Thursday, 29 September 2011

Rake & Trail

So I am thinking I like my design but need to reduce the trial for a safe amount of cornering, long trail make a very stable bike in a straight line but also very hard to get off that straight course.
Typical trail 4-6" , stock XS650 is 4.47"
Stock XS Specs can't remember which model I go this from.

  • Tire Size 26.1"
  • Rake 27 Deg
  • Offset 1.938 "
  • Tree Rake 0.0 Deg
  • Calculated Trail 4.47"

My Options To Reduce the Trail,  

  • Increase OFFSET DISTANCE - Alternate trees with greater offset?
  • Increase Angle Between Neck & Fork
      •  Offset Neck Cups - Possibly make these but think they add trail same as increasing neck angle, opposite of what I want.
      • Raked Tripple Trees - Expensive option to buy & only for HD front ends.

How Does it all work??

Often mentioned in the main stats when looking at a new motorbike, but what do motorcycle rake and trail actually mean and why would you need to know this? Firstly, these terms refer to steering geometry and so rake, trail and offset effect handling and are directly linked to each other, altering one will effect the other. So if you want to alter the handling, cornering ability or straight line stability, rake, trail and offset are some of the first things that you should look at.

Motorcycle rake

Rake can be described as an angle at which the headstock of the motorcycle is inclined when compared against a vertical line drawn perpendicular to the ground. The rake angle effects steering ability, the smaller the rake angle then the easier the bike is to corner but will be less stable in a straight line.

Motorcycle trail

The distance on the ground between a straight line drawn through the center of the front wheel spindle and a line drawn through the center of the headstock axis. The greater the trail distance, then the greated the straight line stability but the harder it is to make the bike corner.

Motorcycle offset

The distance between a line drawn through the centre of the steering stem/ headstock axis and the centre line of the front fork tubes. Typically the offset inversely affects trail, if offset increases then the trail will decrease.
The diagram below illustrates each of the measurements and how they are calculated:
Motorcycle rake, trail and offset
Basic measurements

A bit more in depth look, when neck & forks are nor parallel.  

Your front suspension geometry is defined by the following six variables which are defined as

  • Offset-- Centerline of top of steering neck to centerline of top of fork tubes.
  • Rake-- The angle in degrees of the steering neck from the vertical.
  • Fork Length-- The distance between the top of the fork tubes to the centerline of the axle.
  • Diameter of the front tire.
  • Trail: Distance defined by vertical line from axle to ground and intersect of centerline of steering neck and ground.
  • Raked Triple Trees-- In order to bring trail figures back into line, triple trees with raked steering stems can be used. Expressed in degrees. Usually adjustable in 3, 5, and 7 degrees of rake.
  • Tire Diameter:
Tire Diameter Formula: If you do not know a certain tire diameter use the following formula:

  • Diam Inches = Rim Diam Inches + [ (2 x Aspect Ratio x Section Width) / 25.4]
  • Example: 200/55R18 Tire:
  • 26.7" = 18 + [(2 x 0.55 x 200)/25.4]

  • Front Tire inchesDiameter of front tire. 21" front typically 27.60". 19" front typically 26.10". 16" front typically 25.30". Hayabusa front 120/70x17 23.61".
    Rake degreesSteering Neck Rake: Softail 31 to33 deg. FXRS models 29 deg. FXLR 31 deg. Evo Dyna FXD/DX/DXT 28 deg. Evo Dyna FXDL/DWG 32 deg. Evo FLH 26 deg. Sportster 29.6 deg. Choppers 36 deg to 45 deg. FLH series 26 deg. V-Rod 34 deg. Early Hayabusa 24.1 deg.
    Triple Clamp Offset inchesDistance from center of top of steering neck to centerline of top of fork tubes. Typically in the range of 2.4". V-Rod 2.0". FLH series have a negative offset ( Enter -2.455")
    Fork Length inchesLength from to top of the fork tubes to the centerline of the axle. Harley forks are in the 30", 32", 34" or longer range. Length will not change trail if the triple trees are not raked. FLH Series 27.75". V-Rod 30". Hayabusa forks are 28.5".
    Triple Tree Rake degreesFor standard triple trees enter 0 degrees: The FXDWG uses 3 degree trees; V-Rod uses 4 degree trees; FLH series use 5 degree trees. For chassis that have been raked to 40 degrees or more, and too much trail is present, Raked Triple Trees are necessary. Enter the degrees of rake, typically 3, 5, or 7 degrees.

    For the functioning calculator go here:

    Trail & Rake Calculator

    Note: Trail cannot be a negative number as this will adversely affect handling unless you are building a shopping cart. 
    Typical trail figures are between 4 and 6 inches. 
    Various Harley models have the following trail figures: 

    • FXR 4.22". Softail 5.0" to 5.8". 
    • Dyna Models 4.1" to 5.1". 
    • Sportster 4.6". 
    • FLH models 6.2".
    • V-Rod 3.87". 
    • Early Hayabusa 3.8".

    The relationship between rake, trail and offset

    • If you increase the rake, the trail increases
    • If you increase the trail, the rake increases
    • If you increase the offset for both yokes then the trail decreases
    • If you increase the offset for only the top yoke/ triple tree then the trail will increase
    • If you increase the offset for the bottom yoke/ triple tree independently then the trail will decrease

    How to alter a motorcycles rake, trail or offset

    So why would you want to alter these values? Well if you plan to cruise mainly straight roads then you don’t want your arms getting tired by constantly correcting and holding the bike in a straight line, so you would want to increase the trail and decrease the rake. Which is why the American choppers have such long front forks to give great straight line stability. If you wanted handling around corners you would do the opposite to allow you to throw the bike around the corners and of course there is a fine balance to get the best of both. Also such inventions like steering dampers allow you to have a very low rake but keep the bike stable by stiffening the steering movement which you can buy as an after market part. So, love the look of the bike and the performance of the engine but hate the handling? Alter these values.
    You don’t need to be a mechanic or an expert to be able to alter the steering geometry and it can be done in a variety of very easy ways, just remember that you’re the one who has to ride it, so you take your life in your own hands. So for instance with my Yamaha XS750 the front forks were originally quite long, which is pretty useless for a cafe racer, so new shorter forks were put in to lower the rake and to improve the handling around corners.
    Motorbike rake and trail are also not only affected by the frame construction but by a myriad of other factors (which you can alter) such as:

    • Rear suspension – lowering it will increase rake and trail
    • Front forks – shorter forks will decrease rake and trail, also you can alter their position in the yokes to help shorten their distance
    • Aftermarket yokes/ triple trees – alter the offset to then alter the trail without affecting rake
    • Wheel diameter – smaller front wheel will decrease rake and trail, smaller rear wheel will increase rake and trail
    • Tyre thickness – same as the wheel diameter

    More importantly when riding the bike, braking and suspension travel will also dynamically alter the rake and trail, for instance, as you brake hard on the front, suspension will compress under dive thus decreasing trail and rake potentially inducing wobble in the bike. Bare that in mind if you make any alterations as you could end up with negative figures under extreme braking conditions. Additionally heavy luggage or pillion on the rear may also effect your rake and trail measurements slightly – which is why you should always alter your rear suspension preload.
    Also remember that while any alterations may seem small and un-noticeable at slow speed at higher speeds their effect will be magnified, such as reduced straight line stability!

    Data collated from Bikearama & RB Racing, amongst  others.

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