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'''Trail''', or caster, is the ''horizontal'' distance from where the [[Bicycle and motorcycle geometry#Steering axis angle|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. Most bikes have positive trail, though a few, such as the [[Python Lowracer]] have negative trail.
#Redirect [[Trail motorcycle]]
 
[[Image:TrailDIAG2.jpg|thumb|200px|right|The relationship between head angle, rake and trail in a bicycle]]
 
Trail is often cited as an important determinant of bicycle handling characteristics [http://www.phred.org/~josh/bike/trail.html], and is sometimes listed in bicycle manufacturers' geometry data, although [http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=10063 Wilson and Papodopoulos] argue that [[Bicycle and motorcycle geometry#Mechanical trail|mechanical trail]] may be a more important and informative variable.
 
Trail is a function of head angle, [[Bicycle fork|fork]] offset or rake, and wheel size. Their relationship can be described by this formula:<ref>{{cite web
| title = Steering Geometry: What is Trail?
| last = Putnam
| first = Josh
| year = 2006
| url = http://www.phred.org/~josh/bike/trail.html
| accessdate = 2006-08-08
}}</ref>
:<math>\it{Trail} = \frac{R_w \cos(A_h) - O_f}{\sin(A_h)}</math>
where <math>R_w</math> wheel radius, <math>A_h</math> is the head angle measured clock-wise from the horizontal and <math>O_f</math> is the fork offset or rake. Trail can be increased by increasing the wheel size, decreasing or slackening the head angle, or decreasing the [[Bicycle fork#Offset|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)|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.<ref name="Cossalter">{{cite web
| title = THE TRAIL
| last = Cossalter
| first = Vittore
| year = 2006
| url = http://www.dinamoto.it/DINAMOTO/on-line%20papers/Avancorsa/trail.htm
| accessdate = 2006-12-14
}}</ref> 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.<ref name="CossalterBook">{{cite book
| title = Motorcycle Dynamics
| edition = Second Edition
| last = Cossalter
| first = Vittore 
| year = 2006
| publisher = Lulu.com
| isbn = 978-1-4303-0861-4
| page = 234}}</ref> At least one motorcycle, the [[MotoCzysz|MotoCzysz C1]], has a fork with adjustable trail, from 89 mm to 101 mm.<ref name="MotoCzysz">{{cite web
| title = MotoCzysz
| year = 2006
| url = http://www.motoczysz.com/main.php?area=bike
| accessdate = 2006-12-14
}}</ref>
 
==References==
{{reflist}}
[[Category:Definitions]]

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