is often cited as an important determinant of bicycle handling characteristics,^[13][14] and is sometimes listed in bicycle manufacturers' geometry data. Wilson and Papodopoulos^{[citation needed]} argue thatmechanical trail may be a more important and informative variable,^[15] although both expressions describe very nearly the same thing.

Trail is a function of steering axis angle, fork offset, and wheel size. Their relationship can be described by this formula:^[16]

${\displaystyle {\text{Trail}}_{\text{bicycle}}=\frac{R_w\cos (A_h)-O_f}{\sin (A_h)}}$ and${\displaystyle {\text{Trail}}_{\text{motorcycle}}=\frac{R_w\sin (A_r)-O_f}{\cos (A_r)}}$

where${\displaystyle R_w}$ is wheel radius,${\displaystyle A_h}$ is the bicycle head angle measured from the horizontal,${\displaystyle A_r}$ is the motorcycle rake angle measured from the vertical, and${\displaystyle O_f}$ is the fork offset. Trail can be increased by increasing the wheel size, decreasing or slackening the head angle, or decreasing thefork 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 torake 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.^[17] Trail can also vary as the suspension activates, in response to braking for example. As telescopic forks compress due toload transfer during braking, the trail and the wheelbase both decrease.^{[18][self-published source?]} At least one motorcycle, theMotoCzysz C1, has a fork with adjustable trail, from 89 to 101 mm [3.50 to 3.98 inches].^[19]

Mechanical trail is theperpendicular distance between thesteering axis and the point of contact between the front wheel and the ground.^[20] It may also be referred to asnormal trail.^[21] In each case, its value is equal to the numerator in the expression for trail.

${\displaystyle {\text{MechanicalTrail}}_{\text{bicycle}}=R_w\cos (A_h)-O_f}$, and

${\displaystyle {\text{MechanicalTrail}}_{\text{motorcycle}}=R_w\sin (A_r)-O_f}$

Although the scientific understanding of bicycle steering remains incomplete,^[15] we do have a good overall understanding of the interdependent dynamic complexities.^[22] Mechanical trail is certainly one of the most important variables in determining the handling characteristics of a bicycle. A trail of zero may give some advantages:^[23]

• the influence of the position of the center of pressure of wind forces coming from the side is eliminated
• the wheel flop effect (see below) is eliminated

Skilled and alert riders may have more path control if the mechanical trail is lower while a higher trail is known to make a bicycle easier to ride "no hands" and thus more subjectively stable.

Wheel flop refers to steering behavior in which a bicycle or motorcycle tends to turn more than expected due to the front wheel "flopping" over when the handlebars are rotated. Wheel flop is caused by the lowering of the front end of a bicycle or motorcycle as the handlebars are rotated away from the "straight ahead" position. This lowering phenomenon occurs according to the following equation:

${\displaystyle f=b\sin \theta \cos \theta }$

where:

${\displaystyle f}$ = "wheel flop factor," the distance that the center of the front wheel axle is lowered when the handlebars are rotated from the straight ahead position to a position 90 degrees away from straight ahead

${\displaystyle b}$ = trail

${\displaystyle \theta }$ = head angle

Because wheel flop involves the lowering of the front end of a bicycle or motorcycle, the force due to gravity will tend to cause handlebar rotation to continue with increasing rotational velocity and without additional rider input on the handlebars. Once the handlebars are turned, the rider needs to apply torque to the handlebars to bring them back to the straight ahead position and bring the front end of the bicycle or motorcycle back up to the original height.^[24] The rotational inertia of the front wheel will lessen the severity of the wheel flop effect because it results in opposing torque being required to initiate or accelerate changing the direction of the front wheel.

According to the equation listed above, increasing the trail and/or decreasing the head angle will increase the wheel flop factor on a bicycle or motorcycle, which will increase the torque required to bring the handlebars back to the straight ahead position and increase the vehicle's tendency to veer suddenly off the line of a curve. Also, increasing the weight borne by the front wheel of the vehicle, either by increasing the mass of the vehicle, rider and cargo or by changing the weight ratio to shift the center of mass forward, will increase the severity of the wheel flop effect. Increasing the rotational inertia of the front wheel by increasing the speed of the vehicle and the rotational speed of the wheel will tend to counter the wheel flop effect.

A certain amount of wheel flop is generally considered to be desirable.Bicycle Quarterly magazine states, "A bike with too little wheel flop will be sluggish in its reactions to handlebar inputs. A bike with too much wheel flop will tend to veer off its line at low and moderate speeds."^[25]

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

Increasing the length of the fork, for example by switching from rigid to suspension, raises the front of a bicycle and thus decreases its head angle.^[11] Lengthening the fork would have the opposite effect on the rake of a motorcycle, since rake is measured in the opposite direction.

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

Increasing the offset of a fork reduces the trail, and if performed on an existing fork by bending without lengthening the blades, shortens the fork.^[26]

The state ofNorth Dakota (USA) has minimum and maximum requirements on rake and trail for "manufacture, sale, and safe operation of a motorcycle uponpublic highways."^[27]

"4. All motorcycles, except three-wheel motorcycles, must meet the followingspecifications in relationship to front wheel geometry:

MAXIMUM: Rake: 45 degrees; Trail: 14 inches [35.56 centimeters] positive

MINIMUM: Rake: 20 degrees; Trail: 2 inches [5.08 centimeters] positive

Manufacturer's specifications must include the specific rake and trail for eachmotorcycle or class of motorcycles and the terms "rake" and "trail" must be definedby the director by rules adopted pursuant to chapter 28–32."

For other aspects of geometry, such as ergonomics or intended use, see thebicycle frame article. Formotorcycles the other main geometric parameters are seat height and relative foot peg and handlebar placement.

• Bicycle and motorcycle dynamics
• Bicycle fork
• Bicycle frame
• Hub-center steering
• Motorcycle fork

• Motorcycle Steering Geometry
• Bicycle Fork Lengths
• Illustration of a dimension that is sometimes referred to as caster angle on bicycles.
• Motorcycle rake, trail and offset explainedArchived 19 March 2011 at theWayback Machine

• Motorcycle dynamics

• CS1 errors: generic name
• Articles with short description
• Short description matches Wikidata
• Use dmy dates from September 2020
• Wikipedia articles needing clarification from September 2020
• All articles with unsourced statements
• Articles with unsourced statements from December 2020
• All articles with self-published sources
• Articles with self-published sources from February 2020
• Webarchive template wayback links